Commentary for Avodah Zarah 110:9

A Jew is allowed to tread grapes in a winepress with a non-Jew, since the wine does not become yen nesekh until it goes down into the vat. However, a Jew may not pick grapes with the non-Jew for the non-Jew causes the grapes to become impure. When the non-Jew puts the grapes in his impure winepress, the grapes will become impure. If a Jew helps him to do so, the Jew is helping to make produce grown in the land of Israel impure. The reason that treading on the grapes is permitted is that as soon as the non-Jew touches them, they are already impure.

A Jew who works in a winepress while impure is committing a sin, for he is defiling the terumah and tithes and thereby rendering them inedible. Since this is forbidden, another Jew may not even tread in the winepress with him, because that would be aiding a transgressor. However, the other Jew may help this Jew before the process begins by bringing jugs to the winepress and he may help him remove the jugs when the pressing is over. In other words, it is only forbidden to help him while the impure pressing is going on. Before and after it is permitted.

The same rules that were stated above with regards to helping a winemaker who presses his wine while impure, are also true with regards to the impure baker. One may not help him in the baking process, for he causes impurity to the terumah and tithes. However, one may help him after the loaves are already baked.

According to R. Huna, as soon as the juice from the wine press begins to flow down into the vat, it is subject to the laws of yayin nesekh. What this would mean is that if a non-Jew touched the wine in the press as it started to flow down into the vat, all of the wine in both the press and vat would become prohibited. In the Middle Ages, non-Jews were almost always needed to press the grapes due to the large amount of wine they needed to produce and the small number of Jews available to do the work. How Jews worked around this prohibition was the subject of a fascinating study by Haym Soloveitchik.

According to the mishnah, the wine in the winepress does not become yayin nesekh, assumedly under any circumstance, even after it starts to flow. Therefore, one may purchase a winepress from a non-Jew even though he had put his hand into it.

The only way for the wine in the winepress to avoid becoming yayin nesekh (if touched by a non-Jew) is for the press to be stopped up such that the wine does not flow to the vat below. For once the wine flows, it can become yayin nesekh.

This is basically the same difficulty and resolution we read above.

Again, according to the mishnah, only the wine in the vat is yayin nesekh, not the juice in the press, even once it has begun to flow. This contradicts R. Huna.

Here R. Huna admits that his ruling does indeed contradict the mishnah, but defends the contradiction by claiming that he follows the later halakhah. The Talmud quotes a baraita only part of which deals with treading a winepress with a non-Jew. (The baraita includes a mnemonic here and above to facilitate remembering the order of the clauses). The first topic is causing impurity to ordinary produce (non-terumah). If a non-Jew touches grapes after they have been harvested, he causes them to become impure. Note that this has nothing to do with yayin nesekh. A Jew may eat impure food. But there was originally a custom to try to avoid causing produce in the land of Israel to become impure. This seems to have been an extra measure of holiness. Therefore, one should not glean grapes with a non-Jew, nor work with produce with an impure Israelite, for this would be assisting a transgressor.

But originally, there was no problem with treading grapes with a non-Jew in the winepress. Once the non-Jew had picked the grapes, they were already impure. By treading them, the Jew was not aiding in defiling them. Moreover, this halakhah follows the mishnah, according to which the grapes in the press cannot become yayin nesekh. This early halakhah does not follow R. Huna.

Later, the halakhah changed to follow R. Huna. Jews may not tread grapes with non-Jews for this would cause the juice in the vat to become yayin nesekh.