Let’s talk about naturally sweet wines, about the method of their production, about importance of a long, warm sunny fall, and about useful mold.
Naturally sweet wines are wines that are made by natural fermentation, without fortification.
Broadly speaking, the process of conversion of grape juice into wine can be described as transformation of sugar from juice into alcohol by yeast. For dry wine, yeast simply convert all of the sugar into alcohol and die from hunger afterwards. The amount of alcohol depends on the amount of sugar. There is no sugar left into the wine.
How to make wine with presence of both components, alcohol and sugar?
There are several different approaches to make sweet wine.
1. Stop fermentation before all of the sugar is eaten. It is common knowledge that fermentation can be stopped by alcohol supplementation. Although yeast produces alcohol themselves, a large concentration of alcohol is toxic for them. Most strains are die at 15% abv, although there are a few strains with higher tolerance to alcohol. This is the standard approach for fortified wine production. However, since our subject today is naturally sweet wine, this is not the approach we take.
Fermentation can also be stopped with sulfur oxide supplementation (a very common ingredient in winemaking process) or by strong cooling. Both kill yeast. After either treatment, the wine is filtered to clarify from yeast remains. Wine produced in this way has a low alcohol content (5-8% abv) because fermentation was stopped in the middle of the process. Wines made this way would usually be semi-dry or semi-sweet. For example, Asti is a wine made using this way.
2. Another approach is to add sugar or sterile grape juice to dry wine prepared the standard way. Among fine wines, this method is used for production of sweet sparkling wine as a traditional method. However among still wines it is mainly used for non-expensive semi-dry and semi-sweet wines.
3. Finally, the best naturally sweet wines are made from grape with such a high level of sugar that fermentation stops itself at a point while the sugar is still present. Sweet and liquorish still wines are made using this approach. There are four most popular methods to get grape with high enough sugar levels required for it to work. All of these are based on removal of water from the berry, but each method gives the wine a unique additional flavor. Here they are.
–Freezing grape on the vine. Berries are kept on vine until the first frosts. Crystals of water are removed during grape crushing. This is the only way that keeps the wine aroma unchanged, and only fortifies it. Icewine in Canada and Eiswein in Germany are made by this way.
– Drying grape on the vine. Ripe grape are kept on the vine for some time until they lose some water. A warm and sunny fall is especially important for this method, otherwise the berry would be damaged by rot. This way brings an aroma of raisin and dried fruits into the wine. French Sauternes, American Late Harvest wines, German Spalese and Auslese wines are made by this way.
–Drying grape after harvest. Collected whole bunches of grape are partly dried in warm and dry places. Taste modification is similar to the previous method of drying grape on the vine. Grapes for sweet PX Jeres and Italian passito wines are prepared in this way.
–For the best wine, Noble Rot helps.
Actually, it is a mold called Botrytis cinera, which lives on the grape berries. Although it is a single species, winemakers have two names for it. In cool rainy weather, Botrytis can start to grow on berries before they ripen and can damage the harvest. In this case it is called Grey Rot. Upon other conditions, it can become a great helper, and its name will be Noble Rot. When the mold starts to grow on already ripened grape, it prepares the base for the best sweet wines. A warm, sunny autumn with a cool foggy morning are the best conditions for this. The fungi filament makes little holes in the berry skin and water evaporates through them increasing the concentration of sugar and other substances. It also brings an additional aroma of honey and tropical dried fruits to the wine. The best French Sauternes, Hungarian Tokay, German and Austrian Beerenauslese and Trockenberenauslese wines are made from Noble Rot grape.
Perhaps you already noticed that very sweet wines can have low alcohol content, as low as 7% abv. The higher the sugar level becomes, the lower the alcohol level drops. I am talking now about naturally very sweet wines. Although sugar is the favorite food for yeast, excess of sugar inhibits their function – they end up producing some amount of alcohol and then stop working. Sometimes too much of a good thing is not a good thing at all, even for a unicellular organism.