The sugars present in the fruit (and any sugar that is added to the fruit) are fermented by yeast that is added to the mixture. There are natural yeasts present on the skins of fruits, but these are usually not sufficient to carry out the fermentation on their own.
When sugar is fermented by yeast, it is converted into alcohol (ethanol) and carbon dioxide gas is released. You will see bubbles of carbon dioxide gas in the air lock while the fermentation is active. You know the fermentation is over when no more gas is produced.
The fermentation has to take place without oxygen (it is an anaerobic fermentation). If oxygen gets into the system during the fermentation, the alcohol will be converted into acid (this is what happens when you make vinegar, which is acetic acid). Wine that has spoiled because it has been exposed to the air may taste very acidic.
There are lots of bacteria and yeasts around in the air and on the surface of the fruits. They all have the potential to spoil the wine. It is extremely important that these bacteria do not start to grow in the fermenting grape juice. Particular care must be taken with the cleanliness of the equipment and personal hygiene.
All equipment must be sterilised with a solution of sodium or potassium metabisulphite before it is used. Sodium or potassium metabisulphite creates an inert gas called sulphur dioxide when mixed with water into solution. Both the solution made and the gas are actually doing the “sterilizing”. The solution by contacting the surfaces itself, and the gas by pushing out oxygen from inside the container being sterilized.
There are really two distinctive types of wine made from grapes – red wine and white wine. The main difference in the two types is the variety of grape used as raw material and the removal of grape skins in the production of white wine. Grapes contain a number of chemical compounds that all contribute to the flavour and colour of wine. Tannins are one group of compounds that give the wine a bitterness and astringency. The tannins are found in the grape skins, therefore red wines tend to be more astringent than white wines.
Red grape wine is an alcoholic fruit drink of between 10 and 14% alcoholic strength that is made from the fruit of the grape plant (Vitis vinifera). The colour ranges from a light red to a deep dark red depending on the grape variety and the length of fermentation and maturation. There are many varieties of grape used including Cabernet Sauvignon, Grenache, Nebbiolo, Pinot Noir, and Torrontes. The skins of the grape are included in the production of red wine, to allow for the extraction of colour and tannins, which contribute to the flavour.