Effects of processing on the chemistry and antioxidant capacity of endogenous bioactive compounds in peanut kernels and skins
Craft, Brian David
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The overall aims of this work were to ascertain the primary phenolic constituents in peanut kernels and skins, and determine if there antioxidant content and capacities are conserved through industrial roasting processes. The first study conducted revealed that the predominant phenolic compounds in peanut kernels are free/bound p-coumaric acids, which are released from their ester or glycoside forms during thermal treatment. This observed increase in p-coumaric acid levels was affected both by peanut type and processing method. A Spanish high-oleic cultivar had the highest naturally occurring levels of free/bound p-coumaric acids and a high-oleic Runner yielded an increase of ~785% in free p-coumaric acid from raw to oil-roasted sample. The total phenolic contents and antioxidant capacities of processed peanut kernels from the 2007 crop were found to increase in a stepwise fashion from raw < dry-roast < oil-roast for nearly all phytochemical analyses performed. This suggests that although the chemistry of phenolic compounds in peanuts is changing through roasting, their antioxidant potency is preserved and sometimes increased. The second and third studies undertaken involved the chromatographic characterization of low- (LMW) and high-molecular-weight (HMW) phenolic fractions isolated from dry-blanched (DB) and dry-roasted (DR) peanut skins. Catechin, epicatechin, and p-coumaric acid esters were identified in the LMW fraction of DB peanut skins; whereas catechin, free p-coumaric and protocatechuic acids, and a protocatechuic acid ester were found in DR skins. Total phenolics and antioxidant capacities were similar for DB and DR skins. Some of the collected LMW fractions exhibited higher antioxidant capacities than their corresponding crude extracts; suggesting that LMW phenolics contribute to peanut skin’s total antioxidant capacity. Proanthocyanidin (PAC) analyses showed that DR peanut skin tannins have a higher degree of polymerization than DB skins. A greater incidence of PAC polymerization reactions may be a result of the higher temperatures involved in the dry-roasting process. All three studies indicated that processing is altering the chemistry of the phenolics in peanuts and peanut skins, but their antioxidant efficacy is retained for the consumer.
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