Walnut tannins were maximally extracted with absolute methanol and an extraction time of 60min. Storage (21 days at 25°C) 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°C. Roasting (204°C 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°C 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).
ASJC Scopus subject areas
- Agricultural and Biological Sciences (miscellaneous)
- Food Science
- Chemistry (miscellaneous)