Valorization of household food wastes to lactic acid production: A response surface methodology approach to optimize fermentation process

Lactic acid is a valuable compound used in several industrial processes such as polymers, emulsifiers manufacturing, pharmaceutical, and cosmetic formulations. The present study aims to evaluate the potential use of food waste to produce lactic acid through fermentation, both by indigenous microbiota and by the bio-augmentation with two lactic acid bacteria, namely Lactobacillus plantarum BS17 and Lactobacillus casei BP2. Fermentation was studied both in batch and continuously fed anaerobic reactors at mesophilic conditions and a Response Surface Methodology approach was used to optimize the bioprocess performance and determine the environmental parameters (namely pH and time) that lead to the enhancement of lactic acid production during the batch fermentation by indigenous microorganisms. Results revealed an optimum set of conditions for lactic acid production at a pH value of 6.5 and a fermentation period of 3.5 days at 37 °C. Under these conditions lactic acid production reached a value of 23.07 g/L, which was very similar to the mathematically predicted ones, thus verifying the accuracy of the experimental design. This optimum set of conditions was further employed to examine the production of lactic acid under continuous fermentation operation. Furthermore, concentrations of volatile fatty acids and ethanol were monitored and found to be relatively low, with ethanol being the dominant by-product of fermentation, indicating the presence of heterofermentative bacteria in the food wastes. A final step of downstream process was performed resulting in the successful recovery of lactic acid with purity over 90%.