Method for fermenting sugars using genetically engineered yeast

A batch fermentation process for fermenting a starch hydrolysate containing 60-98 weight percent of glucose based on total carbohydrate and 0.3-5% weight percent of isomaltose based on total carbohydrate to a non-ethanol fermentation product, the method comprising:

• a) forming a fermentation broth containing a first portion of a total amount of the starch hydrolysate to be fermented and a genetically modified yeast containing
• i) an exogenous gene encoding a transporter capable of transporting isomaltose, wherein the transporter is a polypeptide with an amino acid sequence having greater than 90% sequence identity to the amino acid sequence of SEQ ID NO:
• 7, SEQ ID NO:
• 20, SEQ ID NO:
• 22, or SEQ ID NO:
• 25;
• and
• ii) an exogenous gene encoding an enzyme isomaltase capable of converting isomaltose to glucose, wherein the gene encodes for a polypeptide having greater than 90% sequence identitity to the amino acid sequence of SEQ ID NO:
• 9;
• b) fermenting the first portion of the starch hydrolysate in the fermentation broth in an initial fermentation step to produce a fermentation product;
• c) feeding the remaining portion of the total amount of the starch hydrolysate to be fermented containing 60-98 weight percent of glucose based on total carbohydrate and 0.3-5% weight percent of isomaltose based on total carbohydrate into the fermentation broth, wherein the fermentation broth on average has a glucose concentration of 50 g/l or less during this step (c);
• d) producing a final fermentation broth containing the fermentation product, wherein the fermentation product is an organic acid;
• and
• e) isolating the organic acid from the fermentation broth.