“The beginning of your grain, your wine and your oil… you shall give to him!” (Devarim 18:4)
It is a positive commandment to separate Terumah from grain, wine and oil, and give it to the Priest. This is called Terumah Gedolah. About this it says “The beginning of your grain, your wine and your oil… you shall give to him!”. According to biblical law there is no fixed amount for this separation, and even one grain of wheat fulfils the obligation for a large quantity, but Chazal said that one must separate more than this, and that a regular person (who is neither stingy nor generous) should give one fiftieth of his produce.
Amongst the roots of this commandment are since that grain, wine and oil constitute the main sustenance of a person, and the entire world belongs to Hashem, it follows that it is correct for a person to remember his Creator for the blessing that He gave him, and that therefore he should separate some of it for Hashem, and give it to His servants who are constantly occupied with the service of Hashem, and it should be given before any person’s hand has touched it or benefited from it. It is for this reason that Chazal said that “(biblically) even one grain of wheat fulfils the requirement of an entire stack!”, as when the focus is on remembering the mastership of Hashem over the article there is no difference between a lot and a little. However, Chazal added a fitting fixed amount in order to further arouse the hearts of the people about the matter, as since man is not spiritual he will not consider a small amount as much as a big one.
Amongst the law of this commandment are that even though the main biblical obligation is on grain, wine and oil, rabinically the obligation applies to anything which is edible for man, is guarded (cultivated), and grows from the ground. Some say that there is a biblical obligation even on other fruit.
This commandment applies biblically only in Eretz Yisrael when Klal Yisrael are living there. One who does not separate Terumah from wine, grain or oil in Eretz Yisrael when Klal Yisrael are living there, has transgressed this commandment, and receives a great punishment.