Conditions promoting effective very high gravity sugarcane juice fermentation

Background: Applying very high gravity (VHG) fermentation conditions to the sugarcane juice (SCJ) bioethanol industry would improve its environmental and economic sustainability without the need for major infrastructure changes or investments. It could enable a decrease in the consumption of biological and natural resources (cane/ land, water and energy) while maintaining acceptable production parameters. The present study attempts to demonstrate and characterise an efective industrially relevant SCJ-VHG fermentation process. Results: An industry-like SCJ-VHG bioethanol production process with 30 and 35 °Bx broth was employed to investigate the efects of both the yeast strain used and nitrogen source supplementation on process yield, process productivity, biomass viability, glycerol concentration and retention-associated gene expression. Process performance was shown to be variably afected by the diferent process conditions investigated. Highest process efciency, with a 17% (w/v) ethanol yield and only 0.2% (w/v) sugar remaining unfermented, was observed with the Saccharomyces cerevisiae industrial strain CAT-1 in 30 °Bx broth with urea supplementation. In addition, efcient retention of glycerol by the yeast strain was identifed as a requisite for better fermentation and was consistent with a higher expression of glycerol permease STL1 and channel FPS1. Urea was shown to promote the deregulation of STL1 expression, overcoming glucose repression. The consistency between Fps1-mediated ethanol secretion and ethanol in the extracellular media reinforces previous suggestions that ethanol might exit the cell through the Fps1 channel. Conclusions: This work brings solid evidence in favour of the utilisation of VHG conditions in SCJ fermentations, bringing it a step closer to industrial application. SCJ concentrated up to 30 °Bx maintains industrially relevant ethanol production yield and productivity, provided the broth is supplemented with a suitable nitrogen source and an appropriate industrial bioethanol-producing yeast strain is used. In addition, the work contributes to a better understanding of the VHG-SCJ process and the variable efects of process parameters on process efciency and yeast strain response. Keywords: Biofuel, Bioethanol, Sugarcane, Saccharomyces cerevisiae, CAT-1, Very high gravity, Process optimisation, Process sustainability
B. Monteiro was supported by the Ph.D. Grant 2011/12185-0 from the Fundação de Amparo à Pesquisa do Estado de São Paulo-FAPESP. P. Ferraz and M. Barroca are supported by the Doctoral Programme in Applied and Environmental Microbiology (DP-AEM) and the FCT by Ph.D. Grants PD/ BD/113814/2015 and PD/BD/113810/2015, respectively. T. Collins thanks the FCT for support through the Investigador FCT Programme (IF/01635/2014). T. Collins and C. Lucas are supported by the strategic programme UID/ BIA/04050/2013 (POCI-01-0145-FEDER-007569) funded by national funds through the FCT I.P. and the ERDF through COMPETE2020-Programa Operacional Competitividade e Internacionalização (POCI). P. Ferraz, T. Collins and C. Lucas were further funded by the project EcoAgriFood (NORTE-01- 0145-FEDER-000009), supported by the Norte Portugal Regional Operational Programme (NORTE 2020) under the PORTUGAL 2020 Partnership Agreement through the European Regional Development Fund (ERDF).
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