Lactic acid production by irradiated Bacillus NF17 and poly-L-lactate biopolymer formation

This study was conducted to manipulate the thermo tolerant, lactic acid-producing bacteria, Bacillus coagulans strain NF17, in the production of L-lactic acid and a bio polymer: poly-L-lactate. The bacterial isolate NF17 kept in the culture collection of Khon Kaen University and could tolerate high temperature and produce lactic acid, was employed in this research work. Cell suspension of isolate NF17 was exposed to gamma irradiation at various doses (1-5 KGy). The irradiated survivors were screened on the basis of forming larger colonies and clear zones than the parent strain NF17 when grown on Glucose-Yeast extract-Peptone (GYP) containing CaCO3. We obtained 55 effective isolates which the isolate L5I2-14(5), designated as K14, was chosen together with the parent strain NF17 for fermentation experiments. Each bacterial strain was inoculated into GYP broth and incubated statically at 50oC with daily pH neutralization. After 5 days of incubation, the isolate K14 and NF17 produced 9.71 g/l and 7.42 g/l of L-lactic acid, respectively with a small amount of D-lactic acid. Lactic acid production from sugar cane molasses by batch fermentation of Bacillus Sp. K14 was carried out in a 7 l jar fermentor containing 5 l of fermentation medium. It was found that 20% molasses with the agitation speed of 100 rpm gave the highest yield of lactic acid. Poly-L-lactic acid was chemically polymerized by bulk polymerization process at 140oC under 40 mmHg conditions. We could obtain the off-white polymer in a small amount of powder form. Improvement the yield of poly-L-lactic acid would be achieved by using polyisoprene-g-polyvinyl monomer to separate lactic acid from the fermenting liquid prior to polymerization processes