Rashi on Numbers 18:30/רש"י על במדבר י״ח:ל׳

1בהרימכם את חלבו ממנו WHEN YE HEAVE THE BEST THEREOF FROM IT — i.e. after you have heaved the תרומת מעשר (the priest’s share in the tithe) from it.

2ונחשב THEN the remainder MAY BE ACCOUNTED by the Levites as profane (of a non-sacred character) in all respects.

3כתבואת גרן AS THE PRODUCE OF THRESHING FLOOR is to the Israelites. [This is not a mere repetition of verse 27 but is stated in order that] you should not say: Since Scripture terms it (the tithe) “Terumah”, — as it is said, (verse 24) “But the tithes of the children of Israel which they heave as a תרומה unto the Lord” — I may conclude that it is all entirely forbidden to the Levites (just as “Terumah” is forbidden to them); Scripture therefore states, “it shall be accounted unto the Levites as the produce of the threshing floor”. How is it with the remainder of the Israelite’s produce after he has set apart the “Terumah”? It is profane (non-holy)! So, too, is the corresponding portion of the Levite’s tithe profane (cf. Sifrei Bamidbar 122).