Xylose enriched ethanol fermentation stillage from sweet sorghum for xylitol and astaxanthin production

Authors: Stoklosa, Ryan; Nghiem, Nhuan; Latona, Renee

Submitted to: Fermentation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2019
Publication Date: 9/23/2019
Citation: Stoklosa, R.J., Nghiem, N.P., Latona, R.J. 2019. Xylose enriched ethanol fermentation stillage from sweet sorghum for xylitol and astaxanthin production. Fermentation. 5(4):1-17. https://doi.org/10.3390/fermentation5040084.
DOI: https://doi.org/10.3390/fermentation5040084

Interpretive Summary: Agricultural feedstocks can provide polysaccharides that can be broken down to individual sugar monomers (e.g. glucose, and xylose) for conversion to biofuels such as ethanol. A facility that would produce these biofuels is termed an integrated biorefinery. In this type of facility, the main product would be ethanol, but other high value chemicals can be produced along with ethanol to improve overall process economics. Most industrial yeast strains can generate high yields of ethanol, but the small profit margins require other high value chemicals be generated to improve revenue. To accomplish this different yeast strains were cultivated in recovered stillage media after ethanol fermentation to produce high value chemicals from xylose. Sweet sorghum was utilized as the feedstock in this research since it can provide sugars in the form of extract juice, and polysaccharides from the remaining stalk material commonly known as bagasse. The sweet sorghum bagasse (SSB) was pretreated using ammonia and then mixed with sweet sorghum juice (SSJ) during enzymatic hydrolysis. After hydrolysis this mixture was then fermented with a common industrial yeast strain to generate ethanol. The resulting fermentation broth was gently boiled to remove the ethanol, and this produced a stillage media enriched in xylose. This stillage was utilized for fermentation by Candida mogii to produce xylitol, a sugar alcohol that can be used as a natural sweetener, and by Phaffia rhodozyma to generate astaxanthin, a natural carotenoid used for supplementation in aquaculture feed. C. mogii could easily grow in the stillage media, but high xylitol production was achieved by recovering the growing cells and resuspending them in a synthetic media supplemented with xylose. On the other hand, P. rhodozyma produced the greatest amount of astaxanthin when it was cultivated in the stillage media after it was detoxified to remove fermentation inhibitory compounds.

Technical Abstract: Developing integrated biorefineries requires the generation of high-value co-products produced alongside cellulosic ethanol. Most industrial yeast strains produce ethanol at high titers but the small profit margins for generating ethanol require that additional high value chemicals be generated to improve revenue. The aim of this research is to establish the feasibility of fermenting xylose to xylitol or astaxanthin using yeast strains with natural xylose metabolism. Pretreated sweet sorghum bagasse (SSB) was hydrolyzed in sweet sorghum juice (SSJ) followed by ethanol fermentation. Ethanol was removed from the fermentation broth by evaporation to generate a stillage media enriched in xylose. Candida mogii NRRL Y-17032 could easily grow in non-detoxified stillage media, but a high xylitol yield of 0.55 g xylitol/g xylose consumed was achieved after recovered cells were resuspended in synthetic media containing supplemented xylose. Phaffia rhodozyma ATCC 74219 could be cultivated in non-detoxified stillage media, but astaxanthin generation was increased 4X (from 17.5 to 71.7 mg/L) in detoxified media. Future processing strategies to boost product output should focus on a two-step process where the stillage media is used as the growth stage, and a synthetic media for the production stage utilizing xylose generated from SSB through selective hemicellulase enzymes.