Assessment of parameters influencing the yeast acid treatment step in an ethanol fermentation process with cell recycling

Ethanol production from sugarcane in Brazil is typically conducted with high density and cell recycling. Before the recycle, it is conducted an acid treatment step where sulphuric acid is added until pH 2 during around 1 or 2 hours. The treatment with acid is used to reduce yeast flocculation, bacterial contamination and to remove intracellular alcohols and acids. This procedure allows high efficiency and productivity; however the phenomena involved during the acid treatment step has been poorly quantified and optimized. In this work, following a Central Composite Design, it was investigated the effect of temperature (24 to 36°C) and pH (1.8 to 3.5) variations on cell viability and yeast flocculation (through deflocculation phenomena) caused by Lactobacillus fermentum ATCC 0559. Experiments were performed using a bench scale fermentor Bioflo 115 of 2 L with stirring, temperature control, pH and capacitance probes. The yeast was obtained from Iracema mill (Iracemápolis - SP, Brazil) and the bacteria from Tropical Institute (Campinas - SP, Brazil). The quantitative method for deflocculation was correlated with the viscosity. Microscopy analysis was applied to double check the previous results. The cell viability was quantified using the technique of coloring with methylene blue and counting in a Neubauer chamber through optical microscope. Results from statistical analysis showed that the deflocculation phenomenon strongly depends on pH. Furthermore, it was observed that capacitance signal, which is correlated with viable cells, decreased according to time and temperature.