Requirements For Being Kosher
Because of wine's special role in many non-Jewish religions, the kashrut laws specify that wine cannot be considered kosher if it might have been used for idolatry. These laws include Yayin Nesekh (יין נסך) -wine that has been poured to an idol; Stam Yainom-wine that has been touched by someone who believes in idolatry or produced by non-Jews. When kosher wine is yayin mevushal ("יין מבושל" - "cooked" or "boiled"), it becomes unfit for idolatrous use and will keep the status of kosher wine even if subsequently touched by an idolater.
While none of the ingredients that makes up wine (alcohol, sugars, acidity and phenols) is considered non-kosher, the kashrut laws involving wine are concerned more with who handles the wine and what they use to make it. To be considered kosher, a Sabbath-observant Jew must be involved in the entire winemaking process from the harvesting of the grapes, through fermentation to bottling. Any ingredients used, including finings, must be kosher. This requirement can exclude certain fining agents, such as casein (which is derived from dairy products; this does not render it non-kosher but rather unfit for consumption with meat), gelatin (which is commonly, but not necessarily, derived from non-kosher animals; even if kosher, as a byproduct of animal slaughter, it renders the wine unfit for consumption with dairy products) and isinglass (which historically came exclusively from non-kosher fish, but currently can be produced from kosher fish as well). Egg whites can be used in the clarification of kosher wine but would not be appropriate for vegan kosher wine.
Wine that is described as "kosher for Passover" must have been kept free from contact with grain, bread and dough.
Read more about this topic: Kosher Wine