Although it is implied, I don't think it specifically states anywhere that before the flood man may ONLY ate fruit, and yet there are suggestions that man was eating meet soon after the fall. In Genesis 4 there are references to Abel making sacrifices of blood - which were usually eaten.
http://ohr.edu/ask_db/ask_main.php/161/Q1/
As you probably know these differ from burnt offerings which weren't eaten.
