Herr M.Sc. Alexander Akermann hat erfolgreich seine Promotion zum Dr.-Ing. mit dem Thema "Entwicklung von Fermentationsstrategien zur stofflichen Nutzung von nachwachsenden Rohstoffen" abgeschlossen. Die BioVT wünscht ihm viel Erfolg im weiteren Lebensweg!
‘Brewers’ spent grain (BSG) is a low‑value by‑product of the brewing process, which is produced in large quantities every year. In the present work, BSG of seven different brewing recipes, both self‑made and industrially produced, was analyzed and classified in terms of the related brewing processes. It was found, that BSG composition (in terms of soluble compounds) is strongly depended on the brewing parameters. Furthermore, BSG was separated into two streams by pressing: a liquid and a solid fraction. For both fractions, bioprocesses were established to convert the liquid substrate (BSG liquor) into lactate with a lactic acid bacterium (Lactobacillus delbrueckii subsp. lactis) and the solid substrate (BSG residue) into ethanol and acetate with a lignocellulolytic and mixed acid fermenting strain (Cellulomonas uda).
‘BSG liquor contains mainly glucose, maltose and long-chain α‑1,4‑glycosidic bond glucose oligomers. These substances should be separated in existing BSG biorefining concepts, as this leads to numerous advantages for the overall process of valorizing BSG. In contrast to the present work, however, in most cases the liquid fraction is not put to any further use, which is why a bioprocess with BSG liquor was developed for the production of lactic acid in relevant concentrations. The process comprises the application of yeast extract, produced from own brewing processes, as the sole supplemented complex constituent and the addition of a technical glucoamylase mixture in a simultaneous saccharification and fermentation process. Kinetic parameters for the final optimized process conditions with the organism L. delbrueckii subsp. lactis were: Maximum specific growth rate µmax = 0.47 h‑1, maximum lactic acid concentration cLac, max = 79.06 g∙L‑1, process yield YP/S = 0.89 gLac∙gSugar‑1, lactic acid production rate qP = 4.18 gLac∙gCDW‑1∙h-1 and productivity P 15 h = 4.93 gLac∙L‑1∙h-1.’
‘Moreover, a kinetic model was established, that could (amongst others) predict lactate formation and cell growth of L. delbrueckii subsp. lactis for three BSG liquors of different origin in the mentioned simultaneous saccharification and fermentation. 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 α‑1,4‑glycosidic bond glucose oligomers. The nutrient content of the fermentation media, based on BSG liquor, varied broadly in terms of total carbohydrates (26.28 g∙L‑1 to 107.17 g∙L‑1), amino acids (1.52 g∙L‑1 to 6.78 g∙L‑1) and proteins (0.16 g∙L‑1 to 0.46 g∙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), 5.0% to 24.8% (total lactic acid concentration) and 1.2% to 13.5% (productivity of the process). 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 was found to be very important for growth and product formation of L. delbrueckii subsp. lactis.’
‘The residue of the pressed, solid BSG was used to optimize fermentations with Cellulomonas uda. Under aerobic conditions, maximum cellulase activities of 0.98 nkat∙ml‑1, maximum xylanase activities of 5.00 nkat∙ml‑1 and cell yields of 0.22 gCells∙gBSG‑1 were achieved. Under anaerobic conditions, enzyme activities and cell yields were lower, but valuable soluble products (organic acids, ethanol) were produced with a yield of 0.41 gProd∙gBSG‑1. The growth phase of the organism was monitored by measuring extracellular concentrations of fluorophores pyridoxin (aerobic) and tryptophan (anaerobic) and by cell count. By combining reductive with anaerobic conditions, the ratio of ethanol to acetate was increased from 1.08 to 1.59 molEtOH∙molAc‑1. This ratio was further improved to 9.2 molEtOH∙molAc‑1 by lowering the pH from 7.4 to 5.0 without decreasing the final ethanol concentration. A fermentation in a bioreactor with 15 w% BSG instead of 5 w% BSG quadrupled the acetate concentration, whilst ethanol was removed by gas stripping. In other fermentations with 25 w% BSG, total product concentrations of up to 36.03 g∙L‑1 could be achieved.’
‘In this work, a holistic concept for the application of BSG in terms of producing bulk chemicals is proposed on the basis of the mentioned prosses, which also comprises the usage of the Brewers‘ yeast side stream. These strategies could contribute to overcome recent global problems, like global warming and greenhouse gas emission, by proposing a circular instead of a linear value creation process for BSG. Furthermore, developed process monitoring, regulation and modelling strategies can possibly be transferred to numerous other substrates and/or organisms in the future, which was already demonstrated in this study for the substrate grass.’