What red wine is the strongest?

Alcohol is produced when yeast consume the fructose in the must and convert it to ethanol -- a form of alcohol -- and carbon dioxide. The process ends when either all the sugar is consumed or the alcohol concentration becomes so high that the yeast can no longer survive and stop producing. Hence, if you start out with must that's high in sugar and you use a variety of yeast that's very hardy and can survive in high alcohol concentrations, you can produce a high-alcohol wine. Zinfandel is a varietal capable of producing very high-alcohol wines. Some Zins have alcohol content as high as 15.5 percent.

Wines that have higher alcohol content than 16 percent are usually "fortified" -- not with vitamins but with additional alcohol produced by other means, usually distillation.

Port, Sherry, Madeira, and Marsala, to name the most common and famous types of fortified wines, usually have alcohol content between 16 and 21 percent.