11-27: Mathematical models of cashew apple bagasse hydrolizate fermentation by Kluyveromyces marxianus CE025 to produce ethanol and xylitol

In the state of Ceará (Northeast of Brazil), the cashew agroindustry has an outstanding role in the local economy and the cashew apple bagasse (CAB) appears as an alternative lignocellulosic waste for ethanol and xylitol production. The yeasts traditionally used in fermentation processes are able to consume hexoses as substrate but not pentoses. Nevertheless, it is important to look for yeasts which also have the capacity to consume this pentose, such as Kluyveromyces marxianus that is capable of transforming D-xylose into useful bioproducts (ethanol and xylitol). Therefore, in this work, a mathematical model of the fermentation of CAB hydrolyzate by Kluyveromyces marxianus CE025 to produce ethanol and xylitol was proposed. Experimental data were collected through specifically designed fermentations in order to identify the parameters of the model. All experiments were performed in a batch-culture reactor, keeping aeration level and pH of the culture medium (4.5) constant. Maximum specific net growth rate (μ_m), biomass productivity (b), the specific rates of xylose uptake (q_s), ethanol and xylitol production (q_E and q_Xy) and overall yields in biomass (Y^G_x/s), ethanol (Y^G_E/s) and xylitol (Y^G_Xy/s) were determined. Parameter values were obtained by a technique of non-linear regression, using the free software SCILAB version 5.1. The kinetic models described satisfactorily the batch fermentation process as demonstrated by the results.