Brewing yeast strains are heterotrophic organisms capable of utilising a wide variety of nutrients to support growth and generate energy. A property of such organisms is that they must be capable of selective uptake. Thus, assimilation of individual nutrients from wort is made complex by the response of yeast to the mixture of components present. As with all cells, specific systems exist in brewing yeast strains to accommodate the uptake of individual or related classes of nutrients. Of particular relevance to brewery fermentation, assimilation of carbohydrates and nitrogenous compounds are highly regulated processes. When presented with a choice of nutrients, yeast cells tend to use first those that are most easily assimilated. Not only are some components utilised in preference to others but also the presence of some nutrients inhibits the utilisation of others. In consequence, uptake of carbohydrates and the various sources of nitrogen present in wort are ordered processes.
Brewing yeasts can utilise a wide variety of carbohydrates; however, there is some variability between individual strains (see Section 4.2.3). Ale strains of S. cerevisiae are able to ferment glucose, sucrose, fructose, maltose, galactose, raffinose, malto-triose and occasionally trehalose. Lager strains of S. cerevisiae are distinguished by being able to also ferment the disaccharide melibiose. S. cerevisiae var. diastaticus can utilise dextrins.
The patterns of uptake of sugars during an ale fermentation of starting specific gravity of approximately 1.040 are shown in Fig. 3.2. Sucrose is utilised first and the resultant hydrolysis causes a transient increase in the concentration of fructose. Fructose and glucose are taken up more or less simultaneously, in the case of the fermentation illustrated, disappearing from the wort after about 24 hours. Completion of assimilation of glucose is followed by uptake of maltose, the major wort sugar. Maltotriose is utilised last after all assimilation of maltose. Higher polysaccharides, the dextrins, are not utilised by brewing strains and these contribute to beer flavour by way of imparting fullness. Attempts have been made to utilise dextrins via two different strategies. First, through introduction of appropriate enzymes into brewing yeast strains by genetic manipulation (Tubb et al., 1981; Goodey & Tubb, 1982; Vakeria & Hinchliffe, 1989; Lancashire et a!., 1989; Hansen et a!., 1990). Further discussion can be found in Section 4.2.4. Second, dextrins may be hydrolysed to assimilable sugars by addition to wort of commercial dextrinase enzymes.
The sequential uptake of wort sugars reflects the genotype of the yeast and ways in which this is expressed by repression and induction and by carbon catabolite inac-
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