Valorization of barley bagasse for lactic acid production using an enzymatic cocktail produced by Aspergillus...

Agro-industrial processes generate significant lignocellulosic waste, composed mainly of cellulose, hemicellulose, and lignin. Repurposing these residues for lignocellulolytic enzyme production offers an economical, sustainable approach to biomass valorization and the generation of bioproducts such as lactic acid. This study aimed to optimize enzyme production by Aspergillus species grown on various agro-industrial residues and apply these enzymes to hydrolyze barley bagasse for lactic acid production. Four Aspergillus strains (A. brasiliensis, A. clavatus NRRL1, A. flavus, and A. terreus) were cultivated in minimal medium (pH 6.5) with 1% (w/v) Barley Bagasse, Bean Straw or Rice Straw, at 30 °C and 120 rpm for five days. Nine different holocellulolytic activities were screened using natural and synthetic substrates. A. brasiliensis exhibited the highest enzymatic activity when grown on bean straw. Optimization of its cultivation led to 10 g/L of reducing sugars after five days at pH 5.5 and 35 °C. Subsequent optimization of barley bagasse hydrolysis resulted in 49 g/L of fermentable sugars when using 20% (w/v) of this substrate concentration, at pH 3.0, and 50 °C for 24 h. This corresponded to a hydrolysis yield of 44% relative to the total carbohydrate content. The released sugars were fermented by Lactobacillus rhamnosus ATCC 7469, producing 21.1 g/L of L-lactic acid corresponding to a fermentation yield of 47% relative to the available sugars and a volumetric productivity of 0.15 g/L/h. This work demonstrates an integrated bioprocess that couples low-cost enzyme production with effective saccharification and fermentation of barley bagasse. This approach underscores its potential for sustainable lactic acid production. Unlike previous studies that rely on commercial enzymes or neutral-pH hydrolysis, this process uniquely combines acid-tolerant enzymes produced by Aspergillus brasiliensis grown on bean straw, a rarely used substrate, with direct fermentation by Lactobacillus rhamnosus ATCC 7469 at low pH. The integration of these specific microbial species and agro-residues represents a distinctive and effective approach to biomass valorization.