One would expect ice wine to have a long history in winemaking, but it does not. In fact, ice wine has only been around for about 200 years. The prevailing theory of its “birth” centers on a German winemaker who was surprised by an early frost. He decided to press the frozen grapes anyway, but separated them from the rest of his vintage so as to avoid ruining everything. To his surprise, the resulting wine was pure and sweet.
Ever since, ice wine has been produced to some degree in all wine producing countries of the Northern hemisphere, including Austria, Germany, Hungary, Switzerland and others.
The difference between ice wine and other dessert wines is that ice wines show a much clearer fruit and varietal character. This is because other sweet wines are made from botrytis (“noble rot”) affected grapes, or with grapes that have been laid out and dried. Because ice wine grapes are healthy at harvest, a good amount of acidity remains, which gives the wine a raciness that other dessert wines generally have to a far lesser degree.
True ice wine is that made from grapes that are kept on the vine until the temperature sinks below -19.4 Fahrenheit or -7 degree Celsius. In the U.S., New Zealand and some other regions, winemakers have started producing simulated ice wine by tossing the grapes into a commercial freezer. These simulated ice wines are considered to be of lesser quality and will typically sell at half the price of ice wine made in the traditional way.
Both methods do employ the same basic idea: a grape is made up mostly of water, and since only the water will freeze at these low temperatures, the sweet grape nectar can be pressed from the grapes while the frozen water remains trapped in the skins.
It is essential to harvest on the first freezing night of the year, because grapes left on the vine to go through a freeze-thaw-refreeze cycle can pick up unwanted flavors. Winemakers are often nervous wrecks by harvest time, as they will have spent night after night waking up repeatedly to check the temperature.
“Keeping the harvest workers ready can be a tedious and embarrassing task” says Franz Heiss, head winemaker of the respected Heiss Winery in Austria. He will often have to wake up a dozen harvest workers for the fifth or sixth time, only to see another night pass without the desired temperatures quite being reached.