Does Numbers 18 Prove Tithing Isn't on Income?

After the discussion of tithes in Leviticus 27 in the context of redemption, tithing isn't discussed again until Numbers 18. Here, the main theme is that tithe was to be the wages and inheritance of the Levites.

Again, we find absolutely no instructions saying "you shall tithe on the following items." Rather, the only indication in this chapter that tithe was to be received in the form of food comes in verse 31:

Numbers 18:30-31
Therefore you shall say to them: ‘When you have lifted up the best of it, then the rest shall be accounted to the Levites as the produce of the threshing floor and as the produce of the winepress. You may eat it in any place, you and your households, for it is your reward for your work in the tabernacle of meeting.