Fermentation is a natural process that has been used for centuries all over the world to create alcoholic drinks.
There are two basic phases in fermentation: alcoholic fermentation and malolactic fermentation.
Fermentation is a complex yet natural process and begins once the sugars inside the grape come into contact with the natural yeasts present on the thin film that covers each grape berry. These yeasts, which are small organisms, feed on the sugars and excrete carbon dioxide and ethanol (alcohol). There are other by-products of the fermentation process such as glycerol, which make the wine rich, esters (aromatic) compounds, aldehydes, and acids. All of these by-products make a significant contribution to the flavour of wine and are mainly responsible for many secondary aromas in the wine.
Alcoholic fermentation will end when all the sugars have been converted to alcohol. If the sugar content of the grapes is particularly high, fermentation will end once the alcohol content reaches a level that stops the action of the yeasts, leading to a strong and sweet wine. Fermentation may also stop if the ambient temperature drops below a certain level preventing the yeasts from working.
Malolactic fermentation occurs after alcoholic fermentation. This is the process whereby malic acid, which can exist in high concentration in grapes and has a green apple taste, is converted into lactic acid by lactic bacteria. Lactic acid is less bitter than malic acid and makes the wine softer. Malolactic fermentation is always sought in a quality red wine. For it to happen a temperature of 20oC has to be maintained.
Winemakers, particularly in the New World, use cultured yeasts for their fermentation. Those in the Old World, and those that have respect for the terroir will rely only on the natural yeasts present on the grapes for greater complexity and flavour.
Temperature is a very important factor in fermentation and fermentation will only begin at a temperature of 12oC. Temperatures above 35-37oC will kill the yeasts causing fermentation to stop. Winemakers regulate the temperature by either cooling or heating the fermentation tanks.
The fermentation process for red and white wines is different.
For white wine the juice alone is extracted from the grapes and fermented. For red wine, the crushed grapes and juices are fermented together. Only red wines are fermented with the skins and seeds and these pass on colour as well as tannins and additional ingredients that provide flavour.