Utilization of protein extract from dairy manure as a nitrogen source by Rhizopus oryzae NRRL-395 for l-lactic acid production

Six levels of crude protein (0.21, 0.42, 0.84, 1.68, 2.52, and 3.36 g/L) and six levels of protein hydrolysates from dairy manure, defined by degree of hydrolysis (DH, 6.9%, 17.2%, 25.9%, 33.8%, 36.1%, and 36.7%), were investigated as the nitrogen source for production of l-lactic acid by Rhizopus oryzae NRRL-395 with respect to the influence of nitrogen source on l-lactic acid yield and the correlation with biomass yield and mycelia morphology. Increases in crude protein from 0.21 to 1.68 g/L led to an increase in l-lactic acid concentration in the culture media from 6.48 to 57.7 g/L. However, further increases beyond 1.68 g/L did not present continuing increases in l-lactic acid yields. The highest biomass yield was obtained at a crude protein nitrogen concentration of 2.52 g/L. Hydrolysates with high DH resulted in high yields of l-lactic acid and biomass. At a nitrogen level of 0.42 g/L (hydrolysates) with DH ranging from 33.8% to 36.7%, the l-lactic acid yield of 0.53–0.56 g/g of glucose was achieved, coupled with a 13–14% yield of fungal biomass.