Mushrooms at the Border
In order to walk on Shabbat more than 2,000 amahs beyond the city border (approximately one kilometer), one must do something before Shabbat called eruv techumim. This involves placing food for two Shabbat meals at a place 2,000 amahs outside of the city. It is then considered as if he has established his residence there for Shabbat and that he may therefore proceed to walk another 2,000 amahs. This eruv procedure, as is the case for other types of eruvin — such as eruvei chatzeirot, eruvei mavo’ot and eruv tavshilin — is a rabbinical mitzvah and a beracha is recited when making the eruv.
Not everything, however, qualifies as food for this eruv, or for an eruv which is made to allow residents of different courtyards to carry objects into the alley into which their courtyards empty. The mishna excludes only water or salt from being considered acceptable foods. A simple reading of our gemara indicates that truffles and mushrooms are also excluded. The reason for their exclusion even when cooked, say the early commentaries, is that people do not generally rely on them as a staple, nor even as an accompaniment to meals, and only occasionally indulge in them. The Rambam goes even further by ascribing their exclusion to their negative nutritional impact.
Despite this consensus of so many major commentaries based on the text before us, the text before the Gaon of Vilna has an "etc." added to the quotation from the mishna, which radically alters the meaning of the gemara. The exclusion of mushrooms, in his text, is limited to the law of maaser sheni — the second tithe — which is mentioned in our mishna immediately following the law of the eruv. The Torah sets down special rules for what one may purchase in Jerusalem with the money from the redemption of maaser sheni. These rules preclude mushrooms because they do not grow from the earth but are only fungi. As far as eruv is concerned, he concludes, once they have been cooked into an edible state they qualify as food.
This innovative approach of the Gaon of Vilna is elaborated upon in another footnote on our daf of gemara, that of Rabbi Betzalel of Regensburg, and is mentioned by the Mishna Berura (366:23) as well.
- Eruvin 27a