One set of laws that has been discussed here in Massekhet Pesahim addresses how coming into contact with tuma (ritual defilement) affects the status of people, objects or food, particularly with regard to the Temple and the Temple service.
On our daf , Rabbi Yohanan is quoted as teaching a surprising rule. According to him, if grapes have become tameh (ritually defiled) they can be pressed out in very small amounts – less than a beitza (the size of an egg) – and the juice will still be appropriate for use as part of the Temple sacrifices.
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. According to Rabbi Yohanan’s opinion, when do these liquids become ritually impure? This occurs only when one squeezes them, and prior to this the juice remains pure even if the grape was impure. And when one squeezes them, there is less than the minimum measure of grape flesh that would transfer ritual impurity, as food can impart ritual impurity only if it is at least an egg-bulk in size.
Upon hearing this ruling from Rav Aha bar Rav Avya, Rav Hisda responded “Who will listen to you and your teacher, Rabbi Yohanan? Those grapes were tameh – where did the tuma go to?!”
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.
Another way of looking at it is whether the tiny storage areas that hold the liquid within the fruit must be seen as an integral part of the fruit, or if they can be seen as separate compartments – questions that cannot be answered definitively through testing or similar methods.