Applied Microbiology and Biotechnology, Vol.102, No.7, 3411-3424, 2018
The role of Saccharomyces cerevisiae in stabilizing emulsions of hexadecane in aqueous media
During downstream operations involved in the purification of hydrophobic biofuels produced by microorganisms, undesired stable emulsions may be formed. Understanding the mechanisms behind this stability is a pre-requisite for designing cost-effective strategies to break these emulsions. In this work, we aimed at increasing our knowledge on the mechanisms responsible for stabilizing yeast-containing oil-in-water emulsions. For this purpose, emulsions containing hexadecane and different yeast-based aqueous phases were prepared and analyzed for phase separation, surface charge density, particle size, and rheology. First, we observed that compounds present in fresh tablet baker's yeast contribute to emulsion stability. In order to eliminate this effect, we generated stocks with this yeast in the laboratory, and compared its performance with an industrial fuel ethanol strain, namely Saccharomyces cerevisiae PE-2. We confirmed that the presence of yeast cells enhances emulsion stability. The cultivation medium (complex or defined) in which cells are grown, as well as the physiological state of the cells (pre- or post-diauxic), prior to emulsion preparation, influenced emulsion stability. The smaller cell size of tablet yeast probably also contributes to more stable emulsions, when compared to those prepared with yeast cells grown in the laboratory. Baker's and fuel ethanol yeast cells in post-diauxic phase promote the formation of more stable emulsions than those with cells in the pre-diauxic physiological state. Finally, we propose a mechanism to explain the enhanced emulsion stability due to the presence of yeast cells, with electrostatic repulsion between emulsion droplets having the prevailing effect.