why do plastic milk bottles go fat when they are empty?

Question

but if its the case of the bacteria inside the bottle as most of you say. whay does it still do it when you wash the bottle out

Answer 1

I can't say I've noticed this, but could it be that with the top screwed back on (assuming they have screw tops) then the air inside is, at some point warmer, than the air outside and thus the thin plastic bottle expands slightly? But I'm sure some one will give you the definitive answer.

Answer 2

Empty Milk Bottles

Answer 3

It's to do with the bacteria inside the bottle from the milk going off. All you need to do is wash the bottle out and do not put the top back on, then they bottle will not "go fat".

Answer 4

Normally when they are empty, they are out of the fridge in a warmer environment. The warmth makes the bacteria grow, which produce their own gases which expand the bottle. This can be proven by opening the lid and taking a wiff - you can smell the bacteria as rotten milk!

Answer 5

Because of the bacterias left in the bottle by the milk. When these bacterias is exposed to normal room temperatures, they multiply, and this activity heats the air in the bottle very slightly, and heated air expands.

Answer 6

Its because you empty the bottle straight from the fridge, which is cold, but you don't put the empty bottle back in the fridge. The air in the empty bottle expands as the bottle warms up, this then expands the bottle and makes it go fat.

Answer 7

Rotting milk inside the bottle.

Answer 8

Its pretty simple, air expands as it warms up, and shrinks as it gets cooler. When you empty the milk, all the air in the carton is cold, and when you leave it out the air expands. if you put warm air in the carton and then stuck it in the fridge the carton would get all sucked in.