Sometimes I get distracted, and realise I've got a cold cup of coffee on my desk. Will reheating it in the microwave do bad things to it? I've heard people say it will alter the flavour, and some people even say that the effect of the microwave on the coffee will produce medically harmful chemicals. Is this true? Should I just chuck it and make a fresh brew?
Although coffee generally becomes staler with time, heating coffee up in the microwave does not produce any harmful chemicals - as long as the container you are microwaving it in is microwave-safe.
The microwaves emitted inside the microwave are a relatively safe form of radiation that cause the water molecules in the coffee to move faster, thus creating heat. This is relatively harmless.
As far as taste goes, you may consider just making a new coffee, as this will always taste better, in my opinion!
I grind my own beans every morning and use a drip coffee maker to make a big pot. I’ve found that the best way to keep a pot of coffee tasting good is to turn off the hot plate under the coffee immediately after brewing. Let the pot steep for a few minutes for better taste. Despite all the opinion otherwise, I don’t think microwave reheating ruins the coffee at all. Of course, the taste does not improve with age, but less heat overall, and reheating is so much better than leaving the coffee on a heat source. JMHO.