Bottle fermentation
Bottle-fermented wines may also be clarified soon after fermentation. In the transfer process, the bottle-fermented wine is transferred, under pressure, to a second tank, from which it is filtered and bottled. In this case, as with tank-fermented wines, little aging of the wine takes place in contact with the yeast, and sulfur dioxide may be added. The transfer process is widely used in the United States, Germany, and elsewhere.
In contrast, in classic bottle fermentation, or méthode champenoise (“champagne method”), the wine remains in the bottle, in contact with the yeast, for one to three years. During this period of aging under pressure, a series of complex reactions occurs, involving compounds from autolyzed yeast and from the wine, resulting in a special flavour. Bottle-aged wine is rarely transferred, filtered, or rebottled because the addition of sulfur dioxide, required to prevent oxidation, would interfere with the delicate odour so carefully developed by aging. Aged bottle-fermented wines therefore are usually clarified in the bottle. In this process the bottles are placed neck down in special racks at a 45° angle. Each day the bottle is turned to the right and left, inducing the yeast debris within to move down the side of the bottle onto the cork. This process, riddling or remuage, may last from a few weeks to several months. When it is complete, all of the yeast is on the cork, and the bottle is gradually brought to an inverted position of 180°. Mechanical remuage in large containers is widely practiced.
In the traditional procedure, the cork is slowly pulled out, and the pressure within the bottle propels the sediment out of the bottle. In the modern procedure, to prevent undue pressure loss, the bottle temperature is lowered to 10 to 15 °C (50 to 59 °F). The neck of the bottle is placed in a freezing solution and frozen solid. When the crown cap, or cork, is removed and the yeast deposit is ejected, the process is called disgorging, or dégorgement. The bottle is quickly turned to an upright position. When performed properly, disgorging (which is usually mechanized), involves the loss of only 3 to 5 percent of the wine. The bottle is held under pressure while it is refilled.
The filling solution is a small amount of sweetening dosage, usually white wine containing 50 percent sugar. The amount added depends on the degree of sweetness the producer desires. Wines labeled brut, or sometimes nature (a term also applied to a still champagne), are extremely dry (very low in sugar content), usually containing 0 to 1.5 percent sugar; wines labeled extra dry or extra sec, or dry or sec, are sweeter, often containing 2 to 4 percent sugar; semi-dry or demi-sec wines may contain 5 percent or more sugar; and sweet or doux wines have about 8 percent sugar. In commercial practice, there is considerable variation in the exact degree of sweetness described by a specific term. If the dosage does not bring the contents to the desired level, more wine of a previously disgorged bottle is added. The closure, made of cork or plastic, is held in place with a wire netting.
If the wine has been aged for two or three years, the sugar in the final dosage does not ferment, as that in the original dosage did, because few viable yeast cells remain. Even in wines aged for shorter periods, skillful disgorging leaves few viable yeast cells on the sides of the neck of the bottle. Furthermore, the wine lacks oxygen to stimulate yeast growth and is lower in growth-promoting nitrogenous constituents and higher in alcohol than the original wine. The high carbon dioxide content also has a repressive effect on yeast growth. When bottle-fermented wines are fermented very rapidly and disgorged early, however, it is customary to add some sulfur dioxide with the final dosage to repress yeast growth.
In the United States, tank-fermented wines must be labeled “fermented in bulk” or “bulk-fermented.” Bottle-fermented wines may be labeled “bottle-fermented,” but only wines handled by the classic method may be labeled “fermented in this bottle.”
Carbonation
Carbonation is a less involved process but is used infrequently. Carbonated wines have many characteristics of fermented sparkling wines, and this simple physical process is much less expensive. The action of the second fermentation under pressure may produce especially desirable flavour by-products, however, and there is greater prestige value attached to fermented sparkling wines. In some cases, the wines used as a base for the carbonated sparkling wines may be overmature or otherwise inferior to those used for the fermented sparkling wines.
The base wine used for carbonation, like the base wine for fermented sparkling wines, must be well balanced, with no single varietal flavour predominating. Young fruity wines are preferred, and the wine should not contain any trace of off-odour. Since no secondary fermentation takes place, wines of 11.5 to 12.5 percent alcohol content are used. The wine should be tartrate-stable, metal-stable, and brilliant, and the sulfur-dioxide content should be low. For white wines, the colour should be a light yellow.
A variety of techniques have been used for carbonation. Production of carbonation by passing the wine from one bottle to another, under carbon-dioxide pressure, is now seldom employed because of its slowness. Carbonation has been produced in bottles after deaeration, and this technique could be adapted to multibottle operations. Direct carbonation is frequently practiced with cold wine in pressure tanks, and if the stream of gas is finely divided, good carbonation is obtained. Pinpoint carbonation, spraying the wine into a pressure chamber containing carbon dioxide, may also be employed. Following the carbonation procedure, the wine is bottled under pressure. A cork or plastic or crown-cap closure is applied, the label is affixed, and the wine is cased for distribution.
In many countries, there is a higher tax on fermentation-produced sparkling wines than on carbonated sparkling wines. The two types also have different labeling requirements, and the process of carbonation usually must be stated on the label.
There are a few low-level carbon dioxide wines on the market, produced either by fermentation or by carbonation. In Germany and other areas, tank-fermented wines, or “pearl” wines, of about one atmosphere pressure, are produced. In the United States, Portugal, and Switzerland, a number of wines are lightly carbonated at the time of bottling, adding piquancy.
There are a few wines in which the carbon dioxide comes not from alcoholic fermentation, but from malolactic fermentation of excess malic acid in the wine. The vinhos verdes wines of northern Portugal are examples of this type. This fermentation is sometimes responsible for undesirable gassiness in red wines.
Fortified wines
The addition of alcohol during or after alcoholic fermentation produces fortified wines of over 14 percent alcohol, generally called dessert wines in the United States. In most countries, these wines are taxed at higher rates than those of 14 percent or lower alcohol. Fortification has two purposes: (1) to raise the alcohol content sufficiently (usually 17 to 21 percent) to prevent fermentation of all of the sugar and (2) to produce types with a special alcohol character. The alcohol used for fortification is usually (legally required in most countries) distilled from wine. The distillation of the fortifying spirits is made to a high percent alcohol, usually 95 to 96 percent. Industrial alcohol has also been employed in a few countries.
The repressive effect of alcohol on alcoholic fermentation increases rapidly as the alcohol content is raised above 14 percent, particularly in the presence of sugar. To secure prompt cessation of fermentation, the added alcohol must be rapidly and uniformly mixed with the fermenting must, and this is accomplished by stirring or mixing with compressed air.
In the most simple type of fortification, the initial fermentation is allowed to proceed nearly to, or all the way to, completion. The resulting wine is usually subjected to a baking process, as in Madeiras and California sherries, lasting for one to four months, at 58 to 65 °C (136 to 149 °F). If the wine is low in sugar content, heating will change the flavour and colour of the wine only slightly; with greater sugar content, a more caramelized flavour, typical of sweet Madeiras and sweet California sherries, is produced.
When white must is fortified during fermentation, the resulting wine is sweet, the degree of sweetness depending on the original sugar content of the must and the time of fortification. Some types, fortified early, produce very sweet wines. Muscatels, produced in many countries, are often of this type.
Red sweet wines, such as port, are more difficult to produce. Although the grapes must be fermented on the skins to extract colour, the fermentation cannot be continued for long if the requisite sugar is to remain in the finished wine. One method of securing sufficient colour is to use grape varieties containing large amounts of pigments in their skins. The skins and juice are sometimes heated to about 65 °C (149 °F) to extract colour.
The flor sherries, such as the dry or fino-type sherry produced in Spain, are a special type of dessert wine. The base wine is fortified to about 15 percent alcohol, and a special alcohol-tolerant film yeast develops as a film on the wine surface. Acetaldehyde, an aldehyde, is one of the flavour products produced by this procedure. Following this process, the alcohol content may be further raised to 16–18 percent. By adjusting the oxygen content, the flor yeast may be induced to develop and produce acetaldehyde in a submerged culture, a process used commercially in California.
Marsala, a type of dessert wine produced in Sicily, has a dark amber colour and burnt sugar flavour, derived from the addition of grape juice that has been cooked and reduced to about one-third its original volume.
Dessert wines aged for only short periods lack the complex flavour of those dessert wines aged in small oak cooperage for at least two to four years. During aging, white wines gradually darken in colour, while red wines become less red and more amber. Flavour becomes more complex and mellow as wood flavour is extracted from the container, various substances in the wine become oxidized, and complex compounds of acids and alcohol are formed. If the wood containers are stored in warm, dry rooms, more water than alcohol is lost, and the alcohol content of the wine increases. This effect is common in dessert wines of the south of Spain. At lower storage temperatures and normal humidity, there is little change and sometimes even a slight decrease in alcohol content.
In the production of certain wines, special character is achieved by blending wines of different ages, a technique often used for port blends. By varying the proportion of the various wines, a range of types varying in colour and flavour may be produced. The blending may be performed continuously, as in the solera system common in Spain. This process involves a series of casks graduated according to the age of the wine each contains. One or more times each year, a portion of wine, usually 10 to 25 percent, is taken out of the oldest cask. This is replenished from the next oldest containers, and these in turn from younger containers. After a number of years, depending on the portion withdrawn each year and the number of years since the start, the average age of wine in the oldest container no longer changes. This process is called a fractional-blending system.