Maaser Sheni, Chapter Three, Mishnah Nine
As we have learned many times, maaser sheni can be redeemed only outside of Jerusalem. Once in Jerusalem it must be eaten. However, this applies only to pure maaser sheni. Impure maaser sheni, which cannot be eaten and must be redeemed, can be redeemed in Jerusalem.
Our mishnah deals with what is done with the impure maaser sheni produce which was redeemed after it had already been brought into Jerusalem. While this produce is no longer maaser sheni (because it was redeemed) it was maaser sheni when it was brought into Jerusalem, and generally one cannot take maaser sheni out of Jerusalem once it has been brought in. The question is: does this prohibition still apply?
The mishnah mentions principal defilements and secondary defilements. I dont think it is necessary to explain these concepts here, just to say that the defilement caused by principal defilements is more serious than that caused by secondary defilement.
Second tithe which was brought into Jerusalem and became unclean, whether it became unclean by a principal defilement or by a secondary defilement, whether it became unclean within [Jerusalem] or outside:
1) Bet Shammai says: it must all be redeemed and be eaten within except that which became unclean by a principal defilement outside.
2) But Bet Hillel says: it must all be redeemed and may be eaten outside except that which became unclean by a secondary defilement within.
Section one: Bet Shammai says that in all the cases referred to in the mishnah, the maaser sheni must be redeemed and then eaten inside Jerusalem, except for the case of maaser sheni that was made impure by a principal defilement before it came into Jerusalem. This produce was seriously defiled before it came into Jerusalem and therefore the walls of Jerusalem dont absorb it. This means that coming within the walls of Jerusalem doesnt count as far as prohibiting it from leaving Jerusalem. In the other cases, the walls do absorb the maaser sheni and it cannot be taken out.
Section two: Bet Hillel rules more leniently. In all of these cases the redeemed maaser sheni may be brought out of Jerusalem except where the maaser sheni contracted a lesser degree of impurity within Jerusalem. This produce was pure when it was brought into Jerusalem and only contracted a lesser degree of impurities. Therefore, it has to be eaten in Jerusalem. However, in all cases where the maaser sheni became impure before coming into Jerusalem and in cases where it contracted a serious type of impurity within Jerusalem, it can be brought out of Jerusalem and eaten there.