Kinetic Model for Simultaneous Saccharification and Fermentation of Brewers' Spent Grain Liquor Using Lactobacillus delbrueckii Subsp. lactis

Brewers' spent grain (BSG) liquor can be obtained by pressing fresh BSG and was recently proposed as a fermentation medium. A kinetic model was established, that can predict lactate formation and cell growth of Lactobacillus delbrueckii subsp. lactis with three BSG liquors of different origin in a simultaneous saccharification and fermentation approach. The kinetic model assumes a multi-substrate dependence of glucose and several nitrogen-containing molecule classes on cell growth, substrate uptake, and lactate formation. Furthermore, the model contains terms for the enzymatic degradation of 1,4-alpha-bond glucose oligomers. The nutrient content of the fermentation media, based on BSG liquor, varied broadly in terms of total carbohydrates (26.27 g center dot L-1 to 107.17 g center dot L-1), amino acids (1.52 g center dot L-1 to 6.78 g center dot L-1), and proteins (0.16 g center dot L-1 to 0.46 g center dot L-1). The deviation between experiment and simulation was - in relation to the large differences in media composition - quite small and ranged from 1.6% to 12.9% (final cell density) and from 5.0% to 24.8% (total lactate concentration). Both the experiment and the simulation showed that serine and alanine play an essential role in the metabolism of the organism. In general, the ratio of carbohydrates to amino acids in the fermentation medium is very important for L. delbrueckii subsp. lactis. Therefore, the model can contribute to control the bioprocess and assess fermentation kinetics for unknown BSG liquors.