STUDIES ON MICROBIAL PRODUCTION OF LACTIC ACID BY BATCH FERMENTATION

Abstract

The present thesis work includes detailed comparative studies between the four standard Lactobacillus strains acquired from the culture collection NCIM-National Collection of Industrial Microorganisms, at National Chemical Laboratory Pune India, for their potential of lactic acid production. The study includes liquid state batch fermentation, liquid state fed batch fermentation, studies on batch growth and solid state fermentation. The study not only aims towards the higher production of lactic acid, by the use of carbohydrate rich waste materials such as dairy whey, bagasse, starch, but it also highlights the significance of utilizing inexpensive nitrogen source dried baker's yeast. The solid state fermentation studies have been carried out with an aim to utilize several cheap and abundant agricultural and forestry waste materials such as sugarcane bagasse, wheat bran and pine needles and to investigate their potential to support the lactic acid producing bacterial strains. The solid state fermentation studies till now have involved predominantly fungi, but bacterial solid state fermentation has received lesser attention. Hence the solid state studies for these Lactobacillus strains were aimed to determine their comparative lactic acid production performances on the different bed materials with a supporting production media containing pure sugar or partially substituted pure sugar with carbohydrate rich waste materials. Very few studies have been reported on coculture, that present a detailed account of, its performances that were better than the pure strains, in terms of lactic acid production, through batch, fed batch and solid state fermentation methods. The present work involved the production study, of coculture with different sources of carbon and nitrogen, pH, salts, temperature, neutralizer and agitation etc. With the aim to reduce the use of costly sugars in the bacterial culture development, modified MRS culture media has been used for the growth studies, utilizing cheap carbohydrate rich waste materials such as acid treated bagasse extract under boiling or autoclaving conditions and dairy whey. The coculture was prepared from strain-1 (Lactobacillus delbrueckii) and strain-2 (Lactobacillus pentosus). Coculture showed overall maximum lactic acid production 93.57, 115.56, 114.96 and 116.01WL with pure glucose, lactose, whey substituted glucose and whey substituted lactose respectively. Most suitable pH values for higher production of lactic acid were 6.5, 6.75, 6.0 and 6.75 for strains-1, 2, 3, 4 and coculture respectively. An agitation of 180rpm (during shake flask batch and fed batch fermentations) and a temperature of 370 C was found most suitable for all the strains. However the strain-1 and its coculture, had better production at 40° C. The cheap nitrogen source, dried baking yeast, produced higher lactic acid concentrations at its 20 g/L input, which were found competitive with some of the synthetic nitrogen sources such as meat extract. In the fed batch fermentations also, coculture reached overall maximum lactic acid production around 162 g/L. The solid state fermentation studies had the overall maximum lactic acid production of 54.25 g/L with coculture on wheat bran bed material. The batch growth studies indicated maximum cell biomass growth and pH drop with coculture on the modified MRS media as compared to other strains. Hence it can be concluded from the present studies that, coculture can be potentially beneficial for the lactic acid fermentation industries