Enhanced functional properties of tannic acid after thermal hydrolysis

Abstract

Thermal hydrolysis processing of fresh tannic acid was carried out in a closed reactor at four different temperatures (65, 100, 150 and 200°C). Pressures reached in the system were 1.3 and 4.8MPa at 150 and 200°C, respectively. Hydrolysis products (gallic acid and pyrogallol) were separated and quantified by HPLC. Gallic acid and pyrogallol production was increased (p⩽0.05) when heated to 150°C. Pyrogallol production reached (p⩽0.05) up to 4.6mg/ml at 200°C. Antioxidant activity of processed tannic acid at different temperatures was measured by the Rancimat® method using soybean oil. Processed tannic acid at 200°C showed a ∼3-fold increase (p⩽0.05) in induction time from ∼7h for the control. Pyrogallol had higher (p⩽0.05) antioxidant capacity than gallic acid, thus pyrogallol was proven to be one of the major compounds responsible for enhanced antioxidant capacity in processed tannic acid. However, the induction point of processed tannic acid at 200°C was higher than that of synthetic pyrogallol at higher concentration. This shows that pyrogallol alone is not responsible for the antioxidant capacity of processed tannic acid. A disc diffusion assay showed that tested Gram positive pathogenic strains (Listeria monocytogenes and Staphylococcus aureus) were more (p⩽0.05) sensitive (40–51mm) to processed tannic acid than Gram negative pathogens (22.8–26.8mm). Regardless of bacterial species, pyrogallol also showed higher antimicrobial activity than gallic acid. Simple thermal processing of hydrolyzable tannic acid could be used to produce gallic acid and pyrogallol with enhanced antioxidant and antimicrobial capacity.