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

摘要

The optimal pretreatment condition for dairy manure further use obtained through the Box-Behnken experimental design was temperature 120 ℃, H_2SO_4 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.