Because the liquid milk or yogurt packaged in plastic bags, cartons or Tetra Pak (top box) in the market swells due to deterioration or fermentation of miscellaneous bacteria, the bulging of the bags and boxes can be seen with the naked eye, indicating that the milk in the bags and boxes has deteriorated. This is not only a way to identify whether milk has gone bad, but also a sign that it cannot be sold or eaten.

If one bag in a box of milk goes up, it depends on the production date of other milk. If it is still within the warranty period and there is no bag expansion, it should be no problem.