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I brew my coffee with a percolator using finely ground beans and it comes out great when it is still hot. However, I have to brew at least two cups in the machine. So if I make coffee in the morning, the afternoon cup is cold and I have to reheat it.

If the coffee gets cold and then I reheat it in a cup in the microwave, it tastes terrible! Why?

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Is there any chance your cup isn't microwave safe, or the coffee is taking on an aroma from somewhere else over the course of the day? – Jefromi Nov 30 '11 at 19:13
    
So, wouldn't it be a great invention to manage to create a Coffee "maintainer and reheater." Invent it, somebody. Please. – user10294 May 20 '12 at 13:57
    
Here you go: store it sous vide (under vacuum, without atmosphere). That'll at least stop oxidative rancidification, if not hydrolytic rancidification. – Camille Goudeseune May 21 '15 at 15:11
up vote 25 down vote accepted

It has nothing to do with the microwave and everything to do with the volatility of aromas and flavors in coffee. Even coffee kept warm for 4 hours won't taste very good. In my experience, stale brewed coffee results in a more pronounced acidity, if left out to cool, or a woody, muddy, bitter kind of flavor, sometimes with more pronounced acidity, if held warm. If you were to make a straight espresso, it's optimal to drink it within 20-30 seconds. Brewed coffee can usually be held warm around 30 minutes before pronounced deterioration becomes obvious.

A popular Japanese TV show suggested perhaps stirring a pinch of salt in to stale, reheated coffee, which may trick your tastebuds enough into tolerating it. I've found that tactic less than adequate, but your mileage may vary.

There's a ton of information on coffee flavor compounds and other coffee-related research at: http://www.coffeeresearch.org/science/aromamain.htm

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The salt trick works nicely with brewed coffee that has been on heat for too long. It doesn't make the coffee taste good but it gets a bit better.... – froderik Dec 1 '11 at 8:43
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No No No - lot more going on here! I can reheat old coffee on the stovetop and it tastes fine. (even if accidently boiled) I can re-heat coffee brewed just 10 min prior in the MV, and it tastes bitter, burnt. – user17545 Mar 28 '13 at 20:55
    
In my experience adding a bit of hot water after nuking a mug of coffee makes it quite a bit more palatable. Not a good idea in any case, but caffeine emergencies are a real thing.... – Crisfole May 26 '15 at 18:00
    
I've had no issues with reheating my freshly brewed cup of coffee that has just gone a bit cold in the microwave oven. Even the espesso taste fine. That said, I would never reheat even after 10/15 minutes or so. IMHO time is a worse enemy than the MW anyway. Hint: only heat it up to the point that it is drinkable again and mind that the cup will barely heat up. Burning your tastebuds is definitely not a good way of avoiding the awful taste of re-boiled coffee. Coffee is mostly water so it will heat up very quickly. – Maarten Bodewes Feb 7 at 14:31
    
health-wise is there any negative consequences for drinking pre-heated coffee? If so, how long until it's not good for you? – C_B May 4 at 17:35

In addition to Jason's excellent answer, I'll add that if you're leaving coffee out for 4-6 hours before reheating it (from morning to afternoon) then the oils in the coffee have probably gone rancid as well. Coffee oils are highly volatile and require only a few hours to go rancid. This is why press-pot coffee often tastes vile no matter what quality beans you use; the coffee pump is full or rancid oils.

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Well, I think it really is because the coffee extract (espresso) burns if direct heat is applied. If you were to reheat your coffee on a stove in a water bath, the taste may not be so different.

This happens even with freshly made espresso - if the milk isn't hot enough and you want to reheat your coffee, you'd screw up the taste big time.

Another thing is that Arabica is more prone to the burnt taste than Robusta strains (the round beans).

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Though this is your opinion, you offer no justification for it. – razumny Feb 19 '14 at 13:43

Most coffee shop websites say not to reheat coffee. It is because when you brew the coffee, the water temperature is 205F.

I always reheat my coffee for about 10sec -25sec.
for a cup - heavy duty 20-25sec -light 10-15sec
for a mug- 10-20sec.

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Hello han, welcome to the site. Please check on the times again. Some formatting would make you answer easier to understand. You can enter line breaks by hitting "enter" twice. – Stephie Dec 11 '14 at 8:37

I might say that it's perhaps your choice of making coffee in a percolator. A percolator takes boiled water and runs it through coffee grounds. Then it recycles that water and now coffee mixture back through the same coffee grounds. With each successive pass of the liquid it concentrates the coffee/water mixture.

This also changes the coffee grounds as it makes them more and more bitter from the acids that are being released with every successive cycle. Further more the liquid is continually being heated and kept at the boiling point, making for more coffee acid extraction.

This particular choice for coffee making, produces a strong acid component and darker more concentrated flavour. I think it is the very nature of making percolated coffee leads to it being particularly bad tasting when cold. The concentration of oils, acids and other bitter compounds are more noticeable when cold than hot. Also the heat probably makes you take less of it in your mouth, than when it's cold.

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Usually when I reheat coffee in a microwave at the lowest power setting (just one notch above the defrost setting) it's drinkable. At higher settings the taste is completely destroyed.

No question about it - heat destroys coffee. I'm amazed that fast food places and gas stations are able to sell so much boiling hot coffee. It's so bitter and has that rubber aftertaste - hardly resembles real coffee IMHO.

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