Performance of xylose-fermenting yeasts in oat and soybean hulls hydrolysate and improvement of ethanol production using immobilized cell systems

Authors: CORTIVO, PAULO - Federal University Of Rio Grande Do Sul; AYDOS, LUIZA - Federal University Of Rio Grande Do Sul; HICKERT, LILIAN - Federal University Of Rio Grande Do Sul; ROSA, CARLOS - Federal University Of Rio Grande Do Sul; Hector, Ronald - Ron; Mertens, Jeffrey; AYUB, MARCO - Federal University Of Rio Grande Do Sul

Submitted to: Biotechnology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2021
Publication Date: 9/4/2021
Citation: Cortivo, P.R.D, Aydos, L.F., Hickert, L.R., Rosa, C.A., Hector, R.E., Mertens, J.A., Ayub, M.A.Z. 2021. Performance of xylose-fermenting yeasts in oat and soybean hulls hydrolysate and improvement of ethanol production using immobilized cell systems. Biotechnology Letters. 43:2011-2026. https://doi.org/10.1007/s10529-021-03182-2.
DOI: https://doi.org/10.1007/s10529-021-03182-2

Interpretive Summary: Broadening fuel ethanol production from grains and sugar juice from sugar crops to include crop residues would allow for much more renewable fuel to be produced. A major technical barrier is that these residues contain a complex mixture of sugar and include a sugar named xylose, which is not fermented by commercial fermenting yeast. Even yeast engineered to ferment xylose do so poorly. To overcome this problem, new yeast strains need to be identified that can utilize this sugar. In this study, we mined the biodiversity of Brazilian forests to see what nature would offer. A diverse set of yeast strains, originally isolated from plants growing in the Atlantic and Amazon rainforests of Brazil, were analyzed for their ability to convert all sugars in hydrolysates to ethanol and compared to other selected yeast known to ferment xylose. The sugar hydrolysates were prepared from oat and soybean hulls, as these are abundant, under-utilized agricultural commodities. Soybean and oat hulls represent 8-10% and 25-30% of the crop by weight, respectively, and contain a high percentage of sugar. Increasing ethanol production from the sugars available in soybean and oat hulls could provide farmers added value to these crops as well as increasing profit for ethanol producers. The results of this research show that the yeast named Spathospora passalidarum is exceptionally suitable for fermenting the complex sugar mixture present in syrups prepared from oat and soybean hull residues to ethanol and is a promising candidate to be explored for use in industrial bioprocesses for second-generation ethanol production. This research will be of immediate interest to ethanol producers as well as for soybean and oat growers interested in developing new markets for their crop residues.

Technical Abstract: We investigated the fermentation of a mixture of oat and soybean hulls (1:1) submitted to acid (AH) or enzymatic (EH) hydrolyses, both showing high osmotic pressures (> 1,200 Osm•kg-1) for the production of ethanol. Yeasts of genera Spathaspora, Scheffersomyces, Sugiymaella, and Candida, most of them isolated from the Brazilian biodiversity and never tested in bioprocess before, were cultivated in these hydrolysates. Spathaspora passalidarum UFMG-CM-469 showed the best ethanol production kinetics in suspended cells cultures in acid hydrolysate, both under microaerophilia and anaerobiosis. This strain was immobilized in LentiKats® (polyvinyl alcohol) and cultured in (AH) and (EH) and was tested for the effect of supplementation of hydrolysates with crude yeast extract and peptone. The highest ethanol productions were obtained using hydrolysates supplemented with crude yeast extract (AH-CYE and EH- CYE) showing yields of 0.40 and 0.44 g•g-1, and productivities of 0.39 and 0.29 g•(L•h)-1, respectively. The reuse of the immobilized cells was tested in sequential fermentations of AH-CYE, EH-CYE, and a mixture of acid and enzymatic hydrolysates (AEH- CYE) operated under batch fluidized bed, with ethanol yields ranging from 0.31 to 0.40 g•g-1 and productivities from 0.14 to 0.23 g•(L•h)-1. These results grant further research using Spathaspora yeasts for second-generation ethanol production.