Is homemade yogurt reliable?

After interviewing relevant experts, I found that there are several hidden concerns in homemade yogurt:

Self-made conditions are limited, and miscellaneous bacteria take advantage of it.

Generally speaking, yogurt produced by the factory has to go through strict technological process. For example, in the production of solidified yogurt, fresh milk should go through many processes, such as milk purification, standardization, batching, preheating, homogenization, sterilization, cooling, inoculation of starter, filling and sealing, fermentation, cooling and post-ripening. The production of stirred yogurt is slightly different, and the previous process is similar. Then fermenting in a vat, stirring, pulverizing, cooling, filling or mixing with spices and fruits before filling.

Sterilization is an important step in the complex process of yogurt production in factories. The purpose of sterilization is to remove harmful bacteria from raw milk, reduce redox reaction and improve the hardness and texture of milk.

Sterilization should be carried out at 90℃ for 5~ 10 min, and the sterilized mixture must be cooled to 40 ~ 45℃ for later use. Or adopt high-temperature sterilization and heat at 120℃ for about 3~5 seconds. This is beneficial to the preservation of nutrients and reduces the taste of cooking.

Strain preparation and fermentation are the key technologies in yogurt production. In the fermentation process, the starter made of pure culture of lactic acid bacteria was fermented at 42 ~ 45℃, which was the optimum growth temperature for the mixture of Lactobacillus bulgaricus and Streptococcus thermophilus. Inoculation amount, starter activity and culture temperature are the three most important factors in production.

It is impossible to guarantee strict sterilization conditions when making yogurt at home at first. Even if the milk is boiled, all the related utensils for preparing yogurt are disinfected in boiling water, and it is easy to be polluted by other miscellaneous bacteria during the operation. Secondly, yogurt can't be fermented at 42 ~ 45℃ all the time without using a thermostatic container.

"As netizens said, it is not advisable to ferment yogurt in a thermos cup or put the container in the heating pipe or the trunk of a car. First, the temperature is not well controlled. Second, the homemade yogurt becomes solid after several hours of fermentation. If it is not handled in time, it will be frozen in the refrigerator, and miscellaneous bacteria will easily mix in. " Nan Qingxian, a professor at China Agricultural University and a famous expert in animal products processing, said, "It is more dangerous to drink yogurt mixed with more bacteria. Drinking almost will make you sick, especially children. "

Self-purchased strains have hidden concerns.

When sharing the experience of homemade yogurt, people often mention buying yogurt machines and some strains. Unprecedented strains such as Saussurea involucrata, Tibetan mushroom and Caucasian mushroom have appeared on the Internet, which makes it more difficult to identify the original yogurt.

Some strains also put up propaganda slogans such as "probiotics, bifidobacteria, homemade yogurt, QS certified bacteria powder only". In fact, the safety of self-purchased strains, especially those purchased at random on the Internet, cannot be guaranteed. Whether these purchased microbial agents are really QS certification remains to be discussed, and some microbial agents do not even indicate the manufacturer and production location.

Self-purchased strains are usually presented in the form of powder, and ordinary people can't visually see whether the strains are mixed with other miscellaneous bacteria. Due to the influence of temperature, online shopping strains are easy to deteriorate during transportation, which leads to no effect when adding bacterial powder to homemade yogurt. Or yogurt is made of "incorrect" strains, and the naked eye cannot tell whether yogurt is safe or not.

Nan Qingxian said that the microbial inoculum of homemade yogurt should be purchased from regular merchants as much as possible. When buying, be sure to see clearly the ingredients, manufacturer, QS mark and production date of the microbial inoculum.

"If you want to make your own yogurt, it is not recommended to buy online strains. You can choose the big brand yogurt on the market as the strain. Don't buy yogurt made in a small workshop as a strain. Be careful not to reuse homemade yogurt as a strain many times, because there will be mixed bacteria breeding. " Nan Qingxian said.

If you insist on homemade yogurt, you must pay attention to the following points:

1. Use qualified milk for homemade yogurt. You can choose fresh milk or pasteurized milk. Fresh milk should be boiled and cooled to a certain temperature before use. Other sterilized milk such as pasteurized milk does not need to be boiled.

2. Choose famous brand yogurt as strain, put the milk into a sterilized container or directly pour the yogurt strain into a newly opened boxed milk, and put it into a thermostat (usually a yogurt machine) to keep the fermentation process at a temperature suitable for the growth of the strain;

3. The air environment at home does not meet the hygienic requirements of yogurt production. Homemade yogurt should be stored in the refrigerator immediately after it is made and drunk within two days.

4. Homemade yogurt can't be reused as a strain, which is easy to breed miscellaneous bacteria.