Mickey D's uses varieties like the Russet Burbank, which have a nice oval shape and just the right balance of starch and sugar. Excess sugar can cause a fry to have brown spots where it's over-caramelized, leaving a burnt taste and deviating from the uniform yellow-arches color. Just in case, the spuds are blanched after slicing, removing surplus sugar.
Sodium Acid Pyrophosphate
Taters can turn a nasty hue even after they're fried—iron in the spud reacts with the potato's phenolic compounds, discoloring the tissue. The phosphate ions in SAPP trap the iron ions, stalling the reaction and keeping the potatoes nice and white throughout the process. FULL POST