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What happens to the brewed hot tea when it is left in a cup for some time (up to several hours), that spoils the taste of the drink and change its color? What chemical processes lead to such unwanted results and what are the general methods to prevent tea from going bad too quickly?

If there is a significant difference in different tea types' brewing processes, I'd like to know that too.


Assume we make a cup of Earl Grey tea with a spoon of sugar and leave it for a day at room temperature.

I've seen a thin rainbowy layer appear after leaving tea for a few hours (up to a day). I often saw that in teas made from cheap bagged tea. Also the color of the drink becomes distinctively "stale".

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To clarify, I'm interested in chemical processes, like maybe excessive oxygenation of some basic tea component (though I'm not sure if it would fit Cooking.SE). If there's too much air that makes tea go bad, I'd know I should try to keep it in a vacuum container. –  user1306322 Oct 23 '12 at 18:41
    
I'd like to know how to store (and be able to drink) the tea so that it stays good for as long as possible. But mainly this question is about the processes that should be prevented from going on that ruin tea. I'm interested in the nature of such processes. –  user1306322 Oct 24 '12 at 2:45
    
You mentioned in a comment below that you're trying to keep the tea for days, and seeing specific problems (not just bitterness). You might want to edit that into your question; people providing answers aren't going to read every last word down there. Also, are you refrigerating it? Are you leaving the tea leaves in it the entire time? Specific questions get better answers. –  Jefromi Oct 24 '12 at 20:05
    
A friend suggested more questions: are you adding anything to the tea? What kind of tea are you using? (Does it have added flavors or other non-tea ingredients?) Can you describe this colored layer you're seeing? –  Jefromi Oct 24 '12 at 20:11

4 Answers 4

up vote 4 down vote accepted

A rainbowy layer is almost certainly oil. (This is called thin-film diffraction.) Earl Grey uses oil of bergamot for flavoring, and likely cheap teas you've used have oil-based flavorings as well. It's not surprising that the oil eventually separates, and there's not really any way to avoid that without significantly modifying the tea, or using tea that doesn't have quite so much oil.

I would definitely expect the flavor to be stale after it's left for a while - aromatic compounds are by nature volatile, and they'll slowly escape. You might be able to prevent some loss of flavor by putting the tea in something airtight (preferably a completely full container, so there's not even air on the surface) and chilling it, but it's going to be a losing battle. You said the color was stale as well; I don't know exactly what processes would cause that, but in general, the only way you're going to be able to slow down any chemical processes is by chilling the tea.

In the end, the real answer is simply that it's best to drink fresh tea. Your time would probably be better spent getting to where you can make it quickly - for example, get an electric kettle that can rapidly boil a single cup worth of water.

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So no matter how good a container I pour my tea into to take with on a trip, it's still going to spoil as fast as if it was in a cup? –  user1306322 Oct 24 '12 at 20:35
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@user1306322 Like I said, it'll stay fresh better in an airtight container than in an open cup sitting on the counter. And I don't think it's spoiling, it's just losing some flavor. And finally, if your question was about taking tea when traveling, you should have said so. Please post complete information in your questions. –  Jefromi Oct 24 '12 at 20:38
    
It's not about taking tea on a trip, the question is about the chemical processes that ruin its taste and I'm after the methods that prevent those processes. So far I only found out that oil-based flavoring separates from tea. –  user1306322 Oct 24 '12 at 20:43
    
Please re-read my answer, then. I mentioned that the main problem is volatile aromatics escaping, which is preventable to some extent by chilling and by keeping in something completely sealed (so there's no air for them to escape into). –  Jefromi Oct 24 '12 at 21:01

From a text published by the Royal Society of Chemistry on the matter [pdf], Dr Andrew Stapley writes that to brew for long periods

...introduces high molecular weight tannins which leave a bad aftertaste.

The lighter weight tannins provide the colour and flavour of the drink, and require the higher temperatures to infuse properly, but the heavier weighted ones seem to provide the undesirable taste.

I believe this is why microwaving tea also leads to poor results.

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I have found that a back stir after stirring the tea, always eliminates the sour taste and my tea last longer. If you understand how brewing works, (swirling) it makes sense. It's silly, but it works.

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I, too, have been looking for the answer to the OP's question, which was not answered here, btw. I think the process he is referring to is the eventual growth of bacteria in warm tea, but I have yet to find the definitive statement from an official source. I have noticed it in brewed tea that has been sitting too long in restaurants before serving. I do note that the authorities are now recommending that people not make sun tea, and cite the growth of bacteria as the reason. My experience says that refrigeration is the answer to keeping the tea from growing this bacteria.

Also, and this rather mystifies me, some people seem not to be able to recognize the taste of tea that has "gone bad." Others, like the OP and I, recognize it instantly. Sometimes the bad tea has a slightly foamy or fermented look to it.

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Why would bacteria only grow in warm tea? What authorities? What experience? This answer makes a lot of unsubstantiated claims and the second paragraph does not seem to relate to the question. –  Aaronut Jun 16 '13 at 18:28
    
Like I said in my answer, tea goes bad because the aromatic parts slowly escape. It's not a chemical process, but it's still the answer to the question. Bacterial growth will take a lot longer to happen significantly, and is not likely to affect the flavor as much as the fact that the flavor is simply disappearing into the air. –  Jefromi Jun 17 '13 at 0:33

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