הלכה על חלה 4:16

From the laws of the commandment is that which they, may their memory be blessed, said (Mishnah Challah 3:1) that the obligation begins not from the time that the dough is placed into the trough, but from the time of rolling; that is to say that the obligation begins from the time that the water and flour are mixed. And the five types of grain that are obligated in challah are wheat, barley, spelt, oats, and rye; as it is stated (Numbers 15:19), "And it shall be when you eat from the bread of the land" - and only loaves made from these are called bread. And they all combine to [constitute] the measurement [of what requires the separation] of challah. And if one did not separate challah from the dough, he may separate it from the bread; as it says, "when you eat from the bread," which teaches that bread is also obligated. Even if one first kneaded a quantity that was insufficient [to be obligated] in challah, if he then kneaded another batch and mixed the whole loaf into one vessel that has an interior, such that the two combine to the requisite measurement, the vessel combines them [to be obligated in] challah. I heard from my teacher, may God protect him, that this is only if the bread was taken from the oven and put into a vessel that has an interior, but if it was placed on a board or on the floor, or anything else that has no interior, even though they were subsequently placed into a basket, they are already exempt from challah. This is what they meant when they said, "one who removes it into a basket;" it is only if it went from the oven to the basket at the time of [their] removal. And there is no need to say that the oven does not serve to combine [to obligate in] challah (Mishneh Torah, Laws of First Fruits and other Gifts to Priests Outside the Sanctuary 6:16). The bran that is in the dough is counted toward the requisite measurement before it is sifted; but if it is mixed back in after it is sifted, it does not count towards the requisite measurement (see Mishnah Challah 4:6).

From the laws of the commandment is, for example, that which appears in the Gemara (Rosh Hashanah 12a, and see Tosafot, s.v. tanna derabbanan) that the central obligation from Torah writ - and so [too,] of tithes - is from grain, wine and oil alone, since they are the main nourishment of man. But rabbinically, one is obligated also [on] anything that is human food, that is guarded and the growth of which is from the ground. And even though we found that in the Sifrei, they based the thing on a verse, it was only a memory device (asmakhta) - as I have written in the Order of Vayikach Korach (Sefer HaChinukh 295) with the commandment of the tithe. As there they said, "'The first of your grain': just like grain, wine and oil are human food, its growth is from the earth and it has owners - as it is stated, 'your grain' - so too, all that is similar to them is obligated in priestly tithes and tithes." And they, may their memory be blessed, said (Talmud Yerushalmi Challah 4:4 and see Mishneh Torah, Laws of Heave Offerings 2:2, 5) that even though vetch is not human food, it is obligated in the priestly tithe and the tithe, since we eat them in years of famine. And rubia (some have the text, tiah), hyssop and thyme that are planted from the outset for man are obligated in the tithe; and so [too,] all that are similar to them. [If] he planted them for animals - even though he reconsidered and designated them for people while they were attached [to the ground] - they are exempt, since the designation of a man while they are attached is nothing. [If] they grew on their own in a courtyard: if it is a courtyard that protects its fruit, behold, they are obligated - as [when] undifferentiated, they are for people; but if it does not protect, they are exempt. Garden seeds - such as turnip seeds, radish seeds, onion seeds and all that are similar to them - are exempt from the priestly tithe and the tithes, since they are not human food. But nigella is obligated in the priestly tithe and tithes. The seed (berries of) clover, of mustard, of white beans, of capers and of caper buds are exempt, because they are not fruits. About what are these words speaking? When he planted them for the seed. But when he planted them for the vegetable, behold, they are obligated, since they are fruits. And so [too,] the berries of capers are obligated, since they are [its] fruit. Coriander that was planted for the seed is exempt from the priestly tithe and the tithes. If they were planted for the vegetable, one must separate the priestly tithe and the tithe from the vegetable and the seed. And so, [too,] shevet: if he planted it for the seed, it is exempt; if he planted it for the vegetable, it is tithed - [both] the seed and the vegetable. As if he took the vegetable to eat, he separates the priestly tithe and the tithes from it, and then he eats; and when it dries and he gathers its seed, he separates its seed.