Alcoholic beverages are produced following the fermentation of sugars by yeasts, mainly (but not exclusively) strains of the species, Saccharomyces cerevisiae. The sugary starting materials may emanate from cereal starches (which require enzymatic pre-hydrolysis) in the case of beers and whiskies, sucrose-rich plants (molasses or sugar juice from sugarcane) in the case of rums, or from fruits (which do not require pre-hydrolysis) in the case of wines and brandies. In the presence of sugars, together with other essential nutrients such as amino acids, minerals and vitamins, S. cerevisiae will conduct fermentative metabolism to ethanol and carbon dioxide (as the primary fermentation metabolites) as the cells strive to make energy and regenerate the coenzyme NAD + under anaerobic conditions. Yeasts will also produce numerous secondary metabolites which act as important beverage flavour congeners, including higher alcohols, esters, carbonyls and sulphur compounds. These are very important in dictating the final flavour and aroma characteristics of beverages such as beer and wine, but also in distilled beverages such as whisky, rum and brandy. Therefore, yeasts are of vital importance in providing the alcohol content and the sensory profiles of such beverages. This Introductory Chapter reviews, in general, the growth, physiology and metabolism of S. cerevisiae in alcoholic beverage fermentations.

The selection of a brewing yeast strain with the required fermentation and recycling characteristics is critical. The yeast strain will influence the rate and extent of fermentation, the flavour characteristics and the overall quality and stability of the finished beer, and consequently, the economic viability of the brewery. Since high gravity worts can have a deleterious effect on yeast fermentation performance, it is imperative that the strain selected be suitable for this environment, which includes a capacity to withstand high osmotic pressures and elevated ethanol levels. Under controlled in vitro osmotic and ethanol induced stresses, there was a decline in mean cell volume in both lager and ale yeast strains. Whilst significant reductions in viability were observed in the lager strains, the ale strains studied were not affected. Cell surface investigations revealed shrinkage of the yeast cells and crenation of the outside envelope under both stresses, although exposure to ethanol had a more marked effect on the yeast cell surface than sorbitol-induced elevated osmotic pressure.Key words: Cell viability, cell volume, ethanol, high gravity wort, osmotic pressure. -2863(9'8-32In brewing, yeast is recycled and the fact that it is cropped and repitched into subsequent fermentations is one of the differentiating features between brewing and the production of many other alcoholic beverages including Scotch whisky 37 . Consequently, the quality of yeast cropped will not only affect the overall performance of most fermentations but also the quality and stability of the resulting beer. It is important therefore that the factors which influence yeast performance in the brewing process, and particularly in high gravity brewing, are considered, to ensure efficient fermentation and the production of a beer of a consistently high quality and stability. Breweries, the world-over, are continually seeking ways to reduce capital expenditure, labour, utilities, effluent and other operational costs and at the same time ensuring that the quality of their beers remains consistently high. As a result, many are employing the process of high gravity brewing due to its many advantages 39 . Similarly, some breweries have not implemented this process due to its disadvantages 40 . Of significance, are the deleterious effects of this process on yeast fermentation performance 41,43 . Fermentations have been reported to be sluggish due to elevated ethanol levels and high osmotic pressure, resulting in yeast viability and vitality reductions 6 . Osmotic pressure is the force that develops between two solutes of differing concentration separated by a semipermeable membrane 20 . When yeast is exposed to wort it is subjected to an osmotic pressure. Very high osmotic pressures such as those encountered in high gravity worts, may distort yeast metabolism or decrease yeast viability. The extent of the osmotic pressure will depend on the concentration of solutes surrounding the cell 20,23 . It has been shown that increases in wort osmotic pressu...

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The role of nitrogenous components in malt and wort during the production of beer has long been recognized. The concentration and range of wort amino acids impact on ethanolic fermentation by yeast and on the production of a range of flavour and aroma compounds in the final beer. This review summarizes research on Free Amino Nitrogen (FAN) within brewing, including various methods of analysis.

When the cells of a lager brewing yeast Saccharomyces uvarum (carlsbergensis) were grown in minimal media containing sucrose and a non-metabolized sugar sorbitol, significant levels of intracellular ethanol were obtained. Intracellular ethanol concentration decreased as the osmotic pressure of the medium was lowered and the proportion of extracellular ethanol increased. A reduc tion in cell viability occurred when there were high levels of intracellular ethanol. The total amount of glycerol produced increased with increased osmotic pressure, but glycerol diffused out of the cells faster than ethanol.

The Free Amino Nitrogen (FAN) content of wort prescribes efficient yeast cell growth and fermentation performance. FAN consists of the individual amino acids, small peptides and ammonia ions formed during malting, the relative amounts of which vary. In this paper, the individual constituents of FAN were dissected and their effect on both ale and lager fermentations determined. The patterns of amino acid and small peptide uptake and the changes in extracellular protease activity revealed the dynamic environment that develops during fermentation. Lysine and methionine, previously identified as key amino acids in wort fermentation, were investigated further.

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