Cybernetic model for the growth of Saccharomyces cerevisiae on melibiose

Cybernetic modeling has traditionally been used in the modeling of microbial growth on multiple substrates. In this paper, cybernetic modeling has been applied to serve as a model for growth on substrates such as melibiose, which are disaccharides and enzymatically degrade to a monosaccharide mixture in the fermentation broth. The enzyme alpha-galactosidase has been shown to be strongly induced in the presence of galactose and severely repressed by glucose. In the present model, the relative concentration of alpha-galactosidase has been linked to that of the key enzyme for galactose metabolism. The enzymatic degradation process is placed under the control of the cybernetic variables. The maximum rate of melibiose degradation v(m) and the Monod parameters for growth of Saccharomyces cerevisiae on pure glucose and galactose were estimated by batch growth experiments. S. cerevisiae growth on melibiose and a mixture of melibiose and glucose under a variety of preculturing conditions was simulated. Depending on the rate of enzymatic degradation (i.e., the value of v(m)), the cell mass profile for microbial growth on a disaccharide can resemble profiles for growth on a single substrate (melibiose) or can resemble diauxie growth. Experiments indicate that the model is able to accurately predict the cell mass profiles for yeast growth.