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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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  • $\begingroup$ 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. $\endgroup$ – VanJeer Oct 9 '12 at 12:18
  • $\begingroup$ 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. $\endgroup$ – user16893 Jul 21 '15 at 21:15
  • $\begingroup$ @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 $\endgroup$ – aaaaa says reinstate Monica Jul 22 '15 at 15:01
  • $\begingroup$ 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. $\endgroup$ – jamesqf Jul 22 '15 at 17:44
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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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    $\begingroup$ Do the bacteria in milk undergo ethanol fermentation? It's always been my understanding that homolactic fermentation predominates, though that isn't actually based on anything. $\endgroup$ – canadianer Mar 15 '18 at 19:53
  • $\begingroup$ @canadianer no idea, to be honest. I'd be interested in the answer if you look into it though! $\endgroup$ – terdon Mar 15 '18 at 19:55

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