Avodah Zarah 55B

אבל לא בוצרין עמו. ישראל שהוא עושה בטומאה ⁠— לא דורכין ולא בוצרין עמו, אבל מוליכין עמו חביות לגת, ומביאין עמו מן הגת.

but one may not harvest grapes with him. After the harvest the grapes are brought to the winepress, which is ritually impure. This would render the grapes impure, and it is prohibited to render produce of Eretz Yisrael impure. In the case of a Jew who produces his wine in a state of ritual impurity, one may not tread on grapes nor harvest them with him, as it is prohibited to assist transgressors. But once he has produced his wine, one may take the jugs to the winepress with him, as the wine is already impure. And one may also bring the jugs from the winepress together with him.

נחתום שהוא עושה בטומאה ⁠— לא לשין ולא עורכין עמו, אבל מוליכין עמו פת לפלטר.

Similarly, in the case of a baker who makes his bread in a state of ritual impurity, one may not knead or arrange the dough in the shape of bread together with him, so as not to assist in a transgression. But one may take the bread with him to the bread merchant [lapalter].

גמ׳ אמר רב הונא: יין, כיון שהתחיל להמשך, עושה יין נסך. תנן: לוקחים גת בעוטה מן הנכרי, ואף על פי שנטל בידו ונתן לתפוח! אמר רב הונא: בגת פקוקה ומלאה.

Talmud: Rav Huna says: With regard to wine, once it begins to flow toward the exit from the winepress it can become wine used for a libation in idol worship if touched by a non-Jew. The Talmud asks: But we learned in the Mishnah: One may purchase from a non-Jew a winepress in which the grapes have been trodden on, despite the fact that the non-Jew took grapes in his hand and placed them on the pile of grapes that remained to be trodden on? Presumably the wine began to flow once the grapes were pressed, and yet the Mishnah states that the wine is not forbidden. Rav Huna said: The Mishnah is referring to a winepress that is stoppered and full, and therefore the wine does not flow.