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Thin "membrane" is created if I leave cup of tea for hour or more. And it usually sticks to the inner sides of the cup. But I noticed that when I drink tea with jam or syrup then the cup always clean inside as if the syrup contains some kind of acid that remove that tea-thingy.

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    $\begingroup$ I suspect your tap water is pretty hard and this layer is floating limescale particles. $\endgroup$ – andselisk May 14 '19 at 16:28
  • $\begingroup$ @andselisk It happens with my cup of tea too, only that I don't add water, boil the milk, and add the tea leaves. $\endgroup$ – user79161 May 14 '19 at 16:32
  • $\begingroup$ Hold on, so you observe a similarly looking "membrane" on top of just milk (no water)? Or you add milk to the tea? Also, a photo might help. $\endgroup$ – andselisk May 14 '19 at 16:39
  • $\begingroup$ @andselisk but this layer emerge only in tea. It smells like tea, it has similar color. Also I was sure that everyone saw it :/ $\endgroup$ – aiven May 14 '19 at 16:40
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    $\begingroup$ @andselisk looks like I've found the answer cooking.stackexchange.com/a/78500. You was right, it is all about hard water $\endgroup$ – aiven May 14 '19 at 16:45
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The parallel process to deposition of limescale ( that is inhibited by acidic jam) is deposition of products of oxidation of epigallocatechine gallate(EGCG) and similar compounds, that are getting adsorbed on limescale particles and cup walls.

Epigallocatechin gallate (EGCG), also known as epigallocatechin-3-gallate, is the ester of epigallocatechin and gallic acid, and is a type of catechin.

It is found in high content in the dried leaves of green tea (7380 mg per 100 g), white tea(4245 mg per 100 g), and in smaller quantities, black tea (936 mg per 100 g).During black tea production, the catechins are mostly converted to theaflavins and thearubigins via polyphenol oxidases.

EGCG is a polyphenol, vulnerable to oxidation on neutral/alkalic environment, forming insoluble product of intense brown color.

Acidic environment, caused by addition of jam or lemon juice, stabilizes the compound.

Minor natural content of ascorbic acid in fruit juice is protected by abundance of organic acids by similar way.

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  • $\begingroup$ "It does similar thing to ascorbic acid" Do you mean "It does a similar thing as ascorbic acid [does]"/"It does a thing similar to what ascorbic acid does"? $\endgroup$ – Acccumulation May 14 '19 at 19:08
  • $\begingroup$ No. I mean high abundance of organic acids ( citric, tartaric, acetic, malleic ) protects oxidation of minor ascorbic acid, that gets oxidized much slowly in acidic environment. OTOH, if ascorbic acid gets alkalized, it quickly oxidizes by oxygen, turning quite clear solution to intense yellow. But of course, abundance of ascorbic acid would protect EGCG as well. $\endgroup$ – Poutnik May 14 '19 at 19:13
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As @andselisk suggested the problem lied in hard water. And calling that thingy "layer" instead of "membrane" helped to google the problem properly: cooking.stackexchange.com >, www.teamuse.com >

"The scum on the top of the tea is due to hard water (ie calcium carbonate) deposits combining with the tea and reacting with oxygen."

To dissolve that layer we need to rise acid level: add lemon, jam, syrup or even make stronger tea.

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