Toharot, Chapter Ten, Mishnah Four
1) One who puts his grapes [into the wine-press] from the baskets or from what was spread out on the ground:
a) Bet Shammai says: he must put them in with clean hands, and if he puts them in with unclean hands he defiles them.
b) Bet Hillel says: he may put them in with unclean hands and then he may set aside his terumah in a condition of cleanness.
2) [If they are taken] from the grape-pot or from what was spread out on leaves, all agree that they must be put in with clean hands, and if they are put in with unclean hands they become unclean.
Section one: Someone wants to take grapes out of either a basket or from the ground and putting them into the wine press. According to Bet Shammai, he must first purify his hands, for if he touches the liquid that is exuded from the grapes with unclean hands, he will defile them.
Bet Hillel says that the liquid that comes out at this point does not render the grapes susceptible because this liquid is not something he wishes to preserve. The baskets and the ground will just let the liquid seep through. Therefore, the grapes are not susceptible to impurity. When he goes to separate terumah from the wine in the winepress, only then must be make sure he is pure.
Section two: Both houses agree that if he takes the grapes out of a special pot made to hold grapes or off of leaves on the ground, that he must be pure. In this case, the liquid is preserved (the leaves serve to prevent the liquid for being wasted into the ground), and therefore this is liquid that he wants. In this case, the liquid makes the grapes susceptible to impurity.