One-Pot Consolidated Bioprocessing of Waste Potatoes to L-Malic Acid by a Thermotolerant and Metabolically Versatile Aspergillus terreus BD Isolate

Abstract

This study explores a metabolically versatile and thermotolerant Aspergillus terreus BD isolate, which exhibited selective itaconic acid (IA) and L-malic acid (L-MA) production under distinct fermentation conditions at 45°C. An acidic fermentation medium (pH 3.4) with an inorganic nitrogen source favoured predominant IA synthesis. Contrastingly, organic nitrogen supplementation in a near-neutral medium (pH 6.0) diverted the metabolic flux towards L-MA. Relative gene expression analysis supported the differential IA and L-MA production under selective media conditions, showing upregulation of key genes in their respective biosynthetic pathways. Furthermore, the isolate was employed for the valorization of waste potato biomass (WPB), which is a major food waste worldwide. Fermentation of the enzymatic WPB hydrolysate at 45°C generated a substantial L-MA titer (37 g/L) without requiring CaCO3 supplementation. Operating at elevated temperatures reduces cooling demands, minimizes contamination risks, and improves compatibility with saccharification conditions. Eliminating CaCO3 minimizes chemical inputs and neutralization sludge outputs, thereby lowering the environmental burden and downstream complications. Harnessing these significant attributes, the isolate was integrated into a one-pot bioprocess employing simultaneous and semi-simultaneous saccharification and fermentation approaches. Process optimization, including solid loading, pH, inoculum size, pre-hydrolysis time, and enzyme dosage, yielded 60 g L-MA per Kg WPB. Finally, extraction using n-butanol led to the recovery of 58% L-MA. Overall, these findings present A. terreus BD, integrated into an efficient one-pot bioprocess, as a promising platform for the sustainable and economically viable production and recovery of L-MA from starch-rich agro-industrial waste.