Besides being fresh, how much do you know about pasteurized milk?

Author: He Junwei (Shanghai Pasteur Institute, Chinese Academy of Sciences)

The article comes from the Science Academy official account (ID: kexuedayuan)

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When we walk into a supermarket, we will see a wide variety of milk on the counter, including pure milk, high-calcium milk, whole milk, skim milk, etc. However, there is only one kind of milk that is "isolated from the world" and quietly hides in the refrigerator, that is "pasteurized milk".

Pasteurized milk of the same specifications is not only more expensive than room temperature milk, but also has a shorter shelf life. Even so, many people still go to the supermarket to "dig three feet deep" to bring it home. When asked why, the only answer they give is "fresh, delicious".

Yes, pasteurized milk is new, but how much do you know about it? Today, I will explore the history of pasteurized milk with you.

Why is pasteurized milk afraid of "heat"?

The reason why pasteurized milk is so afraid of "heat" is mainly because of its special production process, which uses pasteurization. The goal of pasteurization is to kill pathogenic microorganisms in milk. After pasteurization, the microorganisms in milk are not completely killed, so it still needs to be refrigerated after sterilization. If you are more careful, you will also find that the shelf life of "pasteurized milk" is generally very short.

What is pasteurization?

Pasteurization, also known as low-temperature sterilization, is a sterilization method that uses relatively low temperatures (generally 60-82°C) to kill pathogens without damaging food quality. It is now often used in a broad sense to define heat treatment methods that need to kill various pathogens.

Did you know that pasteurization didn’t originate with milk, but with wine?

In the 18th century, French wine and beer had become popular in Europe, but there was always a problem that troubled them - wine and beer would turn sour if stored for a long time. Barrels of fragrant, sweet wine would eventually turn into "sour juice" that even mice would not want to eat. Some wine merchants even went bankrupt because of this.

It was not until 1856 that a scientist named Pasteur was invited by the owner of a Lille winery to help them find out why the wine had turned sour.

Louis Pasteur (1822-1895): A famous French microbiologist, patriotic chemist, and founder of modern microbiology and immunology. His contribution lies in shifting the study of microorganisms from morphological to physiological pathways, creating microbial physiology. Of course, his contributions are not limited to this, such as successively conquering silkworm disease, chicken cholera, anthrax and rabies. In the list of 100 people who have influenced the course of human history (written by American scholar Michael Hart), Pasteur ranked 12th, just after Einstein, which shows that he has made great contributions to saving and benefiting mankind.

Pasteur was very interested in this problem. He used a microscope to observe and found that there was a "troublemaker" in the spoiled aged wine. Its shape was like a thin stick - yes, it was Lactobacillus.

It is because of their massive reproduction that the wine becomes "sour juice". However, the aged wine that has not deteriorated contains a spherical yeast cell.

Lactobacillus (top) and yeast cells (bottom) under a microscope

In order to further solve the "souring" problem, he began to do experiments himself. This is not as simple as it seems. Sterilization is actually not difficult, just boil it for a period of time, but the result is that the wine is ruined.

Pasteur filled a wire basket with sealed wine bottles and soaked them in water, heating them to different temperatures. His goal was to find a way to get rid of these "troublemakers" while retaining the original flavor of the wine and beer.

After repeated experiments, he finally found a simple and effective method: as long as the wine is placed in an environment of 50-60 degrees Celsius for 30 minutes, the lactobacilli in it can be "caught in one fell swoop". This method is named after Pasteur, the famous "Pasteur sterilization method" (also known as low-temperature sterilization method).

Of course, milk is different from wine, and considering the needs of industrialization, the processing method needs to be as simple and convenient as possible. It is obviously not economical to process milk for 30 minutes like wine. Therefore, food scientists have explored several more efficient and rapid methods to process milk based on the idea of ​​pasteurization.

The difference between pasteurized milk, room temperature milk and ESL milk

Pasteurised milk

Pasteurized milk, also known as pasteurized milk, pasteurized milk, fresh milk, and market milk, is milk made from fresh milk, which has high requirements for the milk source and is processed by pasteurization. The purpose is to kill all pathogenic microorganisms in the milk. Its production process is to heat it at 75-85℃ for 10-15 seconds, and its shelf life is generally no more than 7 days. Due to low-temperature sterilization, on the basis of killing microorganisms, it can retain the natural and original nutrients of milk to the maximum extent.

UHT milk (Ultra High Temperature treated)

UHT milk is also known as room temperature milk, high temperature milk, ultra-high temperature instant sterilized milk, and ultra-high temperature heat-treated milk. Its production process uses UHT sterilization to kill all microorganisms in milk. UHT is a sterilization process for fresh milk. It uses ultra-high temperature instant sterilization at 135-140°C for 4-10 seconds. It does not need to be refrigerated below 10°C and has a shelf life of 1-6 months. Due to the high sterilization temperature, the flavor quality of UHT milk is greatly affected.

ESL milk (Extended Shelf Life)

That is, pasteurized milk with extended shelf life is produced on the basis of improving the sterilization process and improving the hygiene level of filling equipment. It mainly adopts ultra-pasteurization method, 125-130℃, heating for 2.5-4 seconds, and has a shelf life of 10-20 days.

This uses a higher sterilization temperature than pasteurized milk, but a lower sterilization temperature than UHT milk, and has a shorter shelf life. It is between ordinary pasteurized milk and UHT milk, and is essentially still pasteurized milk. ESL milk not only has the fresh taste of pasteurized milk, but also has a lower bacterial count in sterilized ESL milk than pasteurized milk. Its shelf life is longer than pasteurized milk, and the apparent color, aroma, taste, and retention of internal nutrients are very close to traditional pasteurized milk, so it is called the "new generation of pasteurized milk."

What is special about pasteurized milk in terms of nutritional content?

Pasteurization is still used after nearly 200 years, especially in milk. The main reason is that pasteurized milk is "naturally good" and contains almost all the nutrients needed by the human body: protein, fat, rich calcium, vitamins, essential trace elements, etc.

After years of testing, Taiwanese dairy experts have concluded that the lactoferrin content of pasteurized milk is 1,800 times that of normal temperature milk, and that it preserves the active calcium components in milk, which can be described as "self-contained". So how do we explain the "self-contained halo"? The nutritional components of pasteurized milk and normal temperature milk are not much different, but the nutrients in pasteurized milk are "alive", that is, pasteurized milk contains many active substances.

What do active substances do? Our human body is like an "engine". We can only function if we are supplied with raw materials, which are food and water. When the engine is started, it can produce the energy we need for normal life, and of course, active substances are also produced. Energy is what we consume; active substances are a group of "enthusiastic" small teams. For example, if there is a problem with the cell, they will go to repair it; if new cells are produced, they will also participate.

Raw milk contains some active substances, such as alkaline phosphatase, etc. These active substances can directly participate in the body's metabolism. If they are treated with high temperature, these active substances will be "killed". In addition, after UHT sterilization, the immune active protein in whey protein will almost disappear.

Therefore, although pasteurized milk and ambient temperature milk contain the same amount of nutrients, their utilization rate and actual value are worlds apart.

At this point, many people will ask: Wouldn’t it be better to drink freshly squeezed milk directly?

Of course not. You can't drink freshly squeezed milk casually. Raw milk that has not been processed by pasteurization and ultra-high temperature sterilization contains tuberculosis and Brucella, which can cause harm to human health.

Having said so much, I believe you will be able to make your own rational choice between "fresh" and "not fresh", "nutritious" and "convenience".