Effects of processing and storage on walnut (Juglans regia L) tannins

Walnut tannins were maximally extracted with absolute methanol and an extraction time of 60min. Storage (21 days at 25 degreesC) significantly reduced (20-40% reduction) the assayable tannin content in walnuts. The ranges of tannin content in freshly cracked inshell and shelled walnuts were respectively 372-1095 and 363-667mg catechin equivalent per 100g dry weight. The assayable tannin content in inshell and shelled walnuts was 10-20% higher when the particle size was reduced from 2- to 8-mesh. Using 0.5 as compared to 2% (w/v) vanillin as colour development reagent yielded 15-20% higher assayable tannin contents. The assay colour development reached a maximum after 20min of incubation at 25 degreesC. Roasting (204 degreesC for 5min) caused a small (14%) but significant reduction in assayable tannins. Soaking in aqueous alkali solutions was more effective (44-100% reduction) than soaking in aqueous acid solutions (6-76% reduction) in decreasing the assayable tannin content in walnuts. Microwave heating in distilled deionised water resulted in 93-98% reduction in walnut assayable tannins. Blanching at 100 degreesC for 2min was as effective as alkali soaking in significantly reducing the amount of extractable walnut tannins (98% reduction). The tannin content in blanched walnuts was positively related to astringency scores (r = 0.92). (C) 2001 Society of Chemical Industry.