Friday, March 2, 2007
Sweet Wines (Part I)
How do sweet wines get their sweetness? Does a winemaker simply dump sugar into the vat and stir things up?
When it comes to cheap “soda pop” wines this unfortunately is how it’s often done, but in the case of premium wines, wines that come by their sweetness “naturally,” the story is more complex and interesting.
A “naturally” sweet wine is one that derives its sweetness solely from the sugar within the grapes from which it is made. Fermentation is the process in which yeast consumes sugar and produces both alcohol and carbon dioxide gas. If all the sugar is converted to alcohol, the wine is said to be a “dry” (non-sweet) wine. If for some reason a part of the sugar is not consumed by the yeast, then this leftover, or “residual,” sugar gives the wine a sweet taste. Because no sugar has been added, the wine is said to be “naturally” sweet.
But how do you stop the yeast from consuming all the available sugar? One way would be to raise or lower the temperature, but these methods have unacceptable side effects. If you get a fermenting wine hot enough to kill off the yeast the result will be a “cooked” or “burnt” taste, while cooling it does not kill the yeast but rather slows it down and eventually puts it to sleep. As soon as the temperature returns to normal the yeast awakens and goes back to work. The result will be either a dry wine or exploding bottles, or both.
Another way to destroy yeast is to inject sulfur dioxide gas into the fermenting juice. Unfortunately this is a very dicey operation, one that is very liable to leave you with a wine that smells and tastes of burnt match or rotten eggs. Although this would produce a naturally sweet wine, the results would be far from pleasant.
Fortunately there is another way to stop the yeast in its tracks, and that is to raise the alcohol level. Most strains of yeast will permanently call it quits once the alcohol level reaches 15 to 16 percent. So, how do you get to that level of alcohol? You can either add alcohol to the wine or else produce grapes with so much sugar the yeast quits working before all of it can be consumed. In this Sweet Wine (Part I) I will deal with the first case, while in Part II I will discuss the second.
Wines to which alcohol has been added are called “fortified” wines. The added alcohol is in the form of either neutral (tasteless) spirits or a brandy distilled from the same grapes from which the wine is made.
In the old days wines were fortified to help preserve them, although no one knew why it worked. We now know it works because a high alcohol environment retards the growth of the bacteria that turn wine into vinegar. However, in addition to preservation, a useful and very appealing secondary effect – the retention of unconverted sugar – was quickly noted by winemakers and wine drinkers. As wine makers soon discovered, the earlier the fermentation process was stopped, the sweeter the resulting wine.
The best known fortified wines today are Port and Sherry, but there is also Madeira, Marsala, and a bevy of delightful wines the French call vin doux naturel, almost all of which feature the Muscat grape. The alcohol level of fortified wines generally ranges from 17 to 21 percent, and this preserves them, at least to a certain extent. The result is they will “keep” longer, up to several weeks, once they are opened.
So, fortified wines are sweet because a sudden infusion of wine-based alcohol arrests the fermentation process before all the available grape sugar (a form of fructose) can be consumed by the yeast. The result is a high-alcohol wine that tastes good and keeps well. This makes them ideal candidates for use as an aperitif (dry Sherry is a well-known case) or to keep on hand if you want a glass of something sweet with cheese, dessert, or a handful of walnuts (Port works very well here). Left unopened, fortified wines can last for years and years. In the case of the vintage Ports, this can be well over a century.
As I mentioned earlier, another way to make a naturally sweet wine is to produce grapes that have an extraordinarily high level of sugar in them. This is not easy to accomplish. It’s an expensive and labor-intensive business, one that relies on good fortune as much as anything else. I’ll talk about this in Sweet Wine (Part II).