REVIEW   Open Access    

Microbial enzymes: the bridge between Daqu flavor and microbial communities

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  • Baijiu Daqu, a traditional component in the Baijiu brewing process, serves as both a 'saccharifying fermenting agent' and an 'aroma-producing catalyst', embodying a rich historical legacy. Daqu offers a diverse microorganism environment that is crucial for the fermentation of Baijiu. The distinctive flavor profile, a key attribute of Baijiu, is intricately linked to the microflora present in Daqu. To date, research on Daqu has primarily concentrated on the diversity of microbial communities, microbial interactions, flavor characteristics, and biochemical properties. The functional enzyme system in Daqu serves as a crucial link connecting the flavor of Baijiu with the microbial community of Daqu. However, reviews that particularly focus on the role of enzymes in determining the quality of Daqu have not yet been reported. Thus, here the types and production processes of Daqu are initially summarized. Then, the pathways involved in the production of the major flavor substances in Daqu are elucidated, as well as the role and contribution of different functional enzymes in the formation of Daqu flavor. Finally, the current technologies for improving Daqu flavor through microbial inoculation aree discussed, including the advantages, shortcomings, and bottlenecks of microbial inoculation. The findings gained in this study provide valuable information for the efficient production of high-quality Daqu for the brewing of Baijiu.
  • 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

    Zhong Z, Liu T, He K, Zhong M, Chen X, et al. 2024. Microbial enzymes: the bridge between Daqu flavor and microbial communities. Food Innovation and Advances 3(4): 426−437 doi: 10.48130/fia-0024-0041
    Zhong Z, Liu T, He K, Zhong M, Chen X, et al. 2024. Microbial enzymes: the bridge between Daqu flavor and microbial communities. Food Innovation and Advances 3(4): 426−437 doi: 10.48130/fia-0024-0041

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Microbial enzymes: the bridge between Daqu flavor and microbial communities

Food Innovation and Advances  3 2024, 3(4): 426−437  |  Cite this article

Abstract: Baijiu Daqu, a traditional component in the Baijiu brewing process, serves as both a 'saccharifying fermenting agent' and an 'aroma-producing catalyst', embodying a rich historical legacy. Daqu offers a diverse microorganism environment that is crucial for the fermentation of Baijiu. The distinctive flavor profile, a key attribute of Baijiu, is intricately linked to the microflora present in Daqu. To date, research on Daqu has primarily concentrated on the diversity of microbial communities, microbial interactions, flavor characteristics, and biochemical properties. The functional enzyme system in Daqu serves as a crucial link connecting the flavor of Baijiu with the microbial community of Daqu. However, reviews that particularly focus on the role of enzymes in determining the quality of Daqu have not yet been reported. Thus, here the types and production processes of Daqu are initially summarized. Then, the pathways involved in the production of the major flavor substances in Daqu are elucidated, as well as the role and contribution of different functional enzymes in the formation of Daqu flavor. Finally, the current technologies for improving Daqu flavor through microbial inoculation aree discussed, including the advantages, shortcomings, and bottlenecks of microbial inoculation. The findings gained in this study provide valuable information for the efficient production of high-quality Daqu for the brewing of Baijiu.

    • Fermentation is a long-standing food-processing technology through which humans have invented alcoholic beverages that hold an important position in politics, economy, and culture[1]. In Western practices, the fermentation of wine and beer relies on yeast, which requires the prior hydrolysis of starch in the ingredients due to the absence of its own starch-digesting enzymes[2]. However, Eastern techniques have ingeniously circumvented this enzymatic limitation by leveraging the synergistic action of diverse microbial cultures, harnessing a range of enzymes produced by indigenous microorganisms. Therefore, mixed fermentation agents are considered indispensable substances in alcohol fermentation in many regions of Asia, and this mixed fermentation agent is what we now know as 'Jiuqu'[3]. Jiuqu is a time-honored saccharification and fermentation medium used to achieve the decomposition of starch from multiple grains as substrates and the fermentation of alcohol. As early as the Northern Wei Dynasty in China, Jia Sixie recorded the differences and preparation methods of nine types of Jiuqu in 'Qi Min Yao Shu'. Fermentation starters used in liquor production can be generally classified into Daqu, Xiaoqu, and Fuqu based on their raw materials, functions, and production processes[3]. Daqu primarily utilizes barley, wheat, and peas as raw materials and contains a highly complex microbial community, which results in the production of a wide variety of aromatic compounds. In contrast, Xiaoqu mainly uses rice bran or rice flour as the primary substrate and is characterized by a shorter fermentation period, higher yield, lower starter dosage, simpler production process, and lower cost as compared to Daqu[4]. Fuqu, primarily made from wheat bran, is a fermentation starter optimized through microbiological techniques. It reconstructs a simplified microbial community and exhibits strong saccharification capabilities, high efficiency, and low production costs[5]. Among these various types of fermentation starters, Daqu holds a central role as the primary starter in the production of Chinese Baijiu[6].

      Daqu serves as the fundamental structure in the Baijiu production process, with its variety and excellence directly influencing the final product's taste and quality[7]. From the perspective of flavor characteristics, Daqu can be categorized into four types: (1) Jiang-flavor Daqu, such as Maotai, is characterized by a soy sauce flavor, rich body, and lasting aroma[8]; (2) Strong-flavor Daqu, such as Luzhou Laojiao, is characterized by a rich aroma, soft taste, and endless aftertaste[9]; (3) Light-flavor Daqu, such as Fenjiu, is characterized by pure flavor, is sweet and mellow, and refreshing aftertaste[10]; (4) Miscellaneous-flavor Daqu, such as Baiyunbian Baijiu, is characterized by sensory characteristics between Jiang-flavor and strong-flavor Baijiu[11]. The four types of Daqu have different characteristic aroma compounds. The distinctive flavors of Baijiu and the variety of its aromatic volatile compounds can be attributed to the diverse microbial populations within Daqu. These variations in the microbial makeup result in a range of enzymatic profiles within Daqu, which directly affect the synthesis of its flavor compounds[12].

      The quality of fermented foods is strongly influenced by the metabolism of microbial communities, as well as factors like moisture, temperature, and other environmental conditions. However, even with the rapid development of technology today, it is difficult to fully control these environmental factors. Therefore, microbial inoculation technology has been developed to enhance the quality of fermented foods[13]. By inoculating designated fermenting agents, it is possible to improve and guide the fermentation process to achieve the enhancement of targeted product characteristics[14]. At present, inoculation technology has been widely used in the field of fermented foods, such as the inoculation of Lactobacillus in sausages, which has improved the texture and flavor[15].

      The inoculation of Saccharomyces cerevisiae and Acetobacter malorum during the fermentation process of strawberries ensures the correct and rapid acetification for strawberry vinegar[16]. The introduction of Zygosaccharomyces rouxii, Candida versatilis, and Tetragenococcus halophilus into the fermentation process of fish sauce effectively prevents the development of off-flavors[17]. Similarly, for Daqu, which is usually produced in a semi-controlled manner based on experience in family or small workshops, microbial inoculation technology has become a key technology to improve its quality[18].

      To date, research on Daqu has mainly concentrated on the dynamics of microbial communities, flavor attributes, and biochemical properties. However, the functional enzyme system in Daqu is often overlooked. This article starts from the perspective of Daqu's flavor profile, first summarizing the types and production techniques of Daqu. It then examines the pathways for producing the primary flavor substances in Daqu and explores the roles and contributions of various functional enzymes in the development of Daqu's flavor. Finally, the discussion turns to the current techniques for improving Daqu flavor through microbial inoculation, including the advantages, shortcomings, and bottlenecks of microbial inoculation. The findings can provide references for the efficient production of premium Baijiu Daqu.

    • Throughout the Daqu production process, temperature serves as a pivotal factor that influences the assembly of microbial communities, which in turn is a key criterion for classifying Daqu types. The categorization of Daqu is determined by the maximum fermentation temperature recorded throughout its manufacturing cycle[12]. Based on the peak hatching temperature attained through the natural metabolism of microorganisms, the following types of Daqu can be produced[19]: (1) Low-temperature Daqu, reaching peak temperatures of 40 to 50 °C during incubation, usually producing Qing-flavor Daqu, represented by Fenjiu and Erguotou; (2) Medium-temperature Daqu, reaching peak temperatures of 50 to 60 °C during incubation, usually producing Strong-flavor Daqu, represented by Wuliangye; (3) High-temperature Daqu, reaching peak temperatures of 60 to 65 °C during incubation, usually producing Jiang-flavor Daqu, represented by Maotai.

      The production process of Daqu has a long history in China. Daqu is usually made from wheat, barley, and peas through solid-state fermentation, produced through the metabolic activities of natural microbial communities. Six main steps, namely batching, grinding, mixing, forming into blocks, incubation, and storage[12], are commonly involved in the production of Daqu as follows. (1) Batching: When wheat, barley, and peas are used as raw materials for making Daqu, the ratio is usually 5:4:1, 6:3:1, or 7:2:1. (2) Grinding: The degree of raw material grinding affects the quality of Daqu. If it is too fine, it will be too sticky, with small pores inside the Daqu block, poor air permeability, slow microbial growth, and difficulty in dissipating moisture and heat. If it is too coarse, it will be less sticky, with large pores inside the Daqu block, easy to dissipate moisture and heat, prone to premature drying and cracking, poor microbial growth, and other problems. (3) Mixing: The purpose of mixing is to increase the moisture content in the raw materials, promoting the growth and reproduction of microorganisms. The usual water-to-mixing ratio for wheat, barley, and pea mixed Daqu is 40%~45%. (4) Forming into blocks: The general size of the brick-shaped Daqu block is (30~33) cm × (18~21) cm × (6~7) cm. (5) Incubation: The prepared Daqu is stored in the Daqu room, and fermented in stacks at different temperatures according to the type of Daqu. (6) Storage: Once the fermentation process is concluded, Daqu undergoes a period of storage lasting 8 to 10 d with the aim of reducing its moisture level to 15% and allowing the temperature to stabilize at ambient levels. Following this, it is placed in a warehouse for a further six-month storage period[20,21]. Of course, the Daqu production process in different regions will also vary according to the actual local conditions. Figure 1 shows the main production steps of traditional Daqu.

      Figure 1. 

      Process diagram for the traditional production of Daqu.

      The fermentation of Daqu is a long-term spontaneous process, traditionally relying heavily on manual labor, which results in high labor intensity and production costs, but low efficiency. Due to the open nature of manual Daqu preparation, the quality is significantly influenced by operational variability, making it difficult to maintain consistency. To address those challenges, Daqu production is gradually transitioning towards mechanization and automation. Mechanized production has already been applied in steps such as raw material pretreatment and brick shaping. For the traditional manual Daqu-stepping process, some mechanical equipment has already been designed to imitate human foot movements. By utilizing a multi-stage pneumatic cylinder system to step on the Daqu in phases, manual labor has been replaced, and both the quality stability and production efficiency of high-temperature Daqu have been significantly improved. With the advent of Industry 4.0 and the rapid development of artificial intelligence (AI), improvement using AI of Daqu production is gradually becoming feasible. Xiao & Li[22] attempted to integrate ZigBee and WiFi networks using a ZigBee protocol stack to enable real-time monitoring of environmental temperature and humidity and remote data transmission. Thus, an electronic sensing system combined with machine learning has the potential to identify and predict key Daqu parameters and quality. The strength of intelligent technologies lies in their integration of computer science with large datasets, offering a promising solution to the challenges in the brewing industry. These technologies are highly efficient in detection, classification, and prediction, particularly excelling in handling nonlinear relationships, temporal and spatial variability, and multifactorial influences[23]. By applying intelligent technology, it becomes possible to effectively predict fermentation states, regulate the production processes, assess product quality, classify brewing grades, and forecast production outcomes[24].

    • The flavor of Daqu is a key indicator of its quality, and variations in flavor will directly result in different types of Baijiu. The main representative aroma compounds of Jiang-flavor Baijiu have been identified to be tetramethylpyrazine while that of strong-flavor Baijiu is ethyl hexanoate[25]. The principal flavor constituents of miscellaneous-flavor Baijiu include caproic acid, ethyl caproate, heptanone, and butyric acid[11]. At present, the study of flavor compounds mainly relies on technologies such as gas chromatography-mass spectrometry (GC-MS), gas chromatography-olfactometry (GC-O), and gas chromatography-flame ionization detection (GC-FID)[26].
      Zhou et al.[27] conducted a comprehensive analysis and documented a diverse array of 456 volatile compounds in Daqu before 2018. This chemically rich profile encompassesed a spectrum of compounds, such as esters, alcohols, acids, and aldehydes, which are critical for the characteristic flavors of Baijiu. Specifically, the study identified four types of esters, 62 alcohols, 41 acids, and 43 aldehydes, along with other significant contributors to the flavor profile, including 36 pyrazines, 34 ketones, and 34 aliphatic compounds. Additionally, the research highlighted the presence of 25 hydrocarbons, 25 other heterocyclic compounds, 18 furans, and 16 nitrogen-containing compounds, among others. The study also examined the presence of phenolic, ether, sulfur, aromatic hydrocarbon, terpene, lactone, and carbonyl compounds, with counts of 14, 11, 10, 9, 8, 7, and 3 types respectively. Moreover, from the perspective of Daqu types, 255 flavor substances were detected in high-temperature Daqu, 328 flavor substances in medium-temperature Daqu, and 140 flavor substances in low-temperature Daqu. The synthesis of these flavor compounds involves various functional enzymes[28]. Ethyl hexanoate is the main flavor compound in strong-aroma Baijiu, and its synthesis pathway primarily involves the esterification of hexanoic acid and ethanol, catalyzed by relevant esterification enzymes (such as 3-hydroxybutyryl-CoA dehydrogenase, EC: 1.1.1.157)[29]. Glucose undergoes glycolysis to produce pyruvate, which is then converted to acetyl-CoA. Under the catalysis of alcohol acyltransferase, acetyl-CoA reacts with ethanol to form ethyl acetate[30]. Pyruvate dehydrogenase (EC: 1.2.4.1) catalyzes the conversion of pyruvate to acetyl-CoA, making it crucial for the synthesis of ethyl esters and other fatty acid esters. Lactic acid, an important flavor compound in Baijiu, plays a key role in reducing the harshness of ethanol. Lactate dehydrogenase reversibly catalyzes the conversion of pyruvate to lactic acid[31].

      Figure 2 shows the main pathways for the production of the main aroma substances in Daqu. The aroma substances in Daqu mainly come from the decomposition of biological macromolecules (starch, protein, lipids, etc.) under the action of microorganisms and enzymes[32]. Firstly, carbohydrate and energy metabolism are the foundation of microbial growth and also the basis and premise for the production of most flavor substances. The process primarily involves carbohydrate metabolism, glycolytic conversion, pyruvate transformation, and the operation of the citric acid cycle[25]. The intermediate substances produced in these processes are crucial for the production of ethanol and flavor substances. In the TCA cycle, acetyl-CoA is a key precursor for the production of the majority of acetate esters found in Daqu[33]. In addition, acetyl-CoA also plays a crucial role in the metabolism of various flavor intermediates (such as acetoin, fatty acids, etc.)[34]. Owing to the swift advancement in omics-based techniques, our comprehension of the catalytic roles of enzymes within the tricarboxylic acid (TCA) cycle continues to expand. Yi et al.[35] demonstrated the presence of key enzymes involved in the tricarboxylic acid (TCA) cycle, specifically isocitrate dehydrogenase (EC 1.1.1.41), malate dehydrogenase (EC 1.1.1.37), and succinate dehydrogenase (EC 1.3.5.1). These enzymes have been identified in species including Aspergillus oryzae, Thermoascus stipitatus, Aspergillus clavatus, members of the Trichocomaceae family, the phylum Firmicutes, and Emericella nidulans.

      Figure 2. 

      The production pathways of the main aroma substances in Daqu. Green represents the enzymes involved in each stage, while red indicates the aromatic compounds produced.

      Secondly, proteins are also an important source of flavor substances. glutamic acid (Glu), Valine (Val), leucine (Leu), isoleucine (Ile), phenylalanine (Phe), alanine (Ala), tyrosine (Tyr), arginine (Arg), and glutamine (D-Gln) from protein degradation can serve as intermediates for Daqu flavor substances[31]. In addition, pyrazine substances are also an important type of flavor substances, which are mainly produced through the Maillard reaction between sugars and amino acid residues[36]. These alkylpyrazines usually have nutty, baked, and roasted notes, and their formation is related to the acetoin and ammonium (NH3) pathway[37]. Among, tetramethylpyrazine is the representative aroma compound of Jiang-flavor Daqu. In addition, compounds such as guaiacol, tetramethylguaiacol, and benzyl alcohol can provide spicy and clove-like flavors to the qu. Yi et al.[35] discovered 11 metabolic pathways linked to the synthesis of aromatic compounds via transcriptomic analysis, encompassing pathways for the breakdown of various aromatic substrates including aminobenzoate, benzoate, fluorobenzoate, chlorobenzoate, ethylbenzene, naphthalene, biphenyl, styrene, dimethylbenzene, polycyclic aromatic hydrocarbons, and toluene.

      Additionally, the metabolism of esters significantly contributes to the development of Daqu's distinctive flavor profile. The main ester substances in Daqu include ethyl hexanoate, ethyl acetate, ethyl butyrate, ethyl valerate, and ethyl heptanoate, which usually impart fruity, floral, and honey-like flavors to Baijiu[27]. The alcohols and acids produced by ester metabolism are also important components of Daqu flavor. The main acids in Daqu include acetic acid, butyric acid, and hexanoic acid, among which hexanoic acid gives Baijiu a pungent and sour taste. The main alcohol substances in Daqu include n-butanol, isoamyl alcohol, and n-pentanol, among which n-butanol and n-hexanol have an apple-like flavor[38].

      Currently, Daqu manufacturing takes place in an environment that is not fully controlled, with environmental parameters like solar radiation, moisture levels, and thermal conditions exerting substantial influence on Daqu's microbiota and enzymatic profiles, thereby potentially driving the formation of unforeseen flavor compounds[39]. The metabolism of Streptomyces may produce geosmin, which can lead to earthy and moldy flavors in Baijiu[40]. Liu et al.[41] reported the detection of pathogens such as small copper bacteria and fecal enterococci in the finished Daqu and processing environment, particularly in the fermentation room and storage room. The metabolic by-products of these bacteria not only affect the flavor of Baijiu but also people's health. In the entire Daqu production process, enzymes produced by various microbial metabolisms influence the flavor and quality of Daqu. Hence, elucidating the enzymatic pathways involved in the generation of Daqu's flavor substances is a significant area of inquiry for advancing the flavor enhancement of Daqu in subsequent studies.

    • The enzymatic profile within Daqu is instrumental in the fermentation dynamics, with the diversity and concentration of these enzymes are pivotal to the final quality of the product. The main functional enzymes in Daqu include amylase, protease, cellulase, hemicellulase, tannin enzyme, pectinase, phytase, lipase, esterase, lacquer enzyme, etc.[42]. Different types of enzymes play different roles and functions during fermentation. Amylase primarily facilitates the liquefaction and saccharification stages of Daqu fermentation, whereas protease and esterase significantly influences the synthesis of flavor compounds, including ethyl hexanoate[43,44]. The following is an elaboration on different functional enzymes that involved in the production of Daqu (Table 1).

      Table 1.  The sources of enzymes with different functions in Daqu.

      Types of enzymes Microbial sources Refs
      Type Genera
      Enzymes related to carbohydrates metabolism,
      EC 3.2.1.X
      Amylase Bacteria Bacillus; Kroppenstedtia; Leuconostoc; Staphylococcus; Thermoactinomyces [57,62,65,71,83]
      Fungi Aspergillus; Byssochlamys; Penicillium; Rhizomucor; Saccharomycopsis; Thermoascus [50,62,84]
      Glucoamylase Bacteria Leuconostoc; Weissella [85,86]
      Fungi Aspergillus; Byssochlamys; Monascus; Rhizomucor; Rhizopus [84,8790]
      Pectinase Bacteria Enterococcus [85,86]
      Fungi Eurotium; Streptomyces [91,92]
      Glucosidase Fungi Aspergillus; Lichtheimia; Saccharomycopsis; Paecilomyces; Thermoascus [50,89,9395]
      Xylanase Fungi Aspergillus [88]
      Cellulase Fungi Penicillium [50]
      Hemicellulase Fungi Thermomyce [62]
      Enzymes related to proteins metabolism,
      EC 3.4.X.X
      Neutral protease Bacteria Bacillus; Lactobacillus; Staphylococcus [54,57,70,71,96,97]
      Fungi Aspergillus; Eurotium; Lichtheimia; Mucor; Penicillium; Rhizomucor; Rhizopus [44,50,62,87,93,
      94,97]
      Acid proteases Fungi Saccharomycopsis [52,98,99]
      Fibrinolytic enzyme Fungi Rhizopus [97]
      Fibrinogenase Bacteria Bacillus [70]
      Enzymes related to esters metabolism,
      EC 3.1.1.X
      Lipase Bacteria Bacillus [54,71,100]
      Fungi Eurotium; Rhizomucor; Rhizopus [87,89,97]
      Esterase Bacteria Bacillus; Leuconostoc; Staphylococcus; Weissella [65,70,71,85,97]
      Fungi Issatchenkia; Lichtheimia; Mucor; Monascus; Penicillium; Streptomyces; Zygosaccharomyces [44,50,57,66,78,
      89,94,95,101]
      Other enzymes Tannase Bacteria Bacillus [65]
      Fungi Aspergillus; Penicillium [50,93]
      Alkaline phosphatase Fungi Streptomyces [57]
      Phosphate hydrolase Fungi Streptomyces [57]
      Glycosyltransferase Fungi Rhizopus [87,97]
    • Saccharification is the process where polysaccharides (starch, cellulose, hemicellulose, etc.) in grains are hydrolyzed into fermentable sugars by enzymes, marking the beginning of Baijiu fermentation. Daqu plays an important role in this step by providing a variety of enzymes involved in saccharification (Fig. 3).

      Figure 3. 

      The main pathways of saccharification metabolism in Daqu. Green represents hexoses, and blue represents pentoses.

      Carbohydrates in grains (such as rice and sorghum) account for 73.6%−76.6%[45]. The saccharification process is mainly dominated by starch metabolism. Starch can be hydrolyzed by hydrolytic enzymes into dextrin, maltose, and glucose to be utilized by other microorganisms. The main enzymes involved in this process are α-amylase (EC 3.2.1.1), α-glucosidase (EC 3.2.1.3), etc.[46]. α-amylase and α-glucosidase can act on the α-1,4-glucosidic bonds inside the starch and the α-1,4-glucosidic bonds at the non-reducing end of the starch, respectively, thus hydrolyzing starch into small molecular sugars[4749]. Therefore, they exert a significant influence within the Baijiu industry and are also known as liquefaction power (the ability of liquefying enzymes in Daqu to convert starch from a macromolecular state into a low-molecular-weight state) and saccharification power (the ability of saccharifying amylases in Daqu to hydrolyze starch into glucose), respectively. It is widely believed that molds are primarily responsible for breaking down large molecular substances (such as starch) in the raw materials during the initial fermentation stage, providing the essential enzymatic activity for the entire fermentation system[50]. The discovery of a strain of Aspergillus niger with an amylase activity as high as 6,800 U/mL suggests that it could serve as an excellent source of functional strains for γ-amylase and as a co-culture strain for the reinforcement of Daqu[49]. Du et al.[51] reported that Rhizopus oryzae, Aspergillus flavus, and Aspergillus oryzae, which are sources for Daqu production, can produce a large amount of saccharification enzymes and hydrolytic enzymes. Wang et al.[52] reported that Saccharomycopsis fibuligera, an advantageous yeast species in low-temperature Daqu can produce amylase and β-glucosidase, which degraded starch into dextrin, maltose, and glucose, providing nutritional supply for brewing yeast and many other microorganisms involved in Baijiu fermentation. High-temperature Daqu's genes for α-glucosidase, α-amylase, and saccharification enzymes belong to the genera Rhizopus, Aspergillus, and Rhizomucor, which are considered excellent functional strains due to their saccharifying characteristics[53]. Wang et al.[54] purified a glucoamylase from Aspergillus oryzae in Daqu, which exhibits broad substrate specificity, good thermal stability and pH stability, demonstrating excellent potential for industrial application in bioethanol production. Wang et al.[50] reported that the production capacity of saccharification amylase from mold strains in high-temperature Daqu, especially from the genera Aspergillus and Rhizopus was significantly higher than that of other enzymes. Rhizomucor pusillus is an important source of saccharification enzymes in fermented foods such as Jiang-flavor Daqu and light-flavor Daqu[55]. Interestingly, amylase plays a significant role in light-flavor Daqu and is positively correlated with the levels of glycerol, malic acid, and succinic acid. This is crucial for enhancing its biochemical properties, producing unique metabolic products, and creating a distinct flavor[44]. In addition to molds, some bacteria are also contributors to amylase. The genera Thermoactinomyces vulgaris and Thermoactinomyces sacchari have a strong ability to produce amylase, which gives them a strong capacity to catalyze the hydrolysis of starch[56,57]. Bacillus amyloliquefaciens and Bacillus subtilis are aroma-producing strains capable of secreting α-amylase, enabling the rapid fermentation of sugars in raw materials[58].

      In addition to starch, cellulose, and hemicellulose in grains can also be hydrolyzed into sugars by cellulase and hemicellulase for use by other microorganisms[59]. Cellulase and hemicellulase are complex enzymes, mainly including endo-glucanase (EC 3.2.1.4), β-glucosidase (EC 3.2.1.21), β-xylosidase (EC 3.2.1.37), and xylobiose hydrolase (EC 3.2.1.91)[60]. The gene for β-glucosidase in cellulase comes from the genera Aspergillus and Mucor[61]. Thermoascus is a major contributor to the cellulase system[62]. Gou et al.[63] reported that Thermomyces lanuginosus, a thermophilic fungus capable of surviving at temperatures above 60 °C, is an efficient producer of xylanase. Identifying the sources of these key functional enzymes lays the groundwork for future targeted synthesis of these enzymes for the enhancement of Daqu.

    • Proteolytic enzymes, a diverse group of biocatalysts, target peptide bonds within proteins, facilitating their cleavage into smaller peptide fragments and individual amino acids. Proteolytic enzymes catalyze the breakdown of proteins present in the substrate into smaller peptides and amino acids during the fermentation phase of Baijiu production. Metabolites act as a nitrogen supply for the fermentation processes of yeasts and lactic acid bacteria, thereby facilitating the proliferation of these microbes throughout the fermentation period[64]. In addition, some of the protein degradation products, such as amino acids, are themselves flavor substances, while others can serve as precursors for aroma substances. Protease activity can affect the types and amounts of alcohols and organic acids in Baijiu, leading to changes in the quantity and quality of the final esters, resulting in different flavor types of Daqu. Therefore, proteases are indispensable enzymes in the Baijiu production process[65]. It is particularly noteworthy that proteases play a pivotal role in the development of the distinctive Jiang flavor profile in high-temperature Daqu, a critical component in the fermentation process. The enzymatic activity of proteases catalyzes the transformation of flavor precursors, significantly influencing the taste and aromatic characteristics of the end product[44]. Aspergillus flavus is recognized for its significant contribution to the production of acidic proteases in Daqu[50]. Monascus purpureus is capable of producing both acidic and neutral proteases that catalyze the esterification of acids and ethanol. This enzymatic activity is crucial for the transformation of precursor compounds into esters, which are fundamental to the distinctive aroma of Baijiu[66]. Wang et al.[67] successfully broadened the application scope of high-temperature actinomycetes by screening a strain from the Jiang-flavor Daqu that exhibited protease activity as high as 214.99 U. Thermoascus and Rasamsoni have been found to be positively correlated with high protease activity and amino nitrogen content in white Daqu[68]. Bacillus, as one of the most representative and important bacteria in Daqu, also has a strong ability to produce proteases[65,6971]. For example, Bacillus licheniformis shows strong protease activity and can produce aromatic compounds, pyrazines, organic acids, etc.[7174]. Liu et al.[65] reported that the protease strains purified from the strong-flavor Daqu all come from the genus Bacillus. Feng et al.[75] screened a strain of Bacillus pumilus that produces neutral protease from the light-flavor low-temperature Daqu, with a protease activity as high as 202.7 U. In addition to Bacillus, lactic acid bacteria are also major contributors to proteases. The products obtained through protease hydrolysis can participate in the Maillard reaction to produce aromatic substances[76].

    • The lipases present in Daqu are capable of breaking down fats from the raw materials into fatty acids, glycerol, as well as monoglycerides, among others. These compounds offer a source of energy essential for the microbial proliferation and reproduction[37]. In addition, some of the breakdown products of fats (such as fatty acids, monoglycerides, etc.) are of great significance for the taste, aroma, and other qualities of Baijiu. Yan et al.[77] reported that the addition of lipase to yellow water increased the concentration of flavor esters in Baijiu by 32 times.

      Esterases are a class of enzymes that can hydrolyze or synthesize ester bonds in fats. Through the esterification process between carboxylic acids and alcohols, they are capable of generating aromatic esters that enhance the synthesis of flavor compounds in Daqu, consequently influencing the quality of the resulting Baijiu[44]. Esterases are believed to contribute similarly to the formation of the strong-flavors and Jiang-flavors in high-temperature Daqu. Candida have long been considered highly correlated with the esterification power (the capacity of esterases in Daqu to catalyze the synthesis of esters from free organic acids and ethanol) of Daqu[76]. A strain of Issatchenkia orientalis, recognized as an aromatic yeast that produces esterases, has been discovered to participate in the production of ethyl acetate in Baijiu. This yeast strain also exhibits good salt and ethanol tolerance[78]. Bacillus cereus is recognized for its capability to synthesize ethyl acetate, a key aromatic constituent in Chinese Baijiu, via the enzymatic activity of esterases. This production is significant for the distinctive flavor profile of the spirit[79]. It is worth noting that the relative abundance of lactic acid bacteria in Daqu is not as high as that of other dominant bacteria[80]. However, their contribution is significant in shaping the intrinsic qualities of Daqu. Wang et al.[81] identified a diverse group of lactic acid bacteria genera in high-temperature Daqu, encompassing Lactobacillus, Weissella, Pediococcus, Enterococcus, Leuconostoc, and Streptococcus. They are capable of generating a range of enzymatic catalysts, including esterases, which facilitate their involvement in the Maillard process to generate aromatic compounds and also yield substantial quantities of lactate esters' precursors, specifically lactic acid. Lactate esters serve as key precursors in the production of Baijiu, contributing to the enhancement of its mellow and sweet characteristics[80,82].

    • With the development of genomics and proteomics, more functional enzymes have been discovered in Daqu. Isocitrate dehydrogenase, malate dehydrogenase, and succinate dehydrogenase from Aspergillus oryzae, Aspergillus clavatus, and Aspergillus terreus were involved in the formation of Baijiu flavor[35]. Acetyl-CoA not only participated in the biosynthesis of acetate esters but also in the metabolism of flavor intermediates such as fatty acids, ketones, and acetoin[59]. Streptomyces species, which are airborne isolates from the Baijiu production facility, exhibit enzymatic capabilities, including esterase, alkaline phosphatase, and phosphatase hydrolase production. These enzymatic activities significantly contribute to the synthesis of flavor compounds and their precursors in Jiang-flavor Baijiu[57]. The hydrolysis of stored glycogen or oligosaccharides catalyzed by phosphorylases was conducive to ethanol production and various phosphorylases have been widely used in industry due to their economic role in glycosyl transferase reactions[54].

      Currently, research on the quality of Daqu primarily focus on microbial communities, while the role of functional enzyme systems in Daqu, which serve as a crucial link between Baijiu flavor and the structure of the microbial community is not yet well understood. Therefore, efforts should be focused on the identification of key enzymes and their contribution to the quality of Daqu. An integrated multi-omics strategy could be applied to analyze the origin of key functional enzymes during Daqu fermentation, as well as enzymatic properties and substrate specificity etc. Subsequently, the dynamics of biological enzymes in Daqu processing could be mimicked with recombinant enzymes, which would help in understanding the mechanisms that are involved in the quality formation of Daqu and standardizing of Daqu production.

    • In the Baijiu brewing process, the solid-state fermentation undergoes a series of complex and dynamic transformations. These encompass the proliferation of microbial populations, the utilization of nutrients and oxygen, the synthesis and accumulation of metabolic byproducts, fluctuations in temperature, and the evaporation of moisture, all of which contribute to the intricate biochemistry of the fermentation environment[102]. Li et al.[103] discovered that environmental factors exert significant differential impacts on microbial communities during various fermentation stages. In the mesophilic fermentation stage, moisture content and acidity are the primary determinants of microbial composition, whereas in the cooling and maturation stages, core temperature, and pH become the critical factors influencing microbial communities. Furthermore, they noted that bacterial communities might exhibit greater sensitivity to environmental changes than fungal communities. In the natural fermentation of Daqu, the heterogeneity of environmental factors lead to the diversification of Daqu colors, including white, yellow, and black. These different-colored Daqu samples show significant variations in microbial composition, physicochemical properties, enzymatic characteristics, and potential metabolic functions[68]. Fermentation temperature is regarded as a key controlling factor in the production of Daqu, as it influences the growth and death of microbes, thereby determining the succession dynamics of microbial communities[104]. The optimal growth temperatures for different microbial species vary, with molds and yeasts thriving at approximately 30 °C. Therefore, during high-temperature fermentation processes, the growth of these microbes is inhibited. In contrast, bacteria generally tolerate a broader temperature range, with some species capable of surviving between 40 and 60 °C. As a result, the influence of fermentation temperature on bacterial growth is less pronounced as compared to that on fungi[105]. These factors all affect the quality of fermentation to a certain extent[106]. It is currently very difficult to completely control all environmental factors during the fermentation process[86]. Thus, inoculated fermentation has become an important method to enhance the quality of fermentation products. Similarly, in the field of Baijiu brewing, achieving targeted improvement of Baijiu flavor and quality through inoculated fermentation has become a hot topic of current research.

      The research on microbial inoculation of Daqu is still mainly focused on the development of excellent functional strains[13]. Previous research has consistently highlighted Bacillus as a quintessential and significant bacterial genus across diverse Daqu types, underscoring its pivotal role in these fermentation substrates[65,70,71]. The diversity and concentration of Bacillus species are decisive factors for the quality of Daqu, consequently dictating the sensory attributes and aromatic profiles of Baijiu[78]. Bacillus has been established as a principal functional bacterium in the production of Baijiu, possessing a robust capacity to secrete a range of degradative enzymes, including protease, amylase, and saccharifying enzymes. The exceptional capacity for enzyme synthesis in Bacillus confers upon it the ability to generate a diverse array of flavor compounds throughout the Jiang-flavor Baijiu fermentation. As a result, Bacillus has emerged as a favored candidate for inoculated fermentation practices.

      Inoculation with Bacillus licheniformis has increased the amylase activity and the content of tetramethylpyrazine in Daqu, significantly enriching the aromatic compounds in Daqu, successfully regulating the metabolism of the microbial community within Daqu, and enhancing its flavor[106,107]. Daqu inoculated with Bacillus velezensis and Bacillus subtilis exhibited significantly higher liquefaction, saccharification, and esterification powers compared to regular Daqu. Furthermore, the inoculation of Daqu has led to a marked increase in the levels of esters, pyrazines, alcohols, and other volatiles, thereby significantly enhancing the Daqu's overall quality[70,108]. In addition to inoculating Bacillus, some prokaryotes have also become good co-inoculants. Chen et al.[109] inoculated Lactobacillus brevis, which contributed to an enhanced production of flavor compounds during fermentation, consequently reducing the Baijiu fermentation cycle and achieving improvements in fermentation efficiency and quality.

      In addition to prokaryotes, many eukaryotes are also excellent functional strains. The flavor profile and fermentation efficiency of Baijiu are significantly influenced by the activities of wine yeast and ester-producing yeast, which generate key enzymes like lipases, esterases, and acyltransferases[13]. Fan et al.[18] inoculated Wickerhamomyces anomalus and Saccharomyces cerevisiae, and the mixed fermentation of the two yeasts increased the content of flavor compounds in Baijiu particularly ethyl acetate. Li et al.[110] reported that by inoculating Saccharomyces cerevisiae Y7#09 or Clavispora lusitaniae YX3307, the content of ethyl hexanoate in Baijiu was increased, resulting in the enhanced flavor of Baijiu. In addition to yeasts, molds also make a great difference in Baijiu fermentation. It is generally believed that molds are the main contributors to the degradation of large molecular substances (such as starch) in the raw materials during the initial stage of fermentation and provide the basic enzymatic power for the entire fermentation system[106]. Zhu et al.[111] reported that by inoculating Saccharomycopsis fibuligera and Rhizopus microsporus, more pleasant esters, such as isoamyl acetate, and octanoic acid ethyl ester, were produced in the wine medicine, causing the yellow wine to have a richer fruit aroma.

      Compared with the inoculation of a single strain, the inoculation of mixed strains is a more popular choice. The co-inoculation of Bacillus, Pediococcus, Wickerhamomyces, and Saccharomycopsis into Daqu has successfully enhanced the dynamic stability of the microbial community during the solid-state fermentation process of Daqu[112]. The inoculation combination of Bacillus, Saccharomycopsis, and Absidia has had a positive impact on the amylase activity, key volatile compounds, and microbial community diversity in Daqu[113]. The mixed inoculation of Clostridium butyricum, Rummeliibacillus suwonensis, and Issatchenkia orientalis successfully increased the content of butyl acetate and hexanoic acid ethyl ester in strong-flavor Baijiu[114]. Inoculating Aspergillus niger and Saccharomyces cerevisiae altered the microbial community, increased ethanol production, and the content of aromatic compounds in the fermented grains, thereby enhancing the quality of Baijiu[115].

      However, the strategy of enhancing fermentation in Daqu production using mixed microbial cultures still faces several challenges. Firstly, Daqu production is highly dependent on environmental conditions, and the inoculation of mixed microbial communities may lead to fluctuations in its physicochemical properties, thereby affecting the stability of its functional expression and aroma formation. Secondly, there is a lack of unified standards for beneficial microorganisms in Daqu and Baijiu production, with significant variation across different regions worldwide. This highlights the need for evidence-based approaches to improve quality and underscores the importance of establishing standardized protocols to ensure industry-wide consistency. Additionally, the use of mixed functional microorganisms to enhance Daqu production, along with the dynamic changes in microbial community functions during maturation, significantly influences the composition of volatile compounds and enzyme activities. Therefore, before adopting advanced biotechnological methods to tailor Daqu microbial communities for the improvement of Baijiu quality, it is essential to adhere to regulatory standards and conduct comprehensive safety assessments.

    • To date, the Chinese traditional Baijiu industry has developed significantly, but the production of Daqu is still based on an experience-based, semi-open environment, lacking a systematic and standardized production system. This makes it difficult to fully control the quality of Daqu in industrial production and large-scale fermentation processes. The taste profile of Daqu serves as a critical metric for assessing its quality, with the microbial composition within Daqu being the primary determinant of its flavor characteristics. Therefore, research on Daqu primarily focuses on microbial communities, environmental factors, and functional strains. The enzymes within Daqu act as a bridge between the microbial community and the flavor profile of Daqu. By linking the origins of functional enzymes and their contributions to flavor synthesis, a renewed understanding of the relationship between the microbial community and the flavor of Daqu from an enzymatic perspective can be gained. In this research, a comprehensive overview of the types and production processes of Daqu were intitially provided. Subsequently, the biochemical pathways involved in the synthesis of the primary flavor compounds in Daqu were elucidated, emphasizing the roles and contributions of various functional enzymes in flavor development. Finally, contemporary techniques for enhancing Daqu flavor through microbial inoculation were examined, discussing the benefits, limitations, and current challenges associated with these methods.

      There are still many problems and challenges in the development of Daqu. For example, current research on enzymes in Daqu mainly focuses on enzymes from microbial sources, but there is relatively little research on enzymes from environmental sources. A thorough understanding of enzymes from environmental sources is an important basis for achieving standardized production. In addition, the development of strengthened Daqu also faces many challenges. The production of Daqu is multidimensional, and many current studies one-sidedly pursue liquefaction power, esterification power, or fermentation power. However, in fact, the better a certain characteristic is, the higher the quality of Baijiu. Therefore, how to simultaneously suppress the growth of undesirable microorganisms, coordinate the saccharification, fermentation, and flavoring capabilities of strengthened Daqu, and reduce the generation of off-flavors is the future direction and goal for strengthening Daqu. At the same time, with the deepening understanding of functional enzymes in Daqu, using enzyme preparations to achieve targeted strengthening of Daqu is also an important direction for improving the quality of Daqu in the future.

      Finally, Daqu is considered a source of functional species, valuable genes, and functional enzymes. In the future, the integration of multi-omics approaches with cultured microbiota models can facilitate the elucidation of the origin, assembly processes, and functional roles of the microbial communities in Daqu. This will provide a scientific foundation for reconstructing synthetic microbiota, thereby enhancing the quality and stability of Daqu. Through multi-omics strategies, it is possible to selectively screen and engineer key functional microorganisms and their associated enzyme genes, endowing them with more efficient substrate utilization, enhanced nutrient accumulation, the ability to degrade flavor-deteriorating factors, and improved environmental adaptability. Furthermore, the continuous refinement of multi-omics datasets not only provides the basis for rapid and precise absolute quantification of active microorganisms but also lays the groundwork for the future implementation of standardized and efficient intelligent manufacturing processes. This enhanced understanding will facilitate targeted improvements in the flavor profile, quality, and consistency of Daqu.

      • This work was supported by the National Natural Science Foundation of China (32272275).

      • The authors confirm contribution to the paper as follows: writing - original draft: Zhong Z; writing - review & editing: Liu T; resources: Liu J; visualization: Zhong Z; validation: Chen X, Xue Y, Han B, Liu J; data curation: He K, Zhong M; form analysis: Wang D; project administration: Liu J; supervision & funding acquisition: Liu J, Han B. All authors reviewed the results and approved the final version of the manuscript.

      • All data generated or analyzed during this study are included in this published article.

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

      • Copyright: © 2024 by the author(s). Published by Maximum Academic Press on behalf of China Agricultural University, Zhejiang University and Shenyang Agricultural University. 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 (3)  Table (1) References (115)
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    Zhong Z, Liu T, He K, Zhong M, Chen X, et al. 2024. Microbial enzymes: the bridge between Daqu flavor and microbial communities. Food Innovation and Advances 3(4): 426−437 doi: 10.48130/fia-0024-0041
    Zhong Z, Liu T, He K, Zhong M, Chen X, et al. 2024. Microbial enzymes: the bridge between Daqu flavor and microbial communities. Food Innovation and Advances 3(4): 426−437 doi: 10.48130/fia-0024-0041

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