Take my advice, don’t do your own “fermentation project” at home anymore!

In life, people often like to try various DIY projects, from furniture, decoration to food production. Self-creation always brings unique fun. Among them, using fermentation, an ancient bioengineering technology, to make various foods seems to have become a new trend. Homemade wine, homemade yogurt - these seemingly simple operations are attracting more and more families to try. Today, let's take a look at what misunderstandings are easy to fall into when doing "fermentation projects" at home, and what risks they will bring.

It’s too long, so don’t read it. Don’t do it casually at home, or you may get into trouble.

It's just fermentation , why would it explode?

Passion fruit is loved by everyone for its rich aroma and sweet and sour taste, but when you try to seal this tropical delicacy for later use, you may encounter a surprising problem - passion fruit pulp sometimes "explodes" in a sealed bottle. What factors cause this strange phenomenon?

Sealed passion fruit jam explodes (Source: Sina Weibo screenshot)

Passion fruit itself is highly rich in sugars and other organic components that are easily fermented. When the flesh is scooped out of the peel, naturally occurring microorganisms in the air, especially yeasts and bacteria, quickly invade. In the sugar-rich environment of the passion fruit flesh, these microorganisms rapidly multiply and begin to break down the sugars, further triggering the fermentation process. Fermentation is a microbial metabolic activity that produces a variety of byproducts, one of which is carbon dioxide gas.

If you store passion fruit pulp in a completely sealed container, such as a plastic bottle, the carbon dioxide gas will have nowhere to escape, causing the pressure inside the container to gradually increase. When the pressure accumulates to a certain critical point, the container will no longer be able to withstand it and will naturally explode (and splash all over your walls, floor, and even ceiling...). Similar to passion fruit pulp, chili sauce faces the same problem. Chili sauce also contains fermentable sugars and other organic matter, which provides a platform for microorganisms to reproduce and produce gas, so it also has the risk of explosion.

So, how can we avoid this risk? An effective strategy is freezing. Low temperature environment will significantly reduce the activity of microorganisms and almost completely inhibit their metabolic processes. At the same time, low temperature will also inhibit various chemical reactions, thereby further reducing the generation and accumulation of gas. Therefore, freezing can not only effectively control microbial activity, but also extend the shelf life of food.

It is very important to understand the fermentation principles and potential risks of foods such as passion fruit sauce and chili sauce. Only by mastering scientific storage and handling methods, especially how to properly freeze and preserve them, can we effectively reduce these risks and ensure the safety and quality of food.

How come there is methanol when I make my own wine ?

Now everyone knows that fake liquor mixed by unscrupulous counterfeiters contains methanol and cannot be drunk. But did you know that homemade wine can also produce toxic methanol during the fermentation process, posing a safety hazard?

Home-brewed wine may contain methanol (Source: Photo by the author)

People cannot distinguish methanol and ethanol by taste when drinking alcohol, but methanol is different from ethanol. It is a highly toxic chemical that can enter the human body through the respiratory tract, skin, and digestive tract, causing temporary or permanent blindness. Accidental ingestion of 5 grams of methanol may cause serious poisoning symptoms, while more than 12.5 grams may be fatal.

Many people may wonder, since the main component of wine should be ethanol, why does methanol still occur? This is mainly related to the rich pectin in grapes and other fruits (such as citrus peels, hawthorns, dates, apples, etc.). Under the catalysis of pectinesterase, pectin will decompose into methanol and pectic acid. Therefore, the methanol content in wine is directly related to the pectin content in the raw materials used.

Most of the pectin in grapes is concentrated in the grape skins, and the grape skins need to be soaked for a long time in the production of red wine. Therefore, generally speaking, the methanol content is the highest in red wine, the lowest in white wine, and the middle in rosé wine. According to the national standard GB/T 15037-2006, the upper limits of methanol content in red wine, white wine, and rosé wine are 400 mg/L and 250 mg/L respectively. Only wines with methanol content below the standard can be consumed.

In addition to methanol produced by pectin, if you do not use special yeast for fruit wine during the home-brew process, some miscellaneous bacteria will easily multiply, which will also cause the methanol content of home-brewed wine to increase.

How can commercially produced wines contain less methanol than the standard? Generally, we can start from the selection of raw materials and production processes to make wines and fruit wines safer.

In terms of raw material selection, grapes themselves contain pectinase. If they are damaged mechanically and become moldy, their natural pectinesterase activity can be enhanced by the immersion of fruit juice. Such wine will also lead to an increase in the methanol content of the fruit wine. Therefore, the factory will start with material selection and choose fresh, well-matured, and mold-free raw materials for processing, which can reduce the introduction or production of methanol.

At the same time, in industrial-scale wine production, fermentation conditions are usually strictly controlled to minimize the production of methanol and other harmful substances. Advanced production techniques such as distillation are used to effectively remove methanol, so commercial wines usually contain no or only trace amounts of methanol.

The process of home brewing is relatively simple, and the equipment is not as advanced as the industrial level. Due to the lack of strict control over factors such as temperature, acidity, and sweetness during the brewing process, the home brewing environment is usually more unstable than the industrial environment, so it is more likely to produce methanol. In addition, since preservatives such as sulfur dioxide are not used, the quality and safety of home brewed wine cannot be effectively guaranteed. Many media have also reported on the problem of excessive methanol.

Methanol warning! (Source: Screenshot of relevant website)

Can a router make yogurt ? Beware of mixed bacteria

Many people like yogurt very much, but often criticize supermarket products for their high prices and excessive sugar content. This makes home yogurt machines a popular choice, and many people believe that homemade sugar-free and additive-free yogurt must be healthier than commercial products. But is this really the case?

The core function of a home yogurt machine is to provide a stable and constant temperature environment so that yogurt fermentation bacteria (usually thermophilic streptococci and lactobacillus) can grow and ferment under the most suitable conditions. The equipment usually maintains a relatively stable temperature range, generally at 35-45°C, which happens to be the most suitable temperature for most yogurt fermentation bacteria to grow. Yogurt machines usually also have a timing function to ensure the proper fermentation time.

In this environment, fermentation bacteria begin to multiply and break down the lactose in the milk to produce lactic acid. The production of lactic acid lowers the pH of the milk, causing the milk to coagulate, thus forming the smooth yogurt we are familiar with.

But if you don't have a yogurt machine, can you mix leftover yogurt with fresh milk and leave it overnight to make homemade yogurt? In theory, this is indeed a possible method. However, success depends on multiple variables, including but not limited to fermentation conditions and microbial population strengths.

Since the home environment is usually not sterile, the milk added to the leftover yogurt is easily contaminated by other microorganisms, including harmful microorganisms. At room temperature, the yogurt fermentation bacteria cannot form a "population advantage" to inhibit the growth of miscellaneous bacteria. When using a yogurt machine to make yogurt, the yogurt fermentation bacteria will need to reach a "population advantage" in a shorter period of time to inhibit the growth of other harmful or unwanted microorganisms.

There are many microorganisms in milk that has been left at room temperature overnight, including yogurt fermentation bacteria - lactobacillus, and some harmful bacteria. Lactobacillus grows and reproduces slowly at room temperature, while harmful bacteria reproduce quickly. Therefore, if you are not careful, you may not get a bottle of freshly brewed yogurt, but a bottle of "stinky milk".

Some netizens even found that their routers at home would heat up when in use, so they had an idea and placed fermentation bacteria + milk on the router overnight, thinking that this would be a good substitute for the yogurt machine. However, this is not recommended. Although the router can generate heat, the temperature at which it generates heat is not constant. Once the population dominance of the yogurt fermentation bacteria is not formed quickly, the fermentation will fail. If the temperature is too low, before the population dominance of the yogurt fermentation bacteria is formed, other bacteria will also multiply and produce harmful substances, which will remain in the yogurt and cause harm to the human body; if the temperature is too high, the yogurt fermentation bacteria will be killed and cannot continue to ferment into yogurt.

Summarize

Although it may seem interesting to do "fermentation projects" at home, they actually involve multiple complex biochemical processes and potential risks. The home environment often lacks the necessary sterile conditions and professional equipment, making it difficult to ensure the safety and controllability of the microbial fermentation process, and we usually cannot perform quality inspections at home.

More importantly, incorrect fermentation operations may not only cause food to spoil or lose nutritional value, but may also produce toxic substances, posing a potential threat to human health. Therefore, in the absence of professional knowledge and equipment, it is strongly recommended that you do not perform this type of "fermentation project" at home.

References

[1] Yu Qingqin, Zhang Yingchao, Wang Yongmei, Yang Liying, Shi Hongmei. Methanol in wine and fruit wine and its reduction measures[J]. Chinese and Foreign Grapes and Wine, 2019(04):64-67.

[2] Wei Xiaofeng, Yang La, Uwamahoro Francoise, Chen Qiuchong, Li Yumeng, Fang Yulin. Research progress on potential harmful substances in wine[J]. Chinese and Foreign Grapes and Wine, 2018(06):92-96.

[3] Wang Yanhua. Exploring the optimal process conditions for making yogurt with a household yogurt machine[J]. Agricultural Products Processing, 2015(21):28-29.

[4] Wang Yuhong, Li Meijuan, Sun Ruilin, et al. Study on methanol content in home-brewed wine[J]. Preservation and Processing, 2018, 18(5):5.

[5] Zhang Guangfang, Zeng Guifeng, Zeng Lingwen. Detection and analysis of methanol and miscellaneous alcohols in homemade wine[J]. China Food Additives, 2016(5):6.DOI:10.3969/j.issn.1006-2513.2016.05.017.

Planning and production

Author: Denovo Popular Science Writer

Review | Ruan Guangfeng, Deputy Director of Kexin Food and Health Information Exchange Center

Planning | Ding Zong

Editor: Ding Zong