Kinetics of Lactic Acid, Acetic Acid and Ethanol Production During Submerged Cultivation of a Forest Litter-Based Biofertilizer

Fermented forest litter (FFL) is a biofertilizer obtained by anaerobic fermentation of forest litter combined with agricultural by-products. Its production involves an initial one-month solid-state fermentation of oak litter mixed with whey, molasses and wheat bran, followed by a one-week submerged fermentation-called the “activation” phase-during which the solid FFL is fermented with sugarcane molasses diluted in water. This study aimed to evaluate the effects storage duration (6, 18 and 30 months), and temperature (ambient and 29 °C) on the activation phase. For this purpose, pH, sugar consumption and metabolite production dynamics were monitored. Under all experimental conditions, the pH dropped to values close to 3.5, sucrose was rapidly hydrolyzed, and glucose was preferentially consumed over fructose. Fructose was metabolized only after glucose was depleted, suggesting the involvement of fructophilic microorganisms. The time-course evolution of lactic acid (LA) concentration was adequately fitted by the Gompertz model (R2 > 0.970). The highest LAmax concentration (6.30 g/L) and production rate (2.16 g/L·d) were obtained with FFL stored for 6 months. Acetic acid (AA) and ethanol were also detected reaching maxima values of 1.19 g/L and 0.96 g/L, respectively. Their profiles varied depending on the experimental conditions. Notably, the AA/LA ratio increased with the age of the FFL. Overall, sugar consumption and metabolite production were significantly slower at ambient temperature, than at 29 °C. These results contribute to a better understanding of the metabolic dynamics during FFL activation and highlight key parameters that should be considered to optimize future biofertilizer production processes.