How is Wine Made?

Wine is a product produced by the fermentation of naturally occurring sugars in fruits such as apples, pears and berries, although most commonly wine is produced from grapes.

Fermentation is initiated by adding selected strains of yeast. Wild yeasts start the fermentation process, but they only produce about 4% alcohol and impart a yeasty, bitter taste to the wine. Wine yeasts then take over, changing the remaining sugar into alcohol (bringing the alcohol content to about 10%). If left unchecked, vinegar bacteria then take over, turning the wine into vinegar. The winemaker must remove unwanted wild yeasts and vinegar bacteria if he or she is to control the fermentation process and produce quality wine. Potassium bisulphite is added to kill unwanted microorganisms and prevent oxidation.

The sugars that provide the basic substrate for the reaction to occur are found in the grapes. Even though yeast is found naturally on the surface of grapes, “wine yeast” is added to the crushed grape pulp to produce the type of fermentation required for quality wine production. The important characteristic of yeasts is that they metabolise sugars to form alcohol without using oxygen. This process is also referred to as anaerobic respiration. Certain commonly found bacteria are also capable of anaerobic respiration, but with the undesirable result of acetic acid (vinegar) or other souring compounds. If present these bacteria will lead to unpleasant odours and tastes in the wine, hence, the importance of clean, uncontaminated equipment.

The speed of fermentation is dependent on temperature. In many commercial wineries, the temperature of the fermentation tanks is carefully controlled to produce steady temperatures for optimum fermentation. Yeasts only live in temperatures between 4°C and 32°C. The closer to 4°C, the slower they work; the closer to 32°C, the faster they work. White wines are generally fermented in temperatures in the range of 12° to 18°C; reds are fermented at higher temperatures.

A short fermentation period produces a light, inexpensive wine. A longer fermentation time produces a finer quality wine.


- Use red or white grapes.

- Crush grapes and remove stems and skins.

Fermentation time is determined by the type of wine required:

- Sweet wine is produced by stopping fermentation before all of the sugar is converted to acid.

- Dry wine is produced by allowing all sugar to be converted to acid.

Sparkling wine is made several ways, the most natural being to bottle the wine before fermentation is complete and allow fermentation to finish in the bottle.


- Only use red or black grapes.

- Stems may or may not be removed when grapes are crushed.

- Grapes go into the fermenting tank with the skins (and sometimes stems).

- Wine which runs off freely is put into barrels and allowed to mature for varying periods.

- Skins and remaining moisture in the vat can be pressed to retrieve further dark and bitter wine; this can be blended with the wine which was run off earlier to give differing tastes.


Good wine is certainly more than fermented grape juice. The complex flavours and aromas that are vital need to be extracted from the grapes.

Preparing to Ferment

1. Sugar is added. When this comes into contact with yeast it turns from a white powder into a fizzy gas (carbon dioxide), and a simple molecule called ethanol (alcohol). There is much debate as to the quantity of sugar added. It must be a minimum of 10% alcohol by volume which will produce the minimum percentage of alcohol to preserve the wine. On the other hand too much sugar will produce undrinkable syrup. The precise measurement of the amount of sugar is referred to as gravity and is measured using a hydrometer. Wine is usually started with a gravity of between 1090 and 1100 (though some home winemakers start with a kilo of sugar for a gallon of must, and then add up to another 8 oz. later on). The first method gives more control over proceedings. More sugar is added to sweet wines than dry ones, and alcohol will usually end up at around 15%.

2. Tannins are added. This may be in tablet or liquid form.

3. It may be desirable to add some pectic enzyme. This gets rid of haze induced by pectin. It is available as a powder or a liquid. 4. Acid is added. If the finished wine is too acidic it will taste sharp. If it is too alkaline it will taste bland. Winemakers aim to be somewhere between the two extremes. The universal measure of acidity is parts per 100,000 of sulphuric acid (ppt). Wine ranges from 3.0ppt for a light dry wine to 4.5ppt for a sweet full-bodied wine. Winemakers attempt to get the ppt within 0.1ppt of where they want it by adding acid or calcium carbonate, to raise acidity or alkalinity respectively. There are a number of different acids available for this purpose, including:

- Citric

- Malic

- Tartaric

- Succinic

- Tannic

5. Once the must has cooled the yeast is added. Yeasts should be stored at temperatures that do not fluctuate too much, and they need to be fed with yeast nutrients or sugar. They also prefer a slightly acidic environment. There are many different types of yeast to choose from including standard wine yeasts and more specialist ones.  The whole mixture is now stirred and left at a temperature above 15 degrees C to allow it to ferment. The fermenting must is usually strained after a few days. Homemade wines are put in demijohns at this point and fitted with an airlock watch.The mixture is left for weeks to months until the fermentation process stops. This can be determined by:

- A specific gravity of 1005 or less

- No bubbles in the airlock

- The wine begins to clear from the top

The wine is now racked (siphoned) leaving the gunk (lees) behind.

Clearing and Finishing

The aim is to now turn the siphoned wine into crystal clear wine that is bottled and labelled.

- Stabiliser and crushed Camden tablets may be added; after 24 hours no further fermentation will take place, and the wine will be stabilised

- Wine finings can be added to remove any residue within the next week

- The wine is then racked again

- Taste the wine and adjust acidity/alkalinity levels

- Filter the wine

- Pour into wine bottles, cork and leave to mature

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