A model fermentation system was used to define the abilities of 25 Saccharomyces cerevisiae strains, representing the brewing, baking, winemaking and distilling industries, to utilise maltose and sucrose in the presence of glucose and fructose. Three categories of sucrose and maltose utilisers were observed; repressible, constitutive and non-utilisers. In terms of fermentation kinetics, neither high rates of sucrose hydrolysis nor the early onset of maltose utilisation were correlated with reduced fermentation duration in the experimental system used. Instead better positive correlations were found between this parameter and biomass formation (R 2 = 0.62) and rates of maltose or monosaccharide removal (R 2 = 0.87 and 0.82, respectively). Additionally, invertase activity of brewing strains was seen to occur in two forms: cell-associated and non-cell-associated. This survey exposed a number of novel phenotypes that could be harnessed as a means of producing strains with rapid and efficient utilisation of fermentable carbohydrates.

The utilisation of maltose or sucrose by a selection of nine brewing, baking or laboratory strains of Saccharomyces cerevisiae was either repressible, constitutive or absent. Overall fermentation rate showed a good correlation with maximum specific maltose transport rate (R 2 = 0.79), but a poor correlation with maximum maltase activity (R 2 = 0.34), implying that transport rather than hydrolysis of maltose was a rate-limiting step determining fermentation performance. The genetic basis for differences seen between the strains in terms of fermentation kinetics was investigated. All strains were found to possess genomic sequences detectable with probes for the MAL11 (AGT1), MAL31 permease genes and the SUC2 invertase gene, however, the loci present and the pattern of expression of these and other MAL genes varied widely. From comparisons of sugar utilisation and gene expression patterns, constitutive maltose utilisation in strain NCYC 1681 was best explained by AGT1 expression. Such expression of AGT1 was not, however, mirrored by changes in the pattern of expression of the transcriptional activator(s) (MALx3). Patterns of sucrose utilisation were poorly predicted by the kinetics of SUC2 gene expression, indicating that other SUC loci may be of greater importance.

Aims:  To investigate the influence of the choice of yeast strain on the haze, shelf life, filterability and foam quality characteristics of fermented products. Methods and Results:  Twelve strains were used to ferment a chemically defined wort and hopped ale or stout wort. Fermented products were assessed for foam using the Rudin apparatus, and filterability and haze characteristics using the European Brewing Convention methods, to reveal differences in these parameters as a consequence of the choice of yeast strain and growth medium. Conclusions:  Under the conditions used, the choice of strain of Saccharomyces cerevisiae effecting the primary fermentation has an impact on all of the parameters investigated, most notably when the fermentation medium is devoid of macromolecular material. Significance and Impact of the Study:  The filtration of fermented products has a large cost implication for many brewers and wine makers, and the haze of the resulting filtrate is a key quality criterion. Also of importance to the quality of beer and some wines is the foaming and head retention of these beverages. The foam characteristics, filterability and potential for haze formation in a fermented product have long been known to be dependant on the raw materials used, as well as other production parameters. The choice of Saccharomyces cerevisiae strain used to ferment has itself been shown here to influence these parameters.