Effect of pH on Fermentation Characteristics and Protein Degradation by Rumen Microorganisms In Vitro

Abstract

The artificial rumen was used to study the effect of pH on rate of fermentation of a pelleted concentrate and corn silage diet. The pH of the artificial rumen contents was maintained by infusion of artificial saliva and automated addition of sodium carbonate. Characteristics of fermentation were measured for 5 days after a 14-day equilibration. Rate of volatile fatty acid production decreased from 80 to 50 mmoles/day when pH was decreased from 7 to 5. Rates of production of acetic, butyric, isobutyric, and isovaleric acids decreased, whereas those of propionic, valeric, and caproic increased when pH decreased from 7 to 5. The ratio of acetic to propionic acid was 4.3 at pH 7 and .9 at pH 5. Methane production was low at pH 5.5 and ceased at pH 5. Net hydrogen production increased from a trace at pH 5.5 to 20 mmoles/day at pH 5. The ammonia concentration in culture fluid was 4 to 6 mmoles at pH 6 and 7 but was .2mM at pH 5.5 and 5. Low ammonia concentrations were accompanied by decreased free amino acid concentrations and by decreased protease and deaminase activity of washed bacterial cells. Proteolytic organisms, in concentrations of .5 to 1 X 108 cells per milliliter at pH 6 and pH 7, could not be detected (less than 104 cells/ml) at pH 5.5. Proteolytic organisms apparently were washed out of continuous culture at pH below 6.