I have a biter, she's broken skin more than once in undoubtedly aggressive attacks, not confused about food smells. She was very aggressive with the older rat I had when I got the biter to the point I wouldn't house her with the older. (the older had a tumor on her leg and wasn't able to fight back, the newbie (I got to befriend the older when her sister died) would even steal food from her a stash it where the handicapped rat couldn't get it, that didn't fly with me).
long story short, some rats just have crappy attitudes.
My biter is still around, I've given her a second chance, and she's now housed with two other healthy rats and she seems a little calmer nowadays. She was very aggressive with the newer two at first but they're all friends now. She hasn't bit me in a while but she will NOT be held, and when I have to pick her up, usually for cleaning out the cage, she shrieks and fluffs up. She just hates being touched.
The best technique I know of is spoon-feeding them pudding, or something they cant grab, at the door and they'll come to trust you. I'm thinking this is why I don't get bit anymore. That or she just learned the repercussions are less than enjoyable... but I think the positive reinforcement does more than 'punishment', but without a negative, positive has no context.