The laws of Borer – separating – on Yom Tob differ from those that apply on Shabbat. It is well-known that separating different items that are mixed together is allowed on Shabbat only if the following three conditions are met: one removes the desirable items from the undesirable items, and not the other way around; one separates by hand, and not with an instrument especially designed for separating (like a sieve); one separates for immediate purposes, as opposed to separating items in the morning to prepare a dish that will be served later in the afternoon.
On Yom Tob, however, different rules apply. Although it is forbidden even on Yom Tob to separate with an instrument designed for separating, the other restrictions do not apply on Yom Tob as they do on Shabbat. The Shulhan Aruch (Orah Haim 510) writes that on Yom Tob one is allowed to separate as long he does so in a manner that entails the least amount of Tirha (exertion), which means that one must separate the smaller portion from the larger portion. If the mixture contains more desirable items than undesirable items, then one separates the undesirable items, as this is easier and less cumbersome. Conversely, if there are more undesirable items than desirable items, then one removes the desirable items. Furthermore, as the Hafetz Haim (Rav Yisrael Kagan of Radin, 1839-1933) notes in his Mishna Berura (510:7) and Bei’ur Halacha, separating is allowed on Yom Tob even in preparation for a meal that will be served later that day. Since food preparation is allowed on Yom Tob, and one may cook or bake food that will be served later in the day, one may likewise separate in preparation for a food to be served later. Of course, it is forbidden to separate on Yom Tob for the next day, unless the next day is Shabbat and he had made an Erub Tabshilin, just as is the case regarding cooking and baking. But it is entirely permissible to separate on Yom Tob, in the manner described, even in preparation for a meal that will be served later that day.
These Halachot are codified by Hacham Ovadia Yosef in Hazon Ovadia (p. 76) and in Yalkut Yosef (p. 205 in the English edition; listen to audio recording for precise citation).
Summary: One may separate foods mixed together on Yom Tob by removing the smaller portion. Meaning, if the mixture consists of more undesirable items than desirable items, then one should remove the desirable items from the mixture, and vice versa. This may be done even in preparation for a meal that will be served later in the day. The only restriction is that one may not use an instrument especially designed for separating (such as a sieve).
