Toughening In Blanched Asparagus : Identification Of Phenolic Compounds And Evidence For A Free Radical Mechanism

Abstract

Prior research in this laboratory established apparent nonenzymatic toughening occurs in heat treated asparagus tissue. The present work attempted to identify the phenol compounds involved and to discern the mechanism(s) of this reaction. Toughness (Warner-Bratzler shear) of blanched (no detectable per-oxidase or polyphenol oxidase activity) spears stored at 22°C increased significantly over a 4 day period. Reverse phase-high pressure liquid chromatography (RP-HPLC) of methanolic tissue extracts indicated 7 major phenolic peaks. HPLC retention times and Fourier transform-infrared spectroscopy tentatively identified the extracted phenols as: (1) 4-hydroxybenzoic acid; (2) caffeic acid; (3) vanillic acid; (4) syringic acid; (5) p-coumaric acid; (6) syringaldehyde; and (7) ferulic acid. A significant decrease in the concentration (HPLC peak area) of 1, 2, 5, 6 and 7 occurred after 4 days storage. In vitro studies with homogenized (blanched and unblanched) asparagus tissue indicated a significant decrease in the concentration of added phenols. Unblanched tissue produced greater changes. The site of added phenol incorporation (a significant increase in fluorescence intensity) was determined to be the vascular bundle region. Metal ion chelation (EDTA), addition of iron (Fe2+) and reduction of metal ions using L-ascorbic acid affected added phenol utilization variably. Only the addition of a mixed antioxidant (BHA, PG, citric acid) served to significantly decrease utilization of the added phenols by blanched tissue blends, therefore implicating a free radical mediated phenol coupling mechanism. ESR spectroscopy detected the ascorbate radical anion in the blanched homogenate.