R. Ala b. R. ‘Awia sat before R. Joshua and he sat and said in R. Johanan's name: If grapes are defiled, one may tread them out less than an egg in quantity at a time, and their wine is fit for libations (on the alter). This proves that he holds that the juice is indeed stored up;
The juice of the grape is contained by its skin, according to this view, and not part of the whole.
[consequently] when is it [the juice] defiled? When he expresses it; [but] when he expresses it, its standard quantity [for defiling] is absent. If so, [he can tread] as much as an egg too, for we learned: If a man unclean through a corpse squeezes out olives or grapes exactly as much as an egg in quantity, they are clean? . . .
Said R. Hisda to him, Who needs you and R. Johanan your teacher: whither then has their uncleanness gone? This proves that he holds that the juice is indeed absorbed, and since the [solid] eatable is defiled, the juice too is defiled. And do you not hold that the juice is stored up? he replied. Surely we learned: If he who is unclean through a corpse squeezes out olives and grapes exactly as much as an egg in quantity, they are clean.
More than this amount – an egg’s worth of liquid – is subject to being defiled.
To what is this like? To terumah of mulberries and grapes which were defiled, which is not permitted to him either for eating or for burning.’ — but surely it may be eaten too, for if he wishes, he can tread them out less than an egg at a time? — Said Raba: It is (not allowed as) a preventive measure, lest he come to a stumbling-block through them.
While one may find a technical way around a problem, it is still not permitted because of the possibility of coming to error accidently; i.e. eating the defiled grapes absentmindedly. They become like a stumbling block before the blind. (Lev. 19:14)