Fermentative valorisation of agri-food residues: optimisation of Lactiplantibacillus paraplantarum fermentation conditions for the production of platform organic acids
Abstract
Organic acid fermentation is a key route for integrating second-generation (2G) biomass into biorefineries and promoting industrial symbiosis between agri-food residues and platform-chemical production. However, plant matrices such as tomato pomace (TP) and spent coffee grounds (SCG) show limited carbon accessibility and contain phenolic and lipidic compounds that may inhibit bacterial growth. This study uses hydroalcoholically pretreated TP and develops an iterative strategy for organic acid production using Lactiplantibacillus paraplantarum as a biofactory, while evaluating selectivity towards lactate. Quantitative proton nuclear magnetic resonance was used to monitor lactic acid (LA) and acetic acid (AA) titres and apparent glucose-equivalent conversion (aGEC) into these acids. The aGEC is a comparative, non-stoichiometric index rather than a closed carbon yield, because it includes buffer-derived acetate. Response surface methodology identified 44.7% TP replacement, 79.6 h and an initial pH of 5.77 as the basal fermentation conditions (4.73 ± 0.26 gL-1 LA; aGEC ≈ 6%). Nitrogen-source optimisation and pH buffering increased LA to 7.68 ± 0.56 and 14.52 ± 0.76 gL-1 and aGEC to ≈ 10% and ≈ 26%, respectively. The final combined intensification treatment comprised cellulase pretreatment, a modified incubation atmosphere and CaCO 3 as an additional buffering system. It increased aGEC to ≈ 85% but did not significantly increase LA. Acetate accumulated substantially, and the simultaneous modifications prevented attribution of the response to any individual factor. Transfer of the basal conditions optimised for TP to SCG was feasible only at moderate substitution levels. At 44-60% replacement of standard glucose, SCG maintained comparable aGEC values of approximately 28-29% and produced 16.92-15.87 gL-1 LA, whereas higher substitution reduced both aGEC and LA production. This sequential framework distinguishes total-acid accumulation from LA selectivity and shows the limited transferability of residue-specific fermentation conditions.