Wine Basics - Winemaking
Since the juice of almost all grapes is the same pale, almost clear colour, white wines can be made from any colour of grapes. If using black grapes to make a white wine, the winemaker must separate the juice from the skins immediately after pressing and before the fermentation stage so that no colour is picked up from the skins. Some examples of white wine made from black grapes are Champagne, blanc de noirs and white Zinfandel. Most whites are actually made from green grapes. There is, therefore, no hurry to separate juice from skins since there is no danger of them picking up colour, but most winemakers prefer to separate anyway, to prevent too much tannin leaching from the skins into the juice.
Though there is an insignificant group of black grapes whose flesh and juice are actually dark, called "teintures" or "inkers" in English, almost all red wines extract their colour only from the skins of black grapes. Red wines, therefore, can only be made from black (or red) grapes. There are however, some red wines, predominantly old-world styles, where white grapes are blended in with the red grapes. Several appellations in the northern Rhone Valley such as Cote Rotie, Hermitage and Cornas allow for a maximum of 15% Viognier blended in with Syrah to make a very perfumed and aromatic red. In those wines, the white actually intensifies the purple of the Syrah. The most famous example of this type of blending is Chateauneuf du Pape where no fewer than five white varietals are permitted under the AOC designation; this has become a bit of a fad in Australia. And in Tuscany, Trebbiano is still allowed under DOC law to be blended in with Chianti although the percentages permitted are being reduced regularly.
The grape
To begin to understand how any wine is made we must first look at the composition of the grape. If you take any grape- black, red or green- and slice it through the middle it looks much the same.
The Composition
The stalk is woody and full of a substance called tannin. Tannin is a preservative with a bitter flavour. Tannins also add to the structure of a wine and have a great effect on its overall balance.
The skin is covered in a whitish bloom, which is a dusting of wild yeasts and bacteria. The skin itself contains more tannin and, in black grapes, a colouring pigment. Red wines are left on their skins to "macerate" and absorb the colour from the skins and some tannins.
The pips contain bitter oils and are never desirable in white winemaking. They are generally removed in the crushing/destemming process, but may make it into the pressing, which is gentle enough to not press them into the free-run juice.
The pulp (Exo, Meso and Endocarp) is the most interesting part of the grape in white winemaking. Over 70% of the pulp is just water, but the remaining 30% contains most of the elements which give a wine character, flavour and interest. These elements include various fruit sugars and acids, trace elements absorbed from the earth and pectin, a gelling agent. The pulp of almost all grapes is the same colour: a pale green/yellow.
Since the juice of almost all grapes is the same pale, almost clear colour, white wines can be made from any colour of grapes. If using black grapes to make a white wine, the winemaker must separate the juice from the skins immediately after pressing, and before the fermentation stage, so that no colour is picked up from the skins. Some examples of white wine made from black grapes are Blanc de Champagne Noirs (made from Pinot Noir and Pinot Meunier grapes) and white zinfandel. Most whites are actually made from green grapes. There is, therefore, no hurry to separate juice from skins since there is no danger of them picking up colour, but most winemakers prefer to separate them anyway, to prevent tannins from leaching from the skins into the juice. Though there is an insignificant group of black grapes whose flesh and juice are actually dark, known as 'teintures' or 'inkers', almost all red wines extract their colour only from the skins of black grapes. Red wines therefore, can only be made from black (or red) grapes, although there are some very famous red wines that allow for white grapes to be part of the blend, i.e. Cote Rotie, Cornas, Hermitage and Chateauneuf du Pape, which is allowed to use up to 5 different white grapes in its cuvee.
The grape is the fruit of the vine and it develops in bunches as a consequence of the fecundation of flowers. The grape berry is fixed to the stem and this part, because of the substances contained in it and that would negatively affect the taste of wine, is eliminated before pressing the grape by means of a process called destemming. The grape berry's appearance is usually round or stretched and it is covered by the skin whose thickness varies according to each variety and it can also represent 10% of its weight. The skin is rich in pectin, cellulose, aromatic substances and polyphenolic components- generally defined as tannins- responsible for the color in red wines, as well as for the structure and astringency. Both aromatic substances and polyphenols contained in the skin can be extracted by means of maceration in the must- the grape juice produced by pressing- and the quantity of extraction is proportional to the time of maceration. These polyphenols are soluble in alcohol- which is produced during the fermentation of must- and to a lesser extent, in water as well. For this reason the skins of red berried grapes are being macerated in the must with the explicit purpose of giving color to red wines and to extract aromas and tannins.
Grape juice, both of white berried species and red berried species, has a greenish yellow color. Therefore in case the maceration of the skin in the must is completely avoided, it is possible to obtain white wines with red grapes, such as in the case of many classic method sparkling wines produced with Pinot Noir. Likewise, a short maceration of the skins- usually of few hours- can be used for the production of rose wines with red berried grapes. It should be however considered that the quantity of colorant substances contained in the skin of red berries grapes is different according to each variety and therefore every red grape will have proper colorant capacities and qualities. The skin of the grape is covered by a waxy-like substance- which can be easily observed in red berried grapes for its whitish color- on which can also be present yeasts, naturally present in the air, that will start alcoholic fermentation as they will get in contact with the juice. The pulp of the grape berry is rich in water, sugar- present in variable quantities between 15% and 25% of the total matter - acid, pectin, mineral elements, vitamins and nitrogenous substances. The concentration of these elements varies according to the inside zone of the berry.
By observing a cross-section of a grape berry, there can be individuated three distinct zones in which the pulp has different consistencies and concentrations of substances whose quantity varies according to the ripeness level (Figure). The area, which is just under the skin- the external part-, is rich in tannins and aromatic substances, contains about 30% of sugar and about 20% of acids. In the middle part is found the highest concentration of sugar- about 40%- and about 30% of acids. In the inner part are found pips and here is also found the highest concentration of acids- about 50% - and about 30% of sugar. Pips are rich in polyphenols- of pretty astringent nature- that will be extracted during the pressing of berries and that will get mixed to the must. In pips are also found fatty substances that can be used for the production of oil.
As a consequence of the pressing of the berries is obtained the must, which is made of many liquid and solid substances present in the many zones of the pulp and of the skin. Water is the fundamental element and represents about 70-80% of the must, sugar 15-30%, acids 0.5-1.5% and other substances such as minerals, vitamins, polyphenols, aromatic components, pectin, nitrogenous substances, enzymes and microorganisms (yeasts, bacteria and molds). The main sugars of the must are fructose and glucose. Every gram of sugar contained in the must produces- because of the fermentation- about 0.67 grams of alcohol and therefore by measuring the quantity of sugar in the must it is possible to assess the alcoholic percentage in the wine at the end of fermentation. For example, in case a litre of must contains 20 grams of sugar, the alcoholic percentage in the wine will be of about 13.4% (20 × 0.67 = 13.4). The most important acids present in the must are tartaric acid, malic acid and citric acid. Tartaric acid- typical in the grape- certainly is the most important one and it is present in the highest percentage in relation to the others. Polyphenols- usually called tannins and present in variable quantities and according to many factors including climate, environment, grape variety and winemaking practices- are responsible for the taste of wine as well as for the color and astringency in red wines. Among the most important polyphenols components are mentioned anthocyanins, responsible for the color in red wines, and flavones, which play an important role in the color of white wines.