I'll try a milder cheese, but I usually use the Medium Cheddar by Tillamook, and it's not very strong. If you taste a small amount the sodium citrate plain, how does it taste to you? Perhaps we are just super tasters...
I am having a similar problem with my Mac and Cheese, and am using the bare minimum sodium citrate possible. My sodium citrate (also from Will Powder) looks like Chris's (the correct one), but there is a definite sour/bitterness to the mac and cheese that's pretty obvious. It's barely tolerable but really kind of strange tasting. Nobody else has this "off" taste in their mac and cheese?