Why does this milk pouch bulge or bloat?

Normally milk packets should not bulge or bloat. Sometimes the packets may bulge or bloat if not handled properly.
1. Generally, the milk is packed in plastic covers only. Some may be using paperboard covers also for storing the milk. If the quality of the paper is not good and if stored for a long time, these covers may become soft. In such cases, the swelling will be observed at the bottom of the packet only. If there is no leakage, the same milk can be consumed.
2. If the milk packet is not properly cooled, there is a chance the air inside the packet may get warm up and there may be swelling. Once you cool it again this swelling will disappear. Unchilled milk will give scope for microorganisms to grow and they may release gases and the milk may get spoiled.
3. This is the important reason for bulge or bloat. While packing, if the milk is not properly handled microorganisms in the milk may get multiplied. These microorganisms will produce gases and these gases will cause swelling. As time passes on the taste and colour of the milk may change. It is better not to consume such milk.
It is always advisable to use the milk within one or two days after it is packed. Generally, all standard companies will give the expiry date and one should not use it after that expiry date. If you observe that swelling. I recommend not using that milk even though there is no change in the flavour and taste.

Normally the milk inside the packets should not bulge or bloat either and bulging is seen, it might happen due to variance of temperature inside liquid where some portion is extremely in cold condition than the remaining part. This might be attributable to poor handling system.
1) The most preferred way of storage of milk is in the plastic pouch though some milk units might opt for paper version for packaging. In the paper pouch, cases of swelling might be noticeable due to variable temperature of the milk content inside the packet due to attainment non uniform temperature. This case is observed due to poor thermal conductivity of the paper pouch leading to swelling in some regions.
2) if the liquid inside the packets is not cooled uniformly, the air entrapped inside the milk content, there is the possibility of the air of getting warm up and this would cause swelling. This can be tackled with the restoration of proper cooling and in that situation, swelling will disappear. Microorganisms might develop due to non uniform cooling of the milk inside the package.
3) With the time, microorganisms would further multiply exponentially causing gas formation of unpredictable nature. This might be detrimental to the health of the consumers.Formation of gas would cause swelling apart from deterioration of taste and colour of the packaged milk.
Once the packaging process is completed, it should be consumed prior to its expiry date which is indicated on the external portion of the packets. Normally, it is consumable within the forty eight hours from the date of packaging.
Branded Diary Companies such as Amul, Mother's Diary, Sudha Milk etc put the date of expiry on the packets for the safety of consumers. In any case, swelled contents should not be consumed even though there is no change in their flavour and taste charecterstics.