Your question is asked by the Bach on the Tur, at the end of Siman #319, DH Mesanenet.

1) The Bach gives an explanation based on a Halachah cited by Mishneh Berurah (OC 319:12, and Bi'ur Halachah there, DH u'Meni'ach) in the name of Rishonim, that even if someone has two Ochalin in front of him, the one which he desires to eat now is considered as Ochel, and the one which he does not want now is considered as Pesoles. Therefore, the Gemara was concerned that since the yolk is required now for coloring, and the white is not required now, the yolk should be considered as Ochel and the white as Pesoles.

2) To answer this question, the Gemara said that the yolk is needed only for coloring. Therefore, the above Din does not apply, since the yolk is not fully considered an Ochel in such a strong way that it can abolish the white -- which is a real Ochel and not just for coloring -- from being considered an Ochel. The Din that the item that one needs now is considered an Ochel and the other item is Pesoles only applies when one actually eats the first item, not when it is merely a coloring.

3) The white that remains above is still considered an Ochel because it can be removed from the other Pesoles that remains at the top of the Mesanenet, and can be eaten on its own. Therefore, the white is considered as Ochel because one actually eats it, and the yolk is considered as Ochel because the fact that it is used for coloring purposes is sufficient to make it into an Ochel even though it is not actually required for eating. This is why Rashi writes that they are both considered as Ochel. The yolk is not capable of taking away the status of Ochel from the white.

Kol Tuv,

Dovid Bloom

1) From the Bach I cited above it appears that one eats the white. However, the Mishnah Berurah (219:58) writes that it is not intended to eat the white. The Mishnah Berurah writes that nevertheless this is not considered as selecting Ochel from Pesoles (which would be forbidden, since one is doing this with a Kli) because the yolk is also not intended for eating but merely for giving color. The Mishnah Berurah writes this more clearly in the Bi'ur Halachah to 319:3 (DH Hayu), where he says that both the yolk and the white are never intended for eating. The white mixes in with the waste of the mustard and one does not want to eat it. The yolk falls down for coloring, not for eating.

2) What the Mishnah Berurah writes is rather surprising, because Rashi (beginning of 140a) writes that both the yolk and the white are Ochel. How can the Mishnah Berurah say that one does not want to eat either of them? The Shulchan Aruch ha'Rav (319:21) gives an explanation based on the Gemara on 134a that states that the reason why one is permitted to put the egg in the mustard filter is that this does not look like "Borer." Graz He explains that it is only considered "Borer" when one takes one item from another because he wants to eat the first item, not the other item. With the yolk and the white, this is not the case. One does not want to eat the yolk, only to use it for color. Since the yolk and white are both essentially Ochel because they are both edible, it follows that the selection process that one is doing is not considered Borer since one is not doing this for eating but merely for color. If one selects one Ochel from another Ochel only with the intention of making a good color, this is not a process of selection but rather a process of coloring.

3) Therefore, even though one discards the white and eats the yolk with the mustard, one has not performed selection but rather one merely colored the mustard, and it does not look as if one has done "Borer." This is also stated in the aforementioned Bi'ur Halachah -- that since one never prepares the yolk and the white for eating, this is not Borer.

Kol Tuv,

Dovid Bloom

1) I posed this question to a Gadol. However, I asked the question a different way around. I started off with what Rashi writes that both the yolk and the white are considered food. Then I cited the Mishnah Berurah that the white "does not stand to be eaten." I asked how can Rashi say that the white is food if it does not stand to be eaten?! The Gadol answered, "b'Vadai Omed l'Achilah" -- "The white certainly stands to be eaten."

2) Again, this seems a surprising answer. How can one say that the white is ready to eat, if the Mishnah Berurah says it is not?

It seems that the crucial thing to keep in mind in this Sugya is that some of the phraseology is relative. The point is that both the white and the yolk will actually be eaten. Especially in those days, people did not throw away whites and yolks of eggs for no reason. However, when the Mishnah Berurah writes that the white does not stand to be eaten, he does mean that it will not be used at all, but he means that it is not the choice food.

3) We see this slightly more clearly in the Chayei Adam (Hilchos Shabbos 16:10). He writes that one is allowed to put the egg in the filter even though the white remains on top and "it is well-known that the white is not good for eating." If so, the Chayei Adam asks, this should be considered Borer! He answers that this is not Borer because one does not want to eat the yolk either, but rather only use it for coloring.

4) What we are really saying is that as far as eating is concerned, the best way to eat an egg is to beat the yolk and the white together. If the yolk and the white are used separately, neither part is actually choice food. The yolk is used for coloring and the white is not such good food. However, these two minuses cancel each other out, and one is not considered better than the other. Therefore, there is no problem of Borer because Borer applies only when one item is more wanted than the other and one is called Ochel and the other Pesoles. In our Gemara, both are equally Ochel even though neither is such a good Ochel, but certainly both will be eaten eventually.

5) We can now answer the original question. The yolk is good, but since it is used only for coloring this stops it from being considered more of an Ochel than the white. Since the white will also be eaten, even though it is not such good food, it is also considered an Ochel. This is why Rashi writes that they are both Ochel. Since the yolk and the white are equal, there is no prohibition of Borer involved by selecting one from the other.

Kol Tuv,

Dovid Bloom