The ethanol production by a laboratory yeast strain, X2180-lB, was less than half that by an alcohol yeast,
YOY655, in a molasses medium containing 30% sugars, although X2180-1B produced approximately the same
amount of ethanol as YOY655 in a nutrition medium with the same sugar content. The weak productivity of
X2180-1B in the molasses was ascribed to the limitation of sucrose hydrolysis in the molasses. The invertase
activity of X2180-1B was 0.019 (mmol sucrose/min/mg protein) in the nutrition medium, but substantially
zero in the molasses, while that of YOY655 was 1.75 in the nutrition medium and 1.15 even under the inhibitory
conditions in molasses. External addition of invertase greatly enhanced the ethanol productivity of only
X2180-1B. The inhibitory factors of invertase in molasses were heat-stable and dialyzable substances.