Analysis of polyphenols in apple pomace: A comparative study of different extraction and hydrolysis procedures

Apple pomace is a potential source of bioactive polyphenols. The objective of this study was to compare different methods for extraction of phenolic compounds. The content of free polyphenolic compounds in aqueous-methanol extracts was 5.56 mg gallic acid equivalents (GAE)/g dry weight (DW) pomace, with the major free phenolic compounds identified as chlorogenic acid, quercetin-3-O-galactoside, quercetin-3-O-rhamnoside, and phloridzin. Different order of sequential base and acid hydrolysis of the residue and then extraction by diethyl ether/ethyl acetate yielded 6.82-8.12 mg GAE/g DW of bound polyphenols. The major phenolics in the residue, after aqueous-methanol was subjected to base hydrolysis, were phloridzin, 4-hydroxybenzoic acid, and quercetin-3-O-galactoside, and subsequent acid hydrolysis further released significant amounts of 4-hydroxybenzoic acid and isoferulic acid. Extraction of the residue remaining after aqueous-methanol extraction subjected to acid hydrolysis released mainly 4-hydroxybenzoic acid, protocatechuic acid and isoferulic acid, and subsequent base hydrolysis further released significant amounts of protocatechuic acid, catechin and 4-hydroxybenzoic acid. Extracts obtained after direct acid or base hydrolysis yielded 5.55 mg GAE/g DW and 10.62 mg GAE/g DW, respectively. The absolute amounts of the individual phenolic compounds were different in different fractions after either direct base or acid hydrolysis. Regardless of the hydrolysis procedures, the extractions with higher phenolics contents showed the stronger antioxidant activities, as measured by oxygen radical absorbance capacity (ORAC). This work shows that extraction and hydrolysis procedure have substantial effects on the recovery of phenolics from apple pomace.