According to the first Mishna in the perek (34a), there is a disagreement about the definition of “products” with respect to the nezirut prohibition against eating or drinking grape products. The Tanna Kamma of the Mishna teaches that any part of the grape – including pits and skins – counts toward the amount that is required for the nazir to be held liable – one ke-zayit, an amount the size of an olive. There was an older tradition called the Mishna rishona that required drinking a revi’it in order to be held liable, although Rabbi Akiva insists that if a person dipped his bread in wine, then here, too, the amount was a ke-zayit.
In our Gemara, Rabbi Abbahu quotes Rabbi Yohanan as teaching that the rule of a ke-zayit of wine for a nazir is unique in the annals of halakha. Generally speaking, when we check to see whether there is a certain amount of issur (forbidden food) we judge this based solely on the size of the forbidden food itself. The fact that there is other, permitted food mixed with it is of no consequence. The rules of nezirut are the only place in the Torah where heter mitztaref le-issur – a mixture of permissible and forbidden foods will be seen as adding together to have the full amount required for punishment. Only in the case of nazir does a piece of bread the size of an olive absorb wine and create a full-size forbidden mixture.
According to our Gemara, the source for this rule is the word mishrat (Bamidbar 6:3) in the passage forbidding a nazir to eat any wine products. Mishrat means “soaked” and is understood to forbid something that has absorbed wine. Rabbi Avraham min haHar has a different reading in the Gemara, which derives it from the fact that the passage reads ve-khol mishrat anavim – and all liquor of grapes – which is an inclusive statement.