In the vast landscape of bioethanol production, a significant portion is rooted in the simplicity of sugar-rich feedstocks. Notably, sugar cane, sugar beet, and sweet sorghum emerge as pivotal contributors, delivering sucrose, glucose, and fructose—directly fermentable sugars that account for nearly half of the global bioethanol output.
Unlike more complex processes requiring extraneous enzymes, these simple sugars can be readily fermented by yeast. Saccharomyces cerevisiae, a yeast variety commonly employed in bioethanol production, produces invertase—an enzyme instrumental in hydrolyzing sucrose into glucose and fructose, laying the foundation for efficient fermentation.
Enhancing ethanol yields from sugar juices involves strategic interventions like heat treatment and clarification, crucial for minimizing impurities and mitigating risks associated with bacterial and wild yeast contaminants. Further optimizing the fermentation process, the blending of clarified juice with molasses proves beneficial, not only in bolstering yeast nutrition but also in enhancing overall fermentation performance.
The molasses, a dark brown syrup generated in sugar cane and sugar beet refineries, presents an intriguing aspect of sugar-based bioethanol production. Its quality, influenced by sucrose removal during crystallization and centrifugation, significantly impacts alcohol fermentations. While an increased sucrose removal diminishes molasses quality for ethanol production, it remains a valuable nutritional medium for yeast.
However, challenges arise from some compounds within molasses formed during sugar processing that can potentially inhibit yeast activity during fermentation. Potassium salts, compounds from browning reactions, furfurals, and formic acid are among these inhibitory elements. To address this, certain fuel ethanol plants adopt diluted molasses treated with sulfuric acid and heated to minimize impurities before fermentation.
Two fundamental fermentation systems dominate sugar-based bioethanol production:
- Fed-batch addition of substrate with yeast propagation
- Fed-batch addition of substrate with yeast recycle
The first system involves introducing freshly grown yeast to each fermenter, minimizing bacterial contamination. Controlled addition of sugar substrate follows. In the second system, acid-washed yeast is cyclically introduced to sugar-rich substrate, achieving rapid fermentations with minimal yeast growth. Yeast recycling involves sulfuric acid treatment to curtail bacterial contamination.
Ethanol concentrations in the latter system typically range from 8-10% v/v. Explored by Basso et al (2008), yeast behavior in Brazilian fuel alcohol plants using yeast recycling reveals distillery-resident strains exhibiting heightened stress tolerance compared to cultured strains. These resilient strains hold promise as selected starter cultures, charting new paths in the evolution of Brazilian bioethanol processes.