פירוש על עבודה זרה 109:11

A Jew may buy a winepress and all of the grapes that are being trodden in it from a non-Jew, even though the non-Jew has lifted up the trodden grapes and moved them into a different heap. At this point in their processing the grapes are not yet considered wine, and therefore their being handled by a non-Jew does not make them “yen nesek”, which is forbidden to Jews. In other words, while we might suspect that non-Jews offer up libations with wine, they will not do so with grapes that are not yet fully pressed, nor will they do so with grape juice. Only when the juice has descended into the vat where it will ferment into wine is it considered yen nesek.
When the wine does descend from the winepress into the vat, the wine which is in the vat is prohibited. However, that which remains above in the winepress is still permitted, even though it has been handled by a non-Jew.