Alcoholic fermentations

There is evidence that alcoholic drinks, including beer and wine, were being produced thousands of years before the Christian era, making them among the earliest known examples of the exploitation of microorganisms by humans. Ethanol results from the fermentation process, because the conversion of sugar to carbon dioxide and water is incomplete:

C6H12O6-► 2CH3CH2OH + 2CO2

Although, in principle, wine can be made from almost any fruit juice with a high sugar content, the vast majority of commercially produced wines derive from the fermentation of the sugar present in grapes (Figure 17.1). Such fermentation reactions may be initiated by yeasts naturally found on the grape skin; however the results of such fermentations are erratic and may be unpalatable. In commercial winemaking the must (juice) resulting from the crushed grapes is treated with sulphur dioxide to kill off the natural microflora, and then inoculated with the yeast Saccharomyces cerevisiae, variety ellipsoideus. Specially developed strains are

Red wine is made from red (black) grapes only. Both white and red grapes may be used in white wine, but red grapes must first be deskinned.

In dry wines, most of the sugar is fermented to alcohol; in sweet varieties, some sugar remains un-fermented.

Secondary fermentation


Clarifying, filtration


Figure 17.1 Wine production. The production of red and white wines differ in certain details, but share the main steps of crushing the grapes, fermenting their sugar content into alcohol and ageing to allow flavour development used, which produce a higher percentage of alcohol (ethanol) than naturally occurring yeasts. Fermentation proceeds for a few days at a temperature of 22-27 °C for red wines (lower for whites), after which the wine is separated from the skins by pressing. This is followed by ageing in oak barrels, a process that may last several months, and during which the flavour develops. Malolactic fermentation is a secondary fermentation carried out on certain types of wine. Malic acid, which has a sharp taste, is converted to the milder lactic acid, imparting smoothness to the wine.

Malic acid Lactic acid

A secondary product of malolactic fermentation is di-acetyl, which imparts a 'buttery' flavour to the wine. Spirits such as brandy and rum result from the products of a fermentation process being concentrated by distillation. This gives a much higher alcohol content than that of wines.

Most wines have an alcohol content of around 10-20 percent. Fortified wines have extra alcohol added.



WorG^ Solids



Filtration to remove hops




Bottling/ canning


Figure 17.2 Beer production. The early stages serve to convert the carbohydrate present in the grain into a form that can be fermented by the yeast. The details shown refer to the production of a light, lager-type beer

Beer is produced by the fermentation of barley grain. The procedure varies according to the type of beer, but follows a series of clearly defined steps (Figure 17.2). Grain, unlike grapes, contains no sugar to serve as a substrate for the yeast, so before fermentation can begin, it is soaked in water and allowed to germinate. This stimulates the production of the enzymes necessary for the conversion of starch to maltose ('malting'). An additional source of starch may be introduced during the next stage, mashing, in which the grains are ground up in warm water, and further digestion takes place. The liquid phase or wort is drained off and hops are added. They impart flavour and colour to the finished product and also

Mashing releases soluble material from the grain in preparation for fermentation.

Malting is the process whereby grain is soaked in water to initiate germination and activate starch-digesting enzymes.

possess antimicrobial properties, thereby helping to prevent contamination. The mixture is boiled, inactivating the enzymes, precipitating proteins and killing off any microorganisms. In the next stage, the wort is filtered and transferred to the fermentation vessel where yeast is introduced.

Two species of yeast are commonly used in the brewing process, both belonging to the genus Saccharomyces. S. cerevisiae is mainly used in the production of darker beers such as traditional English ales and stouts, whereas S. carlsbergensis (no prizes for guessing where this one was developed!) gives lighter coloured, less cloudy, lager-type beers. Cells of S. cerevisiae are carried to the surface of the fermentation by carbon dioxide bubbles (top fermenters), while S. carlsbergensis cells form a sediment at the bottom (bottom fermenters). 'Spent' yeast may be dried, and used as an animal food supplement.

Fermentation takes about a week to complete, at a temperature appropriate for each type of yeast (S. carlsbergensis prefers somewhat lower temperatures than S. cerevisiae). Following fermentation, the beer is allowed to age or 'rest' for some months in the cold. Beers destined for canning or bottling are filtered to remove remaining microorganisms.

Beers typically have an alcohol content of around 4 per cent. Small amounts of other secondary products such as amyl alcohol and acetic acid are also produced, and contribute to the beer's flavour. 'Light' or low-carbohydrate beers are produced by reducing the levels of complex carbohydrates. The yeast do not possess the enzymes necessary to cope with these branched molecules, so a supplement of debranching enzymes may be added to aid their breakdown.

Hops are the dried flowers of a type of vine. They were originally added to beer because of their antimicrobial properties, but were soon found to improve the flavour too! Hops are added to the wort after boiling.

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