Demai, Chapter One, Mishnah Four
Because demai, produce which may not have been tithed, is only rabbinically prohibited, some of the rules that apply to tevel, untithed produce which is strictly prohibited by the Torah, do not apply to it.
1) Demai may be used to make an eruv, and to make an [alley] partnership, and they recite a blessing over it, and they make an invitation [to recite Birkat Hamazon] over it, and one may separate [tithes] from it even when one is naked, or when it is twilight [on the eve of Shabbat].
2) And if he took out second tithe from it before the first tithe, it doesnt matter.
3) The oil with which the weaver greases his fingers is liable to [the rules of] demai, but [the oil] which the wool-comber puts on the wool is exempt from [the rules of] demai.
Section one: This section lists things which distinguish demai from produce that has certainly not been tithed. Produce that is untithed cannot be eaten and therefore such produce cannot be used to make an eruv, which allows one to carry on Shabbat from a home to an adjacent courtyard. Neither can it be used to set up an alley partnership, a type of eruv that allows one to carry from courtyard to courtyard through an alley. These subjects were dealt with extensively in Tractate Eruvin. Demai, on the other hand, can be used for an eruv or an alley partnership.
One should not bless over produce that has not been tithed, just as one would not bless over non-kosher food. Neither would one recite an invitation to Birkat Hamazon (a zimmun) over untithed produce. In contrast, one blesses and recites an invitation to Birkat Hamazon over demai (see Berakhot 7:1).
When one separates tithes from produce, a blessing is recited. Hence, the person separating must be clothed. In contrast, no blessing is recited when separating tithes from demai, because it may not be necessary to recite the blessing. Reciting an unnecessary blessing is considered to be taking Gods name in vain and therefore in cases of doubt, no blessing is recited. Since no blessing is recited, the tithes from demai may be separated by a naked person.
Finally, it is forbidden to separate tithes on Shabbat because that is considered fixing, a deoraita (toraitic) prohibition. It is also prohibited to separate tithes from demai on Shabbat, because that is a doubtful toraitic prohibition, and in such cases the law is stringent. However, twilight is itself of doubtful statuswe dont know whether twilight is Shabbat or not. Since it is doubtful whether tithes needed to be separated and it is doubtful whether it is even Shabbat at all, the law is lenient (see Shabbat 2:7)
Section two: Normally, tithes must be separated in the correct order. However, when it comes to demai, this is not critical.
Section three: The final mishnah of the chapter is concerned with soaking or greasing something with oil. The rule is that if the oil is used on a persons skin, then tithes must be separated even from demai, because rubbing oil on ones skin (anointing) is considered like drinking. Therefore, the weaver who uses oil to grease his skin must first separate tithes. However, if the oil is used for vessels (see above mishnah three) then it is exempt. Therefore, demai-oil used by a comber on wool is exempt.