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From Ghana to the dry-hot valleys of China: assessing factors influencing fruit yield in agroforestry species Vitellaria paradoxa after 54 years of cultivation outside Africa

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  • Although distributed across the Sudano-Sahelian region as an agroforestry system tree species, Vitellaria paradoxa has yet to be reported as successfully established outside of Africa, significantly limiting its yield and further exploitation. In this paper, in order to assess a well-established population of V. paradoxa in the Yuanjiang dry-hot valley of China and examine the relationships between morphological-geological factors and fruit yield, we monitored dendrometric traits and fruiting across 844 shea trees located on different aspects, and applied partial least square regression to build a yield model based on dendrometric and geographical variables. Results revealed climatic resemblance of the introduction site in Yuanjiang to the natural habitat in Ghana, and the growth performance and fruit yield were also comparable, but accumulated precipitation of about 600 mm was better for fruit yield when heat accumulation reached about 2,000 °C. Apart from crown diameter (p < 0.05), dendrometric parameters (basal diameter, basal area and tree height) had positively weak relationships with fruit yield. On the contrary, aside from north and northeast aspect, other aspects showed a strong negative influence. The findings presented that growth and productivity of V. paradoxa increased with dendrometric parameters and monthly average temperature on shady and semi-shady slope, providing a theoretical basis for the development of shea tree and construction of agroforestry system in dry tropical areas outside Africa.
  • 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.

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  • Cite this article

    Zhao G, Ranjitkar S, Ayemele AG, Li T, Wang X, et al. 2023. From Ghana to the dry-hot valleys of China: assessing factors influencing fruit yield in agroforestry species Vitellaria paradoxa after 54 years of cultivation outside Africa. Circular Agricultural Systems 3:7 doi: 10.48130/CAS-2023-0007
    Zhao G, Ranjitkar S, Ayemele AG, Li T, Wang X, et al. 2023. From Ghana to the dry-hot valleys of China: assessing factors influencing fruit yield in agroforestry species Vitellaria paradoxa after 54 years of cultivation outside Africa. Circular Agricultural Systems 3:7 doi: 10.48130/CAS-2023-0007

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From Ghana to the dry-hot valleys of China: assessing factors influencing fruit yield in agroforestry species Vitellaria paradoxa after 54 years of cultivation outside Africa

Circular Agricultural Systems  3 Article number: 7  (2023)  |  Cite this article

Abstract: Although distributed across the Sudano-Sahelian region as an agroforestry system tree species, Vitellaria paradoxa has yet to be reported as successfully established outside of Africa, significantly limiting its yield and further exploitation. In this paper, in order to assess a well-established population of V. paradoxa in the Yuanjiang dry-hot valley of China and examine the relationships between morphological-geological factors and fruit yield, we monitored dendrometric traits and fruiting across 844 shea trees located on different aspects, and applied partial least square regression to build a yield model based on dendrometric and geographical variables. Results revealed climatic resemblance of the introduction site in Yuanjiang to the natural habitat in Ghana, and the growth performance and fruit yield were also comparable, but accumulated precipitation of about 600 mm was better for fruit yield when heat accumulation reached about 2,000 °C. Apart from crown diameter (p < 0.05), dendrometric parameters (basal diameter, basal area and tree height) had positively weak relationships with fruit yield. On the contrary, aside from north and northeast aspect, other aspects showed a strong negative influence. The findings presented that growth and productivity of V. paradoxa increased with dendrometric parameters and monthly average temperature on shady and semi-shady slope, providing a theoretical basis for the development of shea tree and construction of agroforestry system in dry tropical areas outside Africa.

    • Vitellaria paradoxa (shea tree) is an economically and ecologically important tree capable of adapting to oligotrophic shallow soils and seasonally dry savanna environments in the Sudano-Sahelian region[14]. In particular, the seeds of V. paradoxa are used to produce 'shea butter' that is used locally as an edible oil and in traditional medicine as well as being commercially exploited for export, thereby earning the name 'the emerald of vegetable oil'[5]. The economic importance of V. paradoxa has prompted researchers to investigate its introduction, silviculture, agroforestry ecosystem, ecology, productivity and exploitation[69].

      Introduction and plantation history of V. paradoxa in China: From December 1963 to February 1964, Premier Zhou En-lai and delegation were shown the fruits and products of V. paradoxa In Ghana, Yi Chen, the Chinese Foreign Minister at the time, brought seeds of V. paradoxa to the botanist Cai Xitao. Based on his field experience in Yunnan, Xi-tao Cai recognized the potential for developing V. paradoxa cultivation trial at Yuanjiang Research Station, which features savanna-like vegetation with regularly burnt grassland and dry thickets. In 1982, the research station closed, and approximately 12 hectares of successfully planted shea trees were handed over to the Yuanjiang County Forestry Bureau[10]. These trees have been growing now for over 54 years.

      Internationally, shea trees is best known for its commercial value estimated at multi millions of dollars across cosmetic, pharmaceutical and chocolate industries[11]. Pouliot & Elias[7] estimated that shea trees can potentially funnel about USD150 million to the local economy and contribute up to 12% of total household income in Burkina Faso. Moreover, shea trees are also an important tree species in the agroforestry system in Africa, which integrates ecological and economic benefits[9,12]. To date, most research has focused on its chemical components, fats, proteins and amino acids[1315]. Studies on V. paradoxa ecology and fruit yield have been carried out in countries where this species naturally occurs[16,17]. Although the natural range of this monotypic species extends across over 20 African countries in a narrow belt of well-drained soils, ranging from the savannas of eastern Senegal to the high plateau of east Africa and into south-eastern Uganda[13,18], V. paradoxa has not yet been reported as successfully established outside of Africa, ultimately limiting its output and development. Therefore, a better understanding of factors influencing the productivity of V. paradoxa planted outside Africa is necessary for cultivation and utilization.

      In order to improve the yield and further accelerate the development of V. paradoxa outside Africa, understanding the relations between fruit yield and geographical condition are important and of significant interest to the scientific community. We aim to develop a model to understand and predict fruit yield in response to the dendrometric and geographical characteristics at the plantation site, and explore how shea butter trees grown outside Africa have adjusted to the introduced area and how selected variables affect fruit yield.

    • The research was carried out at the Yuanjian dry-hot valley, located in Yuanjiang County, Yuxi City, Yunnan Province, China (N: 23.601496, E: 101.970145) (Fig. 1). The altitude is about 450 m and annual sunshine duration is 2,239.5 h. The mean annual precipitation is 780−850 mm, most of it (about 80%−85%) falling in the rainy season (May to October)[19]. The vegetation is similar to 'Savanna vegetation', presenting an expansive grassland with scattered trees and shrubs[20]. Moreover, other cultivated species are mainly Mangifera indica, Borassus flabellifer, Litchi chinensis and Delonix regia.

      Figure 1. 

      Plantation area of shea butter in Yunnan, China. Shea butter distribution in the plantation site were mapped along with fruiting trees and fruiting intensity.

      Temperature records obtained from a meteorological station about a kilometer from the shea butter plantation site show warming trends (particularly after 2000) (Fig. 2a). Average temperature change per decade was +0.19 °C from 1973 to 2018, while after 2000 change per decade was +0.58 °C. The mean temperature at the area was 24.36 ± 0.52 °C from 1973 to 2018, while the minimum temperature was 19.09 ± 0.41 °C and the maximum temperature 30.71 ± 0.7 °C. Average annual precipitation was 851.07 ± 192.99 mm, with precipitation levels peaking from 1991 to 1993 (Fig. 2b). After the year 2000, annual precipitation variation was about 200 mm, with no discernible pattern of increase or decrease.

      Figure 2. 

      (a) Temperature and (b) precipitation trend from weather data between 1973 to 2018 recorded at the Yuanjiang, China meteorological station.

    • Shea butter trees were planted across approximately 8.86 hectare of land. We were informed that originally the trees were spaced 6 m apart when Chinese scientists first introduced the shea trees to Yuanjiang, China. We collected data of all trees on the site and the geo-location of each tree was positioned and mapped precisely (Fig. 1). We measured the DBH, basal diameter, height and crown diameter of the trees. Among all measured trees, some showed different growth characteristics, such as multiple trunks. Such trees referred to as outliers in the Grubbs test (p < 0.01) were removed. Each tree was carefully observed for fruiting condition and recorded. We assigned 'high' for a tree with a higher amount of fruit, 'less' for fewer fruit, and 'medium' for in-between, with 'no fruit' used for the trees that did not fruit in the survey year. Because alternating between high and low fruit yields (or no fruit) in consecutive years is common in large fruiting perennial trees, we accordingly supplemented our observations on fruiting by surveying fruiting conditions of 2018 and 2019 and conducting informal interviews with farmers. Interviewed farmers perform agricultural labor at the Yuanjiang Station year-round. They conduct annual weeding and possess a systematic understanding of the growth habits of shea tree; some of them provided key information regarding the recent growth conditions of the trees and assisted us in fruit yield surveys. We calculated Growing Degree Days (GDDs) using temperature datasets from the same time period. Our best threshold range was between 10 and 25 °C, where for computing the GDDs, we used 18 °C as the optimal base temperature. For each year of observation, the total amount of heat accumulation after bud formation till fruit maturity was estimated. Heat accumulation and precipitation data were plotted against fruit yield data to determine the heat accumulation and precipitation required for fruit growth in the plantation site shea tree[20]. The Kruskal-Wallis test was conducted to check if differences between dendrometirc characteristics of recorded trees yielded a different amount of fruit.

      We selected five sites within the plantation area across east-, north-, west- and south-facing slopes as well as the hilltop. Three fruit-containing shea trees at each of the selected sites were randomly marked for recording. Next, three trees were randomly selected from each sample site to collect all the fruit on each tree and measure fruit yield per tree. Then, the fruits of different trees were weighed on scales separately. We selected five different sample sites to evenly distribute study points facing all ordinal directions with the consideration that the different sunlight times could affect fruit yield.

    • The relationship between fruit yield to dendrometric and geographical factors was evaluated using partial least square (PLS)[21,22]. Fruit yield from 15 trees were used as response variables (Y), while tree DBH, basal diameter, height, crown diameter, solar radiation index (SRI), slope, and monthly temperature during active growth period of shea tree during observation years were used as predictors (X). Monthly temperature for each site was calculated using lapse rate method. The temperature lapse in the studied mountain is approximately −0.6 °C for each 100 m elevation gain. The SRI was calculated as absolute radiation index (ARI) is related to the latitude of the surface and thus appropriate to compare the solar radiation received by areas located at sites of the same latitude. In this paper, fruit yield parameters refer to the fresh weight of fruit per tree weighed in the field. The number of significant PLS factors to build the model was determined by a cross-validation method in which 25% of randomly selected data were used to validate the calibration model used by 75% of the data set. The Q2 cumulated index measures the global goodness of fit and predictive quality of the models. Ideally, the cumulated index should be close to 1, while a lower value suggests that the yield varies significantly depending on the selected predictor variables. Calculated R2Y and R2X correspond to correlations between the explanatory (X) and dependent (Y) variables with the components generated by PLS regression. A parameter called Variable Importance in the Projection (VIP) was assessed to determine the importance of different variables in the model both with respect to dependent and other independent variables[23]. VIP scores are based on a weighted sum of squares of the PLS loading and calculated for each variable taking into account the amount of explained Y variance. The major advantage of this is that there will be only one VIP-vector, summarizing all factors and Y-variables, thereby enabling us to identify predictor variables influencing the prediction models. Independent variables with VIP ≥ 0.8 have been suggested to discriminate between relevant and irrelevant predictors for explaining fruit yield[2426]. Standardized model coefficients were used to interpret the results of the regression[26].

      PLS regression first determines latent factors, a variant of principal components, to reduce the dimensionality of the independent (and dependent) variables[24]. Partial least square regression is thus beneficial for highly auto-correlated variables, such as closely related data collected from the field. It then uses these factors as independent factors in a linear model to explain variation in the dependent variables[27]. Additionally, PLS models confer several advantages, such as their capability to handle collinearity, detect outliers, analyze multiple responses and easily interpret visual data[28,29]. As some predictor variables in our study are derived from primary measurements, it is legitimate to expect collinearity problems in the data, which leads to poor prediction performance.

    • We found that the mean annual accumulated temperature during 1973 to 2018 was 8,853.1 ± 201.1 °C when daily mean temperatures above or equal to 10 °C were combined. Heat accumulation increased in recent years compared to earlier decades, indicating very likely changes in the phenology. Similarly, number of rainy days recorded ranged from 81 to 170 d for the same period. Number of days receiving rainfall was found to be increasing in recent years compared to earlier decades. During field work, we observed well established shea trees at the Yuanjiang Research Station and collected dendrometric characteristics of shea trees and are presented in Table 1 (n = 794).

      Table 1.  Tree and site characteristics in the study site.

      StatisticHeight (m)DBH (cm)Basal
      diameter (cm)
      Crown
      diameter (m)
      Altitude (m)
      Minimum5.217.217.52.4451
      Maximum14.540.352.510.6509
      Mean9.328.232.46.1487
      Standard deviation (n-1)1.74.95.91.110.1
      Number of trees (n) = 794.

      Of the 794 trees measured in 2018, 395 trees were not bearing fruit. The remaining 399 trees bore fruit, and we categorized these trees based on the amount of the fruit produced. In the 'less' category, we recorded 273 trees that had few fruits. In the 'medium' and 'high' categories, we recorded 79 trees that bore a significant amount of fruit and 47 trees that bore a higher number of fruits, respectively. In 2019, 12 trees were felled and out of the remaining 782 trees, 365 were not bearing fruit. One hundred and seventy three trees bore 'less' fruit, while in 'medium' and 'high' category 142 and 102 trees were recorded (Table 2). We recorded phenological events in the shea tree, which showed that flower bud appeared during early April, fruit appeared during late May and matured by September (Fig. 3). Leaf phenology began in February and the whole of this period was referred to as the active growth period of shea tree in Yuanjiang Research Station. Fruit transverse diameter measured 29.3 ± 0.9 mm, while longitudinal diameter was 33.8 ± 1.06 mm (n = 20). We selected 15 trees randomly to study fruit yield, and the fruit yield ranged from 6.0 to 76.8 kg per tree (average 33.95 ± 21.31 kg) in 2018 and from 16.25 to 86.0 kg per tree (average 42.3 ± 22.61 kg) in 2019.

      Table 2.  The number of fruiting trees at different levels in 2018 and 2019.

      Year'no fruit''less''medium''high'Total
      20183952737947794
      2019365173142102782
      In 2019, 12 trees were felled.

      Figure 3. 

      The response of phenological stage to heat accumulation and precipitation. Variation in color reflects change in the phenological stages from flower to fruit.

    • We recorded the phenology stage of shea trees, sampled from the whole plantation area. Each phenology stage was plotted against the median value of number of days required to reached that particular stage (Fig. 3). Standard deviation for each stage was calculated and plotted. Heat accumulation and total precipitation during different phenological stages was plotted to understand their relationship. Phenology development stages closely follow heat accumulation, indicating about 350 °C was necessary for bud formation, about 900 °C for fruit formation, about 1,700 °C for color change in the fruit (beginning of biological maturation of the fruit) and above 2,000 °C for fruit maturation (Fig. 3). Similarly, total accumulated precipitation of about 600 mm was good for fruit maturation when heat accumulation reached about 2,000 °C.

    • The Kruskal-Wallis test revealed that apart from crown diameter, there was no significant difference in dendrometric characteristics between different categories of fruiting conditions as well as trees with 'no-fruit' (Table 3). Results indicated that differences in the fruiting condition can be attributed to crown diameter, while remaining tree measurements were not significantly related to the fruiting condition.

      Table 3.  Summary of Kruskal-Wallis test showing dendrometric characters for various fruiting conditions.

      Variablep-value (2018)p-value (2019)
      BA (m2)0.380.09
      Basal diameter (cm)0.110.12
      Crown diameter (m)0.04*0.02*
      DBH (cm)0.380.09
      Height (m)0.230.49
      * Significant at p < 0.05.

      A correlation analysis between all the dendrometirc variables revealed that tree height was correlated with DBH, basal area and basal diameter. Similarly, DBH was highly correlated with basal diameter and basal area. All correlations were significant at p < 0.05. Remaining variables were not correlated with each other (Table 4). A multicollinearity test among the explanatory variables indicated that DBH had highest VIF value (Table 4) and could affect the results of the regression. Although PLS regression deals with collinearity issues, we removed DBH from the PLS regression. However, monthly temperatures were retained in the PLS analysis although high correlation among monthly temperatures were well established.

      Table 4.  Correlation matrix of dendrometric and geographical variables (explanatory and response) used in PLS regression and results of multi-collinearity test among explanatory variables.

      VariablesBABasal DCrown DDBHHeightSlopeSRIYield
      BA0.850.490.930.77−0.440.180.46
      Basal D0.260.920.84−0.170.210.55
      Crown D0.310.23−0.43−0.420.50
      DBH0.76−0.350.290.50
      Height−0.260.240.42
      Slope−0.180.14
      SRI0.27
      VIF12.7711.642.8017.574.051.911.85
      VIF6.516.332.593.601.781.82
      Variables in bold represent the response variable while all others were explanatory variables. Variable highly correlated (> 0.65) are italicized. BA, basal area; D, diameter; DBH, Diameter at breast height; SRI, solar radiation index. VIF stands for variance inflation factor that quantifies the severity of multicollinearity.
    • We generated PLS models using 14 dendrometric, geographical and fruit yield variables. Out of 30 replicated models, we selected nine models based on the regression coefficient (R2) value (Fig. 4). However, we selected best model that showed least RMSE and higher R2. The model described 74% of the variation in the predictor block (R2X) that in turn explained 67% of the variations in fruit yield parameters (R2Y). The predictive quality of the models according to cross-validation was substantially high as indicated by the Q2 value (52%). The median value of Q2 suggested that the fruit yield varied, depending on several of the selected predictor variables. The goodness of model fit statistics show R2 = 0.67 and RMSE = 11.58.

      Figure 4. 

      PLS analysis showing VIP and model coefficient.

      PLS analysis revealed that most of the variables used in the model were important variables contributing to the model and explaining fruit yield. Among these important variables, crown diameter was the most influential variable in explaining fruit yield. The standard coefficient of the model revealed that the larger the crown diameter, the higher the fruit yield per tree (Fig. 4). Similarly, basal diameter, basal area and tree height also positively affected the fruit yield.

      Regarding geographic variables, slope was a major influential factor in the model, though it showed a strong negative influence. The model revealed that the monthly average temperature during the active growth period of shea tree was important for fruit yield. February and March temperatures were not important as revealed by a VIP value less than 0.8, while average temperatures of other months were important. All temperature variables except that of July positively affect the fruit yield (Fig. 4). However, the magnitude of the contribution of temperature variable were not high compared to dendrometirc variables.

      When considering all nine selected models, six models revealed crown diameter as the most influential variable (Fig. 4). However, basal diameter was important in all nine models. However, basal diameter influence the models least compared to crown diameter. Across all the model dendrometirc variables shows consistency in the influence direction (positive or negative), while other variables show difference across these models. Hence, dendrometric variables were more reliable.

    • To our knowledge, we presented a healthy growth and well established small population of shea trees outside of Africa. It is not clear whether the individuals introduced to Yuanjiang originated from a single mother tree or from a heterogeneous population in Ghana. However, they have grown well, and now 54 years later, the plantation produces a significant amount of fruit. We documented 844 and 832 strong and healthy shea trees in 2018 and 2019 respectively. According to a respondent at the Yuanjiang Research Station, the total number of shea trees planted in 1964 exceeded 1,800. Tree counts in 1981 and 2010 documented 1,702 and 1,074 trees, respectively[10], and by 2019, only 832 trees remain, with their total area reduced from 12 ha to 8 ha. Though trees have grown, produced viable seeds that drop and regenerate in the area, it is not allowed to expand[30]. Caretakers and farmers tell us that disease and wind damage are responsible for the death of some trees.

      In addition to their fruit production, these V. paradoxa trees have important historical and research values. Most of the shea trees planted in 1964 reached fruiting stage after 8 years, and the fresh weight of fruits per tree is comparable to V. paradoxa growing in Burkina Faso, West Africa[16]. Fruit yields at the Yuanjiang Research Station are comparable to those of African countries; however, fruit size , is relatively large compared to those reported from Ghana[31,32]. Although the current plantation site is comparatively smaller, dendrometric characteristics varied across different elevations, aspects and slopes; such variation is typical in hilly terrain and attributable to micro-climatic variations and soil conditions[16,27,33]. Buba et al.[34] reported such variation in dentrometric characteristics from different parts of Africa, including within the same as well as different populations and in different land uses. Several studies mentioned the relationships between dendrometric characteristics and fruit production in shea tree as well as other trees[16,35,36]. Studies have shown that tree diameter at breast height, tree height, basal diameter, basal area and crown diameter are the most commonly used measurements[33,37]. However, in our study, no differences were detected for all dendrometric variables except for crown diameter in fruiting condition. These tree characteristics explain the growth and phenology of V. paradoxa at the Yuanjiang Research Station.

      Several studies in Africa focus on the fruit yield of shea butter trees while examining tree growth characteristics. These studies have shown either weaker relationships between dendrometric characteristics and fruit yield, or stronger relationships, particularly with tree height, DBH and crown area/diameter[16,35,38]. Our findings were consistent with African research regarding the influence of dendrometric characteristics, such as crown diameter, basal diameter and basal area, on fruit yield[16,17,35,39]. In addition, we analyzed geographical factors, as Yunnan Province is a mountainous region, and factors such as slope and temperature difference with elevation might play important roles in tree growth and fruiting[40,41]. These geographical factors contributed to the model; however, strong correlation could not be found. Temperature across the active growing period shows a weak influence in the model. Fruiting from only two seasons and temperature estimation that is used in the model might not fully explain the influence of temperature on the fruit yield. Our model indicates that the higher the temperature during the early growth period, fruit yield would be higher and high temperature during fruit development and maturation might cause less fruit yield. However, in our model influence of temperature is not very strong, therefore, further detailed study on phenology of shea tree and climatic influence is required to develop climate based fruit models. In our model, slope is influential, this might be attributed to moisture and wind distribution because soil water mostly drains downward in slopes that could minimize water availability affecting fruit yield[31,42]. Wind intensity is comparatively stronger at higher elevations potentially causing flower and fruit drop before harvest, and pre-harvest winds are reported to reduce the fruit yield of individual V. paradoxa trees by half or even more in Africa[38,43]. Finally, other unknown factors may exist that influence shea fruit yield beyond ones considered in this study[43].

      As we only had access to two-years worth of data, more longer-term data would be useful to quantify the inter-annual variability in V. paradoxa fruit yields. Variations in fruit yield have been noted by the caretaker of the plantation at the Yuanjiang Research Station. Inter-annual variability has been commonly reported for V. paradoxa and may be attributable to weather variability in recent years[1,38,44]. We also noticed approximately 50% of the trees in the plantation not bearing fruit in the survey years. The caretaker also mentioned that some trees bear fruit in alternating years, which could be due to different ecological gradients than in the natural range of shea tree. It is thus critical to understand ecological, physiological or other factors that may be responsible for bearing fruit in alternating years[43]. Hence, research regarding detailed phenology of shea trees in Yuanjiang Research Station will provide a theoretical basis and scientific reference for shea management and possibly pave the way for their introduction and planting as a species useful in retarding desertification in tropical dry valley areas. Understanding these dynamics will be key for effective management and developing commercial-level extraction of the product from shea trees in China.

      In Yuanjiang, shea trees are planted at only one site, precluding the opportunity to compare growth characteristics and fruit yields across different climatic conditions. Our data could not support or explore the causes of such phenomena, as the climatic conditions and elevation of the plantation area is within the comfort zone of shea trees. Studies carried out in the natural range of shea trees have reported that temperature has less influence compared to the pattern of rainfall in the fruiting of this tree, and results from a past study showed that shea productivity in Cote d'Ivoire increased with mean monthly rainfall[43,45,46]; however, heavy rainfall cause flowers to drop, decreasing total fruit yield[47]. Allal et al.[48] reported that shea tree distribution is likely to expand during the warming period in Africa, which is linked to effects related to current climate change. Even the small naturalized population in Yunnan could expand its niche in the context of climate change. Yuanjiang in its adjoining areas in Honghe Prefecture will experience a novel climate by 2050[49], potentially seeing increased dry and warm conditions. Shea trees could be a valuable economic tree in such a harsh climate and it would be an important tree for the management of dry-lands. Moreover, recent literature recommends selecting species based on economic and ecological value to enhance restoration efforts[50,51]. Shea trees could thus fill an important gap in land management planning. Promoting alien tree species for restoration might be debatable, but researchers such as Davis et al.[51] argued considering economic and ecological value rather than the origin of the selected tree for land management. Finally, no harm has been noted yet with regards to the shea tree plantation at the Yuanjiang Research Station, and its economic value is self-evident.

      Fostering a small population of shea trees, classifying it as a dry-land management species, and exploiting its economic potential are all appropriate goals, particularly on the north and northeast slopes of low altitudes in dry-hot valleys. The research results from this study can be used to produce management guidelines for shea butter plantations. Including those enabling shea trees to have dendrometric traits (such as crown diameter) that are positively connected with high fruit yields. Although the identity of the mother trees or tree populations at Yuanjiang is unknown, high-yielding healthy trees can be selected for the collection of seed materials for further plantation and management. Furthermore, future research can continue based on this theoretical foundation and scientific reference for the introduction, planting, management of germplasm resources and construction of agroforestry ecosystems in shea trees to bolster ecological and economic benefits in dry tropical areas.

    • The authors confirm contribution to the paper as follows: study conception and design: Zhao G , Wu L , and Ranjitkar S; data collection: Li T, Wang X, Qin H, Ayemele AG, and Han X; data analysis: Ranjitkar S, Cunningham AB, and Zhang S; draft manuscript preparation: Zhao G, and Ranjitkar S. All authors reviewed the results and approved the final version of the manuscript.

      • This research is supported by the Yunnan Provincial Science and Technology Department (202003AD150004), The Yunnan Provincial Key Laboratory Project (E03L081261), Postdoctoral Research Program in Yunnan Province and the CGIAR research program on climate change (FTA-FP5). We would like to acknowledge support from the farmers at Yuanjiang Research Station for their participation and cooperation in the study. We thank Prof. Sheng-ji Pei for providing planting history information and initial research advice. Dr. Peter Lovett is also thanked for his advice.

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

      • 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 (4)  Table (4) References (51)
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    Zhao G, Ranjitkar S, Ayemele AG, Li T, Wang X, et al. 2023. From Ghana to the dry-hot valleys of China: assessing factors influencing fruit yield in agroforestry species Vitellaria paradoxa after 54 years of cultivation outside Africa. Circular Agricultural Systems 3:7 doi: 10.48130/CAS-2023-0007
    Zhao G, Ranjitkar S, Ayemele AG, Li T, Wang X, et al. 2023. From Ghana to the dry-hot valleys of China: assessing factors influencing fruit yield in agroforestry species Vitellaria paradoxa after 54 years of cultivation outside Africa. Circular Agricultural Systems 3:7 doi: 10.48130/CAS-2023-0007

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