Effects of Temperature and pH on Salt-Stressed Yeast Cultures in Non-Detoxified Coconut Hydrolysate

Brazil has made significant progress in ethanol production from sugarcane juice. However, in terms of water, food, and environmental safety, this production can be optimized by replacing raw food with agricultural waste, giving rise to second-generation ethanol (2G). Among these biomasses, coconut residue stands out in tropical countries like Brazil. This study aimed to fractionate coconut husk through alkaline pretreatment with calcium hydroxide (20% w/w) followed by enzymatic hydrolysis with NovozymesTM Ctec2 and Htec2. The conversion efficiency of polysaccharides into monosaccharides was 19.3% for cellulose and 7.3% for hemicellulose. The inhibitors acetic acid (∼3.0 g/L) and formic acid (∼0.8 g/L) were also observed in the hydrolysate, to which 17.5 g/L NaCl was added to evaluate yeast cells under salt stress. The wild strain Wickerhamomyces sp. UFFS-CE-3.1.2 and the industrial yeast Saccharomyces cerevisiae PE-2 were inoculated into this non-detoxified high-salt hydrolysate and analyzed through a Central Composite Design (CCD). The independent variables were the effects of pH and temperature on yeast metabolism. Assuming cellular growth and sugar consumption as responses to the experimental design, pH of 7.0 and temperature of 40°C were defined as the best cultivation conditions for subsequent fermentation kinetics. As a result, the wild yeast showed the highest biomass production, but it did not produce ethanol. In contrast, the industrial strain fermented glucose, displaying an ethanol yield of 0.24 gethanol/gsugar. Thus, coconut husk can be promising biomass for 2G ethanol production, even though it requires pretreatment and hydrolysis optimization to increase sugar concentration and reduce inhibitors in the hydrolysate.