from a lab in sector B that has a cowgirl and a catgirl. If you haven't finished their quest, you must complete it, talk with the chef again, and you'll be able to progress the milk quest by going to the cowgirl lab.
In that case talk again with the chef that gave you the milk quest and you should be able to progress. I had to do it this way to prevent the quest progressing when you don't want it to.
