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Someone once told me that reheating cold coffee releases poisonous chemicals in the coffee which makes it taste bad.

Browsing the internet I can't find any conclusive results. Some say it tastes bad, others that it causes high blood pressure and yet others that there is no harm in it at all.

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I'm wondering why there aren't similar warnings not to overboil the coffee in the first place? Surely the same chemical reactions would take place? – Oddthinking Aug 18 '11 at 14:53
It's sure as heck dangerous to your taste buds. Yuck! – dmckee Aug 18 '11 at 16:30
If you're not US Citizen, drinking reheated coffee will make you vomit. – vartec Aug 18 '11 at 22:24
I wonder if toxicology would be an appropriate tag for this question. – dancek Aug 23 '11 at 14:24

2 Answers 2

I spent some more time searching and this is the best I came up with:

Q: I love real coffee and sometimes heat the remaining cold coffee in the microwave. My friend says this causes cancer. Is this just an old wives' tale?

A: I have searched medical data bases spanning the past 10 years, but I can find no suggestion that microwaving coffee is harmful. According to the Coffee News Information Service, the vast majority of studies do not demonstrate a link between coffee consumption and cancer, and they are also unaware of any suggestion that microwaving coffee may be harmful. In fact, antioxidants found in freshly brewed coffee may protect against certain cancers, although one researcher has suggested they should be consumed within 20 minutes after brewing for optimum effect.

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Yes, in some cases

-- depending on how carefully the coffee is reheated, and how sensitive the person is to tannins.

The rule-of-thumb test: if it tastes (unusually) bitter, it's not necessarily good for you.

Bad Coffee at Mulholland Drive by Ricardo Infante, on Flickr Image: Bad Coffee at Mulholland Drive by Ricardo Infante, on Flickr; CC-BY-NC 2.0

It's common knowledge that actively heating brewed coffee causes a bitter taste. This is most commonly observed on a coffee brewer warming plate after some time (note that these plates are specifically designed to keep coffee warm and not spoil it). When reheating cold coffee, it should be quite easy to heat a bit too much. This would naturally cause a bitter, unpleasant taste. This is probably the origin of the belief.

Direct heat—e.g., via a warming plate—should not be applied to brewed coffee, at least not for more than few minutes; continued heating will make the coffee bitter. This issue is primarily a concern for autodrip coffee makers; choose a model that dispenses into an insulated carafe over one that uses a warming plate.

I've heard it said that applying heat to coffee causes tannins to form, but can't find a good source. This site confirms that a higher temperature causes more tannins to form. However, the focus is on brewing causing tannin extraction. This should apply to heating afterwards, too: some coffee bean dust (or other small particles) are bound to end up in the coffee. Coffee brewers don't exactly have microfilters.

It's recommended to avoid large concentrations of tannins, as they have many undesirable effects including liver damage in sensitive individuals. So drinking reheated coffee certainly can be dangerous.

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SO basically you are saying: bad because of tannins? Both the links are broken, the one that talks about more tannins and the one that says they have undesirable effects. As is, this kinda looks like an answer with a pretty picture, but no founded content? Is the data somewhere available that supports this still? – Nanne Jan 9 at 15:19
@Nanne I actually tried to discuss this a bit, so my one-line answer would have been "depends" or "in some cases". I'll update the Wikipedia link, as that one is easy. Information about tannin extraction on heating is probably available somewhere, but I don't have the time to search right now. If the picture bothers you, feel free to remove it. Anyway, I consider this a useful answer since it mentions a probable reason for the belief--allowing the reader to consider the health effects for themselves. – dancek Jan 12 at 8:22
The link you provide with the text "many undesirable effects" doesn't actually say that! The introduction of the wikipedia article actually seems to be about the desirable effects it has! The first step seems "I've heard it said, but can't find a good source", and then going from 'it might have more tannins' to 'tannins are bad' is also disputed, if not (with current link) outright wrong. This might be coming a bit strong at it, I'm sorry, I just don't see the connection between the links and the statements you make. I think this is just the thing we are trying to find out about. – Nanne Jan 12 at 10:14

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