Take another look at the sentence:
After listening for weeks to her boyfriend Steve bragging about his mother's chili, Amy anticipated the first delicious spoonful, the hair floating among the beef and beans, however, killed her appetite.
The hair floating among the beef and beans killed her appetite is a main clause. However is a conjunctive adverb interrupting the flow of the clause. Because however is interrupting, it should have a comma both before and after it.