There is a "film" on the surface of the tea. Is it the tea that is dirty or the teacup that is dirty?

Expert of this article: Yang Chao, PhD in Chemistry, University of Chinese Academy of Sciences

If you like drinking tea, especially black tea, you should have noticed:

When the brewed black tea cools down, a thin layer of tea film will form on the surface of the tea. When a spoon touches the film, it will break apart like floating ice, and the surface of the spoon will also be stained with brown substance.

Image source: Photographed by the author

How is this tea film formed? Today we will solve some problems about "tea film".

When the tea cools down, a layer of film will float on the surface. What is this?

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Some people say that the tea film after the tea is cooled is the oil used for frying tea;

Some say it is the "vegetable oil" of the tea leaves themselves or the "wax" contained in the leaves;

Others say it is floating dust mixed into the tea making process;

…

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Which statement is correct?

As early as 1994, two tea-loving chemists at the Department of Chemistry at Imperial College London, Michael Spiro and Deogratius Jaganyi, also noticed the "tea film phenomenon". Professor Pierce said: "Only by knowing what the tea film is and how it is formed can we effectively eliminate it."

Tea film on the surface of black tea, Image source: Giacomin CE, Fischer P. Black tea interfacial rheology and calcium carbonate[J]. Physics of Fluids, 2021, 33(9):092105-.

Therefore, they analyzed through scanning electron microscope, mass spectrometer and other precision instruments and found that the tea film is not a simple "oil film", but a film formed by the interaction of air, tea polyphenols and calcium carbonate ions on the surface of tea. The tiny particles of insoluble carbonates and hydroxides are attached to the surface of organic matter, and the main component of the tiny insoluble matter is calcium carbonate (content about 15-25%), and the rest is a mixture of complex organic chemicals or minerals.

Calcium ions and bicarbonate ions in water are the key factors inducing the formation of tea film, but only one of calcium ions or bicarbonate ions cannot make the tea film "appear". Both must exist at the same time.

The tea film can sometimes be seen but sometimes not?

In order to explore the conditions for the formation of tea film, these two tea-loving chemists found that when using laboratory distilled water to brew tea, no tea film would appear. This shows that certain components in the water used for brewing tea are the key factors in the formation of tea film.

The biggest difference between laboratory distilled water and ordinary tap water is that tap water contains calcium and magnesium ions. The more calcium ions there are, the easier it is to form tea film. In other words, the higher the hardness of the water used for brewing tea, the easier it is to form tea film.

Smart friends will definitely think that the hardness of drinking water at home can be tested based on the amount of tea film formed!

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In addition to the fact that the hardness of water can affect the formation of tea film, the two chemists also brewed tea in a nitrogen environment to avoid contact between the tea and oxygen in the air. They found that it is difficult to observe tea film in tea brewed in this environment.

Therefore, tea film will only be formed in an environment with oxygen, and the formation of tea film must have occurred an oxidation reaction.

Caroline Giacomin, a doctoral student at the Swiss Federal Institute of Technology in Zurich, also studied the "tea film" phenomenon.

In order to explore the characteristics of this film, she used a double cone interfacial rheometer to analyze the mechanical properties of the tea film from a rheological perspective. By using different concentrations of calcium carbonate solution to brew tea, although the tea film is sometimes invisible to the naked eye, it can be "seen" using a rheometer. This is because:

When the elastic modulus of the tea film is less than the viscous modulus, the tea film appears in a "liquid" form;

When the elastic modulus of the tea film is greater than the viscous modulus, the tea film will appear in a solid form. At this time, if you touch it lightly, the tea film will break.

Why does tea film only appear when brewing black tea?

You may wonder why the lemon black tea you often buy does not have tea film. Is this because of the use of purified water or tea brewed without oxygen? In fact, sugar or lemon juice can make the tea film thinner or inhibit its formation.

However, when drinking milk tea, sometimes you can see the tea film, and sometimes you can't. This is because a small amount of milk will promote the formation of tea film, while a large amount of milk will not allow black tea to form a tea film.

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The type of tea is determined by the way the tea is processed, such as fermentation, oxidation, rolling, steaming and drying.

About two-thirds of tea consumed worldwide is black tea, and the other 30% is green tea. Tea film basically does not form on white tea, yellow tea, green tea or lightly processed oolong tea after cooling, but only on black tea. This is because black tea is fully fermented compared to other types of tea, and its fermentation time is relatively long, and the degree of oxidation is between 70% and 95%.

Is drinking tea leaves harmful to the body?

The "tea film" looks dirty, just like the "oil film" in the puddle next to a gas station, so will it affect your health?

In fact, tea film is just some insoluble small particles attached to the surface of organic matter. It has no taste and no mouthfeel and is harmless to the human body.

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Therefore, there is no need to filter out the tea film once it is formed.

Tips for getting rid of tea mask

Don’t wait for the tea to cool down, drink it while it’s hot;

Use purified water instead of tap water to make tea;

Add some lemon juice and sugar to black tea to make lemon black tea;

Don't drink black tea, switch to other varieties of tea.

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