Avodah Zarah 55 - When do Grapes turn into Wine
Wine used in idol worship - wine of libations - is prohibited by the Torah. The Sages have additionally decreed that ordinary wine of idolaters is also forbidden to Jews for all benefit. Furthermore, they decreed that even a Jew's wine becomes prohibited if an idolater touches it.
Grape juice is in the same category as wine. It does not need to ferment in order to attain the "wine" classification, rather, it needs to have been processed to a point where it can be considered a "finished" product.
One may buy the contents of a trodden winepress from an idol worshipper, even though the idol worshipper takes crushed grapes from the juice with his hand an places then on the mound. That is the because the liquid is not considered "prohibited wine" until it descends into the cistern. One may work together with the idolater treading the grapes, and it is not considered deriving benefit from forbidden one. However, because it was not allowed to increase ritual impurity in Israel, one was not allowed to harvest the grapes together with an idolater.
Art: Pierre Auguste Renoir - Grape Harvesters