Optimal mineral composition of artificial saliva for fermentation and methanogenesis in continuous culture of rumen microorganisms

Abstract

In dual outflow continuous fermenters on a 75 : 25 hay : barley diet, fermentation and gas production by mixed rumen microbes were tested in relation to the concentrations of HPO 4 2−, HCO 3 − and Cl − and the Na +/K + ratio in artificial saliva, by applying a 17-run Franquart design, and by fitting second-order polynomial models. The HPO 4 2−, HCO 3 −, Cl − concentrations and Na +/K + ratio ranged from 0.1 to 4 g l −1, from 0.5 to 7 g l −1, from 0.1 to 0.5 g l −1 and from 0.5 to 15 g g −1, respectively. The major factor was the concentration of HCO 3 −, followed by HPO 4 2− and Na +/K +, while the concentration of Cl − had negligible effects. The volatile fatty acid (VFA) production rate was mostly influenced by changes in pH, from 6.7 to 7.2, mediated by HCO 3 − and HPO 4 2− concentrations. The analysis of the fermentation pattern confirmed the predominant action of HCO 3 −. Butyrate (C4) and branched chain VFA production rates, as well as C4 molar proportion in the fermentation broth were also lowered by high or low values of the Na +/K + ratio. The action of minerals on gas production and protozoa population density was complex and involved to an equal degree several experimental factors with a non-linear behaviour. Protozoa numbers, which varied from 14 to 38 μl −1, were favoured by high Na +/K + ratios. Using response surface analysis, the composition of a mineral base was optimised to simultaneously promote protozoa numbers and methanogenesis in vitro, at 39 μl −1 and 1.20 mmol h −1, respectively. The resulting artificial saliva contained 1.65 g l −1 HPO 4 2−, 4.19 g l −1 HCO 3 −, and 0.22 g l −1 Cl −, and had a Na +/K + ratio of 14.2 g g −1.