Commentary for Avodah Zarah 109:12

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.