For the source text click/tap here: Bava Metzia 54
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Second tithe is the section of produce separated after a farmer has already separated and given teruma of their harvests to the priests and then first tithe to the levis. That second tithe is another tenth of the farmer's harvest.
On years one, two, four and five, it is brought to Jerusalem and eaten there. It is given to the poor on years three and six (year seven is shemita and farms are not harvested).
Because it may not be practical to bring that much produce to Jerusalem, farmers are allowed to redeem their second tithe for money which is spent on food bought and eaten only in Jerusalem. It costs an additional one-fifth to bring that money to Jerusalem. But is this payment required? Is a person punished for eating food paid for with the second tithe money if that person has not paid the additional fee?
TheMishnah (Demai) states The [second tithe of] demai is not subject to [the rules of adding a] fifth. It has no mandated time of removal. It may be eaten by an onen. It may be brought into Jerusalem and taken out again. They may allow a small amount to be lost on the road. One may give it to an am haaretz and consume its equivalent in Jerusalem. [Second tithe money of demai] may be redeemed silver [coins] for [other] silver [coins], copper [coins] for [other] copper [coins], silver for copper, and copper for produce, provided that the produce is again redeemed for money, the words of Rabbi Meir. But the sages say: the produce itself must be brought up and eaten in Jerusalem.
Maimonides states "In the age of Yochanan the High Priest who served after Shimon the Just, the High Court sent emissaries who searched throughout the entire territory of Israel. They discovered that everyone was careful with regard to the great terumah and would separate it. But with regard to the first tithe, the second tithe, and the tithe for the poor, the common people among Israel would be lax and would not separate it. “
Which brings us to the Yochanan kohen Gadol ….He forced Idumaea to convert to Judaism, the first example of conversion imposed by the Jews in their history.