ARTICLE   Open Access    

UPLC-Q-TOF-MS/MS analysis of pomegranate peel polyphenols and preliminary study on the biosynthesis of ellagic acid and flavonoids

  • # Authors contributed equally: Xiaoxian Lu, Zi Ye

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  • Polyphenols are natural substances that are consumed in the daily diet and are found abundantly in a diverse range of fruits and vegetables. Deep eutectic solvent is a new kind of green solvent, which is widely used in the extraction and separation of polyphenols because it is non-toxic and pollution-free, and meets the requirements of environmental protection. This research used ultrasonic-assisted deep eutectic solvent (glycerol/urea 1:2, mol/mol, 30% water content) to extract polyphenols from pomegranate peel with improved extraction efficiency. The extracts were analyzed and identified by UPLC-Q-TOF-MS/MS, and 53 compounds were obtained, of which 10 were ellagic acid and its derivatives, and 32 were flavonoids. These polyphenols exhibit a wide range of pharmacological properties, including, but not limited to anti-inflammatory and antioxidant effects. Long-term intake of these polyphenols can effectively prevent chronic diseases, such as cardiovascular and cerebrovascular diseases, chronic tumors, etc. Polyphenols generally have good water solubility, especially anthocyanin compounds with cationic structures that have the best water solubility. Therefore, the incorporation of polyphenols into water to prepare tea and its regular consumption confers significant health benefits. Compared with acarbose, Punicalagin had a better inhibitory ability on α-glucosidase with a binding affinity of −12.2 kcal/mol and 10 hydrogen bonds were formed, indicating that Punicalagin had a potential anti-diabetic function. Pomegranate peel, a by-product of the pomegranate fruit, is typically discarded as waste in daily operations. However, exploring secondary applications for pomegranate peel not only contributes to cost reduction for enterprises but also promotes environmental sustainability and facilitates sustainable development. The extraction technology of pomegranate peel polyphenols also provides a new idea and method for the deep processing of agricultural products. This work will help to understand the biosynthetic pathways of pomegranate peel polyphenols, to facilitate the development of beverages containing pomegranate peel polyphenols.
  • 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

    Lu X, Ye Z, Jiang L, Wen L, Liu J, et al. 2024. UPLC-Q-TOF-MS/MS analysis of pomegranate peel polyphenols and preliminary study on the biosynthesis of ellagic acid and flavonoids. Beverage Plant Research 4: e028 doi: 10.48130/bpr-0024-0031
    Lu X, Ye Z, Jiang L, Wen L, Liu J, et al. 2024. UPLC-Q-TOF-MS/MS analysis of pomegranate peel polyphenols and preliminary study on the biosynthesis of ellagic acid and flavonoids. Beverage Plant Research 4: e028 doi: 10.48130/bpr-0024-0031

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UPLC-Q-TOF-MS/MS analysis of pomegranate peel polyphenols and preliminary study on the biosynthesis of ellagic acid and flavonoids

Beverage Plant Research  4 Article number: e028  (2024)  |  Cite this article

Abstract: Polyphenols are natural substances that are consumed in the daily diet and are found abundantly in a diverse range of fruits and vegetables. Deep eutectic solvent is a new kind of green solvent, which is widely used in the extraction and separation of polyphenols because it is non-toxic and pollution-free, and meets the requirements of environmental protection. This research used ultrasonic-assisted deep eutectic solvent (glycerol/urea 1:2, mol/mol, 30% water content) to extract polyphenols from pomegranate peel with improved extraction efficiency. The extracts were analyzed and identified by UPLC-Q-TOF-MS/MS, and 53 compounds were obtained, of which 10 were ellagic acid and its derivatives, and 32 were flavonoids. These polyphenols exhibit a wide range of pharmacological properties, including, but not limited to anti-inflammatory and antioxidant effects. Long-term intake of these polyphenols can effectively prevent chronic diseases, such as cardiovascular and cerebrovascular diseases, chronic tumors, etc. Polyphenols generally have good water solubility, especially anthocyanin compounds with cationic structures that have the best water solubility. Therefore, the incorporation of polyphenols into water to prepare tea and its regular consumption confers significant health benefits. Compared with acarbose, Punicalagin had a better inhibitory ability on α-glucosidase with a binding affinity of −12.2 kcal/mol and 10 hydrogen bonds were formed, indicating that Punicalagin had a potential anti-diabetic function. Pomegranate peel, a by-product of the pomegranate fruit, is typically discarded as waste in daily operations. However, exploring secondary applications for pomegranate peel not only contributes to cost reduction for enterprises but also promotes environmental sustainability and facilitates sustainable development. The extraction technology of pomegranate peel polyphenols also provides a new idea and method for the deep processing of agricultural products. This work will help to understand the biosynthetic pathways of pomegranate peel polyphenols, to facilitate the development of beverages containing pomegranate peel polyphenols.

    • Polyphenols exhibit diverse pharmacological activities that significantly contribute to the prevention of degenerative diseases, especially cardiovascular diseases and cancer[1,2]. In addition, polyphenols, such as punicalagin, ellagic acid, and kaempferol, have a good inhibitory effect on α-glucosidase, and have potential efficacy in the treatment of diabetes, especially type 2 diabetes, through the PI3K/Akt pathway[3,4]. The polarities of polyphenols are determined by their polyhydroxyl structure. In general, most polyphenols exhibit high water solubility, particularly anthocyanin components which possess a highly soluble ionic structure[5]. Considering this property, incorporating polyphenols into the water as a beverage or through regular tea consumption represents a favorable strategy for enhancing polyphenol intake among individuals. Pomegranate is an important medicinal and edible homologous fruit[6]. Its by-product, pomegranate peel, is listed in the 2020 Edition of the Pharmacopoeia of the People's Republic of China as a traditional Chinese medicinal herb, which is rich in polyphenolic compounds and exhibits a variety of pharmacological effects, including astringent, antidiarrhoeal, haemostatic, and anthelmintic properties[7,8]. The process of obtaining polyphenols from pomegranate peel, the by-product of pomegranate, offers potential benefits in terms of cost reduction, secondary utilization of the peel, and environmental sustainability[9]. In conclusion, the addition of pure natural polyphenols from pomegranate peel to tea drinking is not only beneficial to promote health, but also can reduce costs due to the abundance of pomegranate peel resources[10,11].

      Deep eutectic solvents (DESs) refer to binary or ternary low-melting mixtures composed of hydrogen bond acceptors, such as quaternary ammonium salts, and hydrogen bond donors, including amides, carboxylic acids, and polyols[12]. Conventional extraction methods often involve expensive reagents and toxic solvents that pose risks to human health and environmental pollution[13]. As a novel green solvent, the use of DES can avoid organic solvent contamination in polyphenol extraction processes and meet the standards of green pollution-free practices[14]. In recent years, DES has gained increasing attention in the extraction of active plant components and has been widely applied in the extraction and separation of plant polyphenols, flavonoids, and other compounds due to its advantages including high extraction efficiency and good recovery yields. Moreover, due to its unique inclusiveness, DES can be combined with various extraction techniques such as microwave-assisted and ultrasound-assisted extractions to further enhance its efficiency. Qin et al. found that when using ultrasound-assisted low-melting solvents for polyphenol extraction from Ligustrum robustum leaves powder became rough with numerous gaps indicating thorough extraction of polyphenolic substances from the herbal material resulting in a higher yield at 101.46 ± 2.96 mg gallic acid equivalent (GAE)/g DW[15].

      In this study, DES-based ultrasound-assisted extraction (DES-UAE) with glycerol/urea 1:2, mol/mol, 30% water content obtained the highest total phenolic content (TPC) from pomegranate peel. The information of components in pomegranate peel was analyzed using a UPLC-Q-TOF-MS/MS instrument. After obtaining information on specific components in the pomegranate peel, further metabolic pathway analysis of these compounds was conducted to explore their possible synthesis pathways in the pomegranate peel, which is conducive to understanding the biosynthesis mechanism of the pomegranate peel. The ability of polyphenols in pomegranate peel to inhibit diabetes-related proteins was further explored and compared with acarbose, which was visually represented in the form of molecular docking. Understanding the synthetic pathway of these components in plants helps to determine the source of these raw materials and ensures the reliability and safety of raw materials. For example, the polyphenols in the pomegranate peel can be used to make drinks, not only to meet the needs of the beverage formula but also as antioxidants. The second is to help assess the safety of these ingredients in beverages and ensure that products with these ingredients comply with relevant regulations and standards. Finally, it is beneficial to optimize the production process, improve production efficiency, ensure the stability and consistency of product quality, and improve market competitiveness. In addition, the polyphenols in pomegranate peel, especially Punicalagin, have a positive effect on the treatment of diabetes, and the development of pomegranate peel polyphenols into hypoglycemic drugs has a good application prospect.

    • Huaiyuan pomegranate from Anhui Province (China), purchased online, is characterized by yellow skin with red tinges and nearly colorless seeds, which have the same luster as crystal. After the pomegranate skin was dried (shade place), the measured water content was 17.4%, which was crushed (280 μm) and stored at room temperature.

      Analytical grade reagents, including Folin-Ciocalteu reagent (Biotechnology grade, 2 M), Na2CO3 (≥ 99.9%), glycerol (99%), urea (99%), and gallic acid (GAE, 98%) were purchased from Macklin (Shanghai, China). Chromatographic grade reagents of acetonitrile (≥ 99.9%) and formic acid (≥ 88%) for UPLC-Q-TOF-MS/MS analysis were provided by Merck (Germany).

    • DES (glycerol/urea) was prepared according to the molar ratio of glycerol to urea 1:2 with a water content of 30%, and the mixture was placed in a hot bath. Stir continuously to obtain a clarified liquid and stored at room temperature.

    • The DES-UAE method was used to extract polyphenols from pomegranate peel. The specific operations were as follows: 0.2 g of pomegranate peel powder was mixed with a certain amount of DES in a conical flask, the ultrasonic input power was set to 500 W and the frequency was set to 40 kHz, then TPC was extracted in an ultrasonic bath (XM-500UVF).

    • The TPC in pomegranate peel was determined by the Folin phenol method. The specific operation was as follows: Folin-Ciocalteu reagent was diluted 20 times with deionized water, and 1800 μL was mixed with 40 μL of the centrifuge sample. Reaction for 5 min at room temperature away from light, followed by the reaction with 1,200 μL of 7.5% Na2CO3 solution in the dark for 90 min at room temperature. After the reaction, the absorbance value was measured at 765 nm, and the TPC was expressed as GAE mg/g DW.

    • To study the types and contents of polyphenols in pomegranate peel, the components were analyzed using a UPLC-Q-TOF-MS/MS instrument. The data acquisition mode was set to MSE continue full scan mode, the scanning range was m/z 50−1,200, and the scanning time was 0.2 s. Collision energies are used in MSE, 6 eV, and 15−45 eV for the low and high energy channels. HCOONa was used for mass spectrometer correction, and leucine enkephrine (positive ion mode m/z 556.2771, negative ion mode m/z 554.2615) was used for real-time mass correction. The ion source parameters for mass spectrometry were as follows: capillary voltage 3.0 and 2.5 kV in positive and negative ion modes, sample cone and offset voltage 40 and 80 V, ion source temperature 120 °C, desolvation temperature 500 °C, and desolvation flow rate 1,000 L/h. Acetonitrile (A) and 0.1% formic acid-water (B) were used as the mobile phases. The column temperature was 35 °C and the sample chamber temperature was 10 °C. The flow rate was 0.3 mL/min. The injection volume was 2 μL. Gradient elution: 0−2 min, 5% A; 2−32 min, 5%−100% A. Mass spectrometry data of polyphenols in pomegranate peel obtained by UPLC-Q-TOF-MS/MS were identified by the UNIFI platform.

    • The molecular docking procedures of the identified compounds against diabetes-related enzymes including α-glucosidase, endothelial nitric oxide synthase (eNOS), angiotensin-converting enzyme (ACE) and aldose reductase (AR) were performed with AutoDock Vina to investigate the binding mode of ligands with the receptors. The crystal structures of α-glucosidase (PDB ID; 5NN8), eNOS (PDB ID; 1M9R), ACE (PDB ID; 1O8A), and AR (PDB ID; 2ACS) were obtained from the protein data bank. Before docking, the ligands and receptors were dehydrated, hydrogenated, charged, and saved in PDBQT format. In docking calculation, the receptor was considered semi-flexible while the ligand was flexible, and the grid matrix of the docking region was scaled to guarantee the full coverage of the protein molecule. The grids were set to (112, 92, 110), (90, 84, 104), (80, 90, 76), and (56, 56, 58), and the docking centers were (5.472, −30.250, 83.096), (14.726, 5.474, 38.000), (37.720, 34.803, 39.035) and (16.618, 29.512, 66.722) for α-glucosidase, eNOS, ACE, and AR, respectively. The best score with the suitable energetically favored conformations was applied for further analysis. The interactions of the identified compounds with amino acid residues of α-glucosidase, eNOS, ACE, and AR were analyzed and represented using ChimeraX.

    • The data collected in this study were recorded and calculated with Microsoft Excel. All experiments and measurements were conducted in triplicate.

    • The utilization of DES for the extraction of polyphenols from pomegranate peel exhibits enhanced permeability compared to conventional solvents, leading to a higher yield of polyphenols[16,17]. The cell wall of plant materials was disrupted through ultrasonic cavitation using the DES-assisted extraction method employed in this study, thereby facilitating the accelerated release and diffusion of polyphenols from cells into solvents[18]. In this study, glycerol/urea (1:2 mol/mol, 30% water content, extraction time 20 min, extraction temperature 30 °C, liquid/solid ratio 70 mL/g) was used as the extraction solvent and the maximum TPC obtained was 81.983 GAE mg/g DW. To reflect the high efficiency of the extraction method adopted in this study, the results of this study were compared with those of other studies. As shown in Table 1, the extraction results of this study were superior to those of Bertolo et al.[16], and Vargas-Serna et al.[19]. However, it was lower than Rajha et al.[20], and Kyriakidou et al.[21], which may be because this study used ultrasound-assisted extraction technology, while Rajha et al. used infrared-assisted extraction technology. The two methods had different influences on plant tissues, leading to the difference in results. The reason why the glycerol/urea system can obtain the highest extraction yield may be related to the structure of glycerol. Glycerol molecules contain multiple hydroxyl groups, which can form hydrogen bonds with polyphenols to promote the dissolution of polyphenols[22]. Alañón et al. highlighted the distinctive advantages of polyalcohol-based DESs in the extraction of phenolic acids and other phenolic compounds from plants[23]. At the same time, compared with sugar, glycerol is not as viscous as sugar, which is also conducive to the extraction of polyphenols[24].

      Table 1.  Comparison of the results of extracting polyphenols from pomegranate peel with different DES.

      DES Extraction condition Quantity of extractives Operation steps Ref.
      Choline chloride : Lactic acid (65%:35%) 25 min of extraction at 45 °C,
      20 g (CC-LAC)/1 g
      4.14 mg EAG mL−1 Pomegranate peels were frozen and lyophilized. [16]
      Choline chloride : Glycerol
      (1:2, mol/mol)
      Extraction time 87 s, and a
      solvent/solid ratio of 60.5 mL/g
      7.98 mg eq of gallic acid/g
      of dry weight
      Pomegranate peels were cut and dried in refractance window (RW) equipment at 80 °C for 4 h. [19]
      Malic acid : Glucose : Glycerol (1:1:1, mol/mol), IR 50 °C, 90 min 152 mg/g DM Pomegranate peel were dried in an oven at 50 °C for 48 h. [20]
      Choline chloride : Glycerol
      (1:11, mol/mol)
      Liquid/solid ratio 47 mL g−1,
      time 70 min, and 30% (v/v)
      water concentration
      272.98 mg of gallic acid equivalents per g of dry matter Pomegranate peels were dried for 48 h at 40 °C. [21]
    • In this study, the UPLC-Q-TOF-MS/MS instrument was used to analyze the polyphenols in pomegranate peel. The system has the characteristics of high resolution and fast analysis speed and can realize the efficient separation and structural analysis of the components. Figure 1 was a total ion chromatogram, revealing the rich and diverse composition of the pomegranate peel. The higher the peak in the figure, the higher the abundance of the compound. A total of 62 compounds were identified, but this study focused on polyphenols in pomegranate peel. Therefore, non-polyphenolic substances such as fatty acids and steroids were removed, and 53 polyphenolic compounds remained, as shown in Table 2. Of these, a total of 10 compounds were identified as ellagic acids and their derivatives, while the remaining 32 compounds belonged to the flavonoid class, including 12 flavonols, six anthocyanins, five dihydroflavones, two dihydrochalcones, two flavanols, and one each of flavonol, dihydroflavonol, and chalcone. Man et al. used the UHPLC-QTOF-MS method to analyze nine varieties of pomegranate peel and a total of 64 compounds were annotated, including 13 flavonols, four flavanones and six flavan-3-ols compounds[25]. Punicalin, galloyl-O-punicalin, chebulagic acid, thymol, olivetonide, rutin, avicularin, hyperoside, phlorhizin, genistein, plantaginin, taxifolin were not detected in this study. Zheng et al. also used the UPLC-Q TOF MS technique to study the components in pomegranate peel and obtained a total of 25 components[26]. The key monomer compound of ellagic acid and its derivatives in the biosynthesis process is gallic acid. However, due to the drying treatment of pomegranate peel at 90 °C in this study, gallic acid itself was unstable, so it was decomposed during the drying process and not detected in the mass spectrum. Fresh pomegranate peel was freeze-dried and pulverized by Man et al. for qualitative analysis using UHPLC-QTOF-MS, resulting in the identification of 21 phenolic compounds, including gallic acid. These findings highlight the significant impact of drying temperature on the presence of gallic acid[27,28].

      Figure 1. 

      Total ion chromatogram of the pomegranate peel extract was obtained using UPLC-Q-TOF-MS/MS in both (a) positive and (b) negative modes.

      Table 2.  Identification of the chemical compositions from pomegranate peel using UPLC-Q-TOF-MS/MS.

      No. tR (min) Molecular formula Measured (m/z) Actual (m/z) Adducts Identification Error (ppm)
      1 8.00 C41H28O26 935.0814 936.0869 [M-H] Casuarinin 2.0
      2 8.17 C28H32O16 623.1600 624.1690 [M-H] 6-Methoxykaempferol 3-O-rutinoside −2.8
      3 8.20 C41H28O26 935.0811 936.0869 [M-H] Casuarictin 1.5
      4 8.21 C34H24O22 785.0843 784.0759 [M+H]+ Pedunculagin 1.4
      5 8.43 C27H30O16 655.1498 610.1534 [M+HCOO] Kaempferol 3,7-diglucoside −2.8
      6 8.52 C20H18O11 433.0776 434.0849 [M-H] Guajavarin −0.1
      7 8.65 C27H32O14 579.1731 580.1792 [M-H], [M+HCOO] Naringin 2.1
      8 8.80 C21H20O11 449.1081 448.1006 [M+H]+ Astragalin 0.6
      9 8.81 C21H20O12 465.1030 464.0955 [M+H]+ Isoquercitrin 0.6
      10 8.87 C21H20O10 433.1135 432.1057 [M+H]+ Apigenin 7-glucoside 1.4
      11 8.90 C21H21ClO12 545.0696 500.0722 [M+HCOO] Delphinidin-3-O-glucoside chloride −1.4
      12 9.04 C15H12O6 287.0561 288.0634 [M-H] Dihydrokaempferol 0.1
      13 9.15 C28H34O15 609.1827 610.1898 [M-H] Hesperidin 0.3
      14 9.32 C7H6O3 139.0390 138.0317 [M+H]+ 4-Hydroxybenzoic acid 0.2
      15 9.46 C34H24O22 783.0705 784.0759 [M-H] Terflavin B 2.3
      16 9.49 C21H20O10 431.0988 432.1057 [M-H] Afzelin 1.0
      17 9.50 C15H10O6 287.0546 286.0477 [M+H]+ Fisetin −1.3
      18 9.71 C15H14O6 291.0865 290.0790 [M+H]+ (+/−)-Catechin 0.8
      19 9.89 C21H24O10 435.1299 436.1370 [M-H], [M+HCOO] Phlorizin 0.5
      20 9.93 C20H18O10 417.0834 418.0900 [M-H] Kaempferol 3-α-L-arabinopyranoside 1.7
      21 10.17 C15H12O6 289.0703 288.0634 [M+H]+ Eriodictyol −1.3
      22 10.30 C15H14O6 291.0977 290.0896 [M+H]+ (+)-Epicatechin 2.7
      23 10.35 C22H26O11 484.1831 466.1475 [M+H]+ Agnuside 3.6
      24 10.82 C30H26O13 593.1298 594.1373 [M-H] Tiliroside −0.4
      25 10.98 C13H8O8 315.0127 292.0219 [M+Na]+ Brevifolincarboxylic acid 5.0
      26 11.14 C27H31ClO16 647.1391 646.1301 [M+H]+ Cyanidin-3,5-diglucoside 2.7
      27 11.21 C10H10O4 217.0470 194.0579 [M+Na]+ Ferulic Acid −0.5
      28 11.24 C16H12O7 315.0508 316.0583 [M-H] Isorhamnetin −0.7
      29 11.31 C34H26O22 787.0997 786.0916 [M+H]+ Tellimagrandin I 1.0
      30 11.63 C26H30O13 551.1776 550.1686 [M+H]+ Neolicuroside 3.1
      31 11.74 C15H12O5 273.0755 272.0685 [M+H]+ Naringenin −1.0
      32 11.89 C15H14O5 273.0775 274.0841 [M-H] Phloretin 2.5
      33 11.93 C27H31ClO15 675.1323 630.1352 [M+HCOO] Pelargonidin-3,5-Di-O-glucoside −1.6
      34 12.15 C15H10O6 287.0549 286.0477 [M+H]+ Kaempferol −0.4
      35 12.35 C48H28O30 1083.0610 1084.0665 [M-H] Punicalagin 1.6
      36 12.39 C17H14O8 391.0661 346.0689 [M+HCOO] Syringetin −2.5
      37 12.87 C30H26O12 577.1362 578.1424 [M-H] Procyanidin B1 1.8
      38 13.06 C14H6O8 300.9995 302.0063 [M-H] Ellagic acid 1.8
      39 13.15 C20H22O8 391.1371 390.1315 [M+H]+ trans−Resveratrol 4'-O-β-D-glucuronide −4.3
      40 14.34 C21H20O6 369.1325 368.1260 [M+H]+ Curcumin −1.9
      41 14.55 C28H24O5 439.1535 440.1624 [M-H] 3,4-Dibenzyl-gallic acid Benzyl Ester −3.7
      42 14.77 C15H11ClO5 329.0181 306.0295 [M+Na]+ Pelargonidin −1.8
      43 15.78 C20H20O14 483.0785 484.0853 [M-H] Gallic acid 3-O-(6'-O-galloyl)-β-D-glucopyranoside 0.9
      44 16.71 C30H26O13 593.1303 594.1373 [M-H] Prodelphinidin C 0.5
      45 18.02 C21H22O9 419.1342 418.1264 [M+H]+ Liquiritin 1.4
      46 19.89 C27H24O18 635.0874 636.0963 [M-H] Corilagin −2.5
      47 20.28 C41H28O27 951.0752 952.0818 [M-H] Granatin B 0.7
      48 21.54 C13H16O10 331.0675 332.0744 [M-H] Glucogallin 1.2
      49 21.72 C20H16O12 447.0564 448.0642 [M-H] Ellagic acid 3-O-α-L-rhamnopyranoside −1.2
      50 22.50 C21H21ClO10 467.0733 468.0823 [M-H] Pelargonidin-3-O-glucoside −3.7
      51 22.79 C9H8O4 181.0492 180.0423 [M+H]+ Caffeic acid −1.6
      52 23.78 C9H8O3 165.0548 164.0473 [M+H]+ p−Coumaric acid 1.4
      53 27.37 C21H21ClO11 483.0705 484.0772 [M-H] Cyanidin 3-O-glucoside 1.2
    • Gallic acid, the monomer that forms ellagic acid and its derivatives, has a variety of pharmacological activities, such as inhibiting thrombin and platelet aggregation, thus acting as a natural thrombin inhibitor[26]. Ellagic acid and its derivatives are one of the main components in pomegranate peel, they have rich pharmacological activities, such as anti-cancer, and antioxidant, especially in the scavenging of free radicals in the body has a strong advantage[7,29]. According to research, pomegranate peel extract had a better DPPH scavenging ability, to avoid oxidative stress caused by disease and ageing[30]. The study conducted by Fu et al. demonstrated the efficacy of ellagic acid in treating androgen-induced hair loss, thereby presenting a novel therapeutic option for individuals experiencing hair loss while also expanding the scope of ellagic acid's applications[31]. As shown in Fig. 2, two molecules of gallic acid are connected by an ester bond to form ellagic acid (No. 38) in Table 2, and ellagic acid is connected with 1 molecule of rhamnose to form glycosides, which is ellagic acid 3-O-α-L-rhamnopyranoside (No. 49). The benzene ring of ellagic acid is connected with one molecule of gallic acid, and the structure of glucose was connected with 1 molecule of gallic acid, forming terflavin B (No. 15). If one gallic acid was connected to each of the two benzene rings of ellagic acid, and then compound punicalagin (No. 35) was formed by connecting one molecule of glucose with one molecule of hexahydroxydiphenoyl (HHDP). Punicalagin is a characteristic compound in pomegranate peel, which is not only high in pomegranate peel but also has pharmacological activities such as nerve protection and diabetes treatment[32,33]. If two molecules of gallic acid are joined together in opposite ways from ellagic acid, the compound HHDP can be obtained. A glucose was connected to the HHDP to form a lactone ring with 10 carbons, and then a gallic acid was connected to the glucose to form compound corilagin (No. 46), and if two gallic acids were connected to glucose, compound tellimagrandin I (No. 29) is formed. On condition that a 6-carbon lactone ring was formed between HHDP with glucose and the glucose was attached to two molecules of gallic acid is pedunculagin (No. 4), or the gallic acid of three molecules was compound casuarictin (No. 3). After entering the body, ellagitannin in pomegranate peel is metabolized by intestinal flora into urolithin A and other substances, among which urolithin A has anti-cancer and anti-aging properties[34,35]. Pomegranate peel is a kind of medicinal material with the same origin as food and medicine. Ellagic acid contained in pomegranate peel has good solubility in water, and because of its rich pharmacological activity, it can play a good role in health care when added as tea in water. Moreover, the way to extract the active ingredients in the pomegranate peel can completely avoid the astringent taste caused by directly using the pomegranate peel, then increasing people's desire to drink.

      Figure 2. 

      Synthesis of ellagic acid and its derivatives.

    • The biosynthetic pathway of gallic acid and anthocyanin, the most hydrophilic flavonoid, was intricately linked to the shikimic acid pathway. The 3-dehydroshikimic acid, generated through the shikimic acid pathway, undergoes subsequent conversion to gallic acid facilitated by the coenzyme NADP and shikimate dehydrogenase. The biosynthetic pathway of anthocyanins differs from that of gallic acid. Firstly, the conversion of 3-dehydroshikimic acid to shikimic acid occurs through the co-interaction of NADPH and shikimate dehydrogenase, facilitating the subsequent production of anthocyanins via the phenylalanine and phenylpropane pathways[36]. Flavonoids and their derivatives constitute a major constituent of pomegranate peel, representing significant polyphenolic compounds. They are commonly present in the daily human diet and exhibit diverse pharmacological activities, encompassing potent antioxidant, antibacterial, anti-inflammatory, and other properties. These attributes confer significant advantages in scavenging free radicals within the body, mitigating oxidative stress reactions, and reducing the susceptibility to associated diseases[37]. The antioxidant effect has a vital function in averting cardiovascular disease, cancer, and neurodegenerative diseases[38,39]. Relevant research has demonstrated the potential application of flavonoid polyphenols in the domain of dermatological treatment[40]. Experimental evidence has substantiated that pomegranate peel extract exhibits antioxidative, anti-inflammatory, and anti-apoptotic properties in murine models[41]. The simultaneous discovery of the significant advantages in solubility, resistance to decomposition, and high stability exhibited by polyphenols further enhances their potential for application in various fields[42].

      The extraction of flavonoid polyphenols from pomegranates led to the identification of nine distinct classes of compounds derived from the 2-phenylchromogen parent nucleus. The only flavonoid identified in this study is apigenin 7-glucoside (No. 10), as illustrated in Fig. 3. Flavonol compounds, particularly kaempferol (No. 34) and its derivatives, are found in abundance with a total of 12 different types. These flavonol compounds undergo methylation or glycosylation reactions to generate various derivatives. For example, there are only two compounds that undergo only 3'-hydroxymethylation, such as isorhamnetin (No. 28), and four compounds that undergo 3'-hydroxyglucosylation, such as kaempferol 3-α-L-arabinopyranoside (No. 20), which all contain one glycoside, with the only difference being the type of glycoside. And two compounds contain two glycosides. In addition, compound fisetin (No. 17) and its two derivatives are methylated at positions 3 and 5. Naringenin (No. 31) is postulated as the precursor of dihydroflavone, while the addition of hydroxyl, methoxy, or one or two sugar units leads to the formation of No. 21, No. 45, No. 13 and No. 07. Simultaneously, dihydrokaempferol can be synthesized through the introduction of a hydroxyl group at the third position of No. 31. Pomegranates contain two distinct flavanols, namely (+)-catechin (No. 18) and (+)-epicatechin (No. 22), which exist as optical isomers. Of the anthocyanin compounds, pelargonidin (No. 42) was the starting material, and the other five compounds have undergone 1 to 2 methylation and glycosylation reactions. The biflavonoid compounds include procyanidin B1 (No. 37) and prodelphinidin C (No. 44). In addition, this research identified chalcones and their reduced dihydrochalcones, such as neolicuroside (No. 30) and No. 32 and No. 19. The flavonoids present in pomegranate peel are highly suitable for tea preparation due to their exceptional water solubility and thermal stability. These compounds exhibit potent antioxidant activity, effectively scavenging free radicals within the body, thereby mitigating oxidative stress response, safeguarding cells against oxidative damage, and combating chronic diseases. Consequently, utilizing pomegranate peel as a raw material for tea confers significant health benefits to the human body[43].

      Figure 3. 

      Synthesis of flavonoid and its derivatives.

    • The molecular docking analysis of the α-glucosidase, eNOS, ACE and AR inhibitory potentials of the identified compounds above was performed using AutoDock Vina. The identified compounds showed binding affinities ranging from −6.1 to −12.2 kcal/mol against α-glucosidase, as displayed in Fig. 4. Of the 53 identified compounds, as well as acarbose (No. 54), a positive control used to treat diabetes, the highest binding affinity against α-glucosidase was recorded as −12.2 kcal/mol for Punicalagin (No. 35). As shown in Fig. 5 and Table 3, 10 hydrogen bonds were formed between Punicalagin (No. 35) and α-glucosidase, while acarbose only formed eight hydrogen bonds with α-glucosidase. From the results of molecular docking, Punicalagin had a better ability to inhibit α-glucosidase than acarbose. Yi et al. also carried out molecular docking experiments of punicalagin and α-glucosidase, and obtained the result of intermolecular energy of −7.99 kcal/mol[32]. The treatment of diabetes by acarbose relied on its structure similar to oligosaccharides, reversible binding with α-glucosidase, inhibiting the activity of α-glucosidase and achieving hypoglycemic effect. The molecular weight of Punicalagin was higher than that of acarbose, and the inhibition mechanism of α-glucosidase by Punicalagin was different from that of acarbose. The inhibition of α-glucosidase by Punicalagin might be because the polyphenol structure of Punicalagin contained a large number of hydroxyl groups, and the hydrogen bond formed between Punicalagin and the enzyme impeded the function of the enzyme, thus to achieve the role of hypoglycemia.

      Figure 4. 

      Heatmap of the binding energy of the identified compounds and acarbose to α-glucosidase, eNOS, ACE, and AR.

      Figure 5. 

      (a), (b) Molecular interactions of the identified compounds and acarbose against α-glucosidase, (c) eNOS, (d) ACE, and (e) AR. (a) Punicalagin-α-glucosidase; (b) acarbose-α-glucosidase; (c) Punicalagin-eNOS; (d) Punicalagin-ACE; (e) Ellagic acid 3-O-α-L-rhamnopyranoside-AR.

      Table 3.  Molecular docking results of Punicalagin (No. 35), acarbose and α-glucosidase.

      α-glucosidase Punicalagin
      (No. 35)
      Acarbose No. of
      hydrogen bonds
      Bond length (Å)
      ARG437 O18 1 2.076
      THR833 O18 1 1.927
      ARG837 O3 2 3.572, 2.133
      O4 2 2.679, 3.244
      O5 2 2.204, 2.843
      TRP859 O12 1 3.338
      O3 1 3.456
      TYR360 O5 1 1.848
      ARG585 O8 2 3.354, 2.516
      O17 1 3.719
      ARG608 O2 2 3.582, 2.741
      O7 1 3.285
      O14 1 3.150

      The associated proteins that cause diabetes complications are eNOS, ACE, and AR. The interactions of the 53 identified compounds and acarbose with eNOS, ACE, and AR were also analyzed, and it was calculated that the highest binding affinities with −14.3, −13.9, and −13.9 kcal/mol for Punicalagin, Punicalagin, and Ellagic acid 3-O-α-L-rhamnopyranoside (No. 49). The key mechanism of pomegranate peel polyphenols in the treatment of diabetes was to reduce oxidative stress and lipid peroxidation, and to combine with the generated reactive oxygen species, to achieve the role of preventing complications[44]. Punicalagin, a natural compound derived from pomegranate peel had the characteristics of being more effective and safer, not only playing a role in lowering blood sugar, but also having a good therapeutic effect on complications of diabetes, such as liver damage[45,46].

    • In this study, the outer layer of the pomegranate, which was derived from the fruit itself, was selected as the object of study and ultrasonic-assisted extraction technology with glycerol/urea (1:2, mol/mol, 30% water content) was adopted to extract the peel. This method not only improved extraction efficiency, but also reduced energy consumption and environmental pollution. A total of 53 polyphenols were obtained from pomegranate peel extracted by UPLC-Q-TOF-MS/MS, including 10 ellagic acids and their derivatives and 32 flavonoids (such as flavonoids, dihydroflavonoids, flavonols, anthocyanins, etc.). Ellagic acid and flavonoids, as well as their derivatives, all had good anti-inflammatory and antioxidant properties, which meant that they have great potential to prevent chronic diseases, such as cancer, cardiovascular, and cerebrovascular diseases, etc. According to the heatmap and molecular docking results, Punicalagin (No. 35) in pomegranate peel had a good inhibitory effect on α-glucosidase with a binding affinity of −12.2 kcal/mol and 10 hydrogen bonds formed. The polyphenols in pomegranate peel had a good therapeutic effect on diabetes and its complications, and the effective prevention and treatment of these diseases could improve quality of life and health status, and the development of pomegranate peel had a good application prospect in functional food or beverage. In addition, polyphenols also had good water solubility, and the pomegranate peel compounds added to the water as a tea for daily drinking were not only beneficial to individuals' well-being but also conducive to enhancing corporate profitability and improved the utilization of resources. Furthermore, this approach was of paramount importance as it effectively mitigated the environmental pollution caused by pomegranate peel. If the pomegranate peel is discarded at will, the phenolic substances in it will have a certain impact on water and soil. Extracting useful compounds from pomegranate peel could both solve environmental problems and create economic value, in line with the strategy of sustainable development. Therefore, this study provided an effective way for the comprehensive utilization of pomegranate peel, which has good practical significance and application prospects.

    • The authors confirm contribution to the paper as follows: conceptualization: Liu J, Cui Q; data curation: Wen L, Liu J, Cui Q; formal analysis: Lu X, Ye Z; funding acquisition: Liu J, Cui Q; investigation: Lu X, Ye Z, Jiang L, Wen L, Liu J, Cui Q; methodology: Lu X, Jiang L, Liu J, Cui Q; project administration: Cui Q; resources: Ye Z, Jiang L; supervision: Liu J, Cui Q; visualization: Lu X, Jiang L; writing-original draft: Lu X; writing-review & editing: Liu J, Cui Q. All authors reviewed the results and approved the final version of the manuscript.

    • The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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

      • This work was supported by Zhejiang Provincial Natural Science Foundation of China under Grant Nos LQ24H280009 and LQ22H280007, National Natural Science Foundation of China (82204552), Research Project of Zhejiang Chinese Medical University (2022JKZKTS10), National College Students' Innovation and Entrepreneurship Training Program (S202310344074), Zhejiang Xin-miao Talents Program (2023R410043).

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

      • # Authors contributed equally: Xiaoxian Lu, Zi Ye

      • Copyright: © 2024 by the author(s). Published by Maximum Academic Press, Fayetteville, GA. This article is an open access article distributed under Creative Commons Attribution License (CC BY 4.0), visit https://creativecommons.org/licenses/by/4.0/.
    Figure (5)  Table (3) References (46)
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    Lu X, Ye Z, Jiang L, Wen L, Liu J, et al. 2024. UPLC-Q-TOF-MS/MS analysis of pomegranate peel polyphenols and preliminary study on the biosynthesis of ellagic acid and flavonoids. Beverage Plant Research 4: e028 doi: 10.48130/bpr-0024-0031
    Lu X, Ye Z, Jiang L, Wen L, Liu J, et al. 2024. UPLC-Q-TOF-MS/MS analysis of pomegranate peel polyphenols and preliminary study on the biosynthesis of ellagic acid and flavonoids. Beverage Plant Research 4: e028 doi: 10.48130/bpr-0024-0031

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