Maasrot, Chapter Four, Mishnah Four
Our mishnah deals with a person who wants to drink wine that he drew straight from the winepress.
1) One may drink [wine] out of the winepress, whether [it is mixed] with hot or cold water, and be exempt [from tithes], the words of Rabbi Meir.
2) Rabbi Eliezer bar Zadok says he is liable.
3) But the sages say: with hot water he is liable [to tithe] but with cold water, he is exempt.
Section one: In mishnaic times it was customary to mix wine with water before it was drunk. Rabbi Meir rules that mixing wine drawn straight from the winepress with water does not make it liable for tithes, whether it was mixed with hot or cold water.
Section two: Rabbi Eliezer bar Zadok holds that mixing wine does make it liable for tithes, even in a case where the wine was drawn straight from the winepress.
Section three: When he mixes the wine with warm water, he wont pour any left over wine back into the press because that might cause the cool wine in the press to be spoiled. Since he wont pour any back the sages consider its processing to have been completed and therefore, the wine is liable for tithes. If it was mixed with cold water he might put the left over wine back and hence he can drink it without tithing.