Brewer’s spent grain as a self-sufficient feedstock for homofermentative production of optically pure L-lactic acid using Lactobacillus rhamnosus
Coralie Granget, N. Arul Manikandan, K. Amulya, Michal Dabros, Samantha Fahy, Susan M. Kelleher, Keith D. Rochfort, Jennifer Gaughran, Brian Freeland
Abstract
Brewer’s spent grain (BSG), accounting for 85% of the total by-product from breweries, was used as a feedstock for L-lactic acid production in the present study. BSG was enzymatically hydrolysed and promoted to be a self-sufficient feedstock for L-lactic acid production by using it as a sole source of carbon, protein and minerals. Process parameters like glucose concentration (10–20 g/L), glucose-to-protein source ratio (1:1 – 5:1), source of protein and inoculum concentration (3 – 10% v/v) were selected to divert the carbon source and enhance L-lactic acid concentration, which would otherwise result in bacterial biomass production. Yeast extract and whey permeate resulted in high bacterial growth, whereas self-sustained protein (SSP) in BSG resulted in higher L-lactic acid production. Further, the glucose-to-protein source ratio was maintained at its lowest level for better glucose conversion to L-lactic acid. Glucose concentration strongly influenced L-lactic acid production. Therefore glucose concentration in the batch fermentation process was further increased from 60 to 120 g/L. A maximum L-lactic acid concentration of 114.4 g/L and productivity of 5.14 g/L·h was achieved with an initial glucose concentration of 120 g/L, and the rest of the process parameters such as glucose to protein source ratio of 1:1, inoculum concentration of 10% v/v and SSP in BSG were maintained at its optimum level. Finally, L-lactic acid in the fermentation broth was purified and analysed for its similarity with commercially available L-lactic acid using proton-NMR and FTIR spectroscopy. Thus, the present study valorised BSG by producing L-lactic acid under a biorefinery approach.