Cheese whey is excreted from the milk used in the cheese- making process and amounts to approximately 90% of the volume of milk [1]. The principal component of whey is lactose which can be used as the material to generate ethanol for biofuels by lactose-fermenting yeasts [2]. However, the fermentation of lactose in crude whey is ineffective from an economic viewpoint on account of the high dilution and low concentrations of ethanol (25 mg mL-1) achieved, requiring
high energy inputs for distillation [3]. The production costs of
ethanol improve when the concentration of lactose increases from 100 to 120 mg mL-1 [4].
One simple method to elevate the sugar concentration is the supplementation of high amounts of other sugars, such as sucrose. We have also paid attention to thick juice, a concen- trated extract of sugar beet that is obtained after the removal of non-sugar components and used for further processing to sucrose and ethanol [5]. Typically, the juice is diluted fourfold for efficient fermentation at a sucrose level of about
200 mg mL-1 to reach an ethanol concentration of 100 mg mL-1.
But, there was a problem, namely, that yeast cells utilize