Maasrot, Chapter One, Mishnah Seven
This mishnah deals with when wine and oil become liable for tithes.
1) Wine [is liable for tithes] after it has been skimmed [in the lower part of the winepress].
a) Even though it has been skimmed, he may take from the upper winepress, or from the duct, and drink [without taking out tithe].
2) Oil [is liable for tithes] after it has gone down into the trough.
a) But even after it has gone down into the trough he may still take oil from the pressing bale, or from the press beam, or from the boards between the press [without tithing,]
3) And he may put such oil on a cake, or large plate.
4) But he should not put the oil in a dish or stewpot, while they are boiling.
5) Rabbi Judah says: he may put it into anything except that which contains vinegar or brine.
Section one: The wine that is found in the lower part of the winepress must be tithed once the stuff (skins and seeds) has been skimmed off the top. However, even after the wine in the lower part of the winepress has been skimmed, he can still drink from the wine remaining in the upper parts of the winepress because its processing has not yet been completed.
Section two: Oil is liable for tithes once it has drained down into the trough, the lowest part of the olive press.
Section three: Similar to the case of wine, although some of the oil has drained off into the trough, he may still take oil from other parts of the olive press and use it without tithing.
Section four: There are certain rules as to how he can use oil that didnt need to be tithed. He can use this oil to put on cakes and he can put it directly on a plate and eat it without tithing it.
Section five: However, he cannot put it into a boiling dish because anything cooked cannot be eaten without being first tithed. Cooking, which by definition is a more formal way of eating, makes things liable for tithes, and therefore he cant put the oil into a boiling dish. It seems like the mishnah would allow him to put the oil into a dish that has already been cooked because in such a case the oil itself is not being cooked, rather it is just being consumed with cooked food.
Section six: Rabbi Judah holds that regular cooking, at least of oil, does not make the oil liable with tithes, and therefore he can put the oil into a boiling dish. The only thing he cannot put the oil in is a boiling dish that includes vinegar or brine, because by their sharpness these will hurry the cooking process.