The individual and interactive effects of some ionic species on the ethanol vapour pressure in equilibrium with the system, the water activity, and the fermentative performance of Saccharomyces cerevisiae were investigated. The concentrations of the ions Ca2+, Mg2+, and NH4+ were modulated according to a central composite design, within a range of values that can be individually considered optimal for S. cerevisiae. Although the temperature and ethanol and glucose concentrations were kept constant, the initial water activity and ethanol vapour pressure of the systems were remarkably affected by the interactions between the three ions studied. The data concerning the fermentative activity emphasize the role of the physical state of ethanol. In fact, when the ethanol concentration was kept constant, the highest fermentative performances of yeast were obtained when the ethanol vapour pressure, depending particularly on the Ca2+ and Mg2+ interaction, was the lowest in the matrix of values considered.
