MSG

Monosodium glutamate, or MSG, earned its reputation in Asian takeout kitchens across America, but almost all fast-food restaurants use the flavor enhancer to some extent. Interestingly, MSG has no distinct taste itself. Instead, it amplifies other flavors, especially in foods with chicken or beef flavoring, through processes that scientists don't fully understand.

http://recipes.howstuffworks.com/processing-facts-channel.htm