Wine making, or vinification, is the production of wine, starting with selection of the grapes or other produce and ending with bottling the finished wine. Although most wine is made from grapes, it may also be made from other fruit or non-toxic plant material. The basic fermentation process whereby alcohol is produced from the sugar in grapes is very simple, but its chemistry is still not completely understood. As this knowledge increases, winemakers are being helped to improve the quality of their wine.
The composition of grapes is of great importance in determining the quality of the wine produced. Many compounds are carried over from the grape juice into the wine, and other compounds undergo reactions to form the compounds distinctive to wine. It was once common to ameliorate the wine produced with a variety of chemical treatments, but now this is frowned upon. In this new climate, using high quality grapes is essential to producing high quality wines, and grape composition is more important that ever. In general, grapes consist of clear juice (80%), skins (8%), seeds (4.5%), pulp (4.5%) and stems (3%). The skins, seeds, pulp and stems are collectively known as 'pomace'.
Another way of looking at grape composition is in terms of chemical components. However, it is important to realize that the quantity of a given ingredient is not directly related to its importance. For example, vitamins are very important for yeast growth and for fermentation. The major constituents of grapes are carbohydrates,organic acid,phenolic,nitrogenous compound,minerals (ash),vitamins and aroma compound.
The first major constituent is carbohydrates. Carbohydrates are molecules of the general formula Cx(H2O)x, and sugars are a sub-group of carbohydrates. They consist of up to 10 'monosaccharides' - carbohydrates that cannot be broken down into two new carbohydrates. Usually these do not consist of more than seven carbons. Sugars are sweet-tasting, water soluble and good energy sources. Higher carbohydrates generally have structural functions. The most important sugars in grape juice are the two six-carbon sugars glucose and fructose. These are the sugars that make the juice sweet and are fermented to alcohol by the yeast. In addition, small quantities of pentoses (five carbon sugars) and pectins (galacturonic acid polymers) are found. The pectins have no great importance in the juice itself, but if they are not broken down they can create haziness in the wine.
The second major constituent are organic acids Three main organic acids occur in grapes: malic, tartaric and citric acid. Of these, tartaric acid is rare in fruits but the others occur widely. A fourth acid, succinic acid, is formed from yeast metabolism and so is found in wine but not in grapes. The acids give the juice its acidity, and act as aneffective buffer to maintain the pH at around 3.2 - 3.3. They are also important contributors to the flavour balance of the juice and wine, providing the sharp acidity.
The third major constituent are phenolics. The term 'phenolics' refers to a large group of compounds containing at least one phenol group ....