Transcriptional profiling of Brazilian Saccharomyces cerevisiae strains selected for semi-continuous fermentation of sugarcane must.

Brazil played a pioneering role in the global establishment of the sugarcane bioethanol industry. The bioethanol fermentation process currently used in Brazil is unique due to the acid wash and recycling of yeast cells. Two, industrially adopted, wild yeast strains, CAT-1 and PE-2, have become the most widely used in Brazil. How these strains respond to the unique fermentation process is poorly understood. The improved performance of CAT-1 and PE-2 is hypothesised to be related to enhanced stress tolerance. This study presents a genome-wide analysis of the CAT-1 and PE-2 transcriptomes during a small-scale fermentation process that mimicked the industrial conditions. The common and unique transcriptional responses of the two strains to the Brazilian fermentation process were identified. Environmental stress response genes were up-regulated postfermenter feeding, demonstrating the impact of the prior acid wash and high glucose environment. Cell wall and oxidative stress tolerance were subsequently demonstrated to be enhanced for the industrial strains. Conversely, numerous genes involved in protein synthesis were down-regulated at the end of fermentation revealing the later impact of ethanol-induced stress. Subsequently, the industrial strains demonstrated a greater tolerance of ethanol and the disruption of endoplasmic reticulum homoeostasis. This increased ethanol tolerance was finally correlated with an increased unfolded protein response and increased HAC1 splicing.

Yeast selection for fuel ethanol production in Brazil.

Brazil is one of the largest ethanol biofuel producers and exporters in the world and its production has increased steadily during the last three decades. The increasing efficiency of Brazilian ethanol plants has been evident due to the many technological contributions. As far as yeast is concerned, few publications are available regarding the industrial fermentation processes in Brazil. The present paper reports on a yeast selection program performed during the last 12 years aimed at selecting Saccharomyces cerevisiae strains suitable for fermentation of sugar cane substrates (cane juice and molasses) with cell recycle, as it is conducted in Brazilian bioethanol plants. As a result, some evidence is presented showing the positive impact of selected yeast strains in increasing ethanol yield and reducing production costs, due to their higher fermentation performance (high ethanol yield, reduced glycerol and foam formation, maintenance of high viability during recycling and very high implantation capability into industrial fermenters). Results also suggest that the great yeast biodiversity found in distillery environments could be an important source of strains. This is because during yeast cell recycling, selective pressure (an adaptive evolution) is imposed on cells, leading to strains with higher tolerance to the stressful conditions of the industrial fermentation.