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Grape wine is perhaps the most common fruit juice with alcohol. Because of the commercialisation of the product for industry, the process has received most research attention.
The production of grape wine involves the following basic steps: crushing the grapes to extract the juice; alcoholic fermentation; maltolactic fermentation if desired; bulk storage and maturation of the wine in a cellar; clarification and packaging. Although the process is fairly simple, quality control demands that the fermentation is carried out under controlled conditions to ensure a high quality product. The distinctive flavour of grape wine originates from the grapes as raw material and subsequent processing operations. The grapes contribute trace elements of many volatile substances which give the final product the distinctive fruity character. In addition, they contribute non-volatile compounds which impact on flavour and tannins which give bitterness and astringency.
Although yeasts are the principal organisms involved, filamentous fungi, lactic acid bacteria, acetic acid bacteria and other bacterial groups all play a role in the production of alcoholic fruit products. The proportions of each are dependent upon a number of external environmental factors including the temperature, humidity, stage of maturity, damage at harvest and application of fungicides.
The fermentation may be initiated using a starter culture of Saccharomyces cerevisiae. This approach produces a wine of generally expected taste and quality. If the fermentation is allowed to proceed naturally, utilising the yeasts present on the surface of the fruits, the end result is less controllable, but produces wines with a range of flavour characteristics.
Red grape wine:
Red grape wines are made in many African, Asian and Latin American countries including Algeria, Morocco and South Africa.
Product description: Red grape wine is an alcoholic fruit drink of between 10 and 14% alcoholic strength. The colour ranges from a light red to a deep dark red. It is made from the fruit of the grape plant.
There are many varieties of grape used including Cabernet Sauvignon, Grenache, Nebbiolo, Pinot Noir, and Torrontes. The skins of the grape are allowed to be fermented in red wine production, to allow for the extraction of colour and tannins, which contribute to the flavour.
Raw material preparation: Ripe and undamaged grapes should be used. Red grapes are crushed to yield the juice plus skins, which is known as must.
Processing: The crushed grapes are transferred to fermentation vessels. The ethanol formed during this fermentation assists with the extraction of pigments from the skins. This takes between 24 hours and three weeks.
The skins are then removed and the partially fermented wine is transferred to a separate tank to complete the fermentation. The fermentation can be from naturally occurring yeasts on the skin of the grape or using a starter culture.
Fermentation stops naturally when all the fermentable sugars have been converted to alcohol or when the alcoholic strength reaches the limit of tolerance. Fermentation can be stopped artificially by adding alcohol.
Some wines can be drunk immediately. However most wines develop distinctive favours and aromas by ageing in wooden casks.
Packaging and storage: The product is packaged in glass bottles with corks. The bottles should be kept out of direct sunlight. During storage, wines are prone to non-desirable microbial changes. Yeasts, lactic acid bacteria, acetic acid bacteria and fungi can all spoil or taint wines after the fermentation process is completed.
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