Passover begins on Monday, so lots of people are shopping for kosher wine to bring along to the traditional seder meal. I'd venture a bet that many of them, Jewish or not, probably have no idea exactly what makes kosher wines different from all other wines (insert jokes about the sticky sweetness of Kedem and Manischewitz here). There are just two routes to securing a hechsher (the official seal of approval from a religious authority): Having a 100% kosher winemaking operation, or boiling it. Boiling, you say? Here's how it all works.
The first way to make kosher wine - the way that has been going on for millennia - is to ensure that only Sabbath-observant Jews take part in the winemaking process from start to finish, that all equipment from vineyard to bottling is kept free of any non-kosher contaminants, and that anything used in the wine besides the grapes (including finings) is kosher. There are a few other rules about the way grapes are grown and the like, but you get the idea. In addition, for a wine to be kosher for Passover, it can't involve any additives or yeasts grown on grains, given the holiday's prohibition on eating such foods.
Now, an all Jewish staff who can follow all the rules isn't so tough to scare up in Israel, where many fine kosher wines are produced across several viticultural regions, but it can be elsewhere in the world. Besides, most producers of kosher wine worldwide make non-kosher wines as well, with the kosher representing only one part of their business. That's why many producers find it more practical to employ the second method of making wine kosher, which is called mevushal ("boiled") and is done with a religious authority on hand to supervise. That means literally bringing the wine to a boil (or at least 194°F according to modern custom), which makes the flavors and textures in the wine change pretty drastically...and not for the good. Cheap and easy, but you won't make a great wine this way.
Producers of high quality kosher wines that are made mevushal nowadays (for example, Napa Valley's Hagafen or Israel's Recanati) will eschew boiling and instead flash pasteurize, which means that they very quickly heat the wine and immediately cool it back down, which has far less impact on the wine than straight up boiling it does. Once this pasteurization process is complete, the wine is kosher and nothing in the remainder of the normal winemaking process will subsequently make it un-kosher. And to that we toast, L'chaim!
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