I've been making a tart filled with lemon curd once in a while for a couple of years. (Elderly mom's favorite.) The filling always came out a pretty yellow 'lemon curd' color. But the last couple of times I made it, including today, the lemon curd has a pale almost frothy layer on the top and I'm curious as to why. I haven't altered my technique in any way I'm aware of, and the only variable I can think of might be the brand of butter since I tend to buy whatever's on sale. The flavor isn't affected, it's simple a matter of appearance. Can anyone give me an idea of what might be happening here? Thanks!