A. Velasquez, R. Arias, and M. Toneatti 2012. Effect of the type of substrate on the chemical composition and productivity of a protein concentrate of yeast origin. Cien. Inv. Agr. 39(3): 425-434. Extracts of Saccharomyces cerevisiae (Sc) yeast were generated using a biotechnological solid-state fermentation method. The organic material used for fermentation consisted of a variety of fibrous substrates of agricultural origin, and their effect on the chemical composition and productivity of cellular biomass generated was evaluated. The substrates studied included the following: wheat straw, WS; barley straw, BS; chili stubble, CS; oat hull, OH and starch powder, SP (control). For incubation with Sc, 2 g of (dry) vegetable substrate were added to 12 mL of cultivation medium. The fermentation time was seven days under aerobic conditions (pH 5, 26 C). After extraction of the yeast biomass, its chemical composition and productivity were analyzed. The results showed an effect (P <= 0.01) of the type of vegetable substrate fermented on the crude protein (CP) and true protein (TP) contents of the yeast biomass. The highest concentration of CP was obtained by fermentation of the control TSP (treatment with the starch powder substrate) (506.3 g kg(-1) DM), followed by the treatment TOH (474.5 g kg(-1) DM). The productivity of the yeast biomass was affected by the type of substrate fermented (P <= 0.001). The highest value was obtained with TSP (327.10 mg g(-1)), followed by TOH (treatment with the oat hull substrate) (207.08 mg g-1). The lowest efficiency was measured in TCS (156.30 mg g(-1)). On the basis of these results, it may be inferred that an extract of high CP can be obtained from yeast (Sc) by the fermentation of fibrous substrates.