Mineral water has a strange taste? It's not your illusion

Review expert: Zhu Guangsi, member of Beijing Science Writers Association

Water is an important substance to maintain our life. People need to consume about 1500-2000mL of water every day, which is equivalent to about 3-4 bottles of bottled water.

I wonder if you have noticed this situation: if you don’t finish the bottled water in one day, you will smell a strange smell when you open it the next day. What is this smell?

1

Does “plastic migration” change the taste of water?

What causes the smell in water? Many people would think it is the smell of plastic bottles.

A new study led by scholars from the Medical University of Vienna, Austria, shows that if a person drinks plastic bottled water all year round, he or she will ingest nearly 100,000 microplastic and nanoplastic (MNP) particles each year .

Although it sounds scary, it is actually normal for bottled water to contain very small amounts of microplastics and particles, and under normal circumstances, these substances will not affect the taste of bottled water or normal drinking.

Plastic bottles consist of a bottle cap and a bottle body, of which the bottle cap is generally made of polyolefins, while the bottle body is mostly made of PET. Due to its good tensile and impact resistance properties, the bottles made of PET have the advantages of good transparency and anti-permeability.

Source: pixabay

Selina Tisler and Jan Christensen of the University of Copenhagen in Denmark found in an experiment that hundreds of different chemicals could be detected in water placed in plastic bottles after being left for several hours .
To this end, they placed the same drinking water in new bottles, used bottles, and bottles washed in a dishwasher, and analyzed the chemicals in the different bottles after 24 hours. Non-target screening (NTS) was performed by liquid chromatography-high resolution mass spectrometry (LC-HRMS) to identify these compounds. Ultimately, they believed that the chemicals in the plastic could migrate into the water.

Source | sciencedirect

However, the two scientists also pointed out that this does not mean that "water is toxic" ! In fact, plastics contain many substances, such as plasticizers, softeners, and photoinitiators. Although these substances are extremely toxic and dangerous to the human body to varying degrees, bottled water produced in strict accordance with the relevant standards for food contact materials will not cause great harm to human health if it is left for a period of time before drinking. Most of the time, it will only affect its taste !

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There are many factors that change the taste of water!

Obviously, bottled water may have odors in a series of processes, from production and processing to delivery. For example, antimony oxide is used as a catalyst in the processing of polyethylene terephthalate (PET) during the production process. When bottled water is exposed to high temperatures, a small amount of antimony may dissolve into the water, changing the taste of the water . At the same time, if a single PET bottle is used for a long time, antimony may accumulate in the human body, causing toxicity.

For example, during the processing, the traditional cap-making process - injection molding process, mainly uses an injection molding machine to make the cap. In this process, manufacturers not only use certain additives, but also the processing temperature is as high as 240℃ to 280℃. Therefore, some unstable materials will undergo partial chemical changes and produce irritating odors .

For example, PET material is semi-permeable, so even if the bottle is well sealed, foreign odors may contaminate the product . At this point, we need to focus on the source of these foreign odors. If they are non-toxic and harmless, they may only change the taste of the water. However, if they are toxic or harmful substances, they are likely to be harmful to the human body.

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Water bottles exposed to the sun also have a smell!

I wonder if there are readers who, like me, will pay special attention to whether there is a message "avoid direct sunlight" on the packaging bag when buying food with plastic packaging. Why? Don't worry, read on!

PET, also known as thermoplastic polyester, is milky white or light yellow with a smooth and shiny surface. It can be found that PET has a fairly regular structure and is a symmetrical molecular chain with regular linear macromolecules. In addition, it also has a strong crystallization ability. But it is a semi-crystalline polymer. This is mainly because of its molecular chain rigidity and high glass transition temperature, which greatly affects the movement of its molecular chain. It is precisely because of this that the microscopic relaxation movement of its molecular chain plays a key role in the performance of PET!

Molecular structure of PET Source | Wikipedia

Therefore, after relevant processing, it can be transformed into non-crystalline PET (typical non-crystal: glass) with no fixed form. This is actually a process from an ordered to a disordered state, disrupting the ordered molecular structure of PET and rearranging it randomly. In this process, the changes in microstructure and macroscopic physical phenomena often interact and manifest.

First, at the microscopic level, it is mainly manifested in the depolymerization reaction such as thermal degradation under high temperature and pressure, which is susceptible to the effects of acid, alkali and trace amounts of water. After the depolymerization reaction occurs, the macromolecular chain breaks and the relative molecular weight decreases. For example, under long-term high temperature conditions, PET will release some small molecules, such as acetaldehyde . Although acetaldehyde is non-toxic, there will be a special gas that affects the taste! At this time, although the water in the cup does not seem to have changed, it tastes strange.

Source of thermal degradation of PET at high temperatures | Baidu Encyclopedia

Secondly, at the macro level, an overly cold or overheated environment is not suitable for PET bottled water storage. This is because PET bottled water mostly uses PE (polyethylene) bottle caps, and the expansion coefficients of the two materials are different, which can easily lead to seal failure in an overly cold or overheated environment or under a large temperature difference.

Decomposition of polyethylene source | Baidu Encyclopedia

When the temperature rises, the water vapor pressure will increase accordingly, and the sealed PET bottle will expand due to the increase in water vapor pressure inside the bottle. At the same time, as the temperature drops, the water vapor pressure will decrease, and the sealed PET bottle will shrink due to the decrease in water vapor pressure inside the bottle. Such alternating expansion and contraction will eventually lead to the failure of the PET bottle seal, causing the water in the bottle to interact with substances in the air, contaminating the product and causing odor in the water . Therefore, here is a special reminder for all friends: after we open the water in the bottle, we must drink it as soon as possible.