Demai 1:3-4

Demai 1:3

If a person buys produce from an unlearned person to be used for planting or as animal fodder, or flour to tan hides, or oil for fuel or as a lubricant, it is exempt from having tithes taken. Beyond K’ziv (i.e., outside of Israel proper), one need not take tithes from demai (because we do not assume that the produce may have come from Israel). The portion of dough called challah, a mixture of terumah that fell into regular produce, food that was purchased with the money of maaser sheini, and the portion left over from flour offerings – all of these things are exempt from being tithed in the case of demai. When it comes to perfumed oil of demai, Beis Shammai say it must be tithed and Beis Hillel say it need not.

Demai 1:4

Demai (doubtfully-tithed produce) is different from tevel (produce from which tithes certainly have not been taken) in the following ways: demai may used to make an eiruv (joining residents of a courtyard to carry on Shabbos) or a shittuf (joining the residents of several courtyards to their alley); one bentches over it and forms a zimmun; one may separate tithes from it while naked (because one would not recite a bracha on doing so) and at twilight; if one took maaser sheini (second tithe) from demai before taking maaser rishon (first tithe), it doesn’t matter. Demai oil that a weaver puts on his fingers must be tithed but not the oil put on the wool to comb it. (The latter is a lubricant but the weaver derives physical benefit from the former.)