How is white wine made?
The essential steps of winemaking after the grape harvest are pressing, fermentation, ageing, fining and bottling. The fundamental difference in the production of white and red wine is that fermentation takes place after pressing(must fermentation) for white wine and before pressing(mash fermentation) for red wine.
All of the steps and treatment methods mentioned below are permissible under wine and food law. However, not all measures have to be applied, as some are at the expense of the aroma diversity, fineness, depth and balance of the wine.
The time of harvest depends on the ripeness of the grapes, the weather conditions and the desired wine type. How quickly the grapes ripen depends on the grape variety and the weather throughout the year and especially in autumn. The grapes for Prädikat wines require a higher must weight and must therefore hang on the vine for a particularly long time.
The grapes are harvested either by hand or with the help of machines(harvesters). Mechanical harvesting is very fast and requires less labour, which is generally an economic advantage and, if the weather is unstable or bad at harvest time, also a very practical advantage that can even save the wine quality. Often a negative selection takes place beforehand, in which grapes that do not meet the quality requirements are cut to the ground. More time-consuming is the Hand harvest. Its advantage is that only the ripe grapes (or the rotten ones) are selectively harvested in several passes with time intervals. In Germany, hand harvesting is even required by law for wines of higher ratings, and in steep slopes the use of machines is not possible anyway.
After the grapes have been cut from the vine, they are brought to the winery quickly and, if possible, without damage. The size of the collection containers plays a significant role in whether the berries remain intact, as the weight bearing down on the grapes below can already cause their berry skins to partially burst. Sometimes the grapes are cooled during transport to the winery to keep the aromas fresh.
At the winery, additional selection sometimes takes place: unusable grapes are discarded and/or rotten and diseased berries are cut out of the grapes.
Preparation of the pressing
First, the grapes are destemmed: the individual berries are freed from the stems and crests, the grape skeleton (rape). This destemming is not necessary for whole bunch pressing, which produces stronger white wines with a slight tannin.
Afterwards, the berries are lightly crushed to burst the skins so that the escaping juice can release the colour and aroma substances from the skins. (The resulting mash - the mixture of juice, pulp, skins and pips - is, depending on the quality of the grapes and the intended processing time, generally sulphurisedin order to avoid premature, wild fermentation or oxidation.
Through the addition of Enzymes enzymes can be added to increase the flavour and colour yield. The enzymes also make the mash easier to press: The juice comes out of the berries better, and because the pressing pressure can be kept lower, fewer tannins and bitter substances are extracted.
For the actual pressing process (the Pressing) there are several technologies. What they all have in common is that mechanical pressure is applied to the mash, thus separating the juice from the solids. These then remain as pomace.
A basic distinction is made between vertical and horizontal presses; concrete examples are tree or basket presses, screw presses, spindle presses, screw presses, tube presses and pneumatic presses, which allow particularly fine regulation of the pressure.
Preparation for fermentation
After pressing, the must - the liquid obtained by pressing - is re-sulphurised if necessary to avoid uncontrolled fermentation or oxidation, and then clarified. Clarification is done either by simply letting the must stand for a while to allow the lees to settle to the bottom of the tank, or by biochemical and/or physical processes such as the addition of enzymes, filtration or centrifugation. A common method of pre-clarification is flotation: In this process, the must is impregnated with air or nitrogen under pressure and then depressurised again. The resulting fine gas bubbles draw the lees upwards in the tank so that the clear juice can simply be drawn off at the bottom.
By means of several physical processes, the must can be concentrated the must:
- In Reverse osmosiswater is removed from the must so that the relative proportion of the remaining ingredients(extract) increases. In this process, the must is in a chamber that is separated from a chamber with water by a semi-permeable membrane. By increasing the pressure in the must chamber, water molecules migrate from the must chamber into the water chamber.
- During vacuum distillation, water is also removed from the must, so that the relative proportion of the remaining ingredients (extract) increases. The process makes use of the fact that the boiling point of water drops at low pressure. At a pressure of 0.03 bar, water boils (i.e. evaporates) in the must already at a temperature of about 25°C. The resulting vapour is cooled on a cooling rack. The resulting vapour condenses on cooled surfaces.
- Freeze concentration (cryoextraction) follows a similar principle to that of ice wine production: The must is slowly cooled below 0°C until pure ice crystals form from water and the remaining ingredients concentrate. The ice crystals are then separated from the remaining liquid.
If the sugar content of the must is not sufficient for successful or desired fermentation, the must can be chaptalised, i.e. enriched with unfermented or fermented must or with sweet reserve to increase the alcohol or residual sugar content. In the case of Prädikat wines, chaptalisation is prohibited in Germany.
During fermentation, yeasts convert the sugar from the grapes into alcohol and carbon dioxide, producing heat. Yeasts are already present in the must in natural form if they have not been killed by some of the measures described above.
If these natural yeasts are used to ferment the must, this is called Spontaneous fermentation. However, this type of fermentation involves uncertainties because the natural yeasts may not be strong enough to complete the fermentation completely or may produce undesirable chemical compounds that can affect the aroma of the wine or even pose health risks. Therefore, the natural yeasts are often killed by sterilising the must in order to then carry out the fermentation in a controlled manner with pure-breeding ye asts.
Pure-breeding ye asts have certain properties that are known in advance because they have been specially bred for this purpose. This means that they can be specifically selected and used for the fermentation of the must. With appropriately bred yeasts, the aromatic character of the wine can also be extensively influenced (keyword: secondary aromas). The addition of fermentation aids and yeast nutrient salts can support the work of the pure-bred yeasts.
An important factor in fermentation is temperature. Traditionally, the fermentation temperature was more or less self-regulating and depended on the cellar temperature. In modern cellars, the fermentation tanks can be cooled and the fermentation temperature thus actively controlled: the lower the temperature, the slower the fermentation process(cold fermentation). The size of the fermentation tanks also affects the fermentation temperature: the smaller the tanks, the lower the fermentation temperature. The fermentation temperature and the associated fermentation time have a great influence on the style of the wine.
The yeasts usually work until either the sugar present in the grapes (or added) is completely converted into alcohol or the alcohol content becomes so high that the yeasts stop working. They then become sluggish and die off, as they cannot survive in an excessively alcoholic environment. Among the pure-breeding yeasts, however, there are particularly efficient types that can easily reach 16 percent by volume.
If the winemaker wants to produce a wine with residual sweetness, he can stop the fermentation prematurely. This can be done by cooling or by adding sulphur. A third variant is to leave the wine to ferment fully and then add sweet reserve to subsequently increase the sugar content of the wine again. The sweet reserve is unfermented (i.e. sweet) grape must that has been protected from undesired fermentation by sterilisation.
The ageing of the wine, i.e. the maturation after fermentation, usually takes place in stainless steel tanks or in large or small wooden barrels. Other types of ageing are concrete tanks or - rarely - amphorae (although in these cases fermentation usually takes place in the amphora and ageing is mainly oxidative ).
Maturing in stainless steel tanks leads to reductive, fresh and fruity wines. Maturing in large wooden barrels leads to smoother, full-bodied wines, as the constant minimal air supply through the wooden walls causes micro-oxidation, which also makes the wine more durable. The small wooden barrel (Barrique) has the additional property that it releases toasty and smoky wood aromas as well as additional tannins into the wine; the wine thus becomes spicier and more complex.
Instead of putting the wine into the wood, one can recently also put the wood into the wine: Particularly in the New World, a wine with wood aromas may have been aged in stainless steel tanks, but it is given Oak chips have been added to it. This results in the characteristic taste, but the wine has to obtain the smoothness as with an ageing in wooden barrels in a different way with suitable treatment methods. The advantage for the producers: The use of wood chips is quicker and cheaper than ageing in barrique barrels; in addition, the wood influence can be better dosed this way, and the amount of wood needed is significantly less than for the production of a barrel.
If wines are matured in wooden barrels (especially in barriques), they often also undergo biological acid degradation (BSA). This is often also called malolactic fermentation but this process is not carried out by yeasts, but by lactic acid bacteria. These convert the strong malic acid into the milder lactic acid, making the wine smoother and rounder. BSA can also be actively initiated by the targeted addition of lactic acid bacteria if it does not occur on its own.
Fining and stabilisation
After fermentation or at the latest after the end of the ageing period, the young wine is fined, i.e. it is freed from undesirable substances. These are primarily trub substances (yeast residues, etc.) as well as compounds that can lead to turbidity or impair the taste after bottling. These suspended solids are bound with the help of additives that attract them due to an opposing electrical charge. These additives range from animal protein(hen's egg white, casein, gelatine, fish bladder) to chemical fining agents such as bentonite, silica sol, kaolin clay or activated carbon as well as copper sulphate and blood lye salt.
Wines that are particularly rich in acidity can be deacidified can be deacidified by adding calcium or potassium carbonate, which causes tartaric acid to precipitate as salt crystals (tartrate), or by passing them through malolactic fermentation. The crystalline salts of tartaric acid are also colloquially known as tartar. They are odourless and tasteless and completely harmless to health, but in order to precipitate them for aesthetic reasons before bottling, the wine can also be cooled for several days to around freezing point; this process is called cold stabilisation. The addition of metatartaric acid (produced by esterification of tartaric acid) can prevent the formation of tartar for several months after bottling.
The bound suspended matter, precipitated crystals and residues of the fining agents added are removed after completion of the measures by Racking and then filtering and/or centrifuging the wine.
After fining and stabilisation, the young wine is usually sulphurised again to kill microorganisms and prevent oxidation.
To make the wine more durable, ascorbic acid (vitamin C) can be added before bottling. To give young, fruity white wines that are intended for quick consumption an extra freshness, carbonic acid can be added artificially before bottling.
Blending or cuveting takes place in the vast majority of wines. It is not only a matter of combining different grape varieties, but also, in the case of single-varietal wines, of bringing together different origins, vineyards, tanks and/or barrels. As a rule, it is not the grapes or the must that are blended, but only the fully matured young wines. After the assemblage (another synonym for blending), the finished wine finally matures for a certain time in the cellar to harmonise.
Incidentally, in Germany and many other countries, even if a grape variety, origin or vintage is specifically stated on the label, up to 15 per cent may contain wines from other grape varieties, sites or vintages without this having to be indicated.
After a final filtration and the final sulphurisation - which are not necessary if the wine is ready for bottling directly after stabilisation, which includes both measures - the wine is bottled. Sterility is particularly important here to prevent impurities that can lead to secondary fermentation in the bottle, for example.
For this reason, bottling is usually done under protective gas(nitrogen or carbon dioxide). Sometimes, hot bottling is also carried out, in which the wine is heated to 55°C during filling in order to kill germs. However, this process can affect the aroma.