Baking practices subject yeast to several stresses. The most critical phenotypes for baker’s yeast improvement are strains that tolerate high-levels of sucrose, tolerate freeze-thawing stress, rapidly utilize maltose, and produce high levels of CO2.
Dough’s can contain up to 30% sucrose per weight of flour, which applies harsh osmotic stress on yeast cells. Baker’s yeast strains must efficiently utilize maltose, which is originated from flour in lean dough’s. S.cerevisiae strains that have high sucrose tolerance and also rapidly use maltose have been developed and are used commercially. Analysis of sucrose-tolerant and maltose-utilizing yeasts indicated that the gene expression profiles differed between the two yeasts. The expressions of genes involved in accumulation of glycerol and trehalose, which are involved in resistance to osmotic stress, were higher in sucrose-tolerant yeasts.
A desired characteristic for baker’s yeast is the development of strains that utilize the disaccharide melibiose. Raffinose, a well-known component of molasses, is hydrolyzed by yeast invertase to fructose and melibiose. Baker’s yeast does not have the ability to utilize melibiose as it lacks α-galactosidase. This enzyme is present in bottom-fermenting brewer’s yeast strains, and the gene encoding the enzyme from strains has been introduced into laboratory baker’s yeast strains. The resulting strains give increased biomass without alteration of growth rate in fermentations.
One of the most desired properties of baker’s yeast strains is a rapid fermentation rate. Free sugars in dough are sequentially consumed due to transport and catabolite repression regulatory mechanisms. Amylases in the dough release maltose from starch, but many strains of baker’s yeast utilize maltose weakly. Overcoming this bottle neck has long been a desired feature to improve baking productivity.
The popularity of sweet frozen dough’s, has increased significantly in recent years. Improving the freeze-tolerance and freeze-thawing survival of yeasts is a feature that would benefit the production and quality of these bakery products. Torulaspora delbrueckii is highly tolerant to freezing and freeze-thawing, and this yeast has been considered as a primary yeast or helper in frozen-dough products.