CYBERNETIC MODEL FOR THE GROWTH OF SACCHAROMYCES-CEREVISIAE ON MELIBIOSE

Cybernetic modeling has traditionally been used in the modeling of mic robial growth on multiple substrates. In this paper, cybernetic modeli ng has been applied to serve as a model for growth on substrates such as melibiose, which are disaccharides and enzymatically degrade to a m onosaccharide mixture in the fermentation broth. The enzyme alpha-gala ctosidase has been shown to be strongly induced in the presence of gal actose and severely repressed by glucose. In the present model, the re lative concentration of alpha-galactosidase has been linked to that of the key enzyme for galactose metabolism. The enzymatic degradation pr ocess is placed under the control of the cybernetic variables. The max imum rate of melibiose degradation v(m) and the Monod parameters for g rowth of Saccharomyces cerevisiae on pure glucose and galactose were e stimated by batch growth experiments. S. cerevisiae growth on melibios e and a mixture of melibiose and glucose under a variety of preculturi ng conditions was simulated. Depending on the rate of enzymatic degrad ation (i.e., the value of v(m)), the cell mass profile for microbial g rowth on a disaccharide can resemble profiles for growth on a single s ubstrate (melibiose) or can resemble diauxie growth. Experiments indic ate that the model is able to accurately predict the cell mass profile s for yeast growth.