Chemical Composition, Water Sorption Isotherm, and Phenolic Contents in Fresh and Dried Pomegranate Peels

Pomegranate peels were dried by freeze drying at 20°C; air and vacuum drying at 40, 60, and 90°C; and sun drying. The moisture sorption isotherm was measured and modeled using the Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-De Boer (GAB) models. Two solvents (methanol and ethanol) and water were used to extract the phenolic compounds in pomegranate fruit peel. Fresh peels contained 5,990, 4,530, and 8,460 mg gallic acid equivalent (GAE)/100 g dry-peel solids for methanol, ethanol, and water extracts, respectively. The total phenolic content of ethanol extracts of freeze-dried peels was comparable to that of fresh peels (4,900 mg GAE/100 g dry-peel solids), whereas air- and vacuum-dried peels had significantly lower phenolic contents. Peels air dried at 60°C had the highest phenolic content (2,320-4,650 mg/100 g dry-peel solids) compared to samples air dried at 40 or 90°C (1,160-4,480 mg/100 g dry-peel solids), whereas vacuum-dried peels did not show any trends with temperature. In general, methanol had a higher capacity for extracting phenolic compounds from dried pomegranate peels than water, and ethanol showed a low extraction capacity. In all cases, phenolic compounds were significantly lower in ethanol extracts compared to methanol or water extracts (p < 0.05). In addition, phenolic compounds soluble in water and ethanol were more sensitive to all drying methods except freeze drying.