Will fresh milk go bad if left at room temperature? Experts reveal: Two keys to not damaging the flavor and nutritional value of milk

There was once a piece of information circulating on the Internet that fresh milk would not turn sour or spoil if left at room temperature for many days. Is it possible for fresh milk to remain liquid and not spoil after being left at room temperature for many days? Experts say the key is that if fresh milk is almost 100% sterilized, the sterilization temperature and time may exceed the normal label due to "carelessness", which is likely to happen.

Experts reveal: Two keys to not damaging the flavor and nutritional value of milk

Cow's milk is rich in nutrients and has a high water content, so microorganisms can easily reproduce. Pathogens and other bacteria may also be present. Therefore, it must be sterilized to ensure its hygiene and safety. The purpose of "heat treatment" is not only to kill pathogens and inhibit other microorganisms, but also to maintain the flavor and nutritional value of milk. Therefore, the control of heating temperature and time are two important keys.

Chen Juncheng, who used to be a part-time lecturer at the Extension Education Department of Chinese Culture University, said that when buying fresh milk, you should look at the label and you will find the following three common sterilization methods: including "Pasteurization", which has two types, namely "low-temperature long-time sterilization" and "high-temperature short-time sterilization", and the other is "Ultra-Pasteurization", namely "Ultra-high Temperature Pasteurization" (UHT).

The purpose of "heat treatment" is not only to kill pathogens and inhibit other microorganisms, but also to maintain the flavor and nutritional value of milk. Therefore, the control of heating temperature and time are two important keys.

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Three common sterilization methods for fresh milk

1. Pasteurization/Low Temperature Long Time Sterilization (LTLT)

That is, it is heated at a low temperature of 65℃ for at least 30 minutes and refrigerated immediately after sterilization. Although it cannot completely kill the microorganisms, the number of living bacteria can still be maintained at about several thousand; however, it can completely kill pathogenic bacteria and avoid stomach pain after drinking. It is a sterilization method that is more suitable for Taiwanese fresh milk, and the marked refrigerated storage time is 7 days. If this sterilization method is used at room temperature, it may become sour and unfit for drinking in less than a day.

2. Pasteurization/High Temperature Short Time Sterilization (HTST)

The heating temperature is about 70℃ and the time is at least 15 seconds, which can also achieve the same sterilization effect. The marked refrigerated storage time is 7 days.

3. Ultra Pasteurization/Ultra High Temperature Sterilization (UHT)

That is, the temperature is 130℃ plus or minus 2℃, and sterilization is achieved within 3 to 5 seconds, and the marked refrigerated storage time is 2 to 3 weeks.

Q1: Why can milk treated with UHT be stored for a longer time?

A: In fact, the key point is that the number of residual bacteria after ultra-high temperature sterilization is very small, almost sterile. If you accidentally heat the food a few degrees higher during the manufacturing process and extend the sterilization time by a few seconds, you can achieve complete sterilization. In this way, even if it is left at room temperature for several days, it may not easily become sour or deteriorate.

Q2: Why is it labeled as refrigerated for 2 weeks, but there is still a possibility of spoilage during the shelf life?

A: Chen Juncheng said that there is about a 5% chance of spoilage, mainly because the raw material source of raw milk, the bacterial flora and bacterial species of raw milk are often different, and the types of microorganisms are also different, as well as the heat resistance is also different, which are all factors that affect the possibility of spoilage.

[Expert advice]:

Chen Juncheng suggested that although there are currently no domestic regulations to limit the number of bacteria in fresh milk after sterilization, if the government wants to distinguish between ultra-high temperature pasteurized milk and completely bacteria-free long-life milk, it seems necessary for the government to regulate the number of bacteria detected so that consumers can make more informed choices.