Single-step conversion of rice straw to lactic acid by thermotolerant cellulolytic lactic acid bacteria

Abstract

Lactic acid (LA) is an important intermediate product used for numerous industrial applications in food, cosmetic and bioplastic production. To reduce the cost of lactic acid production, lignocellulosic substrates have been extensively investigated as a source of sugars for fermentation. This study aimed to produce lactic acid by thermotolerant cellulolytic lactic acid bacteria (LAB) through a simplified process using rice straw (RS) as the substrate without the addition of any external hydrolytic enzymes. All 20 LAB strains were isolated from RS-buried soil, and the carbon utilization, growth, and cellulase activity of select LAB were evaluated. RS was pretreated with 1.5% (v/v) H3PO4 at 121 °C for 15 min to obtain a hydrolysate containing 1.54 g/L glucose and 0.94 g/L xylose, and toxic chemicals (furfural and hydroxymethylfurfural) were not detected. Among the LAB isolates, the homofermentative Enterococcus spp. SLC45-3 produced 83.9% L-LA with 2% glucose as the sole carbon source. The highest total LA of 2.20±0.03 g/L was obtained after the cultivation of SCL45-3 in RS hydrolysate at 45 °C and 150 rpm and was close to the LA content of 2.06±0.05 g/L obtained with the RS hydrolysate supplemented with untreated RS. The addition of untreated RS enhanced CMCase activity, and during incubation, constant LA production was detected from 36 to 96 h. Thus, SLC45-3 is a potential LAB candidate for L-LA production through the direct bioconversion of RS under moderate temperature without cellulase supplement.