Charles Ellicott Commentary Numbers 18:12

"All the best of the oil, and all the best of the vintage, and of the grain, the first-fruits of them which they give unto Jehovah, to thee have I given them." — Numbers 18:12 (ASV)

All the best ... — Hebrew, all the fat, as in Genesis 45:18.

The firstfruits of them ... — The quantity is not defined. St. Jerome, in his Commentary on Ezekiel 45:0, mentions the fortieth part as a liberal contribution, and the sixtieth as a scanty one, according to Jewish tradition.