The Poskim discuss whether it is permitted to remove a fly that fell into a cup of wine on Shabbat. At first glance, it would seem to be a problem of Borer, especially since one is selecting the P’solet (fly) from the Ochel (wine).
Nevertheless, the great Rabbi Yom Tob Sahalon (1559-1638, Tsfat) establishes a novel principle in the Halachot of Borer, according to which there would be no problem removing the fly. He says a Hidush that “En Borer B’lah”-there is never a problem of Borer when a solid is mixed in a liquid, because they do not constitute a bona fide mixture. The Be’ur Halacha (Rav Yisrael Meir Kagan of Radin, 1839-1933) cites this opinion, and brings the Bet Meir who also takes that position.
However, the Be’ur Halacha continues, citing the Yad Aharon, that many Poskim disagree with this leniency, and therefore, he does not allow one to remove the fly from the wine. Nevertheless, he suggests a solution to the problem, based on his understanding of the Taz (Rabbi David Segal, Poland, 1586-1667), who says that if one removes some of the wine with the fly it is not Borer, since he is selecting Ochel and P’solet together from the mixture. This is also the advice given by the Ben Ish Hai in Parashat B’shalah (Par. 12). Hacham Ovadia, in Hazon Ovadia Vol. 4, concurs that this is the best solution.
The Mishna Berura also applies this solution to skimming fat off of milk. He permits removing the fat, if he also takes out some of the milk with it. Accordingly, this method would also enable a person to remove foam from beer or coffee, along with some of the beverage itself.
Based on this, the VaYizra Yishak permits removing pits in a salad by removing the pits along with some of the salad. He does point out that all this is based on the Mishna Berura’s understanding of the Taz. The Hazon Ish, however, had a different understanding, according to which the Taz would not be a basis for a solution to these situations.
One may remove P’solet from a mixture, if he selects some of the Ochel along with it. For example, one may remove a fly from a cup of wine along with a little bit of the wine.