The tannin theory of methanogenic toxicity

The tannin theory describes the effectiveness of tannic compounds as bacterial inhibitors based on their molecular weight (MW). The inhibition is postulated to result from the hydrogen-bonding of the tannins with bacterial proteins (i.e. enzymes). Oligomeric tannins are expected to be the most effective inhibitors. The oligomeric tannins form stronger hydrogen-bonds with proteins than do their monomers. The high-MW tannins are also reactive with proteins in the bulk solution; however, they are not able to penetrate to bacterial proteins. In this study, tannins were polymerized by autoxidation to colored compounds in order to investigate the role of tannin MW on methanogenic toxicity. The autoxidation of catechin, a tannin monomer, paralleled the expected toxicity as predicted by the tannin theory. The toxicity increased as it was polymerized to oligomeric tannins and decreased as the oligomers were converted to non-toxic high-MW tannins. The autoxidation of green-tea tannins, oligomeric tannins, did not lead to extensive polymerization reactions and no accumulation of high MW tannins was observed. Instead, the toxic green-tea tannins were transformed to colored non-tannic compounds of variable MW, which were non-toxic.