A. Velásquez, and R. Arias. 2013. Effect of substrate on the in vitro protein digestibility of extracts generated by Saccharomyces cerevisiae. Cien. Inv. Agr. 40(3): 503-511. The in vitro crude protein (CP) digestibility (Divcp), true protein digestibility (Dtp) and degradation rate (kd cp) were measured in different protein extracts of Saccharomyces cerevisiae (Sc) yeast. These extracts were generated through a biotechnological method (solid state fermentation). Wheat straw (TWS), barley straw (TBS), chili stubble (TCS), oats hull (TOH) and starch-glucose powder (TSP; control) were used as substrates for fermentation by Sc, and their effect on the aforementioned kinetic parameters in the generated cellular biomass was evaluated. In 12 mL cultivation medium, 2 mL Sc solution inoculum (6.7×10(6) spores mL-1) was added together with 2 g vegetable substrate (dry) and additional nitrogen source, and the mixture was incubated for 7 days at a constant temperature (26°C at pH 5) under aerobic conditions. The average CP content of the yeast biomass (DM) was of 45%. The Divcp and Dtp were analyzed (two phases: pepsin/trypsin-pancreatin) after the extraction of the yeast biomass, and an effect of the fermented substrate on these values (P≤0.001 and P≤0.01, respectively) was observed. The greatest digestibility (Divcp) was obtained with TSP (78.9% CP) and the lowest with TCS (67.1% CP). In addition, differences in the kd cp between the treatments were also observed (P≤0.01). The high contents of CP, Divcp (71.8% on average) and kd cp (16.4% h-1 on average) suggest that Sc yeast biomass has the potential to become an alternative for the production of animal protein supplements.