For the source text click/tap here: Pesachim 33
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Apparently, Rabbi Yohanan holds: The liquid is stored inside the grape, as the juice is not considered to be part of the grape itself but rather stored in the grape as though contained in a receptacle.
Rav Hisda believes that the juice is mivla beli’i – that the juice and the grape are a single entity, so that when the grape becomes tameh, the entire unit becomes defiled.
This disagreement about whether the juice contained within fruit is part of the fruit or a separate entity (incidentally, the same disagreement exists with regard to blood within the human body, under certain circumstances) is not so much a question of the physical reality of the situation as it is how we perceive the relationship. The question is whether the liquid that is pressed out of the fruit (or the body) was independent and is now simply being removed, or if squeezing it out affects a real change in the liquid.
This notion of the juice in the grape or the blood in the body opens up the legal definition of different forms of matter within one organism, and leads us to the chemistry of grape juice, grape skins and taste.