Maaser Sheni, Chapter Two, Mishnah Four
1) Vetches of second tithe may be eaten when still tender.
2) And they may be brought into Jerusalem and taken out again.
3) If they became unclean:
a) Rabbi Tarfon says: they must be divided among pieces of dough.
b) But the sages say: they may be redeemed.
4) [Vetches] of terumah:
a) Bet Shammai says: they must be soaked and rubbed in a state of purity, but may be given as food [to an animal] in a state of impurity.
b) Bet Hillel says: they must be soaked [only] in a state of purity, but they may be rubbed and given as food [to an animal] in a state of impurity.
c) Shammai says: they must be eaten dry [only].
d) Rabbi Akiba says: whatever is done with them may be done in a state of impurity.
Section one: Like the fenugreek we learned about in yesterdays mishnah, vetches are eaten when soft and young. When they grow they are no longer used for human consumption and are given to animals. So when they are young, one can use maaser sheni money to buy them, but not when they age.
Section two: Normally, once maaser sheni has been brought into Jerusalem, it cannot be taken out. Vetches are exceptional since when they age they become animal food.
Section three: Maaser sheni that has become impure can usually be redeemed for money, in the same way that pure maaser sheni can. This is the sages opinion. Rabbi Tarfon holds that maaser sheni vetches that have become impure can no longer be redeemed for money, because it would be disgraceful to redeem them and then feed them to animals. Therefore, what one can do is divide the vetches up and bake them into impure loaves of bread and eat them while they are impure. This was something that was normally done, perhaps to make their flour last a bit longer. In this way the inedible vetches become fit for human consumption. If he wants to mix them in with pure dough, then he would have to put in less than the volume of an egg of vetches into each batch of dough. In this way the dough would remain pure.
Section four: Bet Shammai says that when it comes to terumah vetches, if they are being soaked and rubbed in order to eat them, the person doing so must be in a state of purity. If they are being fed to an animal, the person may be impure.
Bet Hillel holds that only the soaking needs to be done in a state of purity. If he is rubbing them, then according to Bet Hillel he is preparing them for animal food and he need not be pure.
Shammai himself rules that terumah vetches should not be soaked at all, because that would cause them to be susceptible to impurity. Bet Shammai and Bet Hillel both allow one to soak the vetches, as long as one does so in a state of purity. It is interesting that Bet Shammai rules differently from their eponymous ancestor Shammai, and that the latter is stricter than his own school. It seems that we can note a movement toward leniency in this matter, from Shammai who rules most strictly, to Rabbi Akiva, who rules most leniently and allows one to be impure even when soaking terumah vetches. Since they are mostly used for animal food, even the vetches that are used to feed humans are not subject to the rules of terumah and once can always make them impure.