Production of Sweet Wine
A sweet wine is one that contains the residual to unfermented sugar. There are several ways by which sweetness in wines can be produced. They are controlling fermentation, Addition of Reserve juice, the use of late harvest grapes, resins, frozen grapes.
(The yeast is active till the alcoholic percentage of wine reaches 14 to 15 %. After this the alcohol itself kills the yeast stopping any further fermentation. Use of grape juices with higher concentration of sugar allow the fermentation to carry on till the maximum level of alcohol is achieved, still leaving behind unfermented sugar imparting sweetness to the wine.)
Controlled Fermentation: The most common way of slightly sweet wine is to halt fermentation before all the sugar present is converted into alcohol. Riesling, Gewurtzmariner, and chenin blanc of united states and Austria a good examples of such wines. Adding grape spirit before fermentation is over (fortification) also results to fortified sweet wines with high alcoholic percentage.
Reserve Juice: Used by some German wine makers, this process involves addition of un-fermented grape juice to the wine that has been fermented till dry. Though this is not considered a very scrupulous way of obtaining sweet wines.
Late Harvest: The harvesting is delayed taking it into October and November. This extended period concentrates the sugar in the grapes and help to develop a fruity flavor. This allows a great level of residula sugar in the resulting wine.
Rasins: The grape is dried in sun over straw mats. The water evaporates concentrating the sugar in the grapes. This process is used in Cyprus and Greece from centuries. The resulting wine is also very sweet with a rich fruit flavour.
Botrytis: Also known as ‘noble rot’, Botrytis is a fungus that breaks the skin of the grapes allowing the water to evaporate and concentrating the sugars. Wines made from botrytis-affected grapes are one of the best sweet wines in the world. The sweet wine from Sauterne region is the best example of such sweet wines.
Ice Wine: The grapes are frozen and then pressed. This produces only a small amount of juice highly concentrated with sugar as water is frozen. The wine produced from this juice is certainly sweet but not as sweet as the one that is produced from botrytis-affected grapes. This process is used United States, Canada, and other new world wine areas.