Have you ever made yourself a cup of coffee and forgot about it? If you answered no, you’re lying to yourself! If you answered truthfully, you may have noticed before that your coffee tastes bitter upon your return.
But what, in particular, causes this bitter taste and does reheating your coffee weaken the caffeine within the drink.
What is Caffeine?
Caffeine is a natural, water-soluble stimulant found in over 60 plants. Caffeine is commonly found in tea, coffee, and cacao plants.
Once consumed, caffeine is quickly absorbed into the bloodstream. From there, it blocks the effects of adenosine, which is a neurotransmitter that relaxes the brain and makes you feel tired.
One arabica bean contains 1.9 milligrams of caffeine. This means that each gram of coffee contains, on average, 12 milligrams of caffeine. For a set 240ml glass, you can expect to find 240-720 milligrams of caffeine if espresso and 102-200 milligrams if normal coffee.
Does Coffee Lose Caffeine When Reheated?
Data from Ask Jeeves suggests that the above question is asked 130,000 times per year. So, to find the answer, Inverse interviewed one of coffee’s leading chemists, Dr. Christopher Hendon to provide an answer, once and for all.
To summarise the repot, Dr. Hendon states that under almost all recorded circumstances, reheating coffee won’t cause a loss in caffeine content. Hendon goes on to say ‘caffeine is a pretty stable organic molecule’ and due to this, is unlikely to be affected by an increase and/or decrease in temperature. In fact, coffee needs to go through a process known as ‘sublimation’ in order to lose caffeine, which is when a solid turns into vapour. This would require a temperature of 180°C, so almost impossible to achieve accidentally.
When asked about the origin behind the idea of coffee losing caffeine, Hendon instead attributed it as a placebo effect. When you drink your first cup of coffee, you get a buzz. This is largely a result of it being freshly brewed, in the morning. For many, the second cup of the coffee doesn’t feel the same. By this point, you’re likely at work in a busier environment, so it’s harder to enjoy and focus on the coffee.
Even though caffeine isn’t lost when reheated, the process does cause some negative side-effects for its taste and flavour. Hendon went on to explain that as coffee becomes less fresh, it begins to lose its volatile compounds that provide its most desired flavours. Reheating coffee hastens this process, which leads to a dip in its overall flavour – often falsely attributed to a decrease in caffeine.
Why does coffee taste bitter after reheating?
When coffee is reheated, its chemistry changes. Heat results in coffee’s compounds becoming extracted, which provide flavour.
This means that when you reheat coffee, you’re actually over extracting it. This disrupts the flavour profile when you first brewed the coffee, instead resulting in greater acidity.
Do all types of roasts – dark, medium, light – turn bitter upon reheating?
Every single roast will become more bitter upon reheating, to a certain degree. Dark roasts are more likely to provide a pronounced bitter taste. Dark roasts have had more heat applied to them during the roasting process, meaning they contain more of the bitter-tasting caffeic acids than lighter roasts would from the get-go.
The Best Way to Reheat Coffee
If you HAVE to reheat your coffee, Hendon explains that its best to use a microwave as opposed to the stove. The reasoning being that a microwave will heat the coffee quicker, leaving less time for those volatile compounds to escape and alter the flavour of the coffee.
If you want to preserve the flavour of your coffee completely, a temperature-regulating mug or mini-desktop warmer should be your go to.