Improvement of baker's and brewer's yeast by gene technology

Abstract

Polysaccharides such as starch, cellulose or hemicellulose which constitute the largest part of plant biomass are not fermentable by the yeast Saccharomyces cerevisiae. At the moment the conversion of such compounds depends on the addition of enzymes (e.g. α‐amylase, glucoamylase, xylose isomerase, endoglucanase exoglucanase and β‐glucanase) prior to fermentation. Therefore it is of great commercial interest to create new yeast strains which are able to convert polysaccharides of plant biomass directly into fermentable sugars. In a first step we have constructed an amylolytic S. cerevisiae strain which is able to grow on starch as a sole carbon source. This strain was transformed with DNA harbouring the genes for a‐amylase and glucoamylase from Schwanniomyces occidentalis. Both genes are expressed and the gene products are secreted. Since the two S. occidentalis enzymes are especially suitable in brewing, we thus constructed new yeast strains which could be desirable for the production of low carbohydrate beer. The application of an amylolytic baker's yeast in baking could lead to refrainment from a‐amylase enriched flour in certain types of bread manufacturing. The new S. cerevisiae strains excel by high fermentation rates and ethanol tolerance and the newly acquired ability to degrade starch entirely.