Development of a Two-Stage Bioprocess for the Production of Bioethanol from the Acid Hydrolysate of Brewer’s Spent Grain

Bioethanol, an alcohol produced by microbial fermentation, is traditionally produced from sugar-rich plants such as sugar cane, sugar beet and maize. However, there is growing interest in the use of lignocellulose, an abundant and inexpensive renewable energy source, as a potential substitute for the production of biofuels and biochemicals. Yeast Saccharomyces cerevisiae, which is commonly used for ethanol fermentation, cannot cope with lignocellulose due to a lack of lignocellulolytic enzymes and the inefficient functioning of the pentose phosphate pathway. The aim of this research was to isolate yeasts that can efficiently produce bioethanol and valuable byproducts from both glucose and xylose in a two-stage fermentation process using brewer’s spent grains. This approach should maximize sugar utilization and improve the economic viability of bioethanol production while contributing to waste valorization and sustainability. Kluyveromyces marxianus and Candida krusei were identified and tested with different initial concentrations of glucose and xylose. The results showed that both yeasts produced bioethanol from glucose but were inefficient with xylose, yielding valuable compounds, such as 2,3-butanediol and glycerol instead. A two-stage fermentation was then carried out with weak acidic hydrolysate from brewer’s spent grain. In the first stage, glucose was fermented by S. cerevisiae to produce bioethanol; in the second stage, xylose was fermented by K. marxianus and C. krusei to obtain other valuable products.