Interactions of tannins with enzymes: A potential role in the reduced rate of ethanol fermentation from high-tannin biomass

Abstract

It has been shown that the mash prepared from high-tannin varieties of grain sorghum supports a significantly lower rate of ethanol fermentation than that of lower tannin varieties. The basis for the reduced rate of fermentation appears to be nitrogen deprivation, since the addition of nitrogen or the enzyme protease restores a high rate of fermentation. In this investigation, we have examined the tannin-protein interaction for protease with the hydrolyzable tannin, tannic acid, and a purified condensed tannin or proanthocyanidin from sorghum. In addition, we have used the enzyme β-glucosidase for comparison since it has been widely examined in tannin-protein precipitation studies. The results show a concentration effect of tannins for the inhibition of β-glucosidase and protease, and no preferential binding of the substrates used. Enzyme precipitation and enzyme inhibition appear to result from different types of protein/tannin interactions, since water-washing of enzyme-tannin pellets gave much higher activities than unwashed suspensions. The competitor, polyvinylpyrrolidone, completely displaced tannic acid, but not proanthocyanidin, in the tannin/protease complex. The tannic acid/protease pellet showed 50–60 % of the expected activity, whereas, the proanthocyanidin/protease pellet showed only about 30% of expected activity at one-tenth the concentration of tannin. These results suggest that condensed tannins are good candidates for the inhibition of the fermentation of high-tannin varieties.