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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 !

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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. – VanJeer Oct 9 '12 at 12:18
Don't forget to accept answers by clicking the check-mark on questions for which you have received acceptable answers. :) – user560 Nov 11 '12 at 12:56
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 Jul 21 '15 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 – aaaaaa Jul 22 '15 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. – jamesqf Jul 22 '15 at 17:44

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) :

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

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!

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