For instance, if milk goes sour, is it always bad or is it yoghurt or kefir?

The line between “good” and “bad” bacteria is often not as clear-cut as people think. It depends on the specific bacteria involved and the conditions in which they grow.

Good vs. Bad Bacteria in Food
Good bacteria: Some bacteria are beneficial and intentionally used in food fermentation. For example: Lactobacillus and Bifidobacterium in yogurt, kefir, and sauerkraut help digestion and gut health.
Streptococcus thermophilus and Lactococcus species turn milk into yogurt or cheese in a controlled way.

Bad bacteria: Some bacteria cause spoilage or even illness. For example: Pseudomonas and Clostridium species can spoil milk, making it taste and smell unpleasant.
Listeria, Salmonella, and E. coli are harmful bacteria that can grow in improperly stored dairy products and cause foodborne illness.

Sour Milk vs. Kefir/Yogurt
If milk naturally sours due to random bacterial contamination from the environment, it’s not necessarily safe to consume. It may contain unwanted bacteria that cause spoilage rather than beneficial fermentation.

Yogurt and kefir, on the other hand, are made by adding specific strains of beneficial bacteria in a controlled fermentation process. This ensures the growth of good bacteria while keeping harmful ones out.

The Key Difference
Controlled fermentation = good bacteria dominate → safe & healthy (yogurt, kefir).
Uncontrolled spoilage = unpredictable bacteria mix → potentially unsafe (sour milk left out).
If milk sours naturally, it might still be okay in some cases (e.g., traditional raw milk fermentation), but with modern pasteurized milk, it’s usually best to err on the side of caution and not drink it.

Or maybe we can get used to some bacteria and then we tolerate it?

Yes! Our bodies can adapt to certain bacteria over time, which is why some people tolerate foods or environments that others find harmful. This happens in a few ways:

1. Gut Microbiome Adaptation
Your gut microbiome (the trillions of bacteria in your digestive system) changes based on your diet and environment. If you’re regularly exposed to certain bacteria, your body may become better at digesting or tolerating them.
Example: People who frequently consume fermented foods (like kimchi or raw dairy) develop a gut microbiome that helps them digest these foods without issues.
Another example: In some regions, people drink water with bacteria that would cause stomach upset in travelers, but locals have developed a tolerance.

2. Immune System Training
Exposure to bacteria can help “train” your immune system. If you regularly encounter low levels of a particular bacteria, your immune system may learn to tolerate it rather than overreact.
Example: Kids raised on farms or with pets tend to have stronger immune systems and lower allergy rates due to early exposure to diverse microbes.

3. Beneficial vs. Opportunistic Bacteria
Some bacteria can be harmless (or even helpful) in small amounts but become dangerous in large numbers. If your body is used to low levels of a certain bacteria, you may tolerate it better than someone encountering it for the first time.
Example: Some strains of E. coli are a normal part of the gut, but if they overgrow (or if you’re exposed to a harmful strain), they can cause illness.

So, Can We Get Used to Spoiled Milk?
In theory, if you were regularly exposed to sour milk with a stable bacterial composition, your body might adapt. However, random spoilage introduces unpredictable bacteria, including harmful ones, so it’s not safe to rely on adaptation in that case. But with controlled fermentation (like kefir or yogurt), your gut can absolutely adjust and thrive on those bacteria!
It’s all about gradual exposure, balance, and context! 😊

I’ve been experimenting with making yoghurt by keeping milk warm, seeded with yoghurt culture for several days. It tastes very sour, unlike the commercial yoghurt. It’s an acquired taste.