# Why does milk contained in cartons of milk expand?

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 !

• 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. Commented Oct 9, 2012 at 12:18
• Fermentation takes weeks to happen, and you would know it happened if your milk was spoiled. A lot of times what happens is just that the cold air inside the jug warms up and expands enough to bulge out the sides of the carton.
– user16893
Commented Jul 21, 2015 at 21:15
• @user16893 say you have 1L of air at +4C temp (T=277K) and move it at constant pressure to room temp of +24C (T=297K). You should be able to calculate change in volume as PV=nRT, where P=const Commented Jul 22, 2015 at 15:01
• Perhaps the culprit here is the packaging, not the contents. Since you say 'carton', I'm assuming the ones with a square cross section, no? So they're shipped, and on the shelves, packed tightly together and so hold their square shape. Put a single carton in the refrigerator, and the sides slump a bit under the weight of the contents, since for economic reasons they're made no stronger than necessary. Commented Jul 22, 2015 at 17:44

$$C_{6}H_{12}O_{6} \rightarrow 2 C_{2}H_{5}OH + 2 CO_{2}$$