Kosher wine can only be made with kosher ingredients. According to traditional Jewish law, once the grapes are picked and brought to be crushed, only Shabbat-observant Jews can be involved in making the wine. From crushing to bottling, kosher wine must be handled exclusively by observant Jews.
Why, do you ask?
In the past wine was often used by pagans in their offerings to idol gods. When something good happened, wine would be poured on the ground as a symbolic thank you (if you were an idol worshipper, that is).
As idolatry is prohibited for observers of the Jewish religion, Rabbis set up the rules for kosher wine to ensure that Jews never had a glass of wine that had been associated with an idolatrous (i-dol-a-tres) offering, so they required that only Jews be involved in handling kosher wine.
Even after these rules were set up, some people worried that if you had a nice glass of kosher Chardonnay at a Jewish wedding, it’s possible that the non-Jewish waiter or waitress might have spilled some of your Chardonnay in an idolatrous practice, while their back was turned.
Rabbis ruled in order to avoid the possibility of a Jew ever drinking wine that was idolatry-associated, only cooked wine could be served to a Jew by a non-Jew. Mevushal (literally “cooked”) wine has been heated to the point that idol worshippers wouldn’t use it for their nefarious purposes – because boiling wine removes much of the flavor. Today- mevushal no longer entails actually boiling anything. Today, in order for wine to be called Mevushal, the wine is heated up very quickly in a process called flash pasteurization.