The Shabbat prohibition of Borer forbids separating undesirable food from desirable food on Shabbat, except under certain conditions. The Poskim addressed the question of whether this prohibition would apply to removing the inedible peel that surrounds a vegetable or fruit. Would one be allowed, for example, to peel an onion, or a clove of garlic, on Shabbat? On the one hand, peeling entails removing the undesirable substance (the peel) from the desirable substance (the onion), and this situation would thus seemingly qualify as a case of Borer. Peeling would then be permissible only under the conditions that apply to Borer generally. Conversely, however, one might argue that peeling does not constitute Borer because the two substances are not mixed. Borer means separating undesirable food from desirable food in a mixture. In the case of an onion, however, the peel simply covers the edible food, and does not combine with it to form a mixture. One might therefore argue that the Halacha of Borer should not apply to peeling fruits and vegetables.
This issue is subject to a debate among the Halachic authorities. The work Tal Orot held that Borer does not apply to peeling, and one may therefore peel fruits and vegetables on Shabbat without any restrictions. Most other authorities, however, disagree, and held that peeling indeed constitutes Borer, because the peel is attached to the edible food and they are thus considered a "mixture" of sorts. These authorities allow peeling on Shabbat only "Samuch La’achila," just prior to eating the food item or prior to the meal at which it is served. One may not, however, peel an onion in the morning in preparation for Se’uda Shelishit later that afternoon. Since, according to this view, the restrictions of Borer apply to peeling, it is allowed only just prior to the meal. This is the ruling codified by the Rama (Rabbi Moshe Isserles, Poland, 1525-1572) in Siman 321, and is the position taken as well by Rabbi Moshe Halevi (Israel, 1961-2001) in his work Menuhat Ahaba.
The exception to this rule is situations where some pieces of fruit are invariably removed along with the peel. When one peels a mango, for example, small pieces of mango come off together with the peel. This occurs as well when one peels certain kinds of melon. In these cases, peeling would not constitute Borer, since one is considered separating desirable food from other desirable food, rather than undesirable food from desirable food. Therefore, it would be permissible to peel a mango or certain kinds of melon on Shabbat morning for a fruit salad to be served later in the day.
This would not apply to common fruits such as oranges and bananas, where the peel separates cleanly from the fruit, without any pieces attached. One may peel an orange or banana on Shabbat only just prior to the meal, but not in preparation for later in the day.
Summary: It is forbidden to peel an onion, garlic or fruits on Shabbat unless he does so just prior to the meal in which it will be eaten. The exception to this rule is fruits such as mango and certain melons, where pieces of fruit invariably come off together with the peel; these fruits may be peeled even in preparation for later in the day.
