Screening Lactic Acid Bacteria from Thai Agricultural Products and Wastes for Potential Application on Cassava Starch

Abstract

An evaluation for high yield and stereospecificity-producing lactic acid bacteria (LAB) isolated from various sources including the waste from starch factories, dairy products, fermented vegetable and meat products was conducted under nourished conditions with various concentrations of sodium lactate (3-20% w/v) and glucose (3-30% w/v) at pH 2 – 8. A total of 156 purified cultures were obtained from 57 samples of which 94 strains were homofermentative, the rest were heterofermentative. A purified strain from fermented pork (DM3 strain) demonstrated the highest stereospecificity of L-type (92.8%). The 16S rDNA gene sequence suggested that this DM3 strain was closely related to Lactobacillus rhamnosus with a similarity of 99.94%. DM3 provided a maximum yield (89.6%) and productivity (4.79 g/l.h) when grew under the optimal condition, i.e. pH 6.0, 40^oC, an agitation speed of 150 rpm and the initial glucose concentration of 100 g/l. Further study was performed by applying DM3 to produce lactic acid from cassava starch, the least expensive commercially available carbon feedstock in Thailand, using Simultaneous Saccharification and Fermentation (SSF) process. Cassava starch (150-200 g/l) was initially liquefied by α-amylase (0.1% v/w) and subsequently saccharified and fermented, simultaneously, by glucoamylase (0.5% v/w) and DM3 (5% v/v). The yield of lactic acid from DM3 fermentation was 136.8 and 166.2 g/l and the productitivity was 5.70 and 1.39 g/l.h, when the initial glucose concentrations were 150 and 200 g/l, respectively.