BIOCONVERSION OF CHEAPER CARBOHYDRATE SOURCES FOR LACTIC ACID PRODUCTION

Abstract

The present study was undertaken to isolate a potent microbial strain producing lactic acid and to economize the feraientation process by cutting down the cost of production media by selecting a cheaper raw material. The following strategy was undertaken to fulfill the aim of the present study. A thorough survey into the various sources for isolating a potent lactic acid producer led to the isolation of a strain designated as RB-5 and identified to be Rhizopus oryzae. To maximize the lactic acid production ability of the isolated strain, it was subjected to the physical and chemical mutagenesis. After two stages of UV irradiation, a mutant designated as Rhizopus oryzae RBU2-10 was obtained which had the capability of higher levels of lactic acid production. The mutants obtained by chemical mutagenesis did not result in much improvement in lactic acid production ability of the wild type. The distinction between the wild type R. oryzae RB-5 and mutant R. oryzae RBU2-10 was not limited to their lactic acid production ability but also to their morphological features. The various major parameters were analyzed in submerged condition for increasing the lactic acid production ability of the mutagenized strain R. oryzae RBU2-10. Production levels of lactic acid were also evaluated in solid state fermentation (SSF) condition. Of the three supports viz. bagasse, groundnut shell and coconut coir used for the study, bagasse led to higher levels of lactic acid production. But the levels of production obtained under SSF condition were lower as compared to that under submerged condition. in For economization of the fermentation process the synthetic sugars were substituted with cheaper carbohydrate sources mainly with the sugarcane molasses and with various starch based substrates like broken and pest infested rice, partially rotten potatoes, sago and arrowroot starch. The starch based substrates were hydrolyzed prior to their use in the fermentation reactions. Among the substrates used, the enzyme hydrolysed rice starch led to maximum levels of lactic acid production followed by the enzyme hydrolysed potato, arrowroot and sago starch. The various parameters for achieving increased production levels were analysed using enzyme hydrolysed rice starch as the substrate. Whole cells of R. oryzae RBU2-10 were immobilized in order to analyse the system for semicontinuous production of lactic acid. Two types of matrices i.e. calcium alginate and loofa sponge were used for immobilization. The various factors affecting production levels by immobilized mycelia were studied. These observations had indicated that loofa sponge immobilized mycelia had resulted into higher levels of lactic acid production than the calcium alginate trapped cells. In addition, the loofa sponge was considerably more economical and easily available. Extraction of lactic acid was attempted by using various combinations of the earners mainly tri-noctylamine, tri-n-pentylamine and the diluents, methyl isobutyl-ketone, 1-octanol. Trin- octylamine in methyl isobutyketone was found to be better for the extraction of lactic acid. Extractant when finally treated with the aqueous phase containing 4N NaOH in the combinations of 1: 1and 4: 1(extractant :aqueous phase), the earlier combination led into 85-90%recovery of the lactic acid from the fermentation broth.