This thing has become very popular recently! If your child is playing it, please stop it immediately!

Among primary school students everywhere, a more "unconventional" hobby has quietly emerged - raising smelly water .

Stinky water refers to putting all kinds of things in life, including feces, urine, insect corpses, leftovers, fruits, drinks, etc., as long as the primary school students can think of and get them, in a sealed bottle. After a period of time, these substances begin to rot, produce odors, and some even explode. Primary school students are very interested in this phenomenon and even regard it as a kind of cultivation game.

The stinky water raised by Nanyu classmate (Source: Nanyu classmate, authorized)

Let me first state the conclusion:

Don't raise

Raising stinky water is not a fun thing, it will bring safety and health risks, so don’t try it. If your children have it, you can read this article with your children and deal with the stinky water together.

Next, from the perspective of fermentation, we will talk about why the smelly water smells bad, where the danger is, and how to do the fermentation correctly.

Why does smelly water get smellier as it gets older?

The main way to make smelly water is through microbial fermentation. The main reason for the smell is that microorganisms release various volatile compounds when decomposing organic matter. These compounds often have strong odors. Different organic matter produces different gases:

1

Protein breakdown

When microorganisms break down proteins, they produce ammonia, hydrogen sulfide, and various amine compounds. These compounds often have a strong rancid smell. For example, hydrogen sulfide smells like rotten eggs, while amine compounds have the smell of rotting fish.

2

Carbohydrate breakdown

During the fermentation process, carbohydrates are broken down into short-chain fatty acids, alcohols and esters. Although some of these compounds may have aromatic odors, in some cases they can also produce unpleasant sour and fermented odors.

3

Lipolysis

When microorganisms break down fats, they produce volatile fatty acids such as butyric acid, caproic acid and caprylic acid, which are compounds with a strong, sour odor.

Food containing protein, carbohydrates, and fats spoils and produces a foul odor (Source: AI generated by the author)

So, here comes the first point. The reason why the stinky water is so smelly is because a large amount of organic matter is added.

However, it is not enough to just add a lot of organic matter to make the water smelly. Many fermentation processes are carried out in an oxygen-deficient environment. In this case, anaerobic microorganisms (such as certain bacteria and yeast) will carry out fermentation metabolism and produce smelly compounds. For example, in an anaerobic environment, methanogenic bacteria will produce methane and other odorous byproducts.

The second key point is that smelly water requires anaerobic fermentation. Open the bottle frequently to check if it smells bad, as this will affect the fermentation process.

Many fermentation processes are carried out in an oxygen-deficient environment (Source: AI generated by the author)

Of course, the smell of foul water is also related to the type of microorganisms, and different microorganisms produce different metabolites during the fermentation process. For example, some anaerobic bacteria produce volatile fatty acids and other smelly compounds, while yeast produces alcohols and esters during alcohol fermentation, which sometimes also have a strong odor.

Here are two more examples:

Stinky tofu: The odor of stinky tofu mainly comes from ammonia and hydrogen sulfide produced during the fermentation process. During the fermentation process of tofu, the protein is decomposed by microorganisms, producing these compounds with strong odors.

Natto: The bad smell of natto comes from the polyamines (such as putrescine and cadaverine) and ammonia produced by bacteria (mainly Bacillus subtilis) breaking down the protein in soybeans during the fermentation process.

Why does smelly water explode?

During the fermentation process, microorganisms break down organic matter (such as sugars, proteins, fats, etc.) to obtain energy and produce metabolites. In an oxygen-deficient environment, microorganisms perform anaerobic respiration, breaking down organic matter into small molecules and releasing gas as a byproduct. Some common gas-producing fermentations are as follows:

1

Alcoholic fermentation

Yeast breaks down glucose in an anaerobic environment to produce alcohol and carbon dioxide gas.

2

Lactic acid fermentation

Some lactic acid bacteria produce lactic acid and carbon dioxide when they break down sugars.

3

Anaerobic fermentation

Some anaerobic bacteria, such as methanogens, produce methane and carbon dioxide during fermentation.

4

Nitrate reduction

Some bacteria are able to reduce nitrates to nitrogen gas or nitrous oxide, which are released during the fermentation process.

The reason for explosions when fermenting in bottles is that during the fermentation process, microorganisms decompose organic matter to produce gases (such as carbon dioxide and methane). In a sealed bottle, these gases cannot escape, causing the internal pressure to continue to rise. When the pressure exceeds the tolerance limit of the bottle, the bottle will explode. This risk is very high when fermenting quickly and using non-pressure-resistant containers, which will pose a direct safety hazard.

Stop raising stinky water, it can cause serious harm

The above article talks about the principles of keeping smelly water. The purpose is to popularize the fermentation principle of keeping smelly water to everyone. It is really not recommended to keep smelly water. After all, keeping smelly water can cause many harms. Here are a few simple examples:

1

Respiratory tract infections

In the process of keeping the smelly water, a large number of bacteria and fungi multiply. Once the container explodes, these microorganisms will form droplets and enter the air. If people inhale these droplets, bacteria, fungi and other microorganisms will enter the respiratory tract and cause infection. In mild cases, it may cause symptoms such as coughing and sore throat, and in severe cases, it may induce serious respiratory diseases such as pneumonia.

Diagram of fungal infection in the lungs (Source: AI generated by the author)

2

Local infection

If the stinky water accidentally splashes into the eyes and contacts the mucous membrane, it will cause local infection. The eyes are a sensitive part, and contact with stinky water containing a large number of bacteria may cause eye infections such as conjunctivitis, which manifests as redness, swelling, tearing, pain and other symptoms. In severe cases, it may also affect vision.

3

Intestinal infection

If you accidentally ingest liquid or solid matter in stinky water, it can also cause gastrointestinal infections. Smelly water contains a large number of harmful microorganisms, which, after entering the intestines, will destroy the normal intestinal flora, causing gastrointestinal symptoms such as abdominal pain, diarrhea, and vomiting. In severe cases, it may cause more serious intestinal diseases.

4

Mechanical injury

Container explosions not only release harmful microorganisms, but also cause mechanical damage. Flying fragments may cause skin scratches, eye injuries, etc. Especially when glass bottles explode, fragments fly out at high speed, which can easily cause serious cuts or punctures. If fragile parts such as eyeballs are damaged, it may cause vision loss or even blindness.

Finally, even if no one is directly hurt, isn’t it disgusting that a bottle of stinky water explodes?

What is the actual fermentation process like?

Now that we understand the principles and hazards of self-fermented stinky water, let's expand on this and talk about what regular fermentation is all about. Don't think that microbial fermentation can only produce garbage like stinky water. Fermentation engineering is everywhere in our daily lives. Many of the foods and beverages we are familiar with and love are made through fermentation technology.

For example, yogurt, bread, and cheese in breakfast, soy sauce and vinegar in cooking, kimchi in dinner, and even beer and red wine at parties are all the results of fermentation engineering. Fermentation not only gives these foods unique flavors and textures, but also enhances their nutritional value and health benefits.

Of course, the actual fermentation project is not as casual as making "stinky water". The fermentation process requires specific conditions to ensure that microorganisms can grow and metabolize effectively, thereby producing high-quality fermented products.

1

Suitable temperature

Different microorganisms have optimal growth and fermentation activity at different temperatures. Generally, yeast ferments best at temperatures between 20 and 30°C, while lactic acid bacteria perform best in the temperature range of 30 to 40°C.

2

Suitable pH environment

Microorganisms have specific requirements for pH during fermentation. Yeast fermentation usually occurs in the pH range of 4.0-6.0, while lactic acid bacteria grow better in an environment of pH 5.0-6.5. By adjusting the pH of the culture medium, the fermentation efficiency can be optimized.

3

Oxygen requirements

Different types of fermentation have different requirements for oxygen. Aerobic fermentation (such as acetic acid fermentation) requires sufficient oxygen, while anaerobic fermentation (such as ethanol fermentation) requires an oxygen-free environment. Proper ventilation and sealing control are key steps in the fermentation process.

4

Nutritional supply

Microorganisms need nutrients such as carbon sources, nitrogen sources, vitamins and minerals to maintain growth and metabolism. The culture medium should contain enough glucose, amino acids, vitamins and inorganic salts to meet the nutritional needs of microorganisms.

5

Preventing contamination

During the fermentation process, sterile operation must be maintained to prevent contamination by foreign bacteria. Using sterile equipment, sterile culture medium and strict operating procedures are effective measures to prevent contamination.

6

Proper fermentation time

Different fermentation processes require different times. Fermentation times that are too long or too short will affect the quality of the final product. By monitoring the gas produced, acidity and other indicators during the fermentation process, the optimal fermentation time can be determined.

7

Evenly mixed

In order to ensure that the microorganisms are evenly distributed in the culture medium and obtain uniform nutrient supply and metabolite discharge, proper stirring and mixing are usually required during the fermentation process.

8

Stress Management

In some fermentation processes, especially anaerobic fermentation, controlling the pressure inside the container is also key. Excessive pressure will inhibit the growth of microorganisms, while appropriate decompression can improve fermentation efficiency.

Summarize

Primary and secondary school students' interest in keeping stinky water stems from many reasons, including a little mischievous mentality, but also curiosity about the activities of microorganisms in nature, the desire to explore the complexity of the ecosystem, and the fun of participating in experiments and observations. This interest reflects children's natural spirit of exploration and desire to learn, and is a good start for scientific education.

We should not discourage children's curiosity, but we should also take this opportunity to warn them that when doing scientific experiments, they should clarify the purpose of the experiment, understand the basic knowledge and theory, control the experimental conditions, use appropriate equipment, and most importantly, ensure safety. Otherwise, blind attempts will not lead to real knowledge, and reckless operations will bring accidents and troubles.

Planning and production

Author: Denovo, PhD in Analytical Chemistry

Review丨Liang Chengzhu, Researcher at Qingdao Customs Technology Center

Planning丨Ding Zong

Editor: Ding Zong

Proofread by Xu Lailinlin