Fermentation process

DETAILED DESCRIPTION What is claimed is: 1.
A process for the production of ethanol by continuous fermentation which comprises carrying out fermentation upon an aqueous solution of fermentable sugar in a series of fermentation vessels in which the ethanol content of the fermentation medium is progressively increased in each fermentation vessel as the fermentable sugar is consumed therein, the fermentation employing at least two different strains of ethanol-producing yeast,one of which provides a high rate of ethanol production in a fermentation medium containing a relatively low concentration of ethanol and a relatively high concentration of sugar and the other of which provides a high rate of ethanol production in a fermentation medium containing a relatively high concentration of ethanol and a relatively low concentration of fermentable sugar.
2.
The process of claim 1 wherein the aqueous solution of fermentable sugar contains partial starch hydrolysate in an amount of up to about 40 weight percent of the total carbohydrate present, the partial starch hydrolysate undergoing saccharification to fermentable sugar under the influence of saccharifying enzyme produced by the yeast and/or added saccharifying enzyme.
3.
The process of claim 1 wherein the strain of yeast which provides a high rate of ethanol production in a fermentation medium containing a relatively low concentration of ethanol and a relatively high concentration of sugar is Saccharomyces bayanus.
4.
The process of claim 1 wherein the strain of yeast which provides a high rate of ethanol production in a fermentation medium containing a relatively high concentration of ethanol and a relatively low concentration of sugar is Saccharomyces cerevisiae.
5.
The process of claim 1 wherein each different strain of yeast is separately employed in a fermentation vessel and is separately recovered therefrom and recycled thereto.
6.
The process of claim 1 wherein the different strains of yeast are used together in each fermentation vessel and are separated from the last fermentation vessel in the series and recycled to the first fermentation vessel in the series