Late harvest and dessert wines: how the sweetness is kept

Where the sweetness comes from

In a dry wine, yeast converts almost all the grape sugar into alcohol and carbon dioxide. A sweet wine keeps some of that sugar. Good dessert wines do not get their sweetness from added sugar. It comes from grapes that were unusually concentrated to begin with.

Sweetness also needs a counterweight. A wine with 120 g/L of sugar and too little acid tastes syrupy after one sip. So two things are needed: grapes with very high sugar and a way to stop fermentation before it is used up. Our guide to sweetness, acidity and tannin in wine explains why the best sweet wines also need high acidity to stay fresh rather than cloying.

Four ways to concentrate the grape

• Late harvest. Grapes are left on the vine well beyond normal picking. They lose water, so sugar and flavour concentrate. Riesling, Chenin Blanc and Gewürztraminer are common choices.
• Noble rot. In certain misty autumn conditions, a fungus called Botrytis cinerea pierces grape skins. Water evaporates, sugar and acid concentrate and the wine gains honey, apricot and marmalade notes. Sauternes in Bordeaux and Tokaji in Hungary are the classic examples.
• Ice wine. Grapes freeze on the vine in winter and are pressed while frozen. Most of the water stays behind as ice, so only a small amount of very sweet juice runs out. Canada and Germany, where it is called Eiswein, are famous for it.
• Dried grapes. Picked grapes are dried on mats or racks for weeks or months before pressing. Italy's passito wines, such as Vin Santo and Recioto della Valpolicella, are made this way.

How fermentation is stopped

Very sweet juice often stops fermenting on its own. High sugar and rising alcohol together stress the yeast until it gives up, leaving sugar behind. Many noble rot and ice wines finish this way at fairly modest alcohol.

Winemakers can also stop fermentation deliberately:

• Chilling. Cooling the wine sharply makes yeast inactive.
• Sulphur dioxide. A measured addition stops yeast activity. Sweet wines usually carry more SO2 than dry wines, because sugar left in wine is food for any surviving microbe.
• Filtration. Fine filtration removes yeast so the wine cannot restart in the bottle.
• Fortification. Adding grape spirit raises the alcohol above what yeast can survive. This is how port is made. Our guide to how fortified wines are made covers it in detail.

If a sweet wine is not properly stabilised, yeast can restart in the bottle. The result is fizz, haze and sometimes a pushed-out cork.

Sweet wines from warm countries

Noble rot and ice wine need specific cool climates. Warm regions, India among them, rarely get the misty autumns or hard frosts these styles need. Here winemakers use late picking, drying or fortification instead.

Fortified styles suit hot countries well, because the added spirit protects the wine and the style tolerates warmth better than a delicate dry white. Indian wineries make late harvest wines, often from Chenin Blanc. The challenge is the short ripening season. Temperatures rise fast in March, so grapes left on the vine can lose acidity quickly. Picking at the right moment, with enough sugar and enough acid, decides whether the wine tastes lively or flat.

Serving dessert wines

• Temperature. Serve well chilled, around 8 to 12 °C. Warm sweet wine tastes heavy.
• Glass size. Pour small measures, around 60 to 75 ml, into a small tulip glass.
• Keeping. Thanks to sugar and SO2, opened sweet wines usually last longer than dry ones in the fridge.

The usual rule is that the wine should be at least as sweet as the dish, or the wine tastes thin and sour. With Indian desserts, try a late harvest Chenin Blanc with kheer or rasmalai. Mishti doi or a light mango dessert also works. Very rich, syrupy sweets such as gulab jamun can overpower lighter dessert wines. A fortified wine or a noble rot wine stands up to them better. These are suggestions, so try your own pairings.

Dessert wines are a good example of how much control a winemaker has over style. Small decisions about picking dates, fermentation and stabilisation produce very different wines from the same grape.