ITDG 
Interdev Agroalimentaire  

Small-scale production of grape wine

	Type de fiche	Techniques/Technologies
	Mise en œuvre	ITDG(Royaume Uni)
	
	Zone géographique	Global; 
	Mots clés	BOISSON; FERMENTATION; VIN; RAISIN

	Echelle d’action	Groupe
	Public(s) cible(s)	Small- and medium-scale food processors

Contexte
Grape wine is an alcoholic fruit drink of between 10-14% alcoholic strength, made from the fruit of the grape plant (Vitis vinifera). It is perhaps the most common fruit juice alcohol. 
There are several different wine grape varieties, as follows:

Red wine grapes
Cabernet sauvignon, grenache, nebbiolo, pinot noir and torrontes.  

White wine grapes
Airen, chardonnay, palomino, sauvignon blanc and ugni blanc.

The main difference between red and white wines is the inclusion of grape skins in red wine.  The tannins in the grape skins give a dark colour and contribute to the development of flavour.

Because of the commercialisation of the product for industry, the fermentation process has received much research attention and is well understood.

Problématique / Objectifs
Normal grapes have a wide range of micro-organisms on their skin. The main ones involved in fermentation are the yeasts (Saccharomyces cerevisiae), which are probably in the minority.  They convert the sugars in grapes into alcohol and carbon dioxide. However, lactic acid bacteria and acetic acid bacteria are also important as their presence can result in spoilage of the wine.

The type of micro-organisms present and the relative proportions of each is dependent on a number of external factors including the temperature, humidity, stage of maturity, damage at harvest and application of fungicides to the grapes.  

During the production of wine it is essential to encourage growth of the desirable yeasts, while preventing the growth of the others, especially the lactic acid bacteria that will cause the wine to go sour. This is achieved by manipulating the fermentation conditions so that they encourage the growth of Saccharomyces species.

The fermentation can be initiated using a starter culture of Saccharomyces cerevisiae, which is quite easily achieved by adding the yeast to fruit juice.  This approach produces wine of generally expected taste and quality. Alternatively, the fermentation can be allowed to proceed naturally, using the yeasts present on the grapes.  The end result is less controllable, but produces wines with a range of flavour characteristics.  It is likely that the natural method is the one that is most widely practised at home and at the small-scale.

Description/Mise en oeuvre
The production of grape wine involves the following basic steps:
  crushing the grapes to extract the juice; 
  alcoholic fermentation; 
  malto-lactic fermentation (optional);
  bulk storage and maturation of wine in a cellar; 
  clarification and packaging.  

Selection and preparation of raw material
Ripe and undamaged grapes should be used.  Any unripe or over-ripe grapes should be discarded.  The grapes are then crushed to extract the juice. For red wine, the skins are left on the grapes, while for white wine, the skins are removed after crushing.  

Fermentation
Red wine
The crushed grapes are transferred to fermentation vessels where they remain for between 24 hours and 3 weeks, depending upon the desired colour of the final product.  During this time the colour pigments are extracted from the skins.

After this time the skins are removed and the partially fermented wine is transferred to a separate tank to complete fermentation.  This may be a naturally occurring fermentation from the yeasts present in the mixture, or one using a starter culture of Saccharomyces cerevisiae.  In the latter case, the juice is inoculated with a started culture of yeast. Traditionally fermentation was carried out in large wooden barrels or concrete tanks. Modern wineries now use stainless steel tanks as these are more hygienic and provide better temperature control.

The mixture is left to ferment at 25-30 C until all the fermentable sugars have been converted into alcohol.  This may take from a few days to two weeks and varies according to the external temperature and the amount of yeast and sugars present. The higher the temperature, the faster will be the fermentation.  However, if the temperature is too high (over 40 C) the yeast will be inhibited.  During fermentation, a build up of carbon dioxide maintains an anaerobic environment that inhibits the growth of undesirable microbes such as bacteria and moulds.

As soon as the fermentation of sugars to alcohol is complete, it is essential to prevent any further microbiological activity, which will spoil the wine.

Some wines can be drunk immediately. However, most wines develop distinctive flavours and aromas by ageing in wooden casks. 

White wine
The grapes are crushed to yield the juice and the skins are separated out and removed.  Sometimes the juice is clarified by allowing it to stand for 24-48 hours at 5-10 C, by filtering or centrifugation.  Pectolytic enzymes may also be added to accelerate the breakdown of cell wall tissue and to improve the clarity of the juice. Excessive clarification removes many of the natural yeasts and flora, which is beneficial if a tightly controlled fermentation is desired. Long period of settling out encourage the growth of natural flora which can contribute to the fermentation.

The clarified juice is transferred to a fermentation tank where fermentation either begins spontaneously or it is induced by the addition of a starter culture of Saccharomyces cerevisiae. The fermentation of white wine takes place at 10-18 C for about 7-14 days. The low temperature and slow fermentation favours the retention of volatile compounds which are typical of white wines.

Malto-lactic fermentation
A malto-lactic fermentation is not essential, but some wines benefit from this process. It is a natural fermentation that happens about 2-3 weeks after the alcoholic fermentation is finished and lasts for about 4 weeks. It is a lactic acid fermentation initiated by lactic acid bacteria in the wine. The benefits of this process are that it imparts a more mellow flavour to the wine and wines that have undergone the procedure seem more resistant to other microbial damage.

Bulk storage and maturation of wines
Some wines are ready for drinking immediately after fermentation, but most benefit from an ageing period in wooden casks.  During this time the distinctive flavours and aromas develop.

Packaging
After fermentation and ageing the wine is traditionally packaged in sterile glass bottles with corks made from oak (Quercus suber). The bottles should be kept out of direct sunlight.

Résultats/Eléments chiffrés
The following equipment is required:
Fruit crusher
Large fermentation vessels
Thermometer
Bottle washer and steriliser
Bottle filler
Glass bottles and corks

Enseignements
Although the production of grape wine is fairly simple, quality control demands that the fermentation be carried out under controlled conditions to ensure a high quality product. 

There are several variables that can affect the fermentation process and the final quality of wine. The most important factors to control are:
  the clarification and pre-treatment of juice
  chemical composition of juice
  temperature of the fermentation
  the influences of other micro-organisms

Clarification and pre-treatment of juice
Excessive clarification removes many of the natural yeasts and flora. This is beneficial if a tightly controlled fermentation is desired, but less so if the fermentation is a natural one.

Chemical composition of juice
The main reaction in wine making is the conversion of glucose and fructose (sugars) to ethanol (alcohol) and carbon dioxide. The presence of nitrogenous and sulphurous products also contributes to the fermentation. The addition of sulphur dioxide to the grape juice (to inhibit the growth of undesirable micro-organisms) delays the growth of yeast.  If the sugar level of fruits is low, sugar can be added to the fruit juice.

Temperature
At temperatures of 10-15 C the non-Saccharomyces species of yeast have an increased tolerance to alcohol and therefor have the potential to contribute to the fermentation.

Influence of other micro-organisms
Other micro-organisms have the potential to influence (and spoil) wine production at all stages of the process. Prior to harvest, yeasts grow on the surface of grapes. Fungicides are used in an attempt to control this, but they disturb the natural balance of flora, thus making it difficult to carry out a natural fermentation. After the yeast fermentation has finished, other microbes such as the acetobacter species can oxidise the alcohol to produce acetic acid (this is the process used to make vinegar).

It is important that all equipment is sterilised to prevent contamination of the wine.  A weak hypochlorite solution is used for sterilising bottles and fermentation vessels.

Auteur : Sue Azam Ali
Saisie le : 05/12/2001
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