Oktzim, Chapter Three, Mishnah Five
1) Costus, amomum, principal spices, [roots of] crowfoot, asafoetida, pepper and lozenges made of saffron may be bought with tithe money, but they do not convey food uncleanness, the words of Rabbi Akiva.
2) Rabbi Yohanan ben Nuri to him: if they may be bought with [second] tithe money, then why should they not impart food uncleanness? And if they do not impart food uncleanness, then they should also not be bought with [second] tithe money?
Section one: According to Rabbi Akiva these spices and other plant derivatives (some of them are also mentioned in Tevul Yom 1:5) are considered food in that one can use second tithe money to purchase them. As a reminder, second tithe money is supposed to be brought to Jerusalem and used to buy food there (see Deuteronomy 14:26). However, they do not convey food uncleanness, so in this aspect they are considered food. Evidently the reasoning is that these foods aren’t foods but they do improve the taste of other foods. Since they improve taste, one can use second tithe money to buy them. But since they are not eaten on their own, they cannot convey food impurity.
Section two: Rabbi Yohanan ben Nuri doesn’t like this inconsistency. In his opinion, a plant is either considered a food, in which case it can be bought with second tithe money and it would convey food uncleanness. Or it is not a food in which case it can’t be bought with second tithe money and it doesn’t convey uncleanness.