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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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My father drinks a pot of coffee a day and invariably reheats a cup or three during any given day (sometimes multiple times). He's been doing this for years, and no noticeable side effects :) –  mellamokb Aug 18 '11 at 14:04
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Unless you are adding something to the coffee I can not imagine how this would be possible. I think this would have to be limited to someone using a metalic coffee pot and the reheating could then burn any setiments but I do not think its the coffee that would be the problem rather the container it is being reheated in. –  Chad Aug 18 '11 at 14:15
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When in doubt, ask for a citation I'm reheating my coffee very often (anecdotal evidence) and I am still alive (anecdotal evidence) but I never heared a warning before (anecdotal evidence). –  user unknown Aug 18 '11 at 14:17
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@mellmokb, does he smoke too? Because that would prove tobacco is harmless... :-( –  Oddthinking Aug 18 '11 at 14:42
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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

3 Answers 3

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.

http://www.telegraph.co.uk/health/healthnews/4703608/Is-reheated-coffee-a-recipe-for-cancer.html

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Heat is the enemy of taste in coffee, but it would take more than a couple of minutes in the microwave or on the stove to make anything in it become poisonous. Here's a link to a fairly complete list of what's in the cup:

http://www.jlhufford.com/articles/whatiscoffee.asp

If there are any reactions available between any of those compounds at the energy of reheating, I'm not aware of them.

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Care to comment on the down votes? It is a valid answer. –  Chad Aug 22 '11 at 13:30
    
I'm assuming it's because it's a link to a sales site. But it's the first hit in google when searching for "coffee chemical ingredients" and it's a very good way of thinking to approach tackeling this question. So, here you go, have an upvote. –  Wouter Oct 26 '12 at 7:17

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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