Although the name is fairly self-explanatory, sometimes a lede helps.
A dessert wine is one that it sweet to the taste and often is quite high in alcohol. You might think it would go well with a dessert or pudding, but matching a wine to a dessert is far from easy. Often a dessert wine will go better with biscuits or, to be controversial for a moment, a strong cheese.
Dessert wines are usually made from white grapes, but have a deep yellow, gold or even brown colour in the bottle and the glass. Older, richer wines tend to have darker colours.
The major exception to the white-grapes only rule is muscat grapes, which often are white (or green) but can be black or somewhere in between. Small quantities of a red dessert wine are made in the italian Valpolicella region, using Corvina, Rondinella, Molinara grapes, but these wines are made in very limited quantities by specialised producers. Some Eiswein producers are starting to use tannin-free red varieties, such as cabernet franc.
Wine makers use three main tricks to get the sugar- and alcohol- content high enough to make a good dessert wine. The most obvious way is to add sugar to an ordinary wine before it has been fermented (called chaptalisation). Another option is to add brandy after partial fermentation, called fortification, which results in so-called fortified wines.
The next way is to make wine from grapes which are themselves both very ripe and extremely sweet. However, it is rare to get enough sweetness to drive the fermentation yet still retain the richness and sweetness demanded by a good dessert wine. Thus even sweet grapes are often helped along by varying degrees of either fortification or chaptalisation.
The third and final way is the most sought-after: employing some natural technique to extract moisture from the grape after it has ripened, leaving behind a shrivelled grape containing a tiny amount of liquid, but all the sugar and flavour built up during the ripening phase.
Such techniques include allowing the grape to dry out in a hot, dry environment, allowing ice to freeze the moisture out and allowing a fungus to make the skin porous, which permits the wind and sun to dry the grape out through evaporation.
Different styles of wine
Among the fortified sweet wines, sherry and Port are well-known, while proper, vintage Madeira wines, once the only wine to drink, are nowadays quite unusual. Sherry, Port and Madeira usually are not included in the dessert wines category, and are included in their own classifications, reflecting the vast range of styles and tastes available in these brands.
These well-known styles aside, the essential characteristic of a dessert wine is the balance between sweetness and acidity. A cheap, or poorly-made dessert wine is little more than a combination of wine with sugar and acid, and the key elements lie uneasily together. The key trick of the wine maker is to combine enough acidity with just the right amount of sweetness to deliver a harmonious whole.
Sweet and sour is one of the famous taste combinations. It is used extensively in Asian cookery to great effect. However, in that style of cookery, heat from chilli and other spices adds an extra dimension to the experience. In wine terms, the extra dimension is added through alcohol. Alcohol adds a small amount of flavour, but also body, weight and feel as the volatile spirit warms to body temperature and partially evaporates in the mouth and nasal cavities. Further dimensions are added through the flavours generated in specific grape varietals, and the terroir in which they are grown.
Thus the aim of the wine maker is to balance sweetness, acidity and alcohol content with the more subtle flavours of the specific grapes to create a satisfying experience for the drinker.
Unfortified dessert wines include Sauternes and Monbazillac from France, various categories of Tokaji from Hungary, and some of the (Trocken) Beerenausleses from Germany and Austria. For me, however, some of the most enjoyable were the raisin wines from the Italian island DOC of Pantelleria which lies between Sicily and Tunisia.
The sweetest wines from natural grapes are often from the Muscat family, which make moscatel (or muscatel, muscadel or other variations) wines. However, Ortega ( mostly in Canada) and Huxelrebe (usually from Germany) varieties are also used. These grapes naturally produce a great deal of sugar. Unfortunately, in devoting so much energy to the production of sugar, they do not develop much flavour, so the resulting wines tend to be sweet, but bland. Sometimes they are sweet enough to feed the yeasts during fermentation and still have enough sugar left over for the resulting wine to be sweet, but more commonly, the fermentation is stopped early, before the yeasts have converted too much of the sugar to alcohol. Traditionally this was done by adding cheap brandy to the fermentation vessels, but nowadays the added liquid is more likely to be almost pure ethanol.
Muscat de Beaumes-de-Venise is one of the best-known of these fortified sweet wines.
Having said all that, sweet muscatel wines are cheap and cheerful. Asti Spumante, the sweet fizzy wine of a million summer picnics is made from the muscat grape and in the right circumstances can be an ideal drink, though lacking in any real character.
At the top end of the scale, the world's most expensive sweet wines are made by allowing a particular type of fungus to grow on the grape skins. In thin-skinned grapes, this fungal growth quickly makes the skin porous. If the grape is then subjected to sunshine and a hot, dry wind, the moisture in the grape evaporates, leaving behind a sweet, flavoursome residue. In Hungary's Tokaji region, this residue is called Eszencia -- nectar.
Such fungal growth is an accident of nature. The fungus will only establish itself and thrive in warm, moist conditions, but the grapes will only dry out in hot, dry and breezy conditions. This combination of warm and moist in the night and early morning, but hot, breezy and dry in the afternoon and evening is only found in a few places in the world. The largest region in the world where such conditions exist is around Lake Balaton in Hungary and on the Austrian plain close to the Hungarian border. However, there is one small area in France where the conditions are right, and that area, Sauternes, has become synonymous with dessert wines.
There is much more to dessert wines than Sauternes, however. Hungarian viticulturists were making Tokaji wine long before anyone thought Sauternes might be a good place to make sweet wine. Louis XIV described Tokaji as, "the king of wines wine of kings."
In Germany, the wine authorities have named six categories of wine which carry the most prestigious QmP (Qualitätswein mit Prädikät) tag. From the least sweet upwards, these are Kabinett, Spätlese, Auslese, Beerenauslese, Trockenbeerenauslese and Eiswein.
Kabinett and Spätlese (late-harvested) styles can barely be called dessert wines. From Auslese (selection) upwards, however, these wines can become more and more sweet, though with good acidity to balance the sweetness. Beerenauslese means berry selection, while the Trockenbeerenauslese means literally dry berry selection. I guess the dry here means even more of the water has evaporated from the grape, because it is certainly not a dry style of wine. All of these are made in summer from grapes which ripened well and then shrivelled either under the influence of the sun alone, or following infection by the noble rot. The classification is made on the density (specific gravity) of the must prior to fermentation
Eiswein, however, is a completely different beast. Eiswein, or ice wine uses frost to extract water from the grapes. The grapes are left on the vine until the early frosts arrive (nets keep birds and other hungry wildlife away). Once the temperature drops to about -7°C, the grapes can be harvested. Often this is in the early hours of the morning, before sunrise. They need to be kept cold, as each grape now contains a liquid portion which retains all the sugar and flavour and a lump of pure ice, which contains the water. The skin is still intact. The aim then is to lightly crush the grapes, just enough to retain the sweet, flavoursome liquid, but discard the skin, stalk and the packet of ice, before the ice melts once more to dilute the wine.
Some cheaper ice-wines wines are made by harvesting and pressing the grape juice in the normal way, but then partially freezing the resulting juice, to remove a large percentage of the pure water.
In the middle of the range are the raisin wines. These are mostly Italian, but can still be very drinkable. Northern Italy produces a lot of these. The basic technique is to pick the grapes and then leave them to dry either in the sun, or in artificial ovens. This turns them into raisins. The final step is to squeeze the juice out and then ferment that to make wine.
General notes on making sweet wines
As can be seen from the above summary, making sweet wines is complicated and difficult. It is also much more sensitive to many more factors than making ordinary still wines. Because of this, the wine-maker needs to understand every aspect of the process as one single, integrated process, rather than look at ripening, harvesting, vinification and bottling as different and unrelated aspects of wine-making.
The manufacture of dessert wines is much more of a craft even than ordinary wine making.
A quick note on fermenting very sweet wine juice. It's slow. Yeasts don't like too much alcohol (it kills them) or too much sugar (it dehydrates them by osmosis). The top class grape juices referred to above start out with over 30 percent sugar in them. Eszencia from Tokaji has upwards of 50 percent sugar content. The yeasts can convert sugar to alcohol until the alcohol content reaches 14 to 15 percent, at which point the alcohol concentration kills the yeasts and stops further fermentation.
The most concentrated, sweetest juices, such as Eszencia will not ferment beyond about 6 percent alcohol, as the yeasts cannot work well in such sugary liquid.
Dessert wines: matching with food
Dessert wines tend to go better with desserts focussing on fruit and biscuits than with chocolate or cream. A good dessert wine will have a similar balance of sweetness and acid as ripe, fresh fruit. So if your dessert would be complemented by a fresh peach or a fresh strawberry, then it might work with a suitable wine. On the whole, chocolate is very difficult to match with wine. The sweet red valpolicella mentioned in the lede to this piece might stand up, but in general, it's best to avoid wine with chocolate.
Traditionally, the Italian cantucci biscuits are eaten when dipped in a glass of raisin wine, and this certainly is a delightful combination.
Personally I like a glass of sweet wine with some types of cheese. Strong Cheddar and some blue cheeses need a powerful wine to match them. Traditionally guide books suggest robust reds with strong cheese, but a sweet wine can also work well.
One of the simplest desserts is a Sauternes custard which uses a small amount of Sauternes in the recipe. Drink the rest of the bottle with the dessert and it is heavenly. Yes I've done this, and even though the dessert is creamy, the wine works well.
shaogo says: The Mendelsohn vineyard in Napa makes some great dessert wines; but my very favorite is the "Viognier Doux" by Bonny Doon in Napa.
Notes, and further reference
http://en.wikipedia.org/wiki/Muscat_(grape_and_wine)
http://www.bbr.com/region-3715-recioto-della-valpolicella
http://www.wineonline.ie/cgi-local/browse.pl?view_wine=917
http://en.wikipedia.org/wiki/Dessert_wine
http://www.nysaes.cornell.edu/fst/faculty/acree/fs430/notes_bell/05sweet.html
http://www.nysaes.cornell.edu/fst/faculty/acree/fs430/notes_bell/pdf/bellbot.pdf
http://www.cheflaszlo.com/tokajeszencia.html