Beginning with the Mishna on daf 55a, Massekhet Avoda Zara begins to focus on the laws of yayin nesekh – wine forbidden to Jews because it has been sacrificed as a libation to pagan idols. Due to this concern, the Sages forbid all wine with which non-Jews come in contact. The Mishna teaches that as long as the grapes are still in the process of being squeezed in the wine press – even if there are non-Jews who are touching the grapes and placing them in the press to be tread upon – they will not be considered to have become yayin nesekh. The juice is only considered to have become wine when it flows out of the press and into the collection vats.
In the Gemara, Rav Huna argues that the ruling of the Mishna is not final, and that it is the teaching of Mishna rishona – “the first Mishna.” A later rendition of the Mishna prohibits use of the wine as soon as it is squeezed and has begun flowing, even if it remains in the press.
In the continuation of the Gemara on today’s daf Rav Huna is quoted as teaching that the juice in the press remains permitted only if the basket that strains out the pits and sediment is not returned to the press. If the straining basket is put back into the press, then all of the juice in the press will become forbidden.
Many of the commentators argue that in this ruling, Rav Huna is explaining the position of the Mishna rishona, according to which the juice in the press would ordinarily remain permitted. The Ramban, however, understands Rav Huna’s original ruling differently, and the juice in the press, which is not yet considered to be wine, will not become forbidden simply because it flows into the wine in the collection vat. Adding wine from the straining basket to the juice in the press will prohibit all of the juice.