The Mishnah in Sheviis 4:7 asks when one may begin eating fruits of Shemitah. As the Bartenura and Yachin there explain, this means that as the fruits are left “for eating,” one may only eat them when they’re ripe.
In that Mishnah and the succeeding Mishnayos (7-9), the Tanna lists several fruits and their times:
- Figs may be eaten in the field when they have reddened and in the house when they have begun ripening.
- Grapes may be eaten in the field when they begin producing grape juice and in the house when they have begun ripening.
- Olives may be eaten in the field when they produce a reviis of oil per se’ah of olives; they may be squeezed for their oil to be eaten in the field when they produce a half-log of oil per se’ah of olives; and they may be eaten in the house when they produce a third [of the total amount of oil, or of that which is remaining - Yachin].
The Mishnah in each of these cases states that the final stage of ripening listed (Bartenura) is the same as when it is obligated in Ma’aser. It concludes its teachings by saying that for all other fruits their times for eating in Shemittah is the same as when it’s obligated in Ma’aser.
Now, this list of when they’re obligated in Ma’aser is recorded in Ma’aseros 1. That list includes the three fruits mentioned here (figs and grapes in 1:2 and olives in 1:3). So why doesn’t the Mishnah simply say that one may eat underripe fruits in the field and fruits that are obligated in Ma’aser in the house, instead of going through these specific cases and then setting forth these general rules?
For reference, the links above take you to the Mishnayos in question.