Sakura said

Is this normal - long jing?

I recently purchased a long jing tea and I noticed that if I leave the cup overnight with tea in it, there’s an oily film that forms on top?

I was just wondering if this was normal?

I stumbled upon this:

but I’m still a bit unsure. I guess it’s from the oils they use when they pan fire the tea though.

12 Replies

Hi there: Does this happen when you’ve left other teas out overnight?

If this has ever happened with other teas, or if this is the first time you’ve left tea for a long time, I think this might be because of your water. My in-laws have pretty terrible tap-water, and any time we don’t use bottled water or water from their filter pitcher to make tea at their house, the tea always ends up forming a disgusting looking oily film over the top of the liquid if we don’t drink the tea up quickly.It’s pretty shockingly ugly, and it’s hard to do anything but throw the stuff away once it appears.

The only tea that ever has “oil” floating on top of the water at our house is pu’er. This is naturally occurring oils from the tea leaves, and doesn’t have anything to do with firing or not. The oil, however, actually looks like light, lovely swirls- not like the picture on the link you provide. Also, this oil only ever appears in the first few steepings, and quickly subsides. The blog post above describes this happening more and more as they steep, which makes me think that the oil appearance has more to do with steep time (ie: how long the water sits with tea), than with anything else.

The best way to find out for sure is to try steeping your Long Jing with filter water or- better yet- bottled water. Just make sure the bottled water you get isn’t distilled (then it will taste like nothing!). It would also be a good idea to deep-clead whatever you boil your water in to get rid of any hard water build-up/etc.
I have a feeling you won’t see any oils floating on top of the water.

If there is, it makes me think that your long jing was fired with tea oils. Normally, I don’t think the tea is fired with anything- just the tea leaves and heat and an experienced pair of hands. However, with lower quality teas that might not be as flavorful, some mass-growers will fire the leaves and add tea oil to boost the flavor. It’s a quick solution. That’s the only thing that I would guess could cause your issue if it’s not your water.
I personally have never experienced oil floating on the cups with any of my green teas, let alone my long jing, that didn’t come from bad water.

Good luck!

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This is normal if you see the oily film appear overnight. It is not normal if you see any oily stuff floating on your tea you just steeped/brewed.

The oily substance on your overnighted tea is oxidized tannin compound. Tea naturally contains tannin. It is what gives tea its astringency taste. Tannin from tea is perfectly safe to consume. But when tea sits for a period of time (sometimes as little as an hour), the tannin in the tea liquor reacts with oxygen from the air to form the oily stuff you see. These oxidized tannin compound is bad for you. They interrupt with nutrient absorption and irritates your stomach.

This is a perfect example of why we gongfucha drinkers always advocate other tea drinkers to enjoy their tea while it’s still hot. Some people drink tea for its health benefits. Some drink tea for pleasure. But nobody wants the tea they drink to cause harm to their health. Right?

For the same reason mentioned above, I also recommend AGAINST keeping steeped tealeaves overnight to be steeped again the next day . It’s bad for you. Gongfucha drinkers don’t let their tealeaves cool too much inbetween steeps, and we usually don’t re-steep wet tealeaves that’s already cold. These are all traditions taught to us from our elders. In the past tea wasn’t analyzed with science like we do now so the elders couldn’t tell us why. They just learn from experience and drilled these traditions into our head. Now with modern science we know these thousand-year-old wisdoms are no joke.

By the way, Iced tea should be cooled quickly in sealed container to minimize oxidation of the tea liquor. And for those who drinks bottled tea……umm……you figure.

Now, if your see oily substance floating on hot tea that’s just steeped, that could be anything. What says on their website about tea oil used in pan firing is true. But the amount of oil used is very very little, so little that if you were to wipe the pan with a paper towel after the oil is applied you would not see a trace of oil on the paper towel. I have never seen oil floating on any just-steeped pan-fired tea, or any tea before. Safe to drink or not aside, it is not normal.

We always recommend rinsing your tea before you actually steep it, no matter what type of tea and how good/fresh/expensive it is. Even with tealeaves from our well-known partner tea farm that were picked and processed in our presence so we know they are clean and high quality, we still rinse them. It’s another gongfucha tradition. (Maybe i’ll write a blog post to explain this when I have time).

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Sakura said

Here’s a picture of the oil spots I was talking about:

I left it overnight for 2 days, I used filter water and I don’t see the oils on any other of my teas so I think it may be related to this specific tea.

Yes, I can only notice it when left overnight. The tea leaves are green and the water looks normal when hot but overnight the tea leaves turn brown and there’s that oily layer in the morning.

So you think that it’s just oxidation and not a chemical pesticide or something?

No matter how many times I reuse the leaves when hot the oil is still there when it cools.

From what I see in the picture it’s normal. Some teas have more tannin than others. In addition, how a tea is process can make a difference too. So you won’t see this in all of your teas.

Sakura said

Oh right, I’ll put up some more pictures in case.

It’s a beautiful photo, if nothing else. ;)

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Sakura said

Here’s some more pictures:

Newly brewed, no oil in sight:

Newly brewed leaves:

2 day old leaves:


So that’s the normal process, from spotless green-yellow tea to oily brown leaves?

What grade does this long jing look like?

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Yes. that’s the normal process. Tea is just like vegitables. If you leave your cooked vegi for two days, they will turn dark as well. I would be more concerned if you Long Jing didn’t turn brown.

I can’t tell tea grades by looking at pictures. =)

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OK! So, this thread made me really interested in the actual chemistry of what casues tea scum. The main picture you posted looks just like the classic example of tea scum.

Tea scum is basically the result of catechins in tea (not tannins) interacting with calcium in slightly alkaline water. Tannins and tea waxes were once considered possible culprits, but thorough research and debate has teased out the most likely causes.

Catechins are in all teas- there’s nothing you can do to remove them, and why would you? It wouldn’t be tea anymore.
Calcium, however, can be removed with good filters. It is absolutely impossible for tea scum to form if there is no calcium in your water. If you still have scum appearing, then your filter is either getting old, or it isn’t removing the calcium efficiently or effectively.

If you want NO tea scum, then you must:
1) Eliminate calcium
2) Lower your water’s Ph

If you’d like to learn more about the nitty gritty of which bonds to what and at what valency, you should read the research of Mr. Michael Spiro. All the studies I found were either by him or referenced his work heavily. I would link to all the ones I found below, but I think it would be more fun for everyone to read this great summary of the GREAT TEA SCUM DEBATE. (brace yourself: it’s long!!)
(!! MAINPOINT !! are inserted by me to draw attention to main points of the argument)

“In this genre of the seriously lighthearted (or vice versa) comes the latest controversy or storm in a tea cup. The earthshaking question to be addressed is : what causes the scum on tea ? Drs Michael Spiro and Deogratius Jaganyi of the Chemistry department of the Imperial College of Science, Technology and Medicine, London, pioneered this research about a year ago and reported their findings in the 12 August 1993 issue of the august science journal Nature, and with that started the proverbial storm in the teacup.

Spiro and Jaganyi noted that when tea is brewed in hot water, an unsightly scum develops on the surface of the cup and in the teapot. Many of us have seen it in our own teacups and pots but it was left to these two chemists to try and figure out the actual chemical nature of the scum. Using all the high-tech arsenal that they had at their command, such as scanning electron microscopy, matrix-assisted laser desorption, ionization, mass spectrometry (with the almost R K Narayanesque acronym of MALDI) and chemical microanalysis, they concluded that the scum is a base of calcium carbonate on which the polyphenolic constituents of tea are deposited after they are oxidized in air. Scum formation, they concluded, is formally similar to the fermentation process by which the tea manufacturers convert their green tea leaves into black tea–the curing part of the cure, twist and curi (CTC) process.

Reaction was not long in coming. Dr P P Jones demurred with regard to the organic components of the scum and pointed his finger at the tannins rather than the polyphenolics. Tannins are a bit of unwelcome nuisance, as anyone who overbrews strong Indian tea finds out to his distaste. The Chinese tea has less tannins and more phenolics and flavonoids, whose glory has been sung by many scientists in the recent past for their antioxidant properties and their role in reducing cholesterol levels in the blood. This raises the interesting point, not so far addressed by Spiro, Jaganyi, Jones or anyone else, namely the relative scum contents of Darjeeling, Assam, Sri Lankan, Chinese and other teas.

Dr Ralph Lewin, of the Scripps Institution of Oceanography in California, rebutted the Imperial College claim and dismissed calcium carbonate off the scene. Instead, he thought that it is the high melting lipids or the oily coat on the tea leaves which are melted off by the hot water and float as floes (note the oceanographic imagery) on the surface. As the tea is drunk, the wexy layers stick to the sides of the cup. They stain with tea phenolics which go brown on oxidation. When milk is added to the tea, the fat and proteins present there thicken the scum layer. “One can liquefy, saponify and solubilize them with very hot water and detergent ; I do this every day”, says Lewin.

The issue has been joined and the controversy is on : is there calcium carbonate or not ? What about the tannins ? Chemical analysis by the Imperial College group showed calcium carbonate, so why is Lewin not able to see it ? The puzzle became teasing enough and scientifically challenging for other scientists to enter the fray. At this point two Brazilians, Dr Jacinta Enzweiler of the Institute of Geosciences and Dr Marcelo G de Oliveira, of the Institute of Chemistry of the Universidale Estadual de Campinas at Sao Paulo, took a crack at the problem of where that solid film of calcium carbonate comes from.

!! MAINPOINT !! The clue is in the water used to brew the tea. If the water used is hard, it contains dissolved calmum bicarbonate which would decompose upon heating to release carbon dioxide as gas and calcium carbonate. The latter, being insoluble in water, would precipitate as a white solid. This is of course the basis of the scales that line the walls of steam boilers, heating pipes and kitchen kettles. Thus, the primary cause of formation of the scum-film that floats on the tea is the hard water used. The Imperial College scientists used the London mains water which is hard while Dr Lewin must have used soft water to brew his tea. !! MAINPOINT !!

!! MAINPOINT !! In a reinforcement of this conclusion, the Brazilians brewed tea using distilled water, which contains no salts of any kind. No scum layer was formed. When tap water from Milton keynes of Southampton in England was used, scum formed while water from the Sao Paulo mains being soft did not produce it. Indeed, Spiro and Jaganyi had pointed out the role of calcium bicarbonate earlier. They showed that distilled water produced no scum, nor did water containing calcium chloride. The latter does not break up on heating to produce a precipitate as calcium bicarbonate does. Also, if calcium ions are removed from hard water using the complexing agent called EDTA, no scum fomed. !! MAINPOINT !!

Two other minor puzzles are also solved neatly by this calcium carbonate theory. One is the observation that no scum formed with lemon tea. The answer is two-fold. One is that lemon acidifies the tea through its citric acid. The acid will dissolve and decompose the calcium carbonate in the scum layer ; also the citrate present in the lemon juice complexes the calcium and dissolves it, just as EDTA does. The other minor puzzle was the unexpected result that when more tea bags were used to brew the same amount of water, the less was the scum formed ! They suggest the main reason behind this to be the greater degree of acidification more tea, arising from the higher concentration of polyphenolics in the brew. These act in a manner similar to the citric acid of lemon.

Oxygen too plays a role in the formation of the scum. When tea was brewed in the laboratory in a nitrogen atmosphere rather then in air, there was less scum. Conversely, when an oxygen atmosphere was used there was more scum. Thus, an oxidation process is implicated, so that not all of the scum can be either mostly calcium carbonate (as the Brazilians would have it), or the waxy lipids and the tannins (which Lewin and Jones emphasized). These do not use up oxygen the way the phenolics do. Spire and Jaganyi went back to the laboratory, provoked by the various rival theories, and measured the chemicalcontentsofthescum. Itcontained about 3–7% calcium and 24–40% carbon material. The amount expected for a film of calcium carbonate coated with a layer of organics would be 40% Ca and about 12% carbon. !! MAINPOINT !! Thus, the scum material appears to be made up largely of insoluble tea polyphenolics together with some calcium carbonate. It is the former that form the major component, as the elemental analysis would demand. !! MAINPOINT !! Why not waxy lipids ? That is also answered when one uses that terrible concoction called instant tea. In making instant tea, the manufacturer removes the waxy lipids in order to make it soluble in water. But even instant tea seems to produce some scum, so the culprit must be some other organic materials. !! MAINPOINT !! Why not tannins ? Because they do not vary with the oxygen content, as the scum material does.

Spiro and Jaganyi reported something else that was at once rigorous and clinching. They decided to monitor the amount of scum formed at various temperatures. The higher the brewing temperature, the more the scum. From such an experiment, one can derive what is referred to as the “activation energy” of the process, which gives an indication of the amount of energy that needs to be pumped into the system in order that the reaction occurs. Physical processes such as diffusion of molecules in a liquid do not demand much energy, whereas chemical reactions such as oxidation would need activation. !! MAINPOINT !! The experimental value of 34 kilojoules per mole that they determined for scum formation argues for a chemical reaction, and not just wax melting and rising to the top. !! MAINPOINT !!

Apart from the “fun” part, the tea scum experiments provide much skillful experimentation and analysis. They also typify a class of experiments which we may call as kitchen chemisty, that can be done readily by any school student. They do not demand much in terms of facilities, laboratory equipment or cost, are intellectually stimulating and also didactic…. After all, as the NCERT school textbook says, “Science is Doing” and “Science is Fun”."

Science is so much fun!

Sakura said

Except for lab work :(.

Brilliant! @Spoonvonstup -You are going above and beyond with this one. This is all good to know and fits in exactly with all my personal experience brewing tea at home vs. at other people’s homes, with bottled water and without. I think the main point to take away is that a water filter that goes on the tap or in the refrigerator is a great investment. Water does make up almost all of what you end up drinking. Even really good tea can have trouble with weird water.

By the way- if anyone wants a less scientific look at water, check out Lu Yu, the author of the Classic of Tea, and read about his journeys around the country to find the best water in China,

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TeaVivre said

I remember in the past I forgot to pour out the remaining tea after drinking and left it overnight and in the next day also have the same situation.

The material I can found all mentioned that the thing floated like oil is tea glaze. Because the tea contains caffeine, Theophylline and cocoa, after a long steep they will float above.

I don’t suggest drinking tea left overnight. Although there’s no harm, the tea which was left overnight lost vitamin and the protein and carbohydrate in the tea liquor will become the nutrients of bacterium and mould production. If it is in Summer, the tea liquor will changed to bad odour. If it is put more than 24 hours, it is better to pour it out, or it will cause diarrhea if you drink it after 24 hours.

So I think it is a common feature of most tea, not only for long jing. There may be some differences between different teas. In order to prove my opinion, I made a test. I brewed 5 teas, 2 kind of Dragon Well (Long Jing) (a low level had a high level), Bi Luo Chun Green Tea, Tai Ping Hou Kui Green Tea and Puerh Tea.

Below are the pictures of different teas and the tea liquor at different time.
1 Dry tea leaves

2 tea liquor at 24 hours

3 tea liquor at 36 hours

I didn’t find anything like oil. I think it is not other thing but the tea component. After 36 hours there seems something mouldy appeared, so it is obviously the tea been left for a long time will be harmful to body.

So my conclusion is that it has no relation to the grade of Long Jing Tea. If you are interested in how to choose best Long Jing Tea. You can have a look at my article here:

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