Response surface methodology optimisation enhancing lactic acid production from Prosopis africana pods by Rhizopus oryzae

Microbial fermentation of lignocellulosic biomass into lactic acid (LA) has received considerable attention because it ensures the valorisation of wastes and reduces dependence on fossil sources. First, the proximate and phytochemical compositions of Prosopis africana pods (PAP) were determined. Previous biologically-pretreated pods of Prosopis africana were then saccharified using dilute acid hydrolysis following a full factorial design. The factors optimized were acid type (HCl and H2SO4), acid concentration (1 %, 3 % and 5 %), solid loading ratio (5 %, 10 % and 20 %) and reaction time (15, 30 and 60 minutes). Several Rhizopus oryzae isolates were screened for LA production and the most prolific was molecularly identified. The factors affecting LA yield from PAP hydrolysate were screened using a half-factorial design. The significant factors were then optimised using Box-Behnken Design of Response Surface Methodology (RSM). The proximate analysis showed high levels of protein, lipid, ash and carbohydrates. The phytochemical analysis of PAP revealed the presence of flavonoids, alkaloids, saponins, phenols, tannins, glycosides and terpenoids. The hydrolysis conditions of 3 % HCl, 20 % solid loading and 15 minutes hydrolysis time yielded the highest reducing sugars of 42.5 g/L. The most promising isolate, identified as R. oryzae strain AK-22, produced 19.7 g/L after RSM optimization, a 38.1 % increase over yields from non-optimised conditions. These findings are on the biotechnological production of LA from the pods of Prosopis africana, an abundant yet under-utilised tree crop.