Microbial Transformation of Starch to Lactic Acid Derivatives from Potato Processing Industries

Abstract

This study aimed at producing high amounts of lactic acid and its derivatives from a cheap agro processing industry waste i.e. potato starch waste (PSW) in a direct fermentation utilizing a potential GRAS microorganism. For this, various fermented food samples were screened for isolating starch hydrolyzing lactic acid bacteria. A homofermentative strain, Lactococcus lactis, capable of potato starch hydrolysis was isolated from pickled yam. Maximum lactic acid production (8.5±0.5 g/L) by this strain was observed at optimal conditions of pH 5.0, temperature 30ºC and incubation time 48 hrs. in shake flask fermentation in minimal media and 10.1±0.5 g/L in modified MRS media. To refine L-Lactic acid production, a second order polynomial equation with significant R2 (0.9037) showed considerable correspondence between the predicted (13.25 g/L L-lactic acid) and experimental values (15.60 g/L L-lactic acid) after 48hrs. at pH 5.5 and temperature 30ºC. Besides, the production of L- lactic acid, a reduction in chemical oxygen demand, biochemical oxygen demand and suspended solids of 76.9%, 62% and 77% respectively was also achieved in the process in Potato starch waste. To maximize bioconversion and cell stability, high cell density systems such as immobilized cells as well as dialysis sac based bioreactor was fabricated. The maximum lactic acid concentration (18.9 g/L¬), L-lactic acid productivity and yield of 0.79 g/L/h and 1.48 g/g starch was 1.2 and 2.4 times higher respectively in bioreactor than shake flask fermentation in PSW at pH and temperature of 5.5 and 30ºC with operability for upto 3 cycles. L-Lactic acid obtained after fermentation was 89% pure as observed by HPLC which could be further esterified to produce lactic acid esters due to their wide applications in food, cosmetics and pharmaceutical industries. L-lactic acid recovered after fermentation was esterified to produce esters, namely, ethyl lactate and methyl lactate with a yield of 0.69 g/g starch and 0.79 g/g starch respectively. Overall the results conclusively suggest an economically viable process for a microbial conversion of potato starch waste to the production of L-lactic acid and its derivatives. A comparison of economies vis-a-vis similar or related process as well as commercially available products indicate this process to be prospective as the cost reduces to 10 times and 2.4 times in in ethyl lactate and methyl lactate respectively.