l-(+) lactic acid production by Rhizopus oryzae using pretreated dairy manure as carbon and nitrogen source

The optimal pretreatment condition for dairy manure further use obtained through the Box–Behnken experimental design was temperature 120 °C, H2SO4 concentration 1.61% and solid content 1%, among which temperature was the key factor influencing the performance of pretreatment. Under this condition, dry dairy manure could produce 60.8% of reducing sugar and 2.83% of protein. The pretreated dairy manure was then used as the fermentation substrate for producing l-(+) lactic acid by Rhizopus oryzae without any supplement of additional carbon and nitrogen source. The results demonstrated that either l-(+) lactic acid or ethanol produced could reach the highest concentration and yield in two days. R. oryzae was able to utilize pretreated dairy manure to produce l-(+) lactic acid and the highest concentration and yield were 1210.02 mg L−1 and 40.09% at pH 10, respectively, when ethanol as main byproducts was at a lowest level (74.64 mg L−1), whereas at acidic condition the concentration of ethanol could arrive at a higher level (648.15 mg L−1). The pH not only affected the production of l-(+) lactic acid and ethanol but also had an impact on the morphology of R. oryzae. At higher pH values, small and loose pellets of microorganism formed and produced more l-(+) lactic acid, while at low pH, filamentous mycelia were generated and less l-(+) lactic acid was produced. However, pH did not affect the C/N ratio in the fermentation broth since the ratio was observed to vary in a range from 12 to 15 during the experiment.