Hacham Bension Abba Shaul (Israel, 1923-1998), in a famous responsum (Or Le’sion, vol. 2, Siman 31), addresses the situation of a child who ate a very small amount of the food on his plate, and after the meal, his mother wants to return the untouched portion to the container to be stored for the next day’s meal. One of the conditions to allow Borer – separating – on Shabbat is that the separating must be done for immediate use, such as if one separates food from an undesirable substance so it can be served at a meal that is about to take place. In the situation described, however, the mother removes the untouched food from the undesirable food in preparation for the next day’s meal, and the question thus becomes whether this is permissible on Shabbat. Do we consider this a case of Borer, since the untouched food is being separated from the other food on the plate? Or, can we distinguish between the classic case of Borer, where two substances are actually mixed together, and this situation, where the foods are situated on the same plate, but are not mixed together?
Hacham Bension rules that this would, indeed, constitute Borer and be forbidden on Shabbat if it is not done for immediate use. He writes that although in the Shulhan Aruch’s discussion of Borer it refers to a case of foods that are "Me’urabin" ("mixed"), it is clear, in Hacham Bension’s view, that Borer is not limited to situations of foods that are actually mixed together. As long as the foods are close together, as they usually are if they are on the same plate, the laws of Borer apply, and one would not be allowed to take one food from the other for any purpose other than immediate use.
Sephardic Chief Rabbi Shelomo Amar discusses this issue in his work Shema Shelomo (Orah Haim 10), and writes that if the foods are in separate compartments in the plate – like some children’s plates have – then certainly one may remove food from one compartment. In such a case, the foods are clearly separate, and thus taking from one compartment would not constitute Borer. And even in an ordinary plate, Rav Amar writes, there is room to be lenient and allow separating the untouched food, in contrast to Hacham Bension’s stringent ruling. Rav Amar notes that both the Rambam and Shulhan Aruch speak of Borer in the context of items that are mixed together, and the Be’ur Halacha (Rav Yisrael Meir Kagan of Radin, 1839-1933), in discussing the Rama’s ruling (based on the Terumat Ha’deshen) concerning Borer as it applies to different kinds of fish, comments that this refers to fish that are mixed together. Hence, there is a basis to argue that Borer does not apply to different foods that are placed near one another on a plate but not actually mixed with one another. Indeed, Rabbi Moshe Halevi (Israel, 1961-2001), in his work Menuhat Ahaba (vol. 2), rules that Borer applies only when two foods are mixed together.
As for the practical Halacha, then, those who are lenient in this regard certainly have on whom to rely. However, in light of the stringent ruling of Hacham Bension, and as the issue at stake involves a potential Torah prohibition, it is preferable to be stringent and not separate foods on a plate even if they are not mixed together.
Summary: If there are different foods on a plate that are not mixed together, it is preferable not to take one food from the plate unless this is needed for immediate use. However, those who are lenient in this regard have authorities on whom to rely. A common example is when a child ate one food on the plate but not another, and the parent wishes to take the untouched food and return it to the pot or container for the next meal.
