For wine to be considered kosher, Sabbath-observant Jews must supervise and handle the entire winemaking process.
This includes crushing grapes to make wine, which has been a tradition for centuries in Jewish religious culture.
Wine or grape juice, must be certified kosher. Kashrut (Kosher) laws involving wine are concerned more with who handles the wine and what they use to make it.
Because of wine’s special role in many non-Jewish religions, and because wine was used in the sacred service in the Holy Temple in Jerusalem, the kashrut laws specify that wine cannot be considered kosher if it might have been used for idolatry.
When kosher wine is yayin mevushal (“cooked” or “boiled”), it becomes unfit for idolatrous use and will keep the status of kosher wine even if subsequently touched by an idolater.
All ingredients used in this process are also required by law to be certified as Kosher, including yeasts and fining agents that may have come into contact with alcohol during production processes.