D-lactic acid production from orange waste enzymatic hydrolysates with L. delbrueckii cells in growing and resting state

Agro-food industrial residues as carbon source for d-lactic acid production is a potential strategy to enhance economic feasibility of this bioprocess. Waste hydrolysates rich in monosaccharides can be obtained from such residues to finally render high acid yields after fermentation. This paper reports on the fed-batch processing of partially dried Orange Peel Waste (OPW) at low enzyme loadings, increasing the dry solid content from 7.7 to 22% w/w. The hydrolysate, containing >110 g/L fermentable monosaccharides, was compared to a model sugar solution to produce D-lactic acid using Lactobacillus delbrueckii sp. delbrueckii in a stirred tank bioreactor (STBR), in both growing and resting state. The latter mode for the cells proved to be the most productive. A non-structured non-segregated simple kinetic model previously proposed for this bioprocess was successfully fitted to experimental data from model and real OPW hydrolysates, providing several prospective explanations about the bacterium performance.