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Applying beneficial microbes as transplanting dipping and post-transplanting foliar spray led to improved rice productivity

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  • Rice is a popular food in Africa, but current yield achieved is far lower than the yield potential due to abiotic/biotic stresses. We recently demonstrated that commercially formulated products of Bacillus subtilis and Trichoderma asperellum strains increased rice yield when applied as transplant dipping or post-transplanting foliar sprays in Tanzania (Africa). Further experiments were conducted to investigate: (1) synergies in alternate use of B. subtilis and T. asperellum products as dipping or foliar spray in Tanzania, and (2) effects of coating seeds with Serratia nematodiphila in Kenya (Africa). In Tanzania, using formulated B. subtilis and T. asperellum products led to > 100% increase in yield. Furthermore, alternative use of B. subtilis and T. asperellum as transplant dipping or post-transplanting foliar spray led to further yield increase (ca. 32%) over the use of single-organism products at both times. Microbial treatments led to significant reductions in rice blast. In comparison with the fungicide treatment, increased yield with using microbial products appeared to have resulted mostly from improved plant development rather than reduced rice blast per se. At the Kenyan site where current yield is relatively high, the overall yield increase associated with microbial products was limited although statistically significant. Coating seeds with one specific S. nematodiphila strain led to grain yield comparable to applying microbial products as transplant dipping and post-transplanting foliar spray. The present study suggests that the formulated microbial products can significantly improve rice productivity in subsistence farming and should be applied in alternation over time to exploit their synergies.
  • One of the traditional techniques for increasing value and reducing agricultural produce spoilage is drying. Where more expensive alternative storage methods are used, this is especially crucial[1]. Through the addition of one or more energy sources, moisture from a product is removed throughout the drying process[2,3]. The physicochemical characteristics of the fruit are changed by drying, which can improve the flavor and texture of specific foods like raisins and dates[2]. It lowers the product's water activity (aw), and when the aw value drops to less than 0.6, it inhibits the growth and spread of spoiling bacteria[4]. Drying also reduces product weight, which reduces packing, storage, and shipping costs and ensures off-seasonal production[5,6]. The demand for dried fruit is rising globally as people become more health conscious[7].

    Worldwide pomegranate production is steadily rising, although large post-harvest losses are also common, according to reports[8]. When fruit is unsuitable for standard processing methods due to fruit cracking or sunburn, drying is a great way to reduce post-harvest losses because it extends shelf life and can be utilized to reduce food waste[9]. Numerous products, such as pharmaceuticals, snacks, cereals, quick drinks, and other confectionary items, employ dried pomegranate arils[10]. Dried pomegranate arils, also known as anardana, are utilized both medicinally and culinarily in several regions of the world, including India[11]. Dried arils can therefore be quite useful as value-added items that generate revenue. According to research conducted in the Indian Ramban area, anardana trade accounts for at least 41% of all annual household income[11].

    There are many different drying techniques. The most popular are freeze-drying, hot air drying, vacuum drying, and solar drying. Each technique has pros and cons in terms of the final product's quality and how efficiently it uses energy. Pre-drying procedures are frequently used in conjunction with drying. Pretreatment enhances the drying rate, product quality, and energy efficiency of the drying process. Enzymes that cause enzymatic browning, which lowers the product quality, are rendered inactive by pretreatments[1214]. Sensory qualities like color, texture, taste, scent, microbiological activity, and general acceptability are among the criteria that determine the quality of dried pomegranate arils[15]. These elements are crucial because they can have a big impact on customer preferences and, if not taken into full account, can lead to financial losses[16,17].

    Several studies[12,18,19] have looked at the impact of drying and pretreatment techniques on the general quality of the completed product. However, the combined impact of pretreatment and drying on quality was not sufficiently investigated. Reviews on pomegranate fruit often discuss the fruit's chemistry, nutritive value, and pharmacology. Therefore, by evaluating, highlighting, and reflecting on recent studies on pomegranate aril drying, this review seeks to close these gaps. This review paper compared and contrasted several pretreatment and drying setups with an emphasis on product quality.

    Heat, mass, and momentum exchanges all occur simultaneously during the drying of fruit materials in a sophisticated cellular architecture of biological tissue[20,21]. The characteristics of the material that affect the drying process are intricately dependent on size, shape, porosity, moisture content, and time[22]. For instance, the initial moisture level and the bioactive chemicals in pomegranate arils can vary depending on the cultivar and fruit ripeness. The understanding, engineering, and management of the drying process are further complicated by the intrinsic diversity of biological materials[23,24]. The mass, heat, and momentum transfer events that happen during a typical drying process are shown in Fig. 1[25]. Conduction and convection are the most common heat transfer methods, but radiation is typically only employed for high-end items due to its expensive cost[1,26]. Diffusion, capillary action, and bulk flow are only a few of the processes that might transfer mass. These mass transfer mechanisms must adapt to the ongoing physical changes in the material that take place as it dries out[22,27].

    Figure 1.  A visual representation of the drying processes of solid materials.

    Although it is native to Iran, the pomegranate (Punica granatum L.) is widely distributed worldwide[28]. It belongs to the family Lythraceae and is a deciduous shrub. It is a versatile plant that can be found growing in both semi-arid and subtropical climates. Pomegranates, however, need hot summer temperatures to ripen[29]. Pomegranate fruit has a non-uniform round shape and a range of hues depending on the cultivar and fruit development stage, including yellow, green, pink, deep red, deep purple, and black[30,31]. An outsized calyx crowns the fruit. The leading producers worldwide are Peru, Australia, South Africa, and Chile in the southern hemisphere, and India, China, and Iran in the northern hemisphere[32].

    Pomegranate is a one-of-a-kind fruit with distinct edible seeds (arils) that must be extracted by hand (Fig. 2)[33]. An aril is made up of a seed and fleshy, moist tissue surrounding the seed. Color, sweetness, juice content, and hardness of arils vary depending on cultivar and fruit maturity[30,31]. While the arils can be eaten fresh, they can also be made into jams, jellies, coloring agents, juices, vitamins, and anardana (dried arils). They can also be mixed into yoghurts, biscuits, and cereals[34]. Fresh pomegranate arils can be kept at 7 °C for up to 14 d without losing much quality. Dried pomegranate aril has an extremely low perishability, with a potential shelf life of more than 14 weeks in ambient air[35].

    Figure 2.  A typical breakdown of the material balance throughout the drying process for pomegranate aril.

    As demonstrated in Table 1, pomegranate is high in ellagitannins, gallic acids, ferulic acids, anthocyanins, flavonoids, fiber, and minerals like vitamin C, calcium, and phosphorus. Pomegranates' phenolic components and high vitamin C content (Table 1) have attracted the interest of both researchers and consumers due to their health advantages[3638]. As a result of its strong antioxidant activity and nutritional benefits, pomegranate is considered a superfruit. Pomegranate can also be employed in cosmetics and pharmacology due to its phytochemical and antioxidant qualities[39]. Pomegranate fruit extract (PFE) has bioactive elements that have been found to inhibit or prevent various types and levels of cancer[40]. Punicalagic acid, ellagic acid, urolithin, and luteolin are the most important pomegranate components known to have anticarcinogenic characteristics[40,41]. Pomegranate fruit has also been linked to the prevention of diseases such as Alzheimer's, hypertension, and diabetes[42,43]. Pomegranate supplements may also help during or after exercise because they have the potential to speed up hard exercise recovery[44].

    Table 1.  The nutritional composition of 100 g of pomegranate arils.
    NutrientValueUnit
    Water77.9g
    Energy346kJ
    Protein1.67g
    Total lipid fat1.17g
    Ash0.53g
    Total dietary fiber4g
    Total sugar13.7g
    Calcium10mg
    Phosphorus36mg
    Magnesium12mg
    Iron0.3mg
    Potassium236mg
    Sodium3mg
    Zinc0.35mg
    Vitamin C10.2mg
    Vitamin K16.4µg
    Vitamin E0.6mg
    Vitamin B-60.075mg
    Total choline7.6mg
    Folate38µg
    Adapted from United States Department of Agriculture (Agricultural Research Service), FoodData Central[52].
     | Show Table
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    Pretreatment procedures are utilized to improve the drying process's effect on product quality characteristics such as color, flavor, appearance, and some physicochemical aspects[45,46]. Figure 3 depicts the most often used pretreatment procedures[46]. A product is immersed in a chemical solution prior to drying in chemical pretreatment. Physical pretreatments, on the other hand, necessitate a physical alteration of the product. When drying with heat, Maillard reactions might occur, resulting in an unpleasant color change[47]. As a result, pretreatment techniques are critical in many drying applications, including the drying of pomegranate arils[45,48,49]. There is evidence that pretreatment reduces the product's exposure to heat by reducing drying time[50,51].

    Figure 3.  A classification of the numerous pretreatment techniques utilized in the drying process for pomegranate aril.

    Soaking in acidic solutions involves immersing the product to be dried in a hot acidic solution for many minutes before drying. Pretreatment with an acidic solution keeps the product's color and speeds up the drying process. The acidic solution suppresses polyphenol oxidase enzyme activity, slowing the rate of enzymatic browning (Fig. 3). Furthermore, numerous investigations[5355] have documented the retention of nutrients such as vitamin C in acidic solution pretreatment samples. Some acid-sensitive components, on the other hand, can be destroyed or leached away. As a result, while employing this strategy, this effect must be considered. In a pomegranate aril drying research, arils prepared with 3% citric acid had the highest sensory acceptance[56]. Vardin & Yilmaz[57] conducted research on the combined effect of acid blanching and subsequent drying temperature. The authors blanched the arils in 0.1% citric solution for 2 min at 80 ± 2 °C followed by drying at 55, 65, or 75 °C and discovered that drying at 55 °C had the maximum antioxidant capacity[46]. Understanding the connection between soaking in acid (balancing in acid) and the subsequent drying temperature is required to carry out the operation correctly.

    This entails immersing products in an alkaline solution. Alkaline solutions act by dissolving the wax covering on the fruit's surface, removing resistance to moisture transfer and increasing drying rate[58]. As a result, this pretreatment accelerates the drying process. However, the usage of alkaline solutions raises food safety concerns because the residue might be harmful to one's health[46,59]. In addition, although acidic solutions retain vitamin C, alkaline solutions leach it out and destroy it[46]. Samples dipped in ethyl oleate for roughly one minute revealed a considerably reduced drying rate: a 26.9, 28.5, and 27.2% decrease in drying time at drying air temperatures of 55, 65, and 75 °C, respectively, than the control[60].

    The fruit is dipped into a hypertonic solution, such as salt or sugar solutions, in this approach (Fig. 3). Because of the osmotic pressure differential, the hypertonic solution causes water to diffuse out of the fruit tissue[61]. When compared to other drying processes, osmotically pretreated dried products have great rehydration capability and little losses in quality parameters such as color, appearance, and nutrients[62]. Madhushree et al.[63] discovered that osmotic pretreatment (in 50oBrix sugar syrup concentrations) dried arils had high color retention. This could be owing to the samples' reduced exposure to oxygen when immersed in the sucrose solution. A separate investigation on the osmotic pretreatment of pomegranate arils with a 65°Brix sucrose solution revealed a decrease in drying rate in hot air drying at 70 °C compared to untreated control samples[64]. The scientists attributed the longer drying time (lower drying rate) to the creation of a dense sucrose layer beneath the fruit's surface, which created an additional barrier to moisture transfer. To that aim, the osmotic solution concentration must be assessed because it can result in prolonged drying times.

    Gaseous or liquid sulphur solutions have been used as a food preservation method and as a pretreatment in food drying procedures. Typically, sulphur solutions are utilized for their browning properties, both enzymatic and non-enzymatic[65]. In addition, sulfur pretreatment is associated with high vitamin C and A retention after drying, as well as inhibition of spoilage-related microbial proliferation[66].

    More et al.[23] compared physical pretreatments to chemical pretreatments with 1% potassium metabisulphide on arils. It was discovered that arils prepared with potassium metabisulphide had superior nutritional quality as well as improved color, flavor, taste, and overall acceptability (Table 2)[23]. As a result, processing of pomegranate arils with sulfur solutions can result in high-quality dried goods. Despite their anti-browning, antibacterial, antifungal, and nutrient retention qualities, sulphites might be harmful to one's health if the recommended dosage or daily intake is exceeded[67,68]. Furthermore, while the sulfur solutions maintain vitamins A and C, they deplete vitamin B1[65].

    Table 2.  Key findings in pomegranate aril pretreatment and drying studies.
    Pretreatment methodPretreatmentDrying techniqueKey findingsReference
    BlanchingWater blanching at 90 and 100 °CHot air oven dryingBlanched samples had a shorter drying time.[24]
    Water blanching at 80 °CHot air oven dryerBlanched samples had higher phytonutrient retention than unblanched samples.[69]
    Blanching using 0.1% citric solution
    at 80 ± 2 °C
    Cabinet tray dryerDrying process was shorter for blanched samples and there was a higher rate of bioactive compounds.[57]
    Sulphuring1% potassium metabisulphideSolar drying
    Cabinet tray dryer
    Freeze dryer
    Fruit of cv. Ganesh 1% potassium metabisulphide was of the highest quality and the highest acceptance.[23]
    Blanching and SulphuringHot water blanching 85 °C and 0.2% potassium metabisulphateMechanical dryer
    Solar dryer
    Keeping quality of mechanically dried arils was higher than the solar-dried arils.[70]
    Steam blanching, potassium metabisulphide and 0.3% Sulphur fumigationCabinet tray dryerThe highest dried aril quality was obtained from the combination of steam blanching and 0.3% Sulphur fumigation.[71]
    Steam blanching, sulphuring at 0.3%Vacuum dryer
    Hot oven dryer
    Sun drying
    Poly-tent house drying
    Room drying
    Sun drying had the highest moisture content reduction and the highest overall acceptance.[72]
    Hot water blanching 85 °C,
    potassium metabisulphite varying
    from 0.25% to 1%
    Hot air oven dryerThe best treatment was blanching in hot water at 85 °C for 1 min and then dipping the arils in 0.25% potassium metabisulphite.[70]
    Steam blanching, sulphuringSun drying
    Cabinet dryer
    Blanching reduced drying time. Cabinet drying of blanched samples without sulphuring was considered optimum for anthocyanins.[73]
    Acidic solution2%, 3% and 4% citric acidCabinet tray dryer

    3% acidic treatment was found to be the most acceptable.[56]
    Microwave100 and 200 W.Hot air oven dryer200 W pretreatment resulted in minimum energy utilization and drying time.[74]
    100 and 200 WHot air oven dryer200 W had the highest drying rate.[75]
    Osmotic treatmentSugar syrup, freezing at minus 18 °COpen sun drying,
    Solar tunnel dryer,
    Cabinet tray dryer
    Osmotic treatment and cabinet tray dryer produced dried arils with better physicochemical and sensory qualities.[63]
    • 100% pomegranate juice
    • 50% pomegranate and 50% chokeberry juice
    • 50% pomegranate and 50% apple
    • 50% apple and 50% chokeberry
    • 75% apple and 25% chokeberry
    Freeze drying
    Convective pre-drying vacuum microwave finish drying
    Vacuum-drying and freeze drying
    Pomegranate and chokeberry concentrated juice improved the quality of the dried arils.[12]
    Sucrose solutionHot air oven dryingPretreatment increased the drying time of the samples.[64]
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    The dipping or soaking of a product in an alcoholic solution, usually ethanol, is known as alcoholic pretreatment. Ethanol dissolves the cell wall components, which increases moisture loss and thus the drying rate[76]. Several fruits, including melon (Cucumis melo L.) and apples (Malus domestica), have been pretreated in alcoholic solutions before drying[77,78]. However, no investigations on the pretreatment of pomegranate arils with alcohol were reported. This could be owing to the aril's waxy layer, which could impede permeability and hence the efficacy of the alcohol pretreatment[79].

    In this method, the fruit is dipped, submerged, or sprayed with a liquid solution that forms a thin coating layer on the product's surface and then dried (Fig. 3). According to studies, the use of edible coatings can help to retain the color, texture, and nutrient retention of dried items[80]. It is critical to note that the drying pace and dried product quality are affected by the coating thickness, drying method, and coating solution. Most of the research on edible coverings for pomegranate arils has focused on cold preservation and packing.

    Acoustic cavitation is utilized to rupture cell walls using ultrasound pretreatment[81]. According to one investigation on the effect of sonification on osmotic dehydration and subsequent air drying of pomegranate arils, ultrasonography caused a 2-fold and 2.7-fold increase in water loss[82]. The authors hypothesized that ultrasonic promoted cell wall disintegration and enhanced permeability. While using ultrasonic improved color quality, it also reduced anthocyanin content when compared to osmotically dehydrated samples[82].

    Blanching is a pretreatment technique that involves rapidly heating and then cooling a product that will be dried[45]. Blanching can be used to inactivate enzymes that potentially degrade product quality, such as polyphenol oxidase, peroxidase, and polygalacturonase[4]. Unwanted sensory traits in color, flavor, texture, and nutritional aspects are examples[4,83]. Blanching also improves cell membrane permeability, resulting in a faster drying rate[45]. Furthermore, blanching kills bacteria that might cause product spoiling[45,83]. As indicated in Fig. 3, there are several blanching processes, including hot water blanching and modern technologies such as microwave blanching and infrared blanching.

    Adetoro et al.[18] discovered that blanching pomegranate arils in hot water accelerated drying rates compared to unblanched samples. In a separate investigation, the authors found that blanching arils at 90 °C for 30 s and drying at 60 °C had higher total anthocyanin content and radical scavenging activity than blanching at 100 °C for 60 s and drying at 60 °C. In another work, Karaaslan et al.,[69] blanched arils in water at 80 °C for 2 min to investigate the effects of temperature and pretreatment on the arils. The authors discovered that while 75 °C had the fastest drying time, 55 °C had the maximum anthocyanin concentration, phenolic content, and antioxidant capacity. In other studies, pretreatment procedures are combined to produce high-quality dried fruit products.

    Singh et al.[73] conducted a study to evaluate the drying of pomegranate seeds under various drying conditions. They discovered that blanched samples dried faster and had higher acidity than sulphured samples. Furthermore, the anthocyanin concentration of blanched samples was higher than that of blanched and sulphured samples using the mechanical dryer. The authors hypothesized that the reduced anthocyanin concentration seen after sun drying was caused by the long drying hours in the sun. They also suggested that pomegranate arils be dried using blanching rather than sulphuring to achieve the maximum nutritional quality. Sharma et al.[70] investigated ideal methods for drying pomegranate arils by blanching them in hot water at 85 °C for one minute and then immersing them in a solution of potassium metabisulphite with concentrations ranging from 0.25% to 1% for two minutes (Table 2). The highest potassium metabisulphite content resulted in the lowest acidity.

    Thakur et al.[71] used steam blanching for 30 s and 0.3% sulphur fumigation for one hour to standardize pretreatments for dried arils from wild pomegranate. The authors discovered that cabinet drying outperforms solar drying and open sun drying. The solar dryer ranked second in terms of sensory characteristics like texture, taste, and general acceptability. Sun-dried pomegranate arils, on the other hand, exhibited the highest reduction in moisture content and overall acceptance when compared to vacuum drying, hot air oven drying, polytent drying, and room temperature drying, according to Bakshi et al.[72].

    The effect of pulsed electric field treatment on the behavior of microwave-assisted hot air drying of pomegranate arils was examined by Amiali et al.[84]. When compared to drying at 70 °C, they found that pulsed electric field treatment was only advantageous when the subsequent drying process was done at the lowest temperature (50 °C). The lower the temperature, the higher the overall phenolic concentration. Arils treated with a pulsed electric field had a 21.02% higher total phenolic content than untreated arils.

    Various pretreatment procedures on pomegranate arils have been utilized with the goal of conserving physicochemical, physical, and chemical properties or enhancing drying rates. However, pretreatment standardization is currently restricted and underexplored. Table 2 lists some of the methods for preparing dried pomegranate arils.

    Despite the fact that pomegranate is a fruit with numerous health and nutritional benefits, it is now a modest crop with limited marketability. The difficulty in collecting the interior edible seeds (arils) is the greatest impediment to realizing the full potential of this unusual fruit[29,31]. Only manual extraction of pomegranate arils for laboratory scale testing is described in the literature.

    Pomegranates are first cleaned and sorted for uniformity in color, size, shape, and weight before aril extraction[18,85]. All pomegranates should be washed. To avoid introducing bacteria into the arils when the fruit is sliced open, excess water from the fruit surface is dried before cutting[86]. Following that, the fruit is cut along the ridges and the segments are gently pulled apart to form a flower-like structure. The deconstructed pomegranate is then flipped over a bowl of water and gently tapped with a wooden spoon on the skin side. As a result, the arils will begin to come out without being broken. Once all arils have dropped out, the white membranes are skimmed off the surface of the water as it floats, the water is drained, the arils are separated, and the surplus water is gently patted off with a towel. This method involves cutting the pomegranate with a knife, which results in a loss of more than 30% of the arils owing to mechanical damage[86]. As a result, a better approach was required, such as the machinery developed by Schmilovitch et al.[87], which allows opening the fruit without cutting, extracting the arils with minimal damage, separating the arils from extraneous materials, and delivering clean arils to a packaging machine. This technology could be used to produce dried pomegranate on a greater scale.

    The drying time is determined by the pretreatment process, kind, and technique of dehydration used. The dehydration methods that will be investigated in this study are low-temperature drying and high-temperature drying.

    Low-temperature drying methods employ temperatures ranging from subzero to 50 °C[49,88]. These drying processes are time-consuming and are usually utilized for temperature-sensitive goods like herbs. Furthermore, low-temperature drying procedures reduce the risk of scorching the fruit, protecting heat-sensitive components such as vitamin C[89]. Freeze drying, vacuum drying, and sun drying are all low-temperature drying procedures[90]. Pomegranate arils dried using low-temperature procedures such as freeze drying and vacuum drying have a more acceptable look and nutrient composition than most high-temperature drying methods[12,91].

    Freeze-drying (FD) is a low-temperature technology frequently used for drying food samples for high-quality or heat-sensitive products[92]. It is also recognized as one of the most expensive, time-consuming, and energy-intensive procedures in the food industry[93]. It entails removing moisture from food ingredients under low temperature and high vacuum via ice sublimation[48]. Because the product is frequently frozen, it is also known as sublimation drying.

    Adetoro et al.[94] freeze-dried fresh pomegranate arils at a freezer temperature of −80 °C for 96 h to examine the effect of drying procedures on pomegranate arils. The researchers discovered that color, total phenolic compounds (TPC), total anthocyanin content (TAC), and radical-scavenging activity stability differed significantly from the hot air-drying procedure (Table 3). More et al.[23] investigated the effect of drying procedures on the quality of dried pomegranate arils from three varieties. The authors discovered that FD produced the greatest results in terms of color, flavor, taste, and nutritional factors across all cultivars. However, FD stood out due to its prolonged drying duration, 24−48 h, when compared to solar drying (17 h) and hot air drying (10 h). A study on the influence of freeze-drying on the color attributes of 'Assiuty' pomegranate arils revealed that FD had the best color attributes (L* value of 46.50 ± 4.4 and a* value of 13.97 ± 1.23)[95]. Gölükcü[91] discovered that the FD had the maximum phenolic matter content (5580 mg/kg), followed by vacuum, convective, and sun-dried pomegranate arils (Table 3). Caln-Sánchez et al.[96] investigated the chemical composition, antioxidant capability, and sensory quality of pomegranate arils and rind after exposure to FD. The investigators found that FD pomegranate arils retained the most sensory characteristics and punicalagin content. Similarly, Cano-Lamadrid et al.[12] discovered the best sensory profile and sweetness in FD pomegranate arils at 65 Pa for 24 h at −60 °C. The drying kinetics, total bioactive content, in-vitro bio accessibility of bioactive compounds, and color and microstructural features of pomegranate arils were also studied[97]. When compared to alternative drying methods, the FD was shown to be the best approach in terms of final product quality and has been highly recommended by multiple reviewers[48]. FD arils have been demonstrated to have a higher bioactive chemical content, less shrinking, and excellent color quality. The FD for pomegranate arils has a disadvantage in terms of bioactive chemical recovery when compared to other methods, as well as extensive drying times[97]. Furthermore, FD is costly due to high energy consumption and initial investment expenses[98].

    Table 3.  A summary of the many techniques for drying pomegranate arils.
    CultivarDrying methodDrying
    time (h)
    Drying conditionInitial moisture
    content (%)
    Final moisture
    content (%)
    Key findingsReference
    WonderfulFreeze dryer96−80 °C
    5,999.1 Pa
    74.7 (w.b.)FD showed a higher color shift (19.6% ± 2,77%) at week 4 compared to hot air drying at week 0.[94]
    Ganesh, Bhagwa and AraktaFreeze dryer24−48−45 °C79.9 (w.b.)
    80.5 (w.b.)
    78.9 (w.b.)
    9.65−9.9 (w.b.)
    9.8−10.2 (w.b.)
    9.8−9.9 (w.b.)
    Arakta pre-treated with 1% potassium metabisulphide had the highest ascorbic acid concentration (6.81 ± 0.07 mg 100 g−1).[23]
    AssiutyFreeze dryer36−70 °CTA (18.80, 2.80 mg 100 g−1), TP (608.09a, 41.26 mg 100 g−1), DPPH (68.91, 0.72%), and ABTS (2,956.59c, 120 mol trolox
    100 g−1) were all higher in frozen pomegranate arils than in freeze-dried arils.
    [95]
    Mollar de Elche24−60 °C, 65 Pa,81.5 (w.b.)FD showed higher anthocyanin content (646 mg kg−1) compared to osmotic drying and conventional drying.[12]
    Hicaznar36−20 °C, 100 Pa for
    12 h
    −70 °C, 0.26 Pa
    76.96 (w.b.)10 (w.b.)Arils dried using FD had the highest magnesium content (96.17 ± 6.95 mg kg−1), manganese content (0.96 ± 0.05 mg kg−1) and zinc content (3.93 ± 0.07 mg kg−1) compared to sun drying, hot air drying and vacuum drying.[91]
    KebenFreeze dryer57−55 °C77.6 ± 1 (d.b.)20 ± 1 (d.b.)FD had lower bioactive recovery during in-vitro gastrointestinal digestion (TPC of 2.92%, ABTS of 6.12% and CUPRAC of 38.85%) compared to vacuum drying, hot air drying and ultrasound-assisted vacuum drying.[97]
    KebenVacuum dryer10.855 °C77.6 ± 1 (d.b.)20 ± 1 (d.b.)Vacuum drying had the highest bio accessibility recovery of bioactive compounds at 10.32% compared to HAD, FD and ultrasound-assisted vacuum drying.[97]
    HicaznarVacuum dryer3.7
    4.6
    7.8
    75 °C
    65 °C
    55 °C
    85,000 Pa
    78.1 ± 0.2 (w.b.)16 (w.b.)Drying temperatures of 75°C resulted in higher degradation of anthocyanins, phenolic compounds and antioxidant capacity (20.0%, 51.0%, 29.7% ± 0.28% respectively).[99]
    HicaznarVacuum dryer2455 °C
    3 500 Pa
    76.96 (w.b.)10 (w.b.)FD had the highest quality attributes such as TAC-1288.73 mg/kg) compared to VD and HAD however, due to physical changes that were undesirable physical changes, VD and HAD are recommended for pomegranate aril drying.[91]
    Wild pomegranates (cultivar- unspecified)Vacuum dryer1342 ± 2 °CVD had a higher sensorial overall acceptance (17.1) compared to sun drying (16.2) and room temperature (12.6).[72]
    BasseinSun drying1715.73
    (unspecified)
    Drying rate of pomegranate arils is affected by tray load and the recommended tray load is 1.25 kg m−2[73]
    Wild pomegranates (cv. Unspecified)Solar poly-tunnel14014.9−28.4 °C
    48.5%−74% -RH
    0.639−0.944 -wind speed
    Arils dried in the solar poly-tunnel had higher ascorbic acid, anthocyanins, and phenol content, 12.7 mg 100 g−2, 28.12 mg 100 g−2, and 108.60 mg 100 g−2 respectively than open sun drying.[100]
    SefriIndirect solar dryer75
    10
    6
    4
    40 °C
    50 °C
    60 °C
    75 °C
    78 ± 0.1(w.b.)The optimal water activity for drying and storing arils is 0.3684 ± 0.03.[101]
    Wild pomegranates
    (cv. Unspecified)
    Solar tunnel30−45 °CArils from the Karsog location had the highest TSS, sugars, anthocyanin, total phenols and antioxidant activity[102]
    Microwave2.3
    1.3
    0.7
    270 W
    450 W
    630 W
    70.25 ± 0.5(w.b.)10(d.b.)Color changes increased from 6.77−13.11 with an increase in microwave power from 270−630 W and were lower compared to arils dried using HAD.[85]
    HicazMicrowave1.2
    0.6
    0.4
    210 W
    350 W
    490 W
    23.93 ± 1.4 (unspecified)22.2
    (unspecified)
    Based on quality parameters, a microwave drying power of 350 W was recommended for drying pomegranate arils.
    Sweet acid
    (cv. Unspecified)
    Infrared4.3
    2.2
    1.6
    50 °C
    60 °C
    70 °C
    78 ± 0.2 (w.b.)9 ± 0.2 (d.b.)
    22.2
    (unspecified)
    Drying time for infrared drying was less than for HAD.[103]
    HicazHAD2450, 60, 70 °C at
    1.0 m/s air velocity
    To obtain better dried aril quality, 60 °C was recommended for drying pomegranate arils.[104]
    Wild pomegranates
    (cv. Unspecified)
    HAD16.542 ± 2 °CSun drying resulted in a maximum loss of moisture compared to VD, HAD, poly-tent house drying and room drying.[72]
    Wild pomegranates
    (cv. Unspecified)
    HAD1062 ± 2 °CHAD achieved the highest total soluble solids (39.6°Brix) and drying rate compared to solar drying and open sun drying.[105]
    KebenHAD555 °C; with 1.3 m s−1 constant air velocity77.6 ± 1 (d.b.)20 ± 1 (d.b.)In comparison to hot air oven drying, ultrasound-assisted vacuum drying and freeze-drying have higher quality characteristics.[97]
    Bassein seedlessHAD5−660 ± 5 °C. Airflow in
    the dryer was 1.2−1.8 m s−1.
    8.98 ± 0.091For the finest preparation of anardana, blanched samples (with sulphur) should be dried in a cabinet.[73]
    Mollar de ElcheVacuum-
    microwave
    240, 360, 480 W and pressure ranging
    from 4,000−6,000 Pa
    80.4 (w.b.)Arils dried using vacuum microwave drying at 240 W had the highest sensorial scores for odour and aroma at 3.1 and 5.6 respectively.[106]
     | Show Table
    DownLoad: CSV

    Vacuum drying (VD) is the process of subjecting items to low pressure in a vacuum. Because of the low pressure, water has a lower boiling point, allowing samples to be dried at low temperatures. As a result, VD is appropriate for heat and/or oxygen-sensitive items. During VD, heat transmission can occur by conduction, radiation, or microwave energy. VD is distinguished by faster drying times when compared to FD, and the products are not initially frozen as necessary for FD[107]. This low-temperature operation, combined with the elimination of oxygen during vacuum drying, allows nutrients and bioactive components such as phenolic compounds and vitamins to be retained[108,109].

    Ozay-Arancioglu et al.[97] investigated the influence of drying methods on dried pomegranate arils by comparing four distinct drying techniques: FD, VD, ultra-assisted vacuum drying, and hot air drying. They discovered that VD had better antioxidant capacity values than the samples tested for ABTS following FD. According to Gölükcü[91], VD is second only to FD as the finest choice for producing dried pomegranate (Hicazar) arils. Another study found that arils dried at 55 °C had higher phytonutrient levels than those dried at 65 and 75 °C under vacuum conditions[69]. When compared to other drying procedures, vacuum drying produces products with higher levels of phytochemical components. However, drying times range from 7.8 to 24 h at 55 °C, contributing to high costs, and products can only be dried in batches[91,99,110].

    Sun drying is one of the oldest and most used methods of drying. Sun drying is a low-cost, renewable energy-based drying process. In a nutshell, products are laid out on a flat area where they can be fully exposed to the sun for as long as possible. Because the drying process is dependent on solar radiation, the temperature is low and the drying process can be lengthy, taking approximately 15 d for pomegranate arils[60]. Furthermore, exposure to light and oxygen can lead to decreased preservation of substances like vitamin C. Furthermore, solar drying is an uncontrolled process with substantial risks of pest contamination, dust exposure, and product remoistening at night. To increase safety, solar dryers and solar tunnels are proposed to reduce pest and dust contamination[111]. Solar dryers use a contained environment comprised of a transparent or opaque cover, resulting in either direct or indirect drying[112]. The indirect drier system captures solar heat and transfers it to the product drying chamber via a second solar collector. A solar tunnel dryer is typically large in size and has a clear cover (Fig. 4). To regulate drying conditions such as temperature and relative humidity within the tunnel, solar tunnels often require a forced convection facility. A solar tunnel dryer may also include a solar air heater[111]. Solar dryers have the potential to boost drying temperatures, resulting in a quicker drying time[113].

    Figure 4.  This image depicts a sample of wild pomegranate arils being dried in a solar tunnel drier. Reprinted from Thakur et al.[102].

    In a comparison of hot air drying (60 °C), solar tunnel drying, and sun drying by Madushree et al.[63], hot air-dried arils were shown to have the highest quality. However, of all the drying techniques, solar drying had the greatest L* values (lightness), a desired quality. This was due to the comparatively low temperature of sun drying. Additionally, Bakshi et al.[72] discovered that when compared to vacuum drying, oven drying (42 ± 2 °C), and room drying (23 ± 2 °C), sun dried arils had the highest sensory overall acceptance and the lowest moisture content. In their comparative analysis of drying techniques, Singh et al.[73] discovered that hot air-dried pomegranate arils had the greatest anthocyanin and acidity contents. But in hot-air oven-dried samples, undesirable non-enzymatic browning was most pronounced. Sharma & Thakur[100] demonstrated that the quality of arils dried in solar polytunnels was superior to that of arils dried in the open sun (Fig. 4). The ascorbic acid, anthocyanins, and phenols were found to be significantly greater in the sun polytunnel dried arils, according to the authors. They also received superior sensory ratings for color, texture, taste, and acceptability.

    Temperatures above 50 °C are used in high-temperature drying processes[88]. These drying processes are energy intensive, have large operating expenses, and so are costly. These technologies rely on fossil fuels, which pollute the environment where they are generated and utilized, and their continued usage is seriously harming our environment[114]. The drying mechanism is designed such that there is a controlled direct or indirect heat transmission to the product, leading in moisture elimination. These drying procedures may not be suited for some foods because they may induce nutritional breakdown[115]. Furthermore, high temperatures might cause product shrinkage and distortion. Hot-air drying ovens, steam drying, heat pump drying, and spray drying are all examples of high-temperature drying processes.

    Using forced convection, hot air oven drying (HAD) eliminates moisture from materials. Objects dry out through evaporation when hot air is forced through and around the substance. As a result, the dried product's flavor, color, nutrients, and ability to rehydrate may alter unintentionally[97,104,106]. The HAD techniques were shown to have a comparatively high total color change by Ozay-Arancioglu et al.[97]. As shown in Fig. 5, hot air-dried arils were darker than freeze-dried and vacuum-dried arils.

    Figure 5.  Illustration of dried pomegranate arils that have been dried using different methods. (a) Freeze drying, (b) vacuum drying, and (c) hot air drying. Adapted from Ozay-Arancioglu et al.[97].

    Başlar et al.[99] prepared dried aril samples using the hot air-drying process and subjected them to various quality assessments. Fresh aril samples were dried at three different temperatures (55, 65, and 75 °C). According to the authors' findings, high temperatures and short drying times are optimal for retaining valuable food biocomponents. However, whereas bioactive chemical losses increased over time, they degraded faster at higher temperatures. The antioxidant activity, on the other hand, decreased with drying time and was unaffected by drying temperatures. Horuz & Maskan[104] investigated the effect of hot air drying on pomegranate aril cv. Hicaz at three different drying temperatures and compared quality metrics such color, shrinkage, rehydration capacity, and drying time (Table 3). The authors suggested 60 °C for pomegranate aril HAD. The authors also discovered that shrinkage was greater in HAD than in microwave drying. In a second study, researchers discovered that the optimal drying temperature for retaining bioactive chemicals when drying pomegranate arils (cv. Hicaznar) in a hot air dryer was 65 °C[116]. When compared to the sun drying method for anardana made from wild pomegranate, Bhat et al.[105] discovered HAD dried arils with maximum acidity of 13.72%, phenols of 110.7 mg per 100 g, total sugars (24.2%), and reducing sugars (21.2%).

    However, Bakshi et al.[72] carried out a study with lower temperatures in which they studied the influence of different drying processes on the moisture content of dried pomegranate aril (cv. Wild). Lower temperatures were employed to gain insight into the quality of the dried product when compared to the low temperature drying methods used in the study, such as sun drying, poly tent house drying, room drying, and VD. When compared to alternative drying methods, the authors discovered that HAD (42 ± 2 °C ) for 16.5 h and drying in room at normal air (23 ± 2 °C ) for 10−12 d produced in the greatest loss of moisture from fresh arils of wild pomegranate (75.12%).

    Singh et al.[73] evaluated the influence of different drying conditions on the quality of dried pomegranate arils (Bassien Seedless) samples (Table 3). The scientists discovered that sun-drying preserved more MC while drying was faster with a HAD dryer and generally recommended it as a better strategy for preparing dried pomegranate arils.

    HAD drying of pomegranate arils is a standard drying procedure that can be utilized in commercial settings. Although HAD does not generate high-quality goods like FD, it does provide better TSS, TA, and antioxidant capacity stability. Furthermore, although having a higher rate of bioactive component degradation, higher temperatures may result in higher retention compared to approaches such as solar drying due to the short drying times.

    Electrical current is passed through the pomegranate aril during electric drying techniques including ohmic heating. The intrinsic resistance of the aril induces internal heating as the electrical current flows through it[117,118]. Ohmic heating is typically employed for liquid, viscous, and particle-containing foods[119]. Regardless of the meal's densities, food products prepared using this approach are heated quickly and uniformly[120].

    Dielectric techniques, on the other hand, use electromagnetic waves to directly produce heat inside the product, such as microwave, radio frequency drying, and infrared radiation[121]. Dielectric heating results from the conversion of electromagnetic energy to kinetic energy by dipolar molecules oscillating in accordance with the rapidly oscillating electric field[122,123]. Compared to traditional methods like hot air drying, dielectric technologies dry materials more quickly[124]. Additionally, the items are of a higher caliber than those produced by traditional drying techniques.

    Microwave drying (MD) is one of the emerging drying technologies. Unlike other techniques, MD utilizes volumetric heating to rapidly dehydrate the sample material[104]. Some studies[85,104] have indicated that arils desiccated at 150 W microwave power and 58 bar (abs) pressure produced the highest quality arils. In another study, microwave power of 80 W and vacuum pressure of 60 mm Hg provided the highest drying efficiency and qualitative attributes, including color and texture[125]. Horuz & Maskan[104] observed that microwave-dried pomegranate samples had lower levels of shrinkage and bulk density than hot air-dried samples. The authors also noted that microwave drying caused a greater loss of color in terms of total color difference (E) compared to air drying. It was observed that microwave-dried samples had a brownish hue.

    Drying with MD reduces drying time, but essential quality parameters, such as color, are sacrificed. Since the product is directly heated, the lack of heating uniformity during MD, which is difficult to control and could contribute to product burning has been cited as a disadvantage[126].

    Infrared drying is an effective technique of drying in which the product is heated directly without the use of air as the drying medium. In a comparative study of drying methods (hot air drying and infrared drying) to dry pomegranate arils, the authors discovered that infrared drying effectively dried pomegranate arils and that the polyphenol content in arils dried using infrared drying was higher at 50 and 60 °C than in arils dried using hot air drying at similar temperatures[103]. Therefore, pomegranate arils can be infrared-dried at 50 °C for optimal nutrient retention[103]. In contrast, a distinct study dried pomegranate arils under near-infrared vacuum conditions and found that drying at 60 °C and 20 kPa air pressure resulted in optimal colour retention and shrinkage[127].

    Although infrared drying is a rapid drying method, it is challenging to control due to parameters such as infrared intensity and radiation distance, and its energy consumption is unpredictable[26].

    Drying kinetics is the study of how factors that influence the removal of moisture from products during a drying process interact[110]. The drying kinetics of a substance is dependent on its thermal and mass transport properties. Understanding drying kinetics relates to process variables, and therefore aids in identifying suitable drying methods and controlling drying processes[57,96,116]. For optimal operating conditions, drying kinetics can be used to estimate drying time, energy requirements, and drying efficiency[2]. Due to the complexity of the drying phenomenon, however, mathematical models describing the drying kinetic of biological tissues are devised based on the time history of the moisture ratio from a controlled drying experiment[128]. There are numerous mathematical models, but Table 4 provides a summary of the most used models.

    Table 4.  A summary of the mathematical equations that are most commonly used to model the drying kinetics of pomegranate arils is shown here.
    Model nameModel expressionReference
    PageMR = exp(-ktn)[129]
    NewtonMR = exp(-kt)[130]
    Henderson and PabisMR = aexp(-kt)[131]
    Midili et alMR = aexp(-ktn) + bt[132]
    Wang and SinghMR = 1 + at + btn[133]
    Two TermMR = (aexp(-k0 t) + bexp(-k1 t))[134]
    LogarithmicMR = aexp(-kt) + c[135]
     | Show Table
    DownLoad: CSV

    The most common models that best describe the hot air drying of pomegranate arils include the Logarithmic, Midili and Page models[24,99]. Baslar et al.[99] found the Logarithmic model as the best in describing hot air drying of arils at 55, 65 and 75 °C. In another study, it was found the Sigmoid model describing the kinetics of hot air drying of pomegranate arils at similar drying temperatures[116].

    In their study on the infrared drying of pomegranate arils, Briki et al.[103] discovered that the Midili model was the most accurate representation of the drying kinetics. In a similar manner, it was discovered that the Midili model provided the best fit to the experimental data for drying using a combination of infrared and hot air[136]. Another investigation indicated that the Aghbashlo model provided the greatest fit for the data obtained from near-infrared vacuum drying at a temperature of 60 °C[127].

    Based on measurement data from three pomegranate cultivars (cvs. 'Acco', 'Herkaswitz', and 'Wonderful') at 60 °C, Adetoro et al. demonstrated cultivar as another influencing factor in selecting an optimal model. While the drying data of the blanched samples of all the cultivars in this investigation were best fit by the Logarithmic model, the unblanched samples of 'Acco' and 'Herkaswitz' and 'Wonderful' were best fitted by the Page and Midili models, respectively[24].

    The bioactive chemicals retained and drying periods may be affected by the pretreatment process, although frequently the drying kinetics of both untreated and pretreated samples may be described by the same model. The Page and Modified Page were determined to be the best models to fit the drying data of both blanched and unblanched pomegranate arils under vacuum air drying by Karaslaan et al.[69]. In a different investigation, the Page and Modified Page were shown to be the model that best suited the drying data of pomegranate arils that had been bathed in citric acid and dried by hot air[57]. Although the drying rate of pre-treated samples is higher than that of untreated samples, the scientists noted that the same models were found to best reflect the drying kinetics. The Page, Logarithmic, and Midili drying models are the most popular and effective drying models for pomegranate arils employing HAD, MD, and VD. The mathematical models that were utilized to explain the drying kinetics of pomegranate arils are summarized in Table 5.

    Table 5.  A synopsis of the results of mathematical modeling of the kinetics of drying pomegranate arils.
    CultivarDrying methodDrying parametersPretreatmentSuitable drying modelReference
    cv. HicazHAD55, 65, 75 °C0.1% citric acidPage and Modified Page[57]
    cv. HicaznarHAD55, 65, 75 °CSigmoid[116]
    HAD55, 55, 60 °CPage[137]
    HAD50, 60, 70 °CPage[85]
    MD270, 450, and 630 WPage[85]
    cv. HicaznarVD55, 65, 75 °CHot water blanchingPage and Modified page[69]
    HAD45, 50, 55, 60, 65, and 70 °CMicrowaveMidili[75]
    cvs. Acco, Herskawitz and WonderfulHAD60 °CHot water blanching (at 90 and 100 °C, each for 30 s
    and 60 s)
    Logarithmic, Page,
    Midili for unblanched arils
    Midili and Page for blanched
    [24]
     | Show Table
    DownLoad: CSV

    The drying procedure aids in reducing bacterial growth, which can result in reducing spoilage. However, while food is being dried, other changes may occur that degrade its quality. To determine the product's expiration life, chemical, physical, physicochemical, and microbiological changes are monitored[138,139]. These modifications are affected by stowage, environment, and packaging methods. The most significant factor affecting the integrity of stored food products is temperature[140]. Consequently, most shelf-life experiments are designed to evaluate the temperature-time history in relation to changes in product quality[138,141]. To accurately evaluate quality changes and safety, shelf-life evaluations should ideally simulate actual storage conditions[142]. In the case of desiccated goods, the actual storage period is lengthy, and the evaluation of shelf-life can become time-consuming and expensive. When the actual storage time is lengthy for practical reasons, an accelerated shelf-life test or analysis of the worst-case scenario is employed[142]. Ordinarily, the end of shelf life is determined by relevant food legislation, guidelines issued by enforcement authorities or agencies, guidelines issued by independent professional organizations, current industrial best practices, self-imposed end-point assessment, and market data[142].

    Appropriate packaging material can help to reduce quality losses. The packaging and shelf-life tests on dried pomegranate arils are summarized in Table 6. Sharma et al.[70] examined the packaging of dried pomegranate arils with high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP) films. For 12 months, samples were held at 7 °C and ambient (14−39 °C). They found that HDPE retained the most color, total tannins, and acidity while gaining the least moisture. The authors proposed a safe storage time of 6 and 9 months for HDPE packed dried pomegranate arils under ambient and refrigerated conditions, respectively.

    Table 6.  A list of the several types of containers used to store dried pomegranate arils.
    Drying methodDrying temperaturePackaging materialStorage period (months)Shelf-life performanceReference
    MC drier
    Solar cabinet drier
    Open sun
    62−64 °C
    50−55 °C
    18−24 °C
    Aluminum laminated polyethylene pouch, polyethylene pouches and thermoform trays.6Aluminum laminated polyethylene pouches were best for packaging.[105]
    Microwave-vacuum drying38 °CHigh-density polypropylene (HDPP) and aluminum laminated polyethylene (ALP).3−6Pomegranate arils stored showed that ALP is more protective than HDPP.[35]
    Solar tunnel30−45 °CGunny bags, aluminum laminated polyethylene pouches (ALP) and vacuum-sealed aluminum laminated polyethylene pouches (ALPV).12Both refrigerated and ambient storage can securely preserve dried pomegranate samples for 12 months. Best performance was ALP with vacuum and cold storage.[144]
    Hot air dryer60 °CAluminum laminated polyethylene pouch (ALP), polyethylene pouch (PEP), and thermofoam tray (TT) covered in shrinkable polypropylene transparent sheet.6Moisture absorbers aid in the preservation of samples.[100]
     | Show Table
    DownLoad: CSV

    Bhat et al.[105] compared aluminum-laminated polyethylene (ALP, 99.8 g m−2) and polyethylene pouches (93.9 g m−2) each storing 100 g dried pomegranate arils and stored at ambient (15−25 °C). The authors discovered that aluminum laminated polyethylene pouches performed best after a 6-month storage period. Dak et al. [35] compared HDPP and ALP under accelerated shelf-life conditions (38 ± 1 °C and 90% ± 1% relative humidity) and evaluated the correlation between packaging material, storage period, and anthocyanin, phenolics, TSS, TA, and microbial count. The authors estimated that HDPP and ALP have shelf lives of 96 and 187 d, respectively. The above two research confirmed that ALP had the best performance features. This could be owing to the pouches' thickness and the opaque barrier of the aluminum lamination. The use of opaque packaging material may extend the shelf life of pomegranate arils by minimizing photodegradation of components such as carotenoids, flavonoids, and lipids, which can alter qualitative qualities such as aroma, texture, and color[143]. Based on safe consumption criteria, Mokapane et al.[19] proposed a shelf life of 5 months for citric acid pretreatment and dried arils wrapped in kraft paper pouches.

    In a second investigation, Thakur et al.[144] examined the effectiveness of gunny bags, ALP, and ALP combined with vacuum for storing dried pomegranate arils for a period of one year. According to the authors' findings, ALP performed best when performed under vacuum. The various types of containers that are used to store dried pomegranate arils are broken down into categories and shown in Fig. 6.

    Figure 6.  Different types of storage bags for dried pomegranate arils. (a) Aluminum-laminated polyethylene pouches, (b) vacuum-sealed aluminum-laminated polyethylene pouches, (c) gunny bags and (d) transparent polyethylene pouch. From Thakur et al.[144], modified.

    Sharma & Thakur[100] investigated the effect of active packaging on the quality features of dried wild pomegranate arils over a 6-month storage period. Salt or sugar sachets were inserted in ALP pouches or Thermofoam trays that had been wrapped in shrinkable polypropylene transparent film or polyethylene pouches. The researchers reported that for arils dried in a mechanical drier, ALP pouches had the best quality retention of criteria such as ascorbic acid, anthocyanins, total phenols, color, and texture. Furthermore, the inclusion of salt or sugar in active packing aids in the production of high-quality dried arils. However, salt-based active packaging had somewhat higher TA, ascorbic acid content, total sugars, anthocyanin content, and total phenols than sugar-based active packaging.

    The shelf life of dried pomegranate aril can range anywhere from three months to a year depending on the pretreatment, drying procedures and packaging that were used. There is, however, no definitive guideline about the influence that pretreatment and drying procedures have on the quality characteristics of the product when it is being stored. Many studies on the shelf life of dried pomegranate arils focus on examining the influence that different types of packaging material have on the storage life. These studies pay less attention to the physical and microbiological changes that accompany quality alterations. Additionally, a specialized shelf-life testing process and quality standard for dried pomegranate arils can be difficult to locate in the literature.

    While high temperatures have a positive influence on the drying rate, it has a negative effect on the product's texture, color, and nutritional content. Because of this, lower temperatures are often ideal for retaining the pomegranate arils' nutritional value and maintaining their consistency. Freeze dryers offer the best result in this regard. Freeze-drying, on the other hand, is a time- and money-consuming process. When paired with other types of pretreatment, inexpensive procedures such as sun dryers can be adjusted to produce high retention on chemicals that are virtually identical to those obtained in freeze dryers. However, before implementation, it is necessary to examine the intended outcome of the pretreated arils. This is because pretreatments affect both the drying rate and the retention of nutrients. Therefore, in order to completely optimize the process, it is necessary to have an awareness of the interaction that occurs between the pretreatments and the subsequent drying procedure. It is proposed that recommendations be formulated to assist in the manufacture and marketing of dried pomegranate arils that are consistent, nutritious, and hygienically safe.

    The authors confirm contribution to the paper as follows: study conception and design: Ambaw A, Opara UL; project administration and supervision: Ambaw A, Opara UL; draft manuscript preparation: Maphosa B; manuscript review and editing: Ambaw A. all authors reviewed the results and approved the final version of the manuscript.

    Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

    This work is based on the research supported wholly/in part by the National Research Foundation of South Africa (Grant No. 64813). The opinions, findings and conclusions or recommendations expressed are those of the author(s) alone, and the NRF accepts no liability whatsoever in this regard. Research reported in this publication was supported in part by the Foundation for Food and Agriculture Research under award number 434—grant ID: DFs-18- 0000000008.

  • The authors declare that they have no conflict of interest.

  • [1]

    Zenna N, Senthilkumar K, Sie M. 2017. Rice Production in Africa. In Rice Production Worldwide, eds. Chauhan BS, Jabran K, Mahajan G. Cham: Springer. pp. 117–35. https://doi.org/10.1007/978-3-319-47516-5_5

    [2]

    Zeigler RS, Leong SA, Teng PS, CAB International, International Rice Research Institute. 1994. Rice blast disease. United Kingdom: Wallingford CAB/IRRI. 626 pp.

    [3]

    Asibi AE, Chai Q, Coulter JA. 2019. Rice Blast: A disease with implications for global food security. Agronomy 9:451

    doi: 10.3390/agronomy9080451

    CrossRef   Google Scholar

    [4]

    Besi MI, Tucker SL, Sesma A. 2009. Magnaporthe and its relatives. Encyclopedia of Life Sciences. Chichester, UK: John Wiley & Sons. https://doi.org/10.1002/9780470015902.a0021311

    [5]

    Aktar MW, Sengupta D, Chowdhury A. 2009. Impact of pesticides use in agriculture: their benefits and hazards. Interdisciplinary Toxicology 2:1−12

    doi: 10.2478/v10102-009-0001-7

    CrossRef   Google Scholar

    [6]

    O'Callaghan M. 2016. Microbial inoculation of seed for improved crop performance: issues and opportunities. Applied Microbiology and Biotechnology 100:5729−46

    doi: 10.1007/s00253-016-7590-9

    CrossRef   Google Scholar

    [7]

    Rocha I, Ma Y, Souza-Alonso P, Vosátka M, Freitas H, Oliveira RS. 2019. Seed coating: A tool for delivering beneficial microbes to agricultural crops. Frontiers in Plant Science 10:1357

    doi: 10.3389/fpls.2019.01357

    CrossRef   Google Scholar

    [8]

    Peng G, McGregor L, Lahlali R, Gossen BD, Hwang SF, et al. 2011. Potential biological control of clubroot on canola and crucifer vegetable crops. Plant Pathology 60:566−74

    doi: 10.1111/j.1365-3059.2010.02400.x

    CrossRef   Google Scholar

    [9]

    Müller H, Berg G. 2008. Impact of formulation procedures on the effect of the biocontrol agent Serratia plymuthica HRO-C48 on Verticillium wilt in oilseed rape. BioControl 53:905−16

    doi: 10.1007/s10526-007-9111-3

    CrossRef   Google Scholar

    [10]

    Angelopoulou DJ, Naska EJ, Paplomatas EJ, Tjamos SE. 2014. Biological control agents (BCAs) of verticillium wilt: influence of application rates and delivery method on plant protection, triggering of host defence mechanisms and rhizosphere populations of BCAs. Plant Pathology 63:1062−69

    doi: 10.1111/ppa.12198

    CrossRef   Google Scholar

    [11]

    Vidhyasekaran P, Rabindran R, Muthamilan M, Nayar K, Rajappan K, et al. 1997. Development of a powder formulation of Pseudomonas fluorescens for control of rice blast. Plant Pathology 46:291−97

    doi: 10.1046/j.1365-3059.1997.d01-27.x

    CrossRef   Google Scholar

    [12]

    Xu T, Li Y, Zeng X, Yang X, Yang Y, et al. 2017. Isolation and evaluation of endophytic Streptomyces endus OsiSh-2 with potential application for biocontrol of rice blast disease. Journal of the Science of Food and Agriculture 97:1149−57

    doi: 10.1002/jsfa.7841

    CrossRef   Google Scholar

    [13]

    Spence C, Alff E, Johnson C, Ramos C, Donofrio N, et al. 2014. Natural rice rhizospheric microbes suppress rice blast infections. BMC Plant Biology 14:130

    doi: 10.1186/1471-2229-14-130

    CrossRef   Google Scholar

    [14]

    Chou C, Castilla N, Hadi B, Tanaka T, Chiba S, et al. 2020. Rice blast management in Cambodian rice fields using Trichoderma harzianum and a resistant variety. Crop Protection 135:104864

    doi: 10.1016/j.cropro.2019.104864

    CrossRef   Google Scholar

    [15]

    Li H, Guan Y, Dong Y, Zhao L, Rong S, et al. 2018. Isolation and evaluation of endophytic Bacillus tequilensis GYLH001 with potential application for biological control of Magnaporthe oryzae. PLoS One 13:e0203505

    doi: 10.1371/journal.pone.0203505

    CrossRef   Google Scholar

    [16]

    Chen WC, Chiou TY, Delgado AL, Liao CS. 2019. The control of rice blast disease by the novel biofungicide formulations. Sustainability 11:3449

    doi: 10.3390/su11123449

    CrossRef   Google Scholar

    [17]

    Filippi MCC, da Silva GB, Silva-Lobo VL, Côrtes MVCB, Moraes AJG, et al. 2011. Leaf blast (Magnaporthe oryzae) suppression and growth promotion by rhizobacteria on aerobic rice in Brazil. Biological Control 58:160−66

    doi: 10.1016/j.biocontrol.2011.04.016

    CrossRef   Google Scholar

    [18]

    Murunde R, Ringo G, Robinson-Boyer L, Xu X. 2022. Effective biocontrol of rice blast through dipping transplants and foliar applications. Agronomy 12:592

    doi: 10.3390/agronomy12030592

    CrossRef   Google Scholar

    [19]

    Xu XM, Jeffries P, Pautasso M, Jeger MJ. 2011. Combined use of biocontrol agents to manage plant diseases in theory and practice. Phytopathology 101:1024−31

    doi: 10.1094/PHYTO-08-10-0216

    CrossRef   Google Scholar

    [20]

    R Core Development Team. 2019. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. www.R-project.org.

    [21]

    Ali H, Nadarajah K. 2014. Evaluating the efficacy of Trichoderma spp and Bacillus subtilis as biocontrol agents against Magnaporthe grisea in rice. Australian Journal of Crop Science 8:1324−35

    Google Scholar

    [22]

    Sesma A, Osbourn AE. 2004. The rice leaf blast pathogen undergoes developmental processes typical of root-infecting fungi. Nature 431:582−86

    doi: 10.1038/nature02880

    CrossRef   Google Scholar

    [23]

    du Jardin P. 2015. Plant biostimulants: Definition, concept, main categories and regulation. Scientia Horticulturae 196:3−14

    doi: 10.1016/j.scienta.2015.09.021

    CrossRef   Google Scholar

    [24]

    Bulgari R, Franzoni G, Ferrante A. 2019. Biostimulants application in horticultural crops under abiotic stress conditions. Agronomy 9:306

    doi: 10.3390/agronomy9060306

    CrossRef   Google Scholar

    [25]

    Pieterse CMJ, Zamioudis C, Berendsen RL, Weller DM, Van Wees SCM, et al. 2014. Induced systemic resistance by beneficial microbes. Annual Review of Phytopathology 52:347−75

    doi: 10.1146/annurev-phyto-082712-102340

    CrossRef   Google Scholar

    [26]

    Sood M, Kapoor D, Kumar V, Sheteiwy MS, Ramakrishnan M, et al. 2020. Trichoderma: The "secrets" of a multitalented biocontrol agent. Plants 9:762

    doi: 10.3390/plants9060762

    CrossRef   Google Scholar

    [27]

    Radhakrishnan R, Hashem A, Abd Allah EF. 2017. Bacillus: A biological tool for crop improvement through bio-molecular changes in adverse environments. Frontiers in Physiology 8:667

    doi: 10.3389/fphys.2017.00667

    CrossRef   Google Scholar

    [28]

    Poveda J, Eugui D. 2022. Combined use of Trichoderma and beneficial bacteria (mainly Bacillus and Pseudomonas): Development of microbial synergistic bio-inoculants in sustainable agriculture. Biological Control 176:105100

    doi: 10.1016/j.biocontrol.2022.105100

    CrossRef   Google Scholar

    [29]

    Woo SL, Ruocco M, Vinale F, Nigro M, Marra R, et al. 2014. Trichoderma-based products and their widespread use in agriculture. The Open Mycology Journal 8:71−126

    doi: 10.2174/1874437001408010071

    CrossRef   Google Scholar

    [30]

    García-López AM, Recena R, Avilés M, Delgado A. 2018. Effect of Bacillus subtilis QST713 and Trichoderma asperellum T34 on P uptake by wheat and how it is modulated by soil properties. Journal of Soils and Sediments 18:727−38

    doi: 10.1007/s11368-017-1829-7

    CrossRef   Google Scholar

    [31]

    Chowdappa P, Mohan Kumar SP, Jyothi Lakshmi M, Upreti KK. 2013. Growth stimulation and induction of systemic resistance in tomato against early and late blight by Bacillus subtilis OTPB1 or Trichoderma harzianum OTPB3. Biological Control 65:109−17

    doi: 10.1016/j.biocontrol.2012.11.009

    CrossRef   Google Scholar

  • Cite this article

    Murunde R, Ringo G, Robinson-Boyer L, Xu X. 2023. Applying beneficial microbes as transplanting dipping and post-transplanting foliar spray led to improved rice productivity. Technology in Agronomy 3:7 doi: 10.48130/TIA-2023-0007
    Murunde R, Ringo G, Robinson-Boyer L, Xu X. 2023. Applying beneficial microbes as transplanting dipping and post-transplanting foliar spray led to improved rice productivity. Technology in Agronomy 3:7 doi: 10.48130/TIA-2023-0007

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Applying beneficial microbes as transplanting dipping and post-transplanting foliar spray led to improved rice productivity

Technology in Agronomy  3 Article number: 7  (2023)  |  Cite this article

Abstract: Rice is a popular food in Africa, but current yield achieved is far lower than the yield potential due to abiotic/biotic stresses. We recently demonstrated that commercially formulated products of Bacillus subtilis and Trichoderma asperellum strains increased rice yield when applied as transplant dipping or post-transplanting foliar sprays in Tanzania (Africa). Further experiments were conducted to investigate: (1) synergies in alternate use of B. subtilis and T. asperellum products as dipping or foliar spray in Tanzania, and (2) effects of coating seeds with Serratia nematodiphila in Kenya (Africa). In Tanzania, using formulated B. subtilis and T. asperellum products led to > 100% increase in yield. Furthermore, alternative use of B. subtilis and T. asperellum as transplant dipping or post-transplanting foliar spray led to further yield increase (ca. 32%) over the use of single-organism products at both times. Microbial treatments led to significant reductions in rice blast. In comparison with the fungicide treatment, increased yield with using microbial products appeared to have resulted mostly from improved plant development rather than reduced rice blast per se. At the Kenyan site where current yield is relatively high, the overall yield increase associated with microbial products was limited although statistically significant. Coating seeds with one specific S. nematodiphila strain led to grain yield comparable to applying microbial products as transplant dipping and post-transplanting foliar spray. The present study suggests that the formulated microbial products can significantly improve rice productivity in subsistence farming and should be applied in alternation over time to exploit their synergies.

    • Rice (Oryza sativa) is grown in many African countries with substantial increased productivity recently[1], but the overall rice demand in Africa has outstripped local production, requiring 40% imported rice. Tanzania is the most important rice production zone in East Africa; rice is the second most cultivated food and commercial crop in Tanzania after maize. However, rice productivity in Tanzania is still low, with the average yield in the range of 1.5−2.8 t·ha−1, due in large part to the fact that small-scale and low-technology farmers account for ca. 70% of the rice growing area. On these farms, fertilisers are often not used because of the cost, limiting yields. Rice blast caused by Magnaporthe oryzae is another challenge facing rice farmers[24]. Good blast control may be achieved with intensive fungicide programmes, but this is both expensive, particularly for smallholder farms, and undesirable[5].

      The use of beneficial microbes is being promoted for their potential in sustainable agriculture, by exploiting their biocontrol effects against specific diseases and/or plant growth promoting effects. In addition to conventional applications of biopesticides to soils (e.g., drenching) and plants (e.g., dipping, spraying), coating seeds with low amounts of beneficial microbes can be an efficient way to delivering beneficial microbes for improving seed germination, seedling establishment and suppression of root diseases[6, 7]. Treating Brassica napus seeds with biocontrol organisms and fungicides were, however, less effective than soil drench in reducing clubroot caused by Plasmodiophora brassicae[8]. Bio-priming rape seed with Serratia plymuthica led to reduced development of Verticillium dahliae[9]. Coating eggplant seeds with Paenibacillus alvei or a nonpathogenic Fusarium oxysporum strain led to significant suppression of Verticillium wilt[10].

      To control rice blast, biopesticides may be applied as a seed priming treatment, dipping treatment at transplanting, and foliar sprays post-transplanting[1114]. Promising organisms against rice blast include strains from several common sources for biocontrol agents: Bacillus, Trichoderma, Streptomyces and Pseudomonas[12, 13, 15, 16]. Several rhizobacteria showed a good efficacy against rice leaf blast when applied as soil drench or foliar spray but the efficacy varying with application methods[17]. A recent pilot study in Kenya showed that dipping roots of rice seedlings in commercial formulated B. subtilis or T. asperellum products led to reduced blast development and increased yield. Following this pilot study, we conducted further field studies and showed that applying formulated B. subtills or T. asperellum commercial products as transplanting dipping or as post-transplanting foliar sprays led to large increases in rice yield only at subsistence farming low-yield sites (Tanzania) and the benefit of using the same organism product as dipping or foliar spray varied greatly with seasons[18]. Reduction in blast development due to biocontrol was achieved but did not necessarily lead to corresponding increases in yield[14]. It is often recommended to combine biopesticides to achieve better performance, exploiting potential synergies among different microbial organisms. However, such a laudable objective of exploiting synergies has rarely been achieved[19]. Mixed use of formulated B. subtills or T. asperellum products did not lead to additional benefits in terms of blast control or rice yield[18].

      The present study aims to develop strategies of applying commercial biopesticides for rice growers in Africa, extending our previous research[18] in the following three specific aspects on the use of formulated microbial products in rice production. Firstly, alternate use of formulated B. subtills and T. asperellum products as transplanting dipping or post-transplanting foliar spray were assessed for potential synergistic effects in reducing blast and increasing yield. In the previous research[18], the same biopesticide product was used in dipping and post-transplanting foliar spray. Secondly, we reduced the number of post-transplanting foliar sprays from the previous five/six sprays to two sprays. Finally, the effect of coating rice seeds with one S. nematodiphila strain on blast development and rice yield was compared with using microbial products as transplanting dipping and post-transplanting foliar spray. In our previous study, newly formulated product of this S. nematodiphila strain performed similarly to formulated B. subtills and T. asperellum products when applied as dipping or post-transplanting foliar spraying.

    • Experiments were conducted at one site in Kenya for two cropping seasons and three sites in Tanzania for one crop season in 2022, at the same sites as the experiments reported previously[18]. A randomised block design was used for all sites. Within each block, there was one plot (5 m × 5 m) randomly assigned to one of the treatments; planting space was 20 cm × 15 cm. There was a gap of 75 cm between neighbouring plots.

      In Kenya, the study objective was to compare a seed-coating treatment with the formulated product of one specific Real IPM [The Real IPM Co. (Kenya) Ltd., Thika, Kenya] S. nematodiphila strain (code SN01) with three other Real IPM formulated products of specific B. subtilis strain Bs01 (trade name: REGAIN®), T. asperellum strain T-900 (trade name: Sustain®), and the same S. nematodiphila strain SN01 when applied as dipping at transplanting and/or post-transplanting foliar spray. In Tanzania, the main objective was to assess whether there was additional benefit in the alternate use of formulated B. subtilis and T. asperellum products as transplanting dipping or post-transplanting foliar spray. All formulated B. subtilis, T. asperellum and S. nematodiphila products contained 1.0 × 1010 CFUs ml−1 of specific microbes. The formulated S. nematodiphila product and seed coating product with the S. nematodiphila strain were not included in Tanzania as they were not approved for use there.

      For transplanting dipping treatments, seedling roots were dipped in an appropriate product (5 ml formulated product in 1 L water) for 30 min before transplanting. For post-transplanting foliar application, unlike in the previous study[18], microbial products or fungicides were applied only twice with a knapsack sprayer to protect panicles when they were emerging from the boot, about nine weeks after transplanting; two sprays were applied 7 d apart. Each plot received approximately 7 L each time at the same concentration as dipping treatments.

    • A single cultivar (cv. Basmati 370) was used at the National Irrigation Board site (longitude −37°37'2.53" E and latitude −0°49'24.23" S). In this region, the main agricultural activity is monocropping of rice grown in paddies that are irrigated for six months. The soil at the site is of the black cotton soil type (vertisol).

      There were five blocks and nine treatments. There were three formulated products (B. subtilis, T. asperellum and S. nematodiphila), each applied as (i) transplanting dipping only, and (ii) both as transplanting dipping and post-transplanting foliar spray. There were three additional treatments: (i) seed coating with S. nematodiphila only, (ii) water control [dipping in and foliar spray with water], and (iii) fungicide control [post-transplanting spray only − 'Absolute', a mixture of azoxystrobin (200 g·L−1), difenoconazole (125 g·L−1) and hexaconazole (50 g·L−1)]. For the first cropping season, seeds were sown in early February 2022 and transplanted in early March 2022. For the second cropping season, seeds were sown in late August 2022 and transplanted in late September 2022. Fertilisers were applied via broadcasting to the trial on three occasions: muriate of potash at 25 kg per acre, three days after transplanting, and sulphate of ammonia at 50 kg per acre, two and seven weeks after transplanting.

      Number of tillers and the height of the highest tiller for each plant was assessed weekly immediately from transplanting, giving a total nine assessments. In each plot, the same five plants (randomly selected on the first assessment date) were assessed for tiller development at all nine time points. For each plot, time to the first and median flowering was recorded. Rice blast assessment commenced four weeks after transplanting and thereafter weekly on five leaves, one leaf from each of five plants randomly selected on each assessment occasion for each plot. As blast was not severe, only presence of rice blast was recorded. In total there were five blast assessments. For each plot, 10 panicles (heads) were randomly selected, one from each of 10 randomly selected plants, to estimate number of grains per panicle. The 1000 seed weight and unshelled (raw) gross grain yield were obtained for each plot.

    • Experiments were conducted at three sites [Babati (−3.8541° S, 35.5235° E), Kikwe (3.3711° S, 36.8285° E) and Moshi (3.4197° S, 37.3676° E)]; the soil at all three sites is of the black cotton soil type (vertisol). Seeds were sown on 1/12/2021 (Kikwe), 21/12/2021 (Moshi), and 11/01/2022 (Babati). A single cultivar [cv. SARO 5 (TXD 306)] was used at all three sites.

      At each site, there were three blocks and nine treatments. There were two formulated products (B. subtilis and T. asperellum) that were applied individually in two treatments: only transplanting dipping, and both transplanting dipping and post-transplanting foliar spray. There were a further two treatments with alternate use of the two products: B. subtilis dipping/T. asperellum spray, and T. asperellum dipping/B. subtilis spray. Finally, there were three control treatments: two water controls and one fungicide control (SC250ECORE 250 g·L−1 (difenoconazole), applied at 0.5 L·ha−1). One of the two water controls was initially reserved for the S. nematodiphila seed coating treatment; unfortunately, this product was not registered in Tanzania in time for this experiment. Urea was applied through broadcasting as a top dressing six weeks after transplanting; lambda cyhalothrin or deltamethrin was applied to control stalk borers four weeks after transplanting.

      Number of tillers and the height of the highest tiller for each of five plants were assessed three times; five plants were randomly selected for tiller assessment at each time point. At Kikwe, they were measured 6, 14 and 19 weeks after transplanting; at Moshi, 6, 10 and 15 weeks after; and at Barbati, 4, 18 and 28 weeks after. For each plot, times to 30% flowering and to maturity were recorded. Number of seeds per head and percentage of grains with rice blast symptoms were recorded at harvest: five panicles were selected randomly (one from a single plant) for each plot to count number of grains per panicle and with blast symptoms. Finally, both unshelled (raw) and shelled (net) grain yields were obtained for each plot.

    • For all experiments, preliminary repeated measurement analysis of variance (ANOVA) of tiller number and height showed that there were no significant interactions between assessment dates and treatments. Thus, only the tiller number and height on the final assessment date were statistically analysed and presented. All statistical analyses and graphing were conducted in R[20].

    • The data were subject to two stages of ANOVA. For yield, number of seeds per panicle, and days to flowering or maturity, log transformation was applied to ensure that residual errors follow (or closely follow) normal distributions. A logit transformation was applied to the incidence of leaves with blast symptoms before ANOVA. No transformation was needed for the 1000 seed weight, tiller number and height. Firstly, ANOVA was applied to all the treatment data pooled over the two crop seasons where cropping season was treated as a blocking factor in addition to the within-experiment block and treatment factors. When ANOVA indicated significant differences among the nine treatments, the Tukey HSD (honestly significant difference) test was then applied to conduct pairwise comparisons. Secondly, ANOVA was applied to the six treatments involving three microbial products applied as dipping or as foliar spray. In this analysis the six microbial treatments were considered as a two factorial design [three products, and two application strategies (dipping only, dipping + foliar spray) to assess whether three products differed overall in their effects and whether there were interactions between the three products and two application strategies.

    • Similarly a two-stage ANOVA analysis as for the Kenya data were applied. In addition to unshelled (gross) and shelled (net) grain yield, the ratio between the net and gross grain weight was also calculated and analysed. For net gross ratio, log transformation was applied to ensure that residual errors follow (or closely follow) normal distributions. For all other variables, transformation was the same as for the Kenya data. In the first analysis of all nine treatments, the site was treated as a blocking factor in addition to the within-site block and treatment factors. The second ANOVA was applied to the six microbial treatments to determine the effects of multiple treatments and possible synergy from alternate use of Bacillus and Trichoderma products. In this ANOVA, in addition to site and within-site block factors, there were two treatment factors: products used as (i) dipping (Bacillus or Trichoderma), and (ii) as foliar spray (Water, Bacillus or Trichoderma). The main effect of the foliar spray (two degree-of-freedoms, DF) was further decomposed into two single DF orthogonal comparisons: (i) water vs the two microbial products, and (ii) Bacillus vs Trichoderma. The interaction of the dipping factor with the single DF Bacillus vs Trichoderma foliar spray comparison indicates whether there were synergies in alternative us of Bacillus and Trichoderma products over time.

    • Both tiller number and height increased nearly linearly with time and for both variables there were no significant interactions between treatment and time. The overall average final number of tillers per plant across all treatments was 36.7 (± 0.50). The final tiller number differed (p < 0.001) among the nine treatments: all the six treatments with microbial dipping and/or foliar spray led to more tillers than the water control (Table 1). Overall, there were no additional benefits of applying foliar spray in addition to dipping. The three microbial products did not differ from each other irrespective of the application strategy. In contrast, tiller height did not differ among the nine treatments and among the three products; the overall average final tiller height was 116.0 cm (± 1.80).

      Table 1.  Significant pairwise comparisons among the nine treatments applied to rice plants at one site in Kenya across two cropping seasons in 2022, based on the Tukey HSD test. The nine treatments included three biopesticides (Bacillus subtilis, Trichoderma asperellum and Serratia nematodiphila), each applied as dipping at transplanting, and post-transplanting foliar spray as well as dipping, and two controls (water and foliar fungicide spray). The final treatment was seed coating with S. nematodiphila.

      Treatment 1
      (dipping : spraying)
      Treatment 2
      (dipping : spraying)
      Differencesp value
      Tiller number on the final assessment date
      Water controlBacillus : Bacillus−0.1040.0247
      Water controlBacillus : Water−0.1110.0124
      Water controlSerratia : Serratia−0.0990.0383
      Water controlSerratia : Water−0.1140.0094
      Water controlTrichoderma : Trichoderma−0.1070.0189
      Water controlTrichoderma : Water−0.1090.0146
      Number of seeds per panicle (on the natural logarithm scale)
      Trichoderma : WaterSerratia : Water0.1560.0320
      Water controlBacillus : Bacillus−0.2000.0015
      Water controlBacillus : Water−0.1810.0062
      Water controlSerratia : Serratia−0.1840.0049
      Water controlSerratia : Water−0.1880.0036
      Water controlTrichoderma : Trichoderma−0.2170.0004
      Water controlTrichoderma : Water−0.2410.0001
      Water controlFungicide−0.1550.0331
      Incidence of tillers with blast symptoms (on the logit scale)
      Water controlBacillus : Bacillus0.6850.0000
      Water controlBacillus : Trichoderma0.7850.0000
      Water controlSerratia coating0.5540.0000
      Water controlSerratia : Serratia0.6180.0000
      Water controlSerratia : Water0.6080.0000
      Water controlTrichoderma : Trichoderma0.6390.0000
      Water controlTrichoderma : Water0.5320.0000
      Water controlFungicide0.5970.0000
    • On average, it took 79.0 d (± 0.53) to reach flowering onset; the length of this time did not differ significantly among the nine treatments as well as among the three products.

    • Overall, there were 89.1 seeds (± 1.91) per panicle. There were significant (p < 0.001) differences among the nine treatments in the number of seeds per panicle, due primarily to the fact that the water control had fewer seeds than all the other treatments, except the seed-coating (Fig. 1; Table 1). There were no additional benefits of applying foliar spray in addition to dipping. There were no significant differences among the three products irrespective of the application strategy. The grand average 1000 seed weight was 20.1 g (± 0.21) and there were no significant differences among the three products or effects of additional foliar applications.

      Figure 1. 

      Number of grains per head of rice plants, treated by one of the four biopesticides at different times (seed coating, dipping at transplanting, post-transplanting foliar spray, or both dipping and spray) at one site in Kenya over two cropping seasons in 2022. Two controls (water as negative, and fungicide foliar spray as positive) were included for comparison. The bar represents one standard error, and P value is associated with the F-test in ANOVA, indicating the overall differences among the nine treatments.

      The average gross grain yield was 16.1 kg (± 0.39) per plot; there were no significant differences among the nine individual treatments (Fig. 2). However, ANOVA of combined data over all three microbes (including seed-coating) showed that applying beneficial microbes once (seeding coating or dipping) or twice (dipping and foliar spray) led to a significant increase (p < 0.05) in the gross yield over the water control. The average gross yield per plot was 13.9 kg (± 0.78), 16.6 kg (± 0.61) and 16.4 kg (± 0.57) for the water control, dipping or seed-coating, and both dipping and spray treatments, respectively. No additional benefit was obtained when foliar spray was applied in addition to transplanting dipping. There were no significant differences among the three products when applied as dipping or foliar spray.

      Figure 2. 

      Gross yield (kg per plot) of rice plants, receiving one of the four biopesticides applied at different times (seed coating, dipping at transplanting, post-transplanting foliar spray, or both dipping and spray) at one site in Kenya over two cropping seasons in 2022. Two controls (water as negative, and fungicide as positive) were included for comparison. The bar represents one standard error and P value is associated with the F-test in ANOVA, indicating the overall differences among the nine treatments.

    • Across all treatments, about 7.9% of leaves had blast lesions. There were significant (p < 0.001) differences among the nine treatments in the blast incidence – the water control had a higher blast incidence than all the other treatments (Fig. 3, Table 1). Seed-coating or dipping alone managed rice blast as satisfactorily as the fungicide treatment; there were no additional reductions in blast with additional foliar application. The three microbial products did not differ in their effects on blast development when applied as dipping or foliar spray.

      Figure 3. 

      Percentage of tillers with blast symptoms for those rice plants receiving one of the four biopesticides applied at different times (seed coating, dipping at transplanting, post-transplanting foliar spray, or both dipping and spray) at one site in Kenya over two cropping seasons in 2022. Two controls (water as negative, and fungicide as positive) were included for comparison. The bar represents one standard error, and P value is associated with the F-test in ANOVA, indicating the overall differences among the nine treatments.

    • For both tiller number and height there were no significant interactions between treatment and time. The average final number of tillers per plant was 18.8 (± 0.62). The final tiller number differed (P < 0.001) among the eight treatments: all four treatments with both transplant dipping and foliar applications of Bacillus or Trichoderma led to more tillers than the water control (Table 2). Foliar spray in addition to dipping led to further increases (p < 0.05) in the number of tillers; but there were no synergies in alternate use of Bacillus and Trichoderma.

      Table 2.  Significant pairwise comparisons for all eight treatments applied to rice plants at three trial sites in Tanzania in 2022, based on the Tukey HSD test. The eight treatments included six microbial treatments and two controls (water and fungicide foliar spray).

      Treatment 1
      (dipping : spraying)
      Treatment 2
      (dipping : spraying)
      Differencesp value
      Number of tillers (on a log scale)
      Bacillus : BacillusWater control0.3070.0108
      Bacillus : TrichodermaWater control0.3280.0050
      Trichoderma : BacillusWater control0.4030.0002
      Trichoderma : TrichodermaWater control0.2840.0239
      Tiller height (cm – on a log scale)
      Bacillus : BacillusWater control0.2220.0002
      Bacillus : TrichodermaWater control0.1930.0016
      Bacillus : WaterWater control0.1810.0038
      Trichoderma : BacillusWater control0.2690.0000
      Trichoderma : TrichodermaWater control0.1830.0032
      Trichoderma : WaterWater control0.1790.0045
      Days to 30% flowering (on a log scale)
      Bacillus : WaterTrichoderma : Bacillus0.1440.0098
      Trichoderma : TrichodermaTrichoderma : Bacillus0.1310.0256
      Trichoderma : WaterTrichoderma : Bacillus0.1550.0039
      FungicideBacillus : Bacillus0.1790.0005
      FungicideBacillus : Trichoderma0.1930.0001
      FungicideBacillus : Water0.1300.0285
      FungicideTrichoderma : Bacillus0.2740.0000
      FungicideTrichoderma : Trichoderma0.1420.0110
      FungicideTrichoderma : Water0.1190.0614
      Water controlBacillus : Bacillus0.1480.0010
      Water controlBacillus : Trichoderma0.1630.0002
      Water controlTrichoderma : Bacillus0.2430.0000
      Water controlTrichoderma : Trichoderma0.1120.0296
      Number of seeds per pedicle (on a log scale)
      Bacillus : TrichodermaFungicide0.2000.0022
      Trichoderma : BacillusFungicide0.2090.0011
      Bacillus : BacillusWater control0.2480.0000
      Bacillus : TrichodermaWater control0.3240.0000
      Bacillus : WaterWater control0.2000.0002
      Trichoderma : BacillusWater control0.3330.0000
      Trichoderma : TrichodermaWater control0.2030.0002
      Trichoderma : WaterWater control0.2060.0001
      Shelled yield (kg per plot on a log scale)
      Bacillus : TrichodermaBacillus : Water0.4600.0086
      Trichoderma : BacillusBacillus : Water0.5830.0003
      Bacillus : TrichodermaTrichoderma : Water0.5450.0009
      Trichoderma : BacillusTrichoderma : Water0.6690.0000
      Bacillus : BacillusFungicide0.5140.0021
      Bacillus : TrichodermaFungicide0.7370.0000
      Trichoderma : BacillusFungicide0.8610.0000
      Trichoderma : TrichodermaFungicide0.5180.0019
      Bacillus : BacillusWater control0.8820.0000
      Bacillus : TrichodermaWater control1.1050.0000
      Bacillus : WaterWater control0.6460.0000
      Trichoderma : BacillusWater control1.2290.0000
      Trichoderma : TrichodermaWater control0.8860.0000
      Trichoderma : WaterWater control0.5600.0000
      FungicideWater control0.3680.0202
      Incidence of grains with blast symptoms (on the logit scale)
      Bacillus : WaterBacillus : Trichoderma0.8540.0030
      Bacillus : WaterTrichoderma : Bacillus0.6830.0350
      Trichoderma : WaterBacillus : Trichoderma0.7030.0266
      Water controlBacillus : Bacillus1.2500.0000
      Water controlBacillus : Trichoderma1.5800.0000
      Water controlBacillus : Water0.7260.0037
      Water controlTrichoderma : Bacillus1.4090.0000
      Water controlTrichoderma : Trichoderma1.0610.0000
      Water controlTrichoderma : Water0.8770.0002
      Water controlFungicide1.0120.0000

      The average final height was 34.1 cm (± 1.09). The final plant height differed (p < 0.001) among the eight treatments: all six microbial treatments increased plant height over the water control (Table 2). Foliar spraying with Bacillus led to higher plants than Trichoderma (p < 0.05); there were no synergies in alternate use of Bacillus and Trichoderma.

    • On average, it took 66.9 d (± 1.20) to reach 30% flowering. The time to 30% flowering differed (p < 0.001) among the eight treatments, due primarily to the fact that microbial treatments reduced the time to 30% flowering (Fig. 4; Table 2). For instance, the time to 30% flowering for the water control was 73.9 d (± 1.63), compared to 67.7 d (± 2.45) for dipping only treatments. Applying microbial products as foliar spray at the booting stage also reduced (p < 0.01) the time to 30% flowering, which was due primarily to the synergies in alternate use of Bacillus and Trichoderma (Fig. 5): time to 30% flowering was 60.6 d (± 1.95) for the two treatments of using the two products in alternative, compared to 65.5 d (± 2.51) for the two treatments using the single products as dipping and foliar spray. The average time to grain maturity was 95.5 d (± 1.42). The treatment effects on the time to maturity were similar to, but less pronounced than, those on the time to 30% flowering.

      Figure 4. 

      Number of days to 30% flowering of rice plants that had received water/fungicide or one of the two biopesticides as dipping at transplanting only, or both as dipping and as foliar spray at three sites in 2022 in Tanzania. The bar represents one standard error, and P value is associated with the F-test in ANOVA, indicating the overall differences among the nine treatments.

      Figure 5. 

      Net grain yield (kg) per plot (size 5 m × 5 m) of rice plants that had received water/fungicide or one of the two biopesticides as dipping at transplanting only, or both as dipping and as foliar spray at three sites in Tanzania in 2022. The bar represents one standard error, and P value is associated with the F-test in ANOVA, indicating the overall differences among the nine treatments.

    • Overall, there were 107 seeds (± 1.91) per panicle. The eight treatments differed (p < 0.001) in the number of seeds per panicle, due primarily to the fact that the water/fungicide control had fewer seeds than microbial treatments (Table 2). Foliar spraying led to further increases (p < 0.01) in the number of seeds per panicle, due primarily to the synergies in alternate use of Bacillus and Trichoderma: 88.8 seeds (± 1.95) (water control), 108 seeds (± 2.62) (dipping only), 111 seeds (± 2.76) (dipping + spray with the same product), and 122 seeds (± 1.56) (alternate use of the two products). This synergy was highly significant (p < 0.001).

      On average, the net grain yield was 3.13 kg (± 0.167) per plot. There were significant (p < 0.001) differences among the eight treatments (Fig. 5), accounting for 72.9% of the total observed variability. The water control had a lower yield than all the other treatments, and the fungicide treatment had a lower yield than the four treatments with microbial products applied both at dipping and post-transplanting (Table 2). Foliar spraying led to further increases (p < 0.001) in the net grain yield, and there was a significant (p < 0.01) synergy from alternate use of the Bacillus and Trichoderma products. The average net grain yields achieved were 1.51 kg (± 0.123) (water control), 2.77 kg (± 0.210) (dipping only), 3.56 kg (± 0.195) (dipping + spray with the same product), and 4.70 kg (± 0.227) (alternate use of the two products). Overall, the two Bacillus and Trichoderma products did not differ in grain yield when applied as a dipping or foliar spray.

    • On average, the incidence of grain with blast symptoms was 16.1% (± 1.20%). There were significant (p < 0.001) differences among the eight treatments (Fig. 6), accounting for 57.9% of the total observed variability. The water control had a higher blast incidence than the other treatments and the six microbial treatments did not differ in the blast incidence from the fungicide control (Fig. 6 & Table 2). Foliar spraying led to further reductions (p < 0.001) in the blast incidence, and there was a significant (p < 0.05) synergy in blast control from alternate use of the two Bacillus and Trichoderma products. The incidences of grains with blast were 28.9% (± 1.94%) (water control), 15.5% (± 1.62%) (dipping only), 11.4% (± 1.36%) (dipping + spray with the same product), and 8.49% (± 1.24%) (alternate use of the two products). Overall, the two Bacillus and Trichoderma products did not differ in the blast incidence when applied as dipping or foliar spray.

      Figure 6. 

      Percent grain with blast symptom in each plot of rice plants that had received water/fungicide or one of the two biopesticides as dipping at transplanting only, or both as dipping and as foliar spray at three sites in Tanzania in 2022. The bar represents one standard error, and P value is associated with the F-test in ANOVA, indicating the overall differences among the nine treatments.

      Yield increase achieved by the two microbial products was unlikely due entirely to the reduced blast development. All the six microbial treatments did not differ in the blast incidence from the fungicide control, but four microbial treatments where microbial products were applied both as dipping and foliar spray resulted in higher yields than the fungicide control (Table 2). Figure 7 shows grain blast incidence plotted as against grain yield for individual plots. Many microbial-treated plots had much higher yields than the fungicide-treated plots although the blast incidence was similar.

      Figure 7. 

      Rice grain yield against the % grain with blast symptom in each plot of rice plants that had received water/fungicide or one of the two biopesticides as dipping at transplanting only, or both as dipping and as foliar spray at three sites in Tanzania in 2022.

    • We extended our previous research to assess the effects of using commercial microbial products on rice blast and yield in Tanzania (low yield region - subsistence agriculture) and Kenya (relatively high yield and irrigated paddy rice production region). Low yield obtained in Tanzania, compared to that in Kenya, may have resulted from several factors, including variety genetic potential, blast development (higher in Tanzania), soil fertility, irrigation (used in the Kenya) and climatic conditions. Using microbial products led to significant reductions in blast development with the control efficacy similar to the fungicide treatment. In addition to agreeing with the previous study[18] on the increased yield due to microbial products in the subsistence farming region, the present results demonstrated that alternate use of B. subtilis and T. asperellum at transplanting (as dipping) or post-transplanting (as foliar spray) led to further yield increase (ca. 32.2%, 457 kg per ha) over the use of a single microbial product over time. Alternate use of B. subtilis and T. asperellum resulted in about 123.8% (1.04 ton per ha) and 211.0% (1.28 ton per ha) increase in grain yield over the fungicide and the water controls, respectively. Comparison with the fungicide control suggested that increased yield resulted mostly from improved plant development rather from reduced blast incidence.

      All microbial treatments led to significant reductions in the blast incidence and achieved an efficacy similar to the fungicide control, although the blast incidence was lower than previous studies at the same site[18]. This agrees with previous studies demonstrating that other B. subtilis and Trichoderma strains can reduce rice blast development[1416,21]. The blast fungus can colonise roots and lead to systemic invasion and classical disease symptoms on the above-ground plant parts[22]. In addition to direct competition with the blast fungus in the roots, root dipping and seed-coating may affect the pathogen via their effects on rhizosphere and endophyte microbiome. However, we recently demonstrated that rhizoplane and root endophytes of rice seedlings were not much affected by dipping in either B. subtilis Bs01 or T. asperellum T-900 (Xu, unpublished). Reduced blast development could also have resulted from the indirect effect through plant defence responses induced by the applied beneficial strains. Numerous studies have demonstrated that plant defence responses induced by Bacillus or Trichoderma spp. led to improved plant tolerance or resistance against specific pathogens and/or improved plant development[2328].

      Reduction in blast development is, however, not necessarily equivalent to yield gains, especially for low yielding sites in Tanzania: several microbial treatments led to much greater yield than the fungicide control although the blast incidence was similar. At low yielding sites, applying microbial products led to large increases in grain yield as demonstrated previously[18], due primarily to the increased number of tillers per plant and grains per panicle. In addition, microbial treatment, particularly dipping at the transplanting stage, shortened the time to flowering and maturity. Thus, we speculate that increased yield is due largely to improved plant development induced by applied microbes, rather than due directly to reduced blast development. Even at the high yielding site in Kenya, microbial treatments led to an overall significant increase (ca. 20%) in grain yield over the untreated control. However, as the microbial treatments had similar levels of blast development and grain yield as the fungicide control at the Kenya site, it is not possible to exclude the possibility that the yield increase associated with microbial treatments is due entirely to reduced blast development.

      The three formulated products (B. subtilis, T. asperellum and S. nematodiphila strains) did not differ significantly in terms of rice blast development and grain yield at the Kenya site when applied as dipping at transplanting or post-transplanting foliar spray. Similarly, the B. subtilis and T. asperellum products did not differ significantly at three sites in Tanzania. Overall, these results are consistent with our previous study[18] although the previous study did show some-site specific differences between B. subtilis and T. asperellum products in Tanzania. Moreover, seed-coating with one specific S. nematodiphila strain led to a similar effect as transplanting dipping with or without foliar spray. In addition to induced host defence responses, seed-coating may also influence rhizosphere microbiome (as with dipping) that may affect nutrient uptake. Coating seeds with beneficial microbes is an efficient system to deliver beneficial microbes for improving seed germination and seedling establishment[6,7]. Efficacy of seed priming or dipping seedlings with beneficial microbes have been demonstrated in controlled conditions to suppress rice blast development, including B. subtilis and Trichoderma strains[1416,21]. In the present study, we demonstrated the positive effect of seed-coating with beneficial microbes on rice productivity under commercial production conditions.

      Additional foliar spray following a transplant dipping treatment with the same product did not lead to any additional benefit in Kenya. In contrast, additional spray with the same product as for dipping led to additional yield increases (average 313 kg per ha) in Tanzania, though not as much as increases (average 504 kg per ha) achieved by the dipping only treatments. As we argued above, increased yield is likely due primarily to induced plant responses. Thus, additional foliar applications of the same products as used in dipping (around 6-9 weeks after transplant dipping) may have strengthened plant development pathways previously induced by dipping. Such strengthened plant responses may be more important for plants grown under more stressful conditions. The Kenya experiment was conducted in the irrigation area. In contrast, the three Tanzanian experimental sites relied on rain-fed water, and thus plants may have experienced some drought stress at these sites. Indeed, second season crops were planted in the same sites in order to repeat the trials, as we did in Kenya. However, these repeat trials failed to produce grains because of drought in late 2022 in Tanzania. The differing degree of abiotic stress between Kenya and Tanzania experimental sites may thus explain the differences in the effects associated with additional foliar sprays.

      Variable biocontrol efficacies and consistencies have led to the suggestion of using multiple biopesticides simultaneously to exploit possible synergies among biopesticides and hence improve performance as well as consistencies. However agonistic interactions between component microbes have often been observed, and thus synergistic interactions among biopesticides have rarely been achieved[19]. Our previous study[18] demonstrated antagonistic interactions between B. subtilis and T. asperellum when used in a mixture as dipping or foliar spray. Thus, in the present study, we investigated alternate use of B. subtilis and T. asperellum products over time for their effects on rice productivity. To our surprise, alternate use of the two products consistently outperformed the use of single products, leading to an average 32.2% increase in grain yield (458 kg per ha) over the single products. This additional benefit resulting from the alternate use of the two products may have resulted from the fact that the two (B. subtilis and T. asperellum) strains do not induce the same host responses either qualitatively (i.e., inducing different pathways) or quantitively (i.e. same pathways but induced to a different degree). Thus, applying the two products alternately over time may lead to a more complete plant response. Strains from B. subtilis and Trichoderma spp. can induce systemic defence response on many plant species[23, 2631]. When applied in a mixture, direct competition between the two strains may have weakened the induced plant response. In addition, plant responses at a given time might be limited and hence may not be able to fully respond to the simultaneous use of the two strains. To quantify and statistically test the synergy in the alternative use of the two strains is difficult because the exact definition of synergy (hence statistical test) depends on the extent of overlap in induced plant responses by the two microbes and also dose-response relationships[19].

      In summary, all microbial treatments led to significant reductions in rice blast and increased rice grain yield, particularly in the low yielding region in Tanzania. Alternate use of formulated B. subtilis and T. asperellum products at transplanting (as dipping) or post-transplanting (as foliar spray) led to additional yield increase in the low yielding region over the use of a single organism product at both times. Seed coating with one Serratia strain led to blast control and grain yield comparable to applying microbial products as transplant dipping and post-transplanting foliar spray as well as the fungicide control. Thus, present research results support the use of microbial products, particularly alternative use of different microbial products over time, to improve rice productivity in subsistence farming. Future research is needed to exploit potential benefits of combining seed coating with transplant dipping and/or post-transplanting foliar spray.

      • This research is funded by Innovate UK (project number 105665).

      • Xu X and Robinson-Boyer L declare no conflict of interest; Murunde R and Ringo G are employed by Real IPM who produced the three commercial biopesticides evaluated in the present study. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

      • Copyright: © 2023 by the author(s). Published by Maximum Academic Press, Fayetteville, GA. This article is an open access article distributed under Creative Commons Attribution License (CC BY 4.0), visit https://creativecommons.org/licenses/by/4.0/.
    Figure (7)  Table (2) References (31)
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    Murunde R, Ringo G, Robinson-Boyer L, Xu X. 2023. Applying beneficial microbes as transplanting dipping and post-transplanting foliar spray led to improved rice productivity. Technology in Agronomy 3:7 doi: 10.48130/TIA-2023-0007
    Murunde R, Ringo G, Robinson-Boyer L, Xu X. 2023. Applying beneficial microbes as transplanting dipping and post-transplanting foliar spray led to improved rice productivity. Technology in Agronomy 3:7 doi: 10.48130/TIA-2023-0007

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