Kinetics of mixed culture fermentation of multiple substrates

Abstract

Biomass hydrolyzates contain a mixture of hexoses, pentoses and disaccha- rides, the major components being glucose, xylose and cellobiose. These substrates generally are not all utilized with equal efficiency during fermentations with pure cultures, e.g. ethanol production with Saccaharomyces cerevisiae. Ethanol production from such a mixture of substrates can be greatly enhanced by using a mixed culture of organisms selected for their capability to ferment specific components in the substrate mixture. A model system composed of Candida lusitaniae and Pachysolen tannophilua was selected to study the fermentation of mixtures of glucose, xylose and cellobiose. Both organisms are subject to catabolite repression by glucose. In addition, glucose causes permanent repression of cellobiose or xylose utilization as evidenced by reduced growth rates during the cellobiose or xylose phase of multiple substrate fermentations. C. lusitaniae ferments cellobiose to ethanol and utilizes xylose for growth and can ferment it to ethanol but in very low yield. Both xylose and cellobiose are utilized simultaneously. P. tannophilus ferments xylose to ethanol and utilizes cellobiose very slowly for aerobic growth only; xylose represses cellobiose uptake. Ethanol yields by each species on mixtures of sugars generally are additive for each of the substrates indicating that the yield from one substrate is not affected by the presence of other substrates. In mixed culture on multiple substrates, the two yeasts show competititon during the glucose phase; however, the growth of P. tannophilu3 on cellobiose or xylose is inhibited by the presence of an inhibitory metabolite produced by C. lusitaniae. Thus the interactions between the two species growing on mixed substrates is competititon plus amensalism. The maximum ethanol yield of the mixed culture using multiple substrates is lower than expected because there is no production by P. tannophilus from xylose. However, ethanol production could be increased (10%) by using a higher initial cell density of P. tannophilus. Kinetic models for each species were developed and combined to provide a general model for growth and product formation in the mixed culture fermentation. A catabolite repression model adequately described the growth and ethanol production by each species on multiple substrates. A kinetic model for mixed culture fermentation was developed by addition of an inhibition (amensal effect) term to the repression model. The model gave good agreement with experimental data.