Produção de etanol a partir do hidrolisado de milho extrudado: comparação entre as cepas cat-1 e ethanol red de Saccharomyces cerevisiae

Abstract

The growing demand for renewable sources has driven ethanol production and consolidated Brazil among the world’s leading producers. Although sugarcane remains the foundation of the sugar-energy sector, corn has gained prominence as a complementary feedstock, especially in the Midwest, due to the wide availability of the second harvest and its ease of storage. The high starch content of this grain favors its enzymatic conversion into glucose and maltose, a process that can be intensified by pretreatments such as dry extrusion. In this context, there is increasing interest in fermentation processes under very high gravity (VHG) conditions, which operate with concentrations above 250 g/L of glucose. These conditions increase productivity and final ethanol concentration but impose intense osmotic stress on yeast, requiring robust strains. Given this scenario, the present study aimed to understand the fermentative behavior of the industrial strains Saccharomyces cerevisiae CAT-1 and Ethanol Red and to propose an alternative process for ethanol production from extruded corn without additional nutritional supplementation. To this end, the study was structured in sequential stages, beginning with preliminary fermentation assays and advancing to the application of real hydrolysates under high-solids conditions. Initially, kinetic assays were conducted in synthetic YPD medium containing different glucose concentrations (150, 200, 295, and 320 g/L), inoculated with 1 g/L of cells, to assess strain performance under normal gravity (NG), high gravity (HG), and VHG conditions. Ethanol Red demonstrated greater osmotolerance, reaching higher ethanol concentrations in VHG media compared to CAT-1, establishing a basis for subsequent experiments. During characterization of the enzymatic hydrolysate from extruded corn, maltose was detected, indicating incomplete starch hydrolysis. Therefore, complementary assays were performed in synthetic media containing mixtures of glucose and maltose (150 g/L total sugars) to evaluate the strains’ ability to ferment both carbohydrates. CAT-1 was able to assimilate maltose, although less efficiently than Ethanol Red. However, the presence of this disaccharide reduced ethanol production in both strains. Based on these results, hydrolysates produced using different hydrolysis media (citrate buffer, phosphate buffer, and acidified water) were evaluated. Diluted hydrolysates (~250 g/L) were fermented by both strains to investigate the influence of the hydrolysis medium on fermentative performance. The best results were obtained with citrate and phosphate buffered hydrolysates, although ethanol concentrations remained below expectations. Considering the objective of maximizing yield and productivity without supplementation, the effect of increasing initial cell concentration (30 and 50 g/L) was evaluated in diluted hydrolysates. The best results occurred with 30 g/L inoculum in citrate-buffered syrup, producing 120 g/L (CAT-1) and 117 g/L (Ethanol Red). In undiluted syrup (~320 g/L) with 50 g/L inoculum, ethanol concentrations of 129 g/L (CAT-1, yield 0.31 g/g) and 99 g/L (Ethanol Red, yield 0.26 g/g) were obtained. At larger scale in 2-L bioreactors, ethanol concentrations reached 110 g/L (CAT-1) and 84 g/L (Ethanol Red), with efficiencies of 68.5% and 54.3%, respectively. Despite these advances and the demonstrated feasibility of VHG fermentation using extruded-corn hydrolysates, overall efficiency remained below the industrially desirable threshold (≥80%), indicating the need for further optimization.