Optimization of acid and enzymatic hydrolysis of kodo millet (Paspalum scrobiculatum) bran residue to obtain fermentable sugars for the production of optically pure D (-) lactic acid

The global demand for lactic acid (LA) derivatives, requires feedstocks that are economically viable and environmentally sustainable. Kodo millet bran residue (KMBR) was therefore examined as an alternative feedstock. For the production of enantiomeric excess D (-) lactic acid (DLA), from KMBR, the acid hydrolysis (All) and enzymatic hydrolysis (EH) parameters were optimized. The process variables of AH (acid concentration, substrate loading and holding time) and EH (substrate loading, enzyme dosage, pH and temperature) were optimized using response surface methodology (RSM) and artificial neural network converged with genetic algorithm (ANN-GA). The optimal values determined using RSM and ANN-GA for All were acid (HCl) concentration 0.7 and 0.4 mol/L; substrate loading 12.97 and 13% (w/v); and incubation period 12.62 and 12.89 min respectively for both the models. Whereas for EH, substrate loading 11.89 and 11.43% (w/v); enzyme dosage 108.05 and 97.52 (GAU/g); pH 5.49 and 4.59 and temperature 45.31 and 45 degrees C respectively for both the models. The maximum glucose titer obtained at these optimal conditions, employing RSM, was found to be 41.99 g/L and 77.69 g/L (0.32 and 0.65 g of glucose per g of KMBR) for AH and EH respectively. ANN-GA resulted a maximum sugar titer of 44.06 g/L and 81.34 g/L (0.34 and 0.72 g of glucose per g of KMBR) for All and EH correspondingly. The growth of different DLA producer strains was studied using modified deMan, Rogosa and Sharpe (MRS) media employing both the hydrolysates under aerobic and micro-aerophilic conditions. The results of fermentation experiments indicated that Lactobacillus delbrueckii subsp. delbrueckii NBRC3202 as a potent DLA producer (DLA titer, 25.38 g/L; Yp/s, 1.18 g/g; productivity, 0.53 g/L h and optical purity, 97.79%). The present investigation substantiates the potential utilization of economically viable feedstock KMBR for commercial scale DLA production.