Have you ever experienced tea film when making tea?

It was obviously clean when it was first brewed, but after a while, an oil-like film appeared on the surface of the tea soup, sometimes with a rainbow-colored reflection, and it would break when touched. So here comes the question: When brewing tea, there is a layer of "oil". Is it caused by the cup, the tea leaves, or the water?

Tea can be excluded first. Some people may think that the oil film is pesticide residue on the tea leaves, but it is not actually the case. Even if pesticides are used on tea trees currently grown on the market, they use fat-soluble pesticides that are insoluble in water, so they will not dissolve in the tea soup. Some people also think of the special tea-making oil poured into the process of frying tea, but this oil is also insoluble in water. Even if there is some residue after frying certain varieties of tea, it generally cannot be brewed.

Some people have speculated that this layer of oil is formed when the waxy layer on the tea leaves floats to the surface of the water when the tea leaves are brewed with boiling water. This view was later broken by experiments by two chemists, Spiro and Jaganyi. They found that if tea is brewed with distilled water with very few impurities, there will be no oil film.

It’s not caused by the tea. Is it the water?

Through further experiments, Spiro and Jaganyi finally discovered that the formation of the oil film was indeed related to the water in which the tea was made. It was the calcium ions and bicarbonate ions in the water that played a role, and both ions must exist at the same time. These minerals in the water react chemically when they come into contact with the antioxidants and substances in the tea, forming an "oil film" floating on the surface of the tea. The tea film itself is actually just small particles of carbonates and hydroxides that are not dissolved on the surface of organic matter and are not harmful to human health.

The pH of the water will also affect the effect of tea film: the more alkaline the water, the more likely it is that the water will produce tea film when making tea. Tea brewed with distilled water will have almost no oil film, also because the content of these two ions in the water is extremely low.

Scientists also found that tea film will appear on the surface of tea only in an oxygen environment. If the tea is placed in a nitrogen environment, it will be difficult to observe the tea film. This indicates that the formation of tea film is related to the oxidation reaction.

The influence of the cup on the formation of tea film is mainly reflected in the heat preservation effect of the cup.

The formation of tea film is a chemical reaction that requires high temperature for a period of time. A cup that cools quickly will generally not produce a tea film. On the contrary, if your tea cup always produces a layer of oil film, it means that your cup must have a good insulation effect.

If you want to avoid tea film, just do the opposite.

If you don’t want to change to a cup that dissipates heat faster, you can change the tea water to soft water such as purified water or distilled water. At the same time, pay attention to washing the cup frequently to avoid leaving tea stains. The minerals in tea scale are generally rich in calcium ions and magnesium ions, which will induce the production of tea film. Or you can add some lemon juice to the cup to remove the tea film through acid-base neutralization.

Well, now you can put aside your worries and drink tea with peace of mind.

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