In most instances, use of the word yeast in a food context is synonymous with S. cerevisiae, namely, brewer's yeast or baker's yeast. However, as we shall discover, there are other yeasts involved in fermentation processes.

Yeasts are heterotrophic organisms whose natural habitats are the surfaces of plant tissues, including flowers and fruit. They are mostly obligate aerobes, although some (such as brewing yeast) are facultative anaerobes. They are fairly simple in their nutritional demands, requiring a reduced carbon source, various minerals and a supply of nitrogen and vitamins. Ammonium salts are readily used, but equally a range of organic nitrogen compounds, notably the amino acids and urea, can be used. The key vitamin requirements are biotin, pantothenic acid and thiamine.

Focusing on brewing yeast, and following the most recent taxonomic findings, the term S. cerevisiae is properly applied only to ale yeasts. Lager yeasts are properly termed Saccharomycespastorianus, representing as they do organisms with a 50% larger genome and tracing their pedigree to a coupling of S. cerevisiae with Saccharomyces bayanus.

Saccharomyces (see Fig. 1.2) is spherical or ellipsoidal. Whereas laboratory strains are haploid (one copy of each of the 16 linear chromosomes), industrial strains are polyploid (i.e. they have multiple copies of each chromosome) or aneuploid (varying numbers of each chromosome). Some 6000 genes have been identified in yeast and indeed the entire genome has now been sequenced (see

Brewing yeast does have a sex life, but reproduces in production conditions primarily by budding (Fig. 1.20). A single cell may bud up to 20 times, each time leaving a scar, the counting of which indicating how senile the cell has become.

The surface of the wall surrounding the yeast cell is negatively charged due to the presence of phosphate groups attached to the mannan polysaccharides that are located in the wall. This impacts the extent to which adjacent cells can interact, and the presence of calcium ions serves to bridge cells through ionic bonding. Coupled with other interactions between lectins in the surface, there are varying degrees of association between different strains, resulting in differing extents of flocculation, advantage of which is taken in the separation of cells from the liquid at the end of fermentation.

The underlying plasma membrane (as well as the other membranes in the cellular organelles) is comprised primarily of sterols (notably ergosterol), unsaturated fatty acids and proteins, notably the permeases (discussed earlier) (Fig. 1.21). As oxygen is needed for the desaturation reactions involved in the

Fig. 1.21 Membrane structure.

synthesis of the lipids, relatively small quantities of oxygen must be supplied to the yeast, even when it is growing anaerobically by fermentation.

The control mechanisms that drive the mode of metabolism in the yeast cell (i.e. by aerobic respiration or by fermentation) are based on the concentration of sugar that the yeast is exposed to. At high concentrations of sugar, the cell is switched into the fermentative mode, and the pyruvate is metabolised via acetaldehyde to ethanol. At low sugar concentrations, the pyruvate shunts into acetyl-CoA and the respiratory chain. This is the so-called Crabtree effect. The rationale is that when sugar concentrations are high, the cell does not need to generate as many ATP molecules per sugar molecule, whereas if the sugar supply is limited, the yeast must maximise the efficiency with which it utilises that sugar (ATP yield via fermentation and respiration are 2 molecules and 32 molecules, respectively). The significance of this in commercial fermentation processes is clear. In brewing, where the primary requirement is a high yield of alcohol, the sugar content in the feedstock (wort) is high, whereas in the production of baker's yeast, where the requirement is a high cell yield, the sugar concentration is always kept low, but the sugar is continuously passed into the fermenter ('fed batch').

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