Development and optimization of a culture medium for L-lactic acid production by Rhizopus oryzae using crude protein from dairy manure as a nitrogen source

摘要

Experiments were conducted using the crude protein in fresh dairy manure as the nitrogen source in the culture media for Rhizopus oryzae to produce L-lactic acid. Two uniform design experiments were carried out with one for optimizing seed culture while the other for best percent L-lactic acid production. Multiple linear regression and ANOVA analyses were employed to determine the most significant media components. Data from the first uniform design U_6 (6~2 × 3), in which the experimental factors involved included nitrogen concentration (crude protein), spore concentration, and treatment duration, showed that the levels of these components in the optimal condition for the seed culture medium were 2.1 g/L nitrogen, 16 hour culture time, and 105 spore concentration. The biomass weight in the seed medium developed in this study reached 1.32 g/L, which was 48.3% higher than that of the control. The combination of culture time and nitrogen concentration was found to be most significant in influencing the biomass yield. In the second uniform design experiment, flask culture with five factors (glucose, nitrogen from dairy manure, ZnSO_4, KH_2PO_4, and MgSO_4) at eight levels was examined using the uniform design table U_8 (8~5) with the content of L-lactic acid as the evaluating response Y. The results showed that ZnSO_4 had the most influence on L-lactic acid yield, followed by nitrogen and KH_2PO_4. The optimum culture medium in terms of lactic acid production consisted of 240 g/L glucose, 1.26 g/L crude protein, 1.05 g/L KH_2PO_4, and 0.25 g/L MnSO_4, which could achieve a yield of 59.57% (7.1% higher than the control).