Fate of nitrogen during metabolism of whey lactose by Rahnella aquatilis

The rate of intake of peptides and free amino acids by Rahnella aquatilis was studied, as well as the evolution of concentrations of exopolysaccharide, biomass, and several metabolites. For this purpose, eight wheybased fermentation media were tested: hydrolyzed bovine whey under 1) aerobic and 2) anaerobic conditions; hydrolyzed bovine whey with 3) 2.0% (wt/vol) NaCl or 4) 0.5% (wt/vol) NaCl; 5) plain bovine whey; 6) bovine whey permeate; 7) hydrolyzed caprine whey; and 8) hydrolyzed ovine whey. The lower peptide fraction (< 2000 Da) in plain whey, whey permeate, and hydrolyzed whey fermented aerobically was virtually depleted by 24 h. In all experiments, most consumption of free amino acids occurred at initial stages of fermentation. In caprine whey, ovine whey, and bovine whey fermented anaerobically, free Val was present at high levels until the end of fermentation, and the highest production of acetic acid was also observed. In ovine whey, high levels of peptides and free amino acids were recorded, as well as the highest production of several organic acids (except lactic acid) and the lowest viscosity and polysaccharide concentration. The low concentration of peptides and free amino acids in plain whey and whey permeate did not apparently permit active consumption of lactose, whereas lack of molecular oxygen and presence of salt played a strong role toward inhibition of exopolysaccharide production. The qualitative and quantitative profile of the nitrogen fraction likely might have determined the metabolism of lactose and, consequently, conditioned the production of exopolysaccharide and organic acids even though growth of R. aquatilis was not affected similarly.