Post Hummingbird Keyword Density

Hummingbird was the most confusing Google algorithm change for me. I understand it was meant to change the engine itself and refine the entire search process. I do not understand if and how it affected keyword density. I know there are always arguments and differences of opinions on proper keyword density; however, what is the relationship of change between penguin/panda and hummingbird? What is the proper keyword density post hummingbird?