הלכה על בכורות 53:17

From the laws of the commandment is that which they, may their memory be blessed, said (Mishneh Torah, Laws of Heave Offerings 7:1) that an impure priest that ate pure priestly tithe is [punishable] with the death penalty and is lashed for it. But if he ate impure priestly tithe - even though it is [forbidden] with a negative commandment - he is not lashed, as it is not holy. And that which they said (Berakhot 2a) that impure ones eat priestly tithe with the sun setting and they see three medium stars in the firmament - and that time period is like a third of an hour after the setting of the sun. And that which they, may their memory be blessed, said about one who was eating priestly tithe and felt that his limbs shuddered to ejaculate semen, and that which they said about camel drivers. And that which they said (Bekhorot 27a) about the priestly tithe of outside the Land, that it is permitted to a priest that the impurity does not come out to him from his body - such as a boy minor who did not experience an emission or a girl minor who has not [yet] become menstruant. And that which they said (Chullin 130b) that all ignoramuses are assumed to be impure, and so we only give the priestly tithe to priests who know to keep it in purity. And I have also mentioned above (Sefer HaChinukh 159) some of the laws of the impurities in general and in particular.

The commandment is practiced by males and females in the Land of Israel by Torah writ, as it is stated, "when you eat from the bread of the land," but specifically at the time that all of [the people of] Israel are there, meaning to say their majority. [This is], as it is stated, "when you come" - and the explanation came upon this (Ketubot 20a) [to mean] all of you, and not when some of you come. On a rabbinic level [we are obligated] to separate challah [even] outside of Israel, so that the concept of challah not be forgotten by Israel (Mishneh Torah, Laws of First Fruits and other Gifts to Priests Outside the Sanctuary 5:7). And because the obligation is so that it not be forgotten by Israel, we are accustomed to be lenient with it, such that we only separate a kazayit from a large dough, and throw it into the fire. And it is not given to any priest - child or adult. And I have heard that in some places they are accustomed to separate large challah according to the measurement that the Sages gave us for it, and they give it to a minor priest boy, that impurity does not come to him out of his body or to a minor priest girl who has not yet menstruated. And they even give it to an adult priest who has immersed [to purify himself from genital flows]. And even though he is impure due to contact with the dead, they give him to eat it in these places. It also seems that another leniency outside of Israel is that one may deliberately cancel it out by a majority [of permissible matter], as it [is found] in Tractate Yevamot (it should read, Bekhorot 27a) and in other places in the Gemara. And as far as I know, the same does not apply to any other prohibition in the Torah, with the exception of [branches] that fell into the oven on a festival which can have other [prepared] wood added to it to cancel those, as they said (Beitzah 4b), "a person may add wood to them and permit them." And they said that the reason of the thing is because the prohibited matter burns constantly, meaning to say it is something consumed by the fire, and we can therefore be lenient.