Examples of modified brewing strains

In the brewing industry, several traits have attracted the most research attention (Table 9-6). As noted earlier, brewing strains of S. cerevisiae generally do not hydrolyze dextrins and limit dextrins during the beer fermentation. These sugars remain in the beer, and although they may have a positive influence on body and mouth feel characteristics, they also contribute calories. Reducing these dextrins provides the basis for making low-calorie or light beers. Most brewers use commercially available enzymes, usually fungal glucoamylases, that are added to the wort during mashing. Another approach is to use yeast strains that express glucoamylase and that degrade these dextrins during the fermentation. Such strains have been isolated (e.g., S. cerevisiae var diastaticus), but, unfortunately, when used for brewing, the beer quality is poor, due, in part, to the production of phe nolic off-flavors. Even hybrid strains, obtained by mating brewing yeasts with diastatic yeasts, may still produce off-flavors.

Therefore, a better alternative directly introduces glucoamylase genes (encoding for both glucose-1,4 and glucose-1,6 hydrolysis activities) into brewing strains. Accordingly, STA2 genes (from S. cerevisiae var. diastaticus) and GA genes (from Aspergillus niger) have been cloned into brewing strains, and the genes were expressed and the enzymes functioned as expected (i.e., dextrins and limit dextrins were used). Instability problems occurred initially, since the cloned genes were first located on plasmids, but this problem has largely been circumvented either by using more stable, yeast-derived plasmids or by using integration vector systems that result in the cloned genes being integrated within the host chromosome.

Another problem that occurs in beer production is due to the p-glucans that are derived from the cell walls of barley malt. Consisting of p-1,4 and p-1,6 linked glucose polymers, these materials form gels and foul filters, and increase beer viscosity and haze formation.They can be digested by commercially available enzymes, but, like the glucoamylase example described above, genes coding for bacterial or fungal p-glucanases can be cloned into brewing strains such that they are expressed during fermentation.

Table 9.6. Traits targeted for strain improvement.

Trait

Relevant gene1

Function

Dextrin utilization

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