This post is about the much maligned sweet wines. Any kind of wine, red, white or pink; sparkling or still; natural or fortified, can be sweet. So what is a sweet wine anyway? All wine is made by fermenting sugars present in grape juice. When the process is finished there is always some sugar left that did not ferment. This is called residual sugar. In a dry wine there can be as little as one gram of residual sugar in each liter of wine (1g/l). When you get much over 40g/l you have what most people would consider a sweet wine. Some sweet wines can have over 750g/l.
If a wine maker wants to make a sweet wine she has a few ways to prevent the sugars from being fermented into alcohol.
If there is enough sugar in the unfermented grape juice (called must) you will still have enough sugar left in the wine when fermentation stops. This is because the yeast that does the fermentation can only live with a certain amount of alcohol. Different strains of yeast have different alcohol tolerances. If the wine maker is using yeast that has a lower tolerance (either because that is what naturally occurs or because they add it) fermentation will stop with sufficient residual sugar still present. If there is enough sugar in the must, it doesn’t matter what kind of yeast you have. Alcohol will stop the fermentation with a sufficient level of residual sugar. Wines made this way are sometimes referred to as “Natural”. This does not mean the winemaker is part of the natural wine trend (to be discussed later), though they can be.
Since alcohol stops fermentation, a second method is to add alcohol. Wines that have alcohol added are called “fortified”. Fortified wines that are frequently sweet include Port and Madeira.
A third method of stopping fermentation is to chill the wine. While yeast does not die at lower temperatures, it does become dormant. This is the way Asti (FKA Asti Spumante) is made. A problem with this method is if the wine warms up later the yeast can become active again. Unwanted fermentation in a bottle is a bad thing. Not only does the flavor change but bottles can literally explode. This is less of a problem in sparkling wines. The pressure from the dissolved carbon dioxide in sparkling wines tends to suppress fermentation.
Faced with the problem of reactivated fermentation, wineries can use a couple of tools. The first is to use very fine filters to remove the yeast. No yeast, no fermentation. A problem with this method is yeast isn’t the only thing removed. The filters have to be so fine to remove the yeast that they also remove other things that add flavors and body to the wine.
Another method is pasteurization. In pasteurization the wine is heated until it reaches a temperature that kills the yeast. Once again, no yeast means no fermentation. Heating wine, however radically changes its flavor. Some wine makers use a process called flash pasteurization. This process developed in the fruit juice industry (they too are interested in stopping fermentation) heats the wine very quickly and then cools it very quickly. The whole process can take under one minute. While it is generally agreed that flash pasteurization has less of an impact on the flavor of wine than the slower method, it is still controversial.