Thermophilic biocatalysts for one-step conversion of citrus waste into lactic acid

Abstract

Agri-food residues offer significant potential as a raw material for the production of L-lactic acid through microbial fermentation. Weizmannia coagulans, previously known as Bacillus coagulans, is a spore-forming, lactic acid-producing, gram-positive, with known probiotic and prebiotic properties. This study aimed to evaluate the feasibility of utilizing untreated citrus waste as a sustainable feedstock for the production of L-lactic acid in a one-step process, by using the strain W. coagulans MA-13. By employing a thermophilic enzymatic cocktail (Cellic CTec2) in conjunction with the hydrolytic capabilities of MA-13, biomass degradation was enhanced by up to 62%. Moreover, batch and fed-batch fermentation experiments demonstrated the complete fermentation of glucose into L-lactic acid, achieving a concentration of up to 44.8 g/L. These results point to MA-13 as a microbial cell factory for one-step production of L-lactic acid, by combining cost-effective saccharification with MA-13 fermentative performance, on agri-food wastes. Moreover, the potential of this approach for sustainable valorization of agricultural waste streams is successfully proven. KEY POINTS: • Valorization of citrus waste, an abundant residue in Mediterranean countries. • Sustainable production of the L-( +)-lactic acid in one-step process. • Enzymatic pretreatment is a valuable alternative to the use of chemical.

Keywords: Citrus waste; Fed-batch fermentation; Lactic acid; W. coagulans.

Fig. 1

Test of Cellic Ctec2 hydrolysis efficiency on citrus waste. Different concentrations of untreated (raw citrus waste) and mechanically ground biomass were tested. Experiments were performed in duplicate

Fig. 2

Effect of MA-13 and Cellic Ctec2 on citrus waste degradation after 24 h. Values are presented as mean values ± SD (n = 3)

Fig. 3

¹³C CP-MAS NMR spectra of citrus waste (A), citrus waste + MA-13 (B), citrus waste + Cellic Ctec2 (C) and citrus waste + Cellic Ctec2 + MA-13 (D). Peaks of liquid-like, low molecular weight substances, whose intensity is not reliably recorded in CP experiments, are marked by dots. In the insert, an example of deconvolution of the polysaccharide region is reported. NMR spectra were repeated on three independent biological samples

Fig. 4

Growth parameters of MA-13 on citrus waste. A Lactate production (circle) and cell viability (CFU/mL) (triangle). B pH variation (in square). Values are presented as mean values ± SD (n = 3)

Fig. 5

Time course of SSF batch and fed-batch experiments performed in triplicate with MA-13. Consumption of glucose and production of lactate in SSF are shown. A representative curve for each condition is shown in the graph