Enhanced antioxidant capacity and antimicrobial activity of tannic acid by thermal processing

Thermally processed tannic acid (PTA) showed stronger antioxidant capacity and antimicrobial activity by Rancimat® and disc diffusion tests than fresh tannic acid (FTA).Soybean oil treated with PTA had 33–84% longer induction period (IP) of oxidation (8.0–14.7 h) than FTA treated oil (6.0–8.0 h), while untreated oil (control) showed only 5.9 h IP. The IP was increased (p < 0.05) with increased processing time up to 15 min, with no changes thereafter. The PTA samples had strong antimicrobial activity (inhibition zone diameter ⩾12 mm) on 10 tested human pathogens while the FTA samples showed antimicrobial activity only on two strains, Salmonella Typhimurium (ATCC 14028) and Enterobacter sakazakii MSDH over 1- and 2-day incubation periods. 13C NMR spectra showed that PTA had a higher content of hydrolysed aromatic carboxylic acid groups than FTA. The results showed that thermally processed tannic acid for 15 min had about 67% higher antioxidant capacity and about 50% higher antimicrobial activity than its fresh counterpart. Thermal processing could be useful for enhancing antioxidant capacity and antimicrobial activity of hydrolysable polyphenols in natural plants.