Why does milk contained in cartons of milk expand?

Question

In the morning, I went to the fridge to drink some chocolate milk. At night, when I took out the same carton of milk, the packaging seems expanded, like some kind of air is inside. Explain please !

Answer 1

The likeliest culprit here is fermentation carried out by bacteria present in the milk. Fermentation of a sugar, for example glucose, releases carbon dioxide (a gas) :

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

Since this reaction produces a gas, the gas builds up in the milk carton causing it to bulge. This is not, generally, a good sign, make sure to smell the milk before drinking it!

Answer 2

There are probably several reasons. One that I can think of is that spoilage bacteria may have introduced gases that increased the pressure inside the carton. Probably that's why you notice the bulging.