system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do plastic milk jugs always have gross little dried flakes of milk crust around the edge of the cap? No other containers of liquid (including milk-based ones) seem to have this problem.
gpt
I work at a dairy.  I've ran the machines that make the bottles and fill the bottles.  Basically after the milk goes in the cap is for a lack of a better term dropped onto the top of the bottle, pressed down and then screwed tight in basically one quick motion that takes about a second.  When the cap is pressed down excess milk will squeeze out of the top before it screwed on tightly.  After the cap has been attached the filled bottles go through a quick sanitize rinse before they are put into milk crates.  The rinse gets the vast majority of the milk off but the little bit that's left will eventually dry on.  When we fill we fill to the top with no air bubbles left, which you have to do because milk foams when it's being forced quickly into a bottle like that.  When you look at other drinks that come in bottles you'll notice there's a little bit of air at the top, but not so much in a milk jug.  

For the five year olds in the room 😁 milk has a lot of air bubbles in it when you first put it in the bottle so you have to over fill it to top it off..  we give the bottles a bath but sometimes a little gets left behind.