פירוש על עבודה זרה 111:3

This is stricter than above. According to the earlier halakhah, it was forbidden to tread grapes with Jews who did not observe the rules of purity. According to the later halakhah, it was even forbidden to harvest grapes with them, for doing so was considered aiding a Jew in causing the defilement of the portion of terumah that would have to be separated from the grapes.

But in this matter the law became more lenient. No longer was it forbidden to cause ordinary, non-terumah, produce to become defiled. Therefore, one could glean grapes with non-Jews.

The baraita about tithes teaches that even when in the press the liquid can be considered wine once the waste (skins and seeds) has been skimmed off the top. This is even before the liquid flows down to the vat, and thus disagrees with our mishnah.
Rava answers that there is indeed a dispute between the mishnah and the baraita, and it reflects the dispute between the rabbis and R. Akiva. According to the rabbis, the grape juice is not subject to tithes until it descends into the vat. This accords with the mishnah from Avodah Zarah. In contrast, R. Akiva says that it is subject to tithes once it is has been skimmed, even though it is still in the press. This contradicts the mishnah from Avodah Zarah.

The baraita states that even though he has skimmed the waste off of the wine that it is in the vat, he may still drink untithed liquid that comes from the press, which is above the vat. Clearly this baraita considers the skimming that occurs in the vat to be the skimming that makes it liable for tithes and subject to yayin nesekh.

In this version of the baraita, R. Akiva clearly states that it is not considered wine until it has been removed from the vat and put into casks.

The Talmud now accepts this second baraita and reads the first baraita in its light. R. Akiva indeed holds that the liquid is not wine until it is put into the cask and there skimmed.

The Talmud now accepts this second baraita and reads the first baraita in its light. R. Akiva indeed holds that the liquid is not wine until it is put into the cask and there skimmed.

There are now three tannaitic opinions as to when it becomes wine: 1) Mishnah—when it goes into the vat; 2) Anonymous opinion in the baraita—when it goes into the vat and is skimmed; 3) R. Akiva—when it goes into the cask and is skimmed.

There are two resolutions to this difficulty. First of all, it may be that the entire comparison between tithes and yayin nesekh is unjustified. The rabbis were strict when it came to yayin nesekh—the liquid is subject to becoming yayin nesekh as soon as it goes down into the vat, even before it is skimmed. The dispute was only with regard to tithes.
But Rava did use a baraita about tithes in his understanding of the mishnah. Therefore, he must posit that there are indeed three opinions.