Abstract
First-generation bioethanol is primarily produced from sugar-cane and starch feedstock, mainly in Brazil and the USA. However, these feedstocks directly impact the food chain, necessitating alternative sources for bioethanol production. Exploring lignocellulosic biomass has increased bioethanol production due to its abundant availability and polysaccharide components. However, its recalcitrant nature requires pretreatment to facilitate enzymatic saccharification. After pretreatment, the cellulose and hemicellulose components in lignocellulosic biomass can be hydrolyzed by cellulases and hemicellulases enzymes to release fermentable sugars C6 and C5. Norwegian domesticated Ebbegarden kveik yeast was studied for its ability to ferment glucose into ethanol at elevated temperatures. The primary aim was to evaluate the yeast’s capacity to produce ethanol at 42 0C using sugars from two lignocellulosic fractions generated after steam-explosion pretreatment, cellulose-rich cellin, and hemicellulose-enriched wood molasses.
The research used liquid wood molasses from Spruce and Birch, and solid spruce cellin hydrolysates for yeast fermentation using Syringe piston and Blue Sens Gas Analyzer. Spruce molasses was diluted to a certain percentage of dry solids, wherease birch molasses and cellin hydrolysates were directly used for fermentation. Wood molasses were subjected to enzymatic saccharification with the EC200 enzyme, corresponding to 3.2kU /g DS hemicellulase activity. Cellin hydrolysate was fermented with 20µM and 35µM Zn supplementation and 0.09g/L yeast vitamin supplementation to investigate the impact on ethanol yield. Samples were collected before and after fermentation to analyze the glucose and ethanol concentration by HPLC.
Ebbegarden kveik yeast demonstrated efficient utilization of C6 sugars and lesser utilization of C5 sugars in the experiment with various carbon sources. Immobilized yeast beads showed enhanced glucose consumption, indicating protection against hydrolysate inhibitors. Enzymatic saccharification significantly increased glucose content in both birch and spruce wood molasses. Zinc supplementation boosted the glucose consumption rates but did not affect ethanol yield, suggesting complex interactions between substrate composition and yeast metabolism. Additionally, experiments with alginate beads in a 1:5 ratio to the final fermentation volume resulted in a higher glucose consumption rate but lower ethanol yield. It is hypothesized that the impact of toxic compounds in lignocellulosic biomass led to diauxic shifts and accumulation of trehalose. This metabolic shift, consuming most glucose, might be the potential reason for the low ethanol yields, indicating that kveik yeast is sensitive to toxic environmental conditions. The reusability of alginate beads in repeated batch fermentation for lignocellulosic fractions, particularly wood molasses, may not be ideal, as the beads were clogged by microparticles present in them. This study showed that Ebbegarden kveik yeast is not suitable for second-generation bioethanol production at 42 0C. Further studies should explore kveik yeast’s stress response mechanisms and innovative strategies to improve fermentation efficiency in fermenting lignocellulosic biomass.
