Table 4.
The effect of drying techniques on the phytochemical and nutritional properties of pomegranate peel waste.
| Drying Technique | Cultivar | Country | Phytochemical and Nutritional Properties | Key Findings | Reference |
|---|---|---|---|---|---|
| Sun (4 days) and oven drying (overnight at 100 °C) | Baghva, Ruby, Indian white, and Egypt | Sultanate of Oman | Moisture loss (%), pH, and vitamin C | Sun-dried fruit peel had highest water loss, pH, and vitamin C values, with ranges of 64.49–82.52%, 3.5–4.5, and 100–75 mg/100 g dry weight (DW), respectively. | [9] |
| Freeze-drying (20 °C for 96 h), air drying (48, 30, and 24 h), vacuum drying (60 kPa vacuum at 40, 60, or 90 °C for 24, 13, and 4.5 h), and sun drying for 11 h | Helow | Sultanate of Oman | Moisture loss (%) and total phenolics | Freeze-dried peels showed phenolic contents comparable to those of fresh peel (i.e., 4900 and 4010 mg gallic acid equivalent (GAE)/100 g dry peel solids). Vacuum drying ranged from 1200 to 5330 mg gallic acid equivalent (GAE)/100 g dry peel solids extracted in ethanol at 40 °C and in water at 90 °C, respectively. The highest sun-dried peel value of 4080 mg gallic acid equivalent (GAE)/100 g dry peel solids was achieved in water extract. | [85] |
| Microwave drying (up to 9 min) air drying (5 days), sun drying (5 days) silica gel drying (up to 7 days) and hot air oven drying (40 °C for 30 h) |
Unknown | India | Moisture loss (%) | Highest drying weight for pomegranate peel using microwave dryer at 34.67 g. Lowest moisture loss for pomegranate peel reported using a microwave oven dryer at 65.33%. | [86] |
| Oven (40 °C for 48 h) and solar drying (50 °C for 2 h) | Unknown | Egypt | Total phenolic content (TPC) and total flavonoid content (TFC) | Total phenolic content (TPC) highest in oven-dried peel with 17.78 mg gallic acid/g in methanol extract. Total flavonoid content (TFC) values statistically similar for both oven- and sun-dried whole peel at 7.98 and 7.96 mg rutin equivalent (RE)/g, respectively. | [18] |
| Freeze (−88.7 °C for 16 h) and oven drying (40, 50, and 60 °C for 22, 17, and 12 h, respectively) | Wonderful | South Africa | Punicalin, rutin, p-coumaric, +catechin,-epicatechin, hesperidin, vitamin C | Higher punicalin and p-coumaric values at 60 °C. Freeze-dried (FD) peel had higher rutin, +catechin, -epicatechin, and hesperidin values at 4666.03 mg/kg dry mass (DM), 674.51, 70.56, and 16.45 mg/kg DM, respectively. Vitamin C was reported at ˂30 µg ascorbic acid equivalent (AAE)/g dry mass (DM) using an oven drier at 40 °C. | [34] |
| Freeze-drying at −80 °C, ambient temperature of ~25 °C, and oven drying at 50 °C | Unknown | USA | Total phenolic content (TPC), α-punicalin, β-punicalin, ellagic acid, α- and β-punicalagin | Higher total phenolic content (TPC), α- punicalin, and β-punicalin values in freeze-dried peel samples of 96 mg/g gallic acid equivalent, 0.8 mg/g, and 1.6 mg/g, respectively. Higher punicalagin (~38.6–50.3 mg/g) and ellagic acid (~2.8–3.2 mg) levels in the peel fractions. | [87] |
| Freeze-drying (FD) for 48 h; convective drying (CD) at 50, 60, or 70 °C; vacuum–microwave drying (VMD) at 3 different power levels of 240, 360, or 480 W; combined drying (CPD-VMFD) and predrying (CPD) at 60 °C for 90 or 150 min, VMFD with microwave wattage of 360 W. | Mollar de Elche | Spain | Punicalagin isomers (α-PC and β-PC), ellagic acid (EA), and total phenolic content (TPC) | The α-punicalagin and β-punicalagin isomers were highest in fresh peel at 139 and 143 mg/g DW, followed by freeze-dried peel at 113.4 and 98.7 mg/g dry weight (DW), respectively. High ellagic acid (EA) reported in fresh peel at 2.49 mg/g, followed by convective drying (60 °C) and freeze-drying, which were statistically similar at 1.95 mg/g at 1.71 mg/g, respectively. Fresh samples had higher total phenolic content (TPC) values at 125 mg/g gallic acid equivalent, followed by freeze-dried peel at 118 mg/g gallic acid equivalent. | [88] |
| Single-mode microwave oven dryer, microwave hydro-diffusion (MH), and gravity oven dryer (MHG): both at temperatures ˂ 100 °C for ˂5 min, and traditional oven dryer at 40 °C for 48 h | Acco and Wonderful | Italy | Total phenolic content (TPC), free ellagic acid (EA), and ellagitannins (ETs) | Microwave hydro-diffusion gravity oven dryer was reported with high amounts of total phenolic content (TPC) at 190–230 mg gallic acid equivalent (GAE)/100 mL, ellagitannins at 106–110 µg/mL, and radical scavenging activity (RSA) at 480–560 mg ascorbic acid equivalent antioxidant capacity (AEAC)/100 mL. | [89] |
Abbreviations: dry weight (DW), total phenolic content (TPC), total flavonoid content (TFC), gallic acid equivalent (GAE), dry mass (DM), ascorbic acid equivalent (AAE), rutin equivalent (RE), Freeze-dried (FD), convective predrying-vacuum-microwave finish drying (CPD-VMFD), vacuum-microwave finish drying (VMFD), ascorbic acid equivalent antioxidant capacity (AEAC), ellagic acid (EA), α-punicalagin (α-PC), β punicalagin (β-PC), microwave hydro-diffusion (MH), Microwave hydro-diffusion gravity oven dryer (MHG), radical scavenging activity (RSA).