There are number of different wines which come under this category. Often very sweet, you don't really want to drink a lot of it at one time. For this reason you'll see dessert wines sold in the smaller 375ml bottles (as well as in larger bottles). At a recent picnic, the smaller bottle did quite well for eight of us.
While the classification of "dessert" wines can include any number of things, this is where we'll deal with those wines that are affected by "the rot." Not just any rot, however, but the "noble rot,"Botrytis cinerea, a mold which causes the vine disease called grey rot. Some years (but not all), when conditions are exactly right, with warm, sunny afternoons and damp, foggy mornings, the mold doesn't rot the fruit, but affects it in a different way. About 90% of the water in the grape disappears and the grapes shrivel up. Since relatively little of the sugar is lost, you get extremely concentrated and sweet grape juice. These grapes can be harvested and treated specially. Noble Botrytis adds a honeyed, aromatic flavor characteristic of its own to the wine. In the end, what you get is a sweet and, when lucky, an incredibly complex and flavorful liquid that, as it ages, turns from pale yellow to dark gold, maturing and concentrating the flavors.
The most famous of these wines is the French Sauternes, and the most famous French Sauternes is Chateau d'Yquem. It may take an entire vine to produce one glass of this precious liquid which is barrel aged for 3 1/2 years before bottling. But even then, it should not be drunk for at least 20 years! It merely gets better and better and could be drunk after 100 years. One can go on and on, gushing over this, but there is nothing quite like the myriad of intense flavors that come from an aged bottle of this rich, sweet, complicated wine.
Chateau d'Yquem is so good that it stands alone, classified "Grand Premier Cru" (first great growth). Other Sauternes will be classified "premier crus" (first growth) and "deuxiemes crus" (second growth). Sauternes are often comprised of 80% Semillon and 20% Sauvigon Blanc.
Since what is normally lousy weather contributes to the attack of Botrytis, harvesting grapes can continue past the normal end of season, perhaps into December. Many wineries will produce a "late harvest" wine in the manner of the French Sauternes. So while you will find Sauvignon Blanc and Semillon grapes in Sauternes, you can also find, for example, late harvest Riesling or Gewurztraminer. (I drank a late harvest Chardonnay once. Not great, but interesting. And a good way to get rid of essentially what was "rotten" grapes.) The U.S. wines I have seen do not age nearly as long as Sauternes, but will undergo maturation in the bottle for some time.
Other truly great (you decide if they are "better" than Sauternes) sweet dessert wines produced from late-harvest, Botrytis affected grapes include (but certainly aren't necessarily limited to):
German Beerenauslese (BA) and Trockenbeerenauslese (TBA) from the Mosel-Saar-Ruwer and Rheingau regions. They are made from nobly-rotted Riesling grapes. TBA is made from the most highly raisined grapes only and is outrageously sweet. Some say these are every bit as good as any Sauternes (including d'Yquem), and they are a lot rarer, since noble rot strikes Germany far less often than Sauternes.
Wines of the Valpolicella district in Italy. When fermenting raisined grapes fully dry, the result is the very rich-tasting, alcoholic, and long-lasting dry wine, Amarone. If they leave some residual sugar, the wine is called Recioto di Valpolicella.
As we will see, you don't necessarily need Botrytis to create a concentrated
wine. This can also be done by freezing the grapes or by letting them dry
in the sun to some extent. Such wines won't have the Botrytis flavor which
itself is a wonderful component of Botrytis affected wines--so long as
you don't take it to an extreme, for wines overly affected by Botrytis
can taste like shoe polish in early stages. It could take ten or twenty
years to get rid of this problem.
Another popular category of dessert wine is Eiswein (a.k.a. Icewine, although strictly speaking that is, I'm told, a trademark of the Vintners' Quality Association, Ontario, Canada). Eiswein is produced by leaving the grapes on the vine until start to become raisins and until they freeze (technically known as "cryoextraction"). Temperatures of -7C (20F) or below are required. The wine is then pressed, and the shards of (water) ice are removed. The combination of extremely overripe grapes with the concentration resulting from removing the excess water produces an extremely sweet, intense, luscious wine.
Eiswein was originally developed in Germany in the 18th century, and is now produced in several areas along the northern and southern fringes of the world's wine-producing areas, including northern Germany, the northern United States, and New Zealand. However, the biggest production now comes from Ontario, Canada, where Eiswein has become a dominant (and to some, overpriced) part of the wine industry.
In Germany and elsewhere, most Eiswein is made from Reisling, and a few other varieties. In Ontario, most is made from Vidal, a thick-skinned hybrid grape well-suited to the purpose. The result is a thick, fruity wine, with flavors ranging from apricot to fruit salad and tropical fruits. Ontario Eiswein is typically produced with juice at a level of 45 brix (as compared to 22 brix for a table wine). Often a "second pressing" of icewine grapes, with somewhat lower brix levels, is used to make a "Select Late Harvest" wine. The flavors of these "baby icewines" are similar to icewine, but with lower intensity and much lower prices.
Some attempts have been made, in areas not "blessed" with a cold winter, to produce Eiswein artificially, by putting grapes in a freezer. The results are typically described as "good but not great." One reason is that the grapes are usually not left to overripen as much as they are when the "natural" process is used. On the other hand, it is usually a lot cheaper. A particular example of this (so far as the technique, at least) would be "Vin de Glacier" from Bonny Doon Winery in California; literally "Refrigerator Wine" (from a winemaker with a sense of humor).
While an "ice wine" produces concentrated flavors, it does
not, of course, have any of the flavors due to Botrytis, so it certainly
is a different type of product.
There are other ways to get sweet wines:
Stop the fermentation process before the yeasts have consumed all the grape sugars and produced a dry wine. This can be done in at least two ways: