Optimization Of Co-production Of Lactic Acid And Chitosan From Pretreated Corn Cob

Pretreated corn cob is a kind of cellulosic waste left in the industrial production process of xylose, which is made from corn cob hydrolyzed by acid. There has been a great amount of it usually pooled intensively at some place. Nowadays, these cellulosic waste are not in full utilization, but has caused serious environmental pollution. Lactic acid can be used to synthesize polylactic acid, a feedstock chemical for biodegradable plastic which is used as a substitution to lighten environmental pollution caused by plastic. As a result, lactic acid is in great demand. There is a lack of water resource and the water price is rising. In this condition, chitosan as a kind of safe, nontoxic, high efficient microbial flocculant, supplies reliable technical guarantee for the technology of water advanced treatment and reuse. The overall goal of this study was to develop an innovative process for lactic acid and chitosan co-production using pelletized Rhizopus oryzae LY-07 from pretreated corn cob.The process of conversion was studied and optimized, hoping to provide data referrence for the utilization and exploitation of cellulosic waste resource.In this study, cellulases were used to hydrolyze pretreated corn cob, and the cellulosic component in it was decomposed into reducing suger. The influences of some factors, such as the pH,temperature, substrate concentration and enzyme concentration, were researched on the process of hydrolyzation by cellulases, for a higher conversion rate of reducing suger from pretreated corn cob.Hydrolyzate was used to cultivate Rhizopus oryzae LY-07 for lactic acid fermentation.The morphology of Rhizopus oryzae LY-07 cultivated in the hydrolyzate and the condition in which the pellet could form were studied.By experiments of single-factor and orthogonal design,both the fermentation medium and condition were optimized in order to get higher lactic acid yield. Biomass from fermentation was gathered, and then chitosan was extracted by chemical method from it. The processes of removing protein by diluted alkali and deacetylation by concentrated alkali were optimized.At last, the high efficient and biodegradable microbial flocculant was obtained. The results are as follows:(1)The suitable range of pH for the cellulase used in the study was 5-5.4, for it is a kind of acid cellulase. The optimized conditions for cellulase hydrolyzing pretreated corn cob were as follows:pH5.2;temperature of 55℃;enzymatic concentration of 40U/(g pretreated corn cob);substrate concentation of 100 g/L. Under this optimized condition,24h were needed to hydrolyze pretreated corn cob completely, and at that time the concentration of reducing suger in the hydrolyzate reached 27.52g/L, and the ratio of the conversion was 27.52%.(2) The spores germination and the hypha growth of Rhizopus oryzae LY-07 are slow, so yeast extract was used as nitrogen source to accelerate the cultivation. At the same time, the pH of the cultivation medium was adjusted to 2.5 by adding HCl to inhibit the excessively fast growth of the mycelia, avoiding the formation of large clumps of Rhizopus oryzae LY-07.At last, ideal morphology, pellets of 2mm in diameter, was gained.(3)Concentrated hydrolyzate was used to culture Rhizopus oryzae LY-07 for lactic acid fermentation. By experiments of orthogonal design, the optimization of the culture medium was determined:the concentration of the components are 5% reducing suger of the hydrolyzate,0.35% yeast extract,0.04% MgSO4·7H2O,0.0075% ZnSO4·7H2O,0.02% KH2PO4. The optimized the fermentation condition:temperature of 33℃,rotation speed of 140rpm,liquid volume of 50ml,10 pieces of the pledget. Under this condition, the concentration of lactic acid reached 29.3g/L after 72h fermentation, and the ratio of the conversion was 58.6%.(4)Chitosan was extracted from the mycelia by chemical methods.The optimizied condition for removing protein was:5% NaOH,boiling water bath for 2h. The optimized deacetylation condition was also obtained by orthogonal experiment:40% NaOH,110℃for 2h. The concentration of NaOH influenced the deacetylation the most:the higher concentration, the better deacetylation.Offwhite sheet chitosan was obtained at last. The deacetylation ratio reached 86.3%,and extraction yield of chitosan was 5.4% from the dry mycelia. However, because of the presence of CaCO3 in the dry mycelia, the actual yield was comparatively lower than the theoretic one.This study used pretreated corn cob as raw material to co-product lactic acid and microbial flocculant of chitosan by characteristic microorganism, Rhizopus oryzae LY-07.This study can help to lay a foundation of data for forming new techniques of environmental biological technology. Thus, it can supply a new technical approach for the utilization of herbaceous celluloses as resources and new product development. The application of achievements from this basic research not only brings new economic growth point for the biomass utilization in the future, but also drives this field go to further study. After all, this work had great significance in science and environmental technology.