I added an egg yolk almost exactly one year ago this week to a 100% Castile soap. Since it was also the first time I had ever made a 100% Castile soap, it's unfortunately hard for me to say what, if any, benefits the yolk contributed to it. It turned out quite nice, but seeing as how I had nothing similar (but without a yolk) to compare it to, I wonder if it couldn't just have easily been as nice without the yolk, you know?
What I really need to do is make another yolk soap, but with one of my regular soap formulas instead of a formula I've never used before so I can get a better idea of what differences the yolk adds.
In any case, all went smoothly- i.e., nothing funky or unusual happened in my yolk soap. It soaped beautifully and it didn't go rancid or anything like that.
My batch was a 1.1 lb batch using 1 egg yolk. I stickblended the yolk to about 1/2 cupful of raw soap batter that I had removed from my main pot when it was at the emulsification stage, and then I poured the blended yolk/soap mixture back into the pot and finished soaping as usual with nary a problem.
The only thing I'd like to make mention of for those of you who want to try this is that when you add a yolk, make sure to remove the thin membrane that surrounds the yolk first. You don't want to add the membrane to the soap or it will add white lumps or curdles. What I did when I made mine is this: after I first separated the yolk completely intact from the white, I poked a small hole in the thin, transparent membrane that surrounds the yolk and holds it together. I then let the yolk run freely out of the hole into a cup and threw the membrane away.
HTH!
IrishLass