Some evidence that humic compounds are capable of inhibiting enzymatic activity is reported in the literature. The influence of sodium humate on the hydrolysis of a non-soluble potato protein during anaerobic digestion was investigated. The addition of 250 and 1000 mg of humate per litre caused an inhibition of the protein hydrolysis. The inhibition could be characterized mostly as an extended lag-phase of hydrolysis (extended 8 days by 1000 mg of humate per litre). The onset of acidification and methanogenesis was dependent on the release of substrate by hydrolysis. No inhibition was observed in the anaerobic digestion of a mixture of the six most abundant amino acids in potato protein. This indicated that only the hydrolytic stage of anaerobic protein digestion was inhibited by humate.
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