They'll eventually get used to it. Rats are smart creatures, they'll sample little bits of food, and if it dosen't make them feel sick then they'll know it's ok (well, in the wild this is normal, my girls are gorbs and don't care )
Ratties sort through and pick out what they like in their mix, but I always leave what's left in their cage until they eat it. I don't refill the bowls until everything's gone, even the food that they hid/hoarded
my boys never hide their food! sniffs covers his bowl up with his towels but never hides anything (he even keeps his toys in his bowl LoL it's adorable all the balls he can fit in there go in the rest go around neatly he is so cute)
LoL Oh you should have seen it, I was cracking up! He would not let go! I tryed to pull my hand away but he just held on tighter...so I just let him and he placed my hand in the bowl and kinda pushed it down, like it was trying to stuff it in there. >.< Jays so funny!