Alcoholic fermentations, Wine and Beer production
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.
- Fermentation reactions may be initiated by yeasts found naturally on the grape skin; however the results of such fermentations are erratic and may be unpalatable.
- In commercial wine making 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 utilized, which manufacture a higher percentage of alcohol (ethanol) than naturally occurring yeasts.
- Proceeds for a some days at a temperature of 22–27 ◦C for red wines (lower for whites), once that the wine is separated from the skins by pressing.
- This is followed by ageing in oak barrels, a method which will last for several months, and during which the flavour develops.
- Malolactic fermentation may be a secondary fermentation carried out on certain types of wine.
- Malic acid, has a sharp taste, which is converted to the milder lactic acid, gives smoothness to the wine.
COOH-H2OC-H2C-COOH −−−−−−−→ CH3-H2OC-OOH + CO2
Malic acid Lactic acid
- A secondary product of malolactic fermentation is diacetyl, 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 higher alcohol content than that of wines.
- Beer is produced by the fermentation of barley grain.
- The procedure may varies according to thedifferent types of beer, but follows a series of defined steps.
- 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 or wort is drained off and hops are added.
- They impart flavour and colour to the finished product and also 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 step, the wort is filtered and then transferred to the fermentation vessel where yeast is introduced.
- Two species of yeast are mainly 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 Saccharomyces cerevisiae are carried to the surface of the fermentation by CO2 bubbles (top fermenters), while Saccharomyces carlsbergensis cell forms a sediment at the bottom (bottom fermenters).
- ‘Spent’ yeast may be dried, and also used as an animals food supplement.
- Fermentation takes a few weeks to finish, at a temperature appropriate for each type of yeast (S. carlsbergensis prefers somewhat lower temperatures than S. cerevisiae).
- Following fermentation, the brewage is allowed to age or ‘rest’ for few months in the cold.
- Beers destined for canning or bottling are filtered to get rid of remaining microorganisms.
- Beers usually have an alcohol content of around four 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 made by reducing the levels of complicated carbohydrates.
- The yeast don’t possess the enzymes necessary to cope with these branched molecules, thus a supplement of debranching enzymes could be added to assist their breakdown.