Sweet wines are simply those that contain un-fermented sugar (either fructose, glucose, or sucrose).There are several ways to produce a sweet wine. The easiest (and least expensive) way is to simply add sugar (up to 2% to 4%) or sucrose syrup to a dry wine. This results in a wine that is definitely sweet, but not necessarily one that would be acceptable to many con-sumers.A similar and more common approach is to add unfermented juice, preferably from the same grapes used to make the wine. Alternatively, sweet wines can be made by stopping the fermentation before all of the glucose and fructose have been fermented. In some cases, sugar may be added to the juice prior to fermentation, such that when the fermentation is complete, residual sugar (and sweetness) remain. This is one of the more common practices in the United States, and many of the sweet wines from New York state are produced this way.
In either case, arresting the fermentation is the key step. Generally, this is done by rapidly cooling the wine, and then filtering out the yeast. One recent innovation is to encapsulate the yeasts within alginate beads, which are placed inside permeable bags. At the desired time, the bag (containing the yeast) is simply removed (see Chapter 3 for encapsulated cultures). Culture activity can also be stopped by adding alcohol spirits, as is the case for fortified wines (see below). Finally, sweet wines can be manufactured via one of several tradi tional techniques.These are based on concentrating the sugar in the juice or grapes. The juice, for example, can be partially concentrated by heating. In contrast, the grapes can be dried via atmospheric drying, or frozen so that the ice can be removed. However, the most well-known traditional technique for concentrating sugars in grapes is to dehydrate the grapes while they are still on the vine. These so-called botrytized wines are made throughout Europe, with the Sauternes, white wines from the Loire Valley of France, being the most widely prized.
The traditional process for making sweet botrytized wines starts in the vineyard. Grapes left on the vine past their optimum harvesting will invariably be infected by fungi, including the ubiquitous fungal organism Botrytis cinerea. Depending on climatic conditions, there can be two outcomes to this infection. If the climate is sunny and moderately dry during the day but humid during the evening, B. cinerea will grow to just the right extent on the surface of the grape, resulting in what is referred to as "noble" rot (Figure 10-6). If conditions are not "just right," overgrowth of B. cinerea or infection by other fungi can occur, resulting in simply rotten grapes that
have no value.Thus,the manufacture of these wines carries considerable risk.
Ultimately, noble rot growth of B. cinerea results in dehydration of the grapes and concentration of the sugars. Dehydration occurs due to secretion of pectinolytic enzymes by B. cinerea that degrade the pectin-containing cell walls of the grapes. Water evaporation then occurs, provided the atmosphere is sufficiently dry.The decrease in moisture is important not only because it concentrates the solids, but also because it controls growth of undesirable fungi. Eventually, the mold-covered grapes begin to take on the appearance of raisins (moldy ones at that). The moisture level of the grapes will be reduced by about 50% and, even though the mold metabolizes some of the sugars in the grapes, the sugar concentration in the juice is still increased by 20% to 40%.After the grapes are harvested and gently crushed, portions of the free run and the pressed juice are combined.The fermentation is then generally the same as for other white wines, with a temperature range of 18°C to 20°C.At the end of the fermentation, the ethanol concentration will range from 9% to 13%, and the wine will contain as much as 10% total unfermented sugar.
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