Hm. A quick look suggests that apple slices, in particular, might very well
be coated with whey or caseinate. Sliced potatoes may, also. Yuck! Who'd
have guessed?

I have no idea how commonly this is being done, or if it was just an
investigation, but it's a really bad idea!

Weavre

http://www3.interscience.wiley.com/journal/119015194/abstract?CRETRY=1&SRETR
Y=0

ABSTRACT: 
Color analysis on apple and potato slices coated with calcium caseinate or
whey protein solutions showed that the 2 coatings efficiently delayed
browning by acting as oxygen barriers. The antioxidant properties of the
films were realized using a model allowing the release of oxidative species
by electrolysis of saline buffer. Whey proteins were a better antioxidant
capacity than calcium caseinate. Furthermore, addition of carboxymethyl
cellulose (CMC) to the formulations significantly improved their
antioxidative power. Best scavenging of oxygen free radicals and reactive
oxygen species was found for films based on whey proteins and CMC which
inhibited by 75% the formation of colored compounds produced by the reaction
of the oxidative species with N,N-diethyl-p-phenylenediamine.