How does the shelf life of milk come about? When you stand in front of the freezer, you will find some details on the milk carton - the date of manufacture is February 15, 2013, the shelf life is 30 days, and the recommended refrigeration temperature is 4°C. These are some of the commonplace numbers that we often ignore. How are these standards set? Do you know how to pick them?
The ideal way is of course to carry out an actual measurement, but we must first determine the approximate time frame and figure out several factors that affect the shelf life of milk, which are the characteristics of the milk itself, processing technology, packaging forms and freezer storage conditions.
For example, the freezer temperature, the normal storage condition is 7°C, in the accelerated test will be set to 21°C, so that the shelf life obtained by the formula backwards, to obtain the shelf life achievable in practice. At the same time, the normal shelf life test is also conducted to verify whether the results are reliable.
The shelf life of refrigerated milk is not an absolute threshold, as different environmental conditions can cause different chemical changes in the food. If the environment is harsh, such as extremely hot and cold, the shelf life can not be used as an absolute safety standard; conversely, if the requirements of the package is stored normally, and no deterioration problems are found, a day or two after the expiration of the milk is not inedible.
Fresh milk does not mean just milk, because fresh milk exposed to the air for about 2 hours will begin to grow bacteria deterioration, generally within 15 minutes after extrusion of fresh milk to take the freezer refrigeration preservation, and sent to the processing plant for further disinfection. So you buy the so-called fresh milk in the freezer, are sterilized, the shelf life of different milk, but also with the sterilization and packaging methods used by them.
Milk that has a short shelf life and is packaged simply - such as bottled milk - is mostly processed by pasteurization. This is done by heating the milk to 62°C to 65°C and holding it there for 30 minutes, or to 75°C to 90°C and holding it there for 15 to 16 seconds.
After this method of sterilization, the milk, in which the bacterial spores are not inactivated, so to be stored in the freezer at low temperatures, usually 4 ℃, this environment, you can keep fresh for about 1 week.
Another treatment method is called ultra-high temperature sterilization, also known as UHT milk, in which the milk is placed at 135°C to 140°C, heated for 1 to 2 seconds, and then quickly cooled in the freezer. This is commonly used for bagged milk with a long shelf life. The disadvantage is that some of the nutrients in the milk that are not heat-resistant, such as vitamins, will be destroyed, but because the high temperature time is short, the overall impact of this technology on the nutritional value is not significant.
Secondly, aseptic packaging is also an important step, as milk should not be exposed to light and air throughout the process, so that it can maintain its room temperature storage needs and without the addition of preservatives. Adding preservatives can create excess input costs and is likely to disrupt the entire aseptic process. The shelf life of milk after ultra-high temperature sterilization is 1 to 6 months.