Yogurt really helps digestion! But it's not what you think...

Have you seen the news recently about a certain brand of lactic acid bacteria being fined for passing it off as organic food?

In addition to the various "functions" marked in the publicity (weight loss, digestion aid, etc.), there are more and more types of lactic acid bacteria drinks, which often makes us dazzled when purchasing.

In order to know the difference between different lactic acid bacteria, we often look at the ingredient list and find that in addition to raw milk, some of the ingredients include four bacteria: Lactobacillus bulgaricus, Streptococcus thermophilus, Bifidobacterium lactis, Lactobacillus acidophilus, and some include two bacteria...

Everyone is wondering, isn't yogurt made by adding microorganisms to ferment milk? Are all these bacteria used for fermentation?

Speaking of this, anime fans will definitely think of the popular science anime "The Story of Cute Fungi". The freshman Zemu from the Agricultural University has a superpower - he can see various microorganisms with the naked eye. Obviously, as ordinary people, we do not have this superpower.

In fact, we can see that Lactobacillus bulgaricus, Bifidobacterium, etc. listed in the ingredient list are all lactic acid bacteria. They are the "heroes" behind yogurt. How much do you know about lactic acid bacteria?

A list of yogurt ingredients (left); "The Story of Moe Mushrooms" (right)

What are lactic acid bacteria?

Lactic acid bacteria (LAB) is a general term for a type of bacteria that is a prokaryotic organism that can utilize carbohydrates for fermentation and ultimately produce a large amount of lactic acid.

The metabolic type is heterotrophic anaerobic, that is, it survives on existing organic matter in an oxygen-free environment. Most of the metabolites of lactic acid bacteria are beneficial to human health, so they are classified as probiotics.

The history of lactic acid bacteria can be said to be very long. It is recorded that in ancient times, humans used the principle of microbial fermentation in food brewing and processing, and yogurt existed abroad as early as 4,500 years ago. However, it was not until the mid-19th century that French scholar Pasteur discovered the existence of lactic acid bacteria when studying the reasons why wine turns sour.

Since then, scholars have begun to discover other types of lactic acid bacteria, including Streptococcus lactis, Lactobacillus, Bifidobacterium... With the in-depth research of more scholars, the large family of lactic acid bacteria is constantly being discovered.

At present, there are 18 genera and about 200 species of lactic acid bacteria discovered, including Lactococcus, Lactobacillus, Bacillus, Bifidobacterium, etc. They are widely distributed in nature. Lactic acid bacteria have been found in human food, animal feed, soil and even the natural environment. They have their own characteristics in morphology, some are rod-shaped, some are spherical... but their fermentation functions are basically the same.

Morphological characteristics of Lactobacillus and Streptococcus lactis

Source: "Isolation and Identification of Lactic Acid Bacteria in Naturally Fermented Yak Yogurt"

The power of lactic acid bacteria

Yogurt is made from fresh milk. First, the milk is sterilized, then lactic acid bacteria are added to the milk. After fermentation and cooling, we can get the yogurt we drink. How does lactic acid bacteria exert its "power" to make milk sour during fermentation?

We break down the fermentation of yogurt into three steps:

In the first step, lactic acid bacteria begin to rapidly proliferate in milk. When there is no glucose to decompose, lactic acid bacteria will find a way to use the β-galactosidase produced by metabolism.

In the second step, β-galactosidase "cuts" the lactose in the milk into two substances: glucose and galactose;

In the third step, lactic acid bacteria use the newly produced glucose to perform anaerobic respiration. The glucose is broken down into pyruvic acid and water, releasing some energy. The two are combined to synthesize lactic acid, which makes the yogurt sour.

Simply put, the sour taste of yogurt comes from the lactic acid produced after the breakdown of lactose. Therefore, people who are lactose intolerant will not have adverse reactions when drinking yogurt. Lactic acid bacteria are the "heroes" in making yogurt.

Does lactic acid bacteria help digestion?

Many people choose to drink yogurt, partly because it tastes good and they like it, and partly because they think that drinking yogurt helps digestion and defecation. So more and more people drink a cup of yogurt after meals, especially after a greasy dinner, believing that drinking it will make defecation smoother the next day and reduce the burden on the stomach. Can yogurt really regulate the stomach and intestines and aid digestion?

In 2011, the World Gastroenterology Organization (WGO) pointed out that probiotics play an important role in relieving constipation and diarrhea; a few years later, the organization put forward a further view: probiotics have a certain effect in preventing and treating gastrointestinal diseases.

Therefore, scholars and experts have launched a series of academic studies to verify whether lactic acid bacteria in yogurt are beneficial to intestinal health. The results show that the effect of lactic acid bacteria on the human intestine varies from person to person, and more importantly, lactic acid bacteria cannot survive and "settle down" and "reproduce" in the human intestine.

Freshly produced yogurt does contain a large number of live bacteria, but it needs to go through packaging, transportation, and sales before it can reach us. The lactic acid bacteria will die due to temperature changes during the journey; secondly, the lactic acid bacteria enter the intestines and experience a series of "tortures" from gastric juice, digestive juice, and bile. If they are lucky enough to arrive in the intestines alive, they have to compete with other microbial communities gathered in the intestines. Therefore, the probability of lactic acid bacteria in yogurt surviving and successfully multiplying in the intestines is almost zero.

What science says about "yogurt aiding digestion" is actually not the same thing as what people think about yogurt aiding digestion!

During the yogurt fermentation process, lactic acid bacteria break down part of the lactose in the milk and convert it into lactic acid. Lactic acid can combine with calcium to form calcium lactate, which is easier for the human body to absorb.

From this perspective, yogurt is indeed "aiding digestion", so we must look at the efficacy of lactic acid bacteria in yogurt scientifically and not drink it blindly!

Various microorganisms in the human intestine (Source: Reconstructed image from Miyuansu.com)

References

[1] Jin Xinyan, Yan Zhongxin. Isolation and identification of lactic acid bacteria in naturally fermented yak yogurt[J]. Qinghai Journal of Animal Husbandry and Veterinary Medicine, 2021, 51(05): 5-10.

[2] Zmora N, Zilberman-Schapira G, Suez J, et al. Personalized Gut Mucosal Colonization Resistance to Empiric Probiotics Is Associated with Unique Host and Microbiome Features[J]. Cell, 2018, 174(6):1388-1405.

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Audit expert: Wang Guoyi, Postdoctoral Fellow in Food Safety

Tadpole Musical Notation original article, please indicate the source when reprinting

Editor/Heart and Paper