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First of all & to bear in mind at all times, the yeast added during the fermentation process of winemaking eats the natural sugars in the grape, to produce alcohol (& heat).
Sweet wine can be made in 3 ways: interrupting the fermentation process (filtering & fortifying), the adding of sweetness or the sugar levels within the grape itself (dried grapes, Noble Rot & freezing).
INTERRUPTING THE FERMENTATION PROCESS
The fermentation process, where the yeast eats the natural sugars to produce alcohol, can be interrupted by filtering out the yeast some time into the process. This will result in a wine with a certain level of sweetness.
Many of the off-dry & medium wines are made this way, like a Gewurztraminer or Pinot Gris from Alsace, France.
Another method to interrupt the fermentation process that will result in a sweet wine, is to add alcohol, so called fortifying. The high in alcohol % spirit will kill all the yeast, cutting off the fermentation process, leaving a sweet wine. Most yeast will die when the liquid reaches 13-15%.
Port & some Muscat wines from some Mediterranean countries are made this way. You can also find this method used in other parts of the world. The Victoria region in Australia even add a bit of oak & oxidization to the mix, adding coffee & dried fruit notes. General notes for the young wines: intensely perfumed, peach, floral, high in alcohol, low in acidity.
Adding an unfiltered grape juice is used to sweeten the wine after the fermentation process. In Germany this method is called Sussreserve [sweet reserve]. This grape juice can only be made from the same grape the base wine is from. The Sussreserve will balances out the acidity & lowers the alcohol of the base wine. Most German Qualitatswein like Kabinett, Spatlese & Auslese are made this way.
In Spain, the sweet Sherries are made this way. The traditional Sherries are dry, but demand for sweeter versions rose back then, with names like Pale Cream (citrus & almonds), Medium (nuts) & Cream (coffee & nuts). The sweetness is added before bottling.
SUGAR LEVELS WITHING THE GRAPE ITSELF
Like mentioned earlier, yeast dies when the alcohol level of the liquid reaches 15%. Most of the grapes out there don’t have the required sugar levels to rise above this, so other ways need to be found to create a sweeter style of wine.
This can be done by drying the grapes. Grapes can be harvested later (late-harvest) ór can be harvested & laid to rest in the sun. The warmth will let the grapes shrink & evaporate, concentrating the natural sugar levels from the grapes. After this process, the grapes will go through the normal way of wine making.
The Italian sweet wine Recioto & the known Amarone wines are made this way. Also, the Pedro Ximenez Sherry goes through this method. General notes: dried fruit, full bodied, syrupy, high in alcohol.
Another way of raising the sugar levels, is using the method of Noble Rot. A mold called Botrytis Cinerea, attacks perfectly healthy grapes & weakens them. It will make the grapes open-up, that will speed up the evaporating process. This will then make the grapes shrink, concentrating the sugar levels & acidity within the grape. A specific micro-climate to develop Noble Rot is required: perfectly grapes, misty & damp mornings to encourage Noble Rot & warm afternoons to speed up the evaporation. Note that not every year a climate like this can be guaranteed. Also, hand-picking these delicate grapes is essential, reflecting in the price.
Sauternes, Tokaji, Beerenauslese, Trockenbeerenauslese, Vouvray & some Australian ones are made this way. General notes: honey, dried fruits, orange marmalade, mushrooms.
The last way is to freeze the grapes. Again, this micro-climate can’t be guaranteed & yield is low. Canada, Germany & Austria are countries where Icewine/Eiswein can be found. Grapes are held in the vines where in the winter the grapes will freeze. They will then be hand-picked & crushed, leaving intensely sugar-concentrated ice crystals. These will then be melted & processed. General notes: pure, fruity, high acidity, full bodied, syrupy.