I never had a problem with MMF yellowing until just recently. I did a wedding cake for a morning wedding and made it the night before. The cake was creamy white and not the bright white that it usually is. I'm not sure what I did differently. It was the same recipe I usually use. Any ideas what may have happened?
I'm not sure what happened, but maybe try adding in some white-white when you're melting the marshmallows.
MMF is typically an off white, not a true white. Your level of whiteness will depend on the marshmallows and freshness of those. I add white coloring dye to my MMF if I want it to be white white.
Thanks. I guess I've always had colored accents on the white cakes I've made and hadn't noticed that it was really an off white.