Nitrogen levels and yeast viability during ethanol fermentation of grain sorghum containing condensed tannins

Abstract

Selected varieties of sorghum, Sorghum bicolor [L.] Moench, give high crop yields and they also return to favorable energy balance in terms of energy calories produced per cultural energy invested. The brown, condensed-tannin, bird- and mold-resistant varieties illustrate these advantages, but their nutritional value and ability to support the expected rate of ethanol fermentation is significantly lower than that of non-brown sorghums. It has been previously shown that the addition of nitrogen to brown sorghum mash supports a high rate of fermentative metabolism without removing the tannins, and suggested that the basis for the inhibition of ethanol fermentation was nitrogen starvation of the yeast cells. In this investigation, it is demonstrated that the addition of protease enzyme to mash results in an increase in amino nitrogen sufficient to support accelerated rates of ethanol fermentation by yeast cells. Thus, the hypothesis commonly cited in the literature that the presumed inhibitor, condensed tannins, function to reduce fermentative metabolism solely via the binding and precipitation of proteins is rejected. In addition, both pre- and post-cook additions of various nitrogen sources give the same restoration of expected rates of ethanol fermentation. The results from studies on the viability of yeast cells suggest that one major cause of reduced rates of ethanol fermentation in mash prepared from brown sorghum varieties is accelerated cell death probably resulting from nitrogen starvation.