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

Walnut tannins were maximally extracted with absolute methanol and an extra ction 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 respecti vely 372-1095 and 363-667mg catechin equivalent per 100g dry weight. The as sayable tannin content in inshell and shelled walnuts was 10-20% higher whe n 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 assa yable tannin contents. The assay colour development reached a maximum after 20min of incubation at 25 degreesC. Roasting (204 degreesC for 5min) cause d a small (14%) but significant reduction in assayable tannins. Soaking in aqueous alkali solutions was more effective (44-100% reduction) than soakin g in aqueous acid solutions (6-76% reduction) in decreasing the assayable t annin content in walnuts. Microwave heating in distilled deionised water re sulted in 93-98% reduction in walnut assayable tannins. Blanching at 100 de greesC for 2min was as effective as alkali soaking in significantly reducin g the amount of extractable walnut tannins (98% reduction). The tannin cont ent in blanched walnuts was positively related to astringency scores (r = 0 .92). (C) 2001 Society of Chemical Industry.