How much caffeine is in your cup?

Trying to decipher how much caffeine is in one cup of coffee is harder than you think...
26 April 2022

Interview with 

Jonathan, Cambridge University & James Hoffmann




Just how much caffeine is too much? Harry Lewis' father drinks four mugs of the stuff a day, is that too much? 

James - That's a good question. When it comes to caffeine consumption, there are broad guidelines out there. And they'll say for an adult male, it's 300 milligrams of caffeine a day. The tricky bit is that you don't know how much caffeine is in the cup of coffee that you're drinking right now. You don't know how much coffee is in the raw coffee beans. You don't know how good a job you've done of getting the caffeine out of the beans, but I would say 4 cups of coffee, if it's good, quality coffee will take you around 300 - 350 milligrams. Cheaper coffee has a higher caffeine content so if it's three cups of instant coffee you're probably there quite comfortably.

Harry - Caffeine has historically had it pretty bad in the press. But research suggests that moderate coffee consumption, anywhere between 2-5 cups a day could be more healthful than harmful. It's been linked to a lower likelihood of type-2 diabetes, heart disease, liver cancer, Parkinson's disease and depression; seeing as it stimulates dopamine pathways in the brain. If that's the case, what are the negatives of drinking coffee?

James - I think that the primary issue is gonna be sleep and I think we understand more and more about the importance of sleep. Good quality sleep is essential for health in a way that I don't think we truly understood until relatively recently. If caffeine is interrupting your sleep, it really is a problem that has long lasting and serious health effects and I think it's definitely one to worry about. I think we've probably been a bit blasé historically about our conception of caffeine and tied it into feeling productive and getting the job done and all of that kind of stuff. I think that's the first big indicator. Different people genetically do deal with caffeine differently. There are slow caffeine metabolizers and there are fast caffeine metabolizers, so the half life of caffeine in your body will vary based on your genetics. There's no single hard and fast rule.

Harry - There are subsections of the population that this rule doesn't really apply to. The story is a little different, for example, in pregnant women. For this cohort, there's a reduced recommended intake. It's less than 200 milligrams of caffeine a day; That's on average, roughly 2 cups. That's because high caffeine consumption is associated with complications, including miscarriage and stillbirth. Maybe we should know how much caffeine differs between each mug of coffee we drink. With that in mind, I took it up with my friend, Jonathan in the chemistry department at Cambridge University.

Jonathan - You've made me read up on this topic quite a lot, and I actually appreciate it because it's quite interesting once you get into the nitty gritty of it. It's interesting that in the year 2022, people still debate on how best to get the caffeine out of the coffee. Very timely, we're very timely indeed.

Harry - Here's the plan. I have 3 samples of espresso and hopefully we're gonna see if there is a difference between brands in caffeine concentration. Now, if we were to do this to the highest standard, Jonathan says we would need hazardous chemicals and some pretty skilled expertise. I've managed to twist his arm and drop quite a lot of that scientific scrutiny, but bear in mind, he really did not give in willingly.

Jonathan - The way we split these samples up is we'll make sample number 1 to be Hot Numbers, which is a local coffee shop. Sample number 2 will be a large scale coffee chain, and sample number 3 will be another local coffee shop. The first thing that we do is we will make a dilution of these because the coffees are quite dark in colour and we want them to be as translucent as we can these solutions in order to get our light through them eventually.

Harry - We're using a pretty fancy bit of kit. It's a UV spectrometer. What does a UV spectrometer do?

Jonathan - With this we can basically shine some light through a liquid and we can see at what wavelength of light the particles in this liquid absorb or emit.

Harry - Right. I'm assuming that the more of this molecule, in our case caffeine, there is the more...

Jonathan - …bigger, the absorbance of light will be at that wavelength. Yes.

Harry - And we'll see that on this computer here.

Jonathan - Yes. That will look like a peak. Like a mountains.

Harry - In goes sample 1, sample 2, and finally sample 3. In fact, I've told a bit of a white lie, because we actually did run a lot more tests. Mainly because Jonathan was attempting to refine our results, by the end though, we were left with 3 graphs representing our three espressos.

Jonathan - With samples 1 and 3, we see generally that they show higher levels of absorbance at the wavelength that we would expect caffeine to absorb at.

Harry - And then with sample two, what do we see?

Jonathan - Sample 2 shows a lower general absorbance at that specific wavelength, which could suggest that there's less caffeine in that sample than the other two.

Harry - From the data that we've got, it's not perfect by any stretch of the imagination, but we have an absorbance of 0.4 for sample 2, and for samples 1 and 3 it's 1.4 and 1.5 respectively. It's 3x bigger. Maybe 4x bigger, almost.

Jonathan - Sample number 2 came from a big chain coffee house, whereas samples number 1 and 3 were from local coffee shops.

Harry - Now our experiment isn't precise. It would probably start a brawl over at a peer-reviewed journal. That's because we didn't extract the rest of the gubbins from the coffee, due to us needing those dodgy chemicals. When the light shines through our sample, it is possible that some of the rubbish left behind in the coffee is absorbed at the same wavelength as caffeine. In short, we need to take our results with a large pinch of salt. But our results do make sense as James explains.

James - Historically, you've seen a bunch of studies done on the British high street, coffees from all the major chains and really quite broad caffeine levels, found in those drinks big variations from relatively low to really surprisingly high because caffeine is not a simple thing. It exists in nature primarily as an insect repellent. The higher you grow coffee, the less need there is for the plant to produce caffeine. Unless you know the altitude that your coffee was grown at, it's hard to make a very good guess that the caffeine content, different species produce different caffeine contents. Then in the brew process, caffeine's highly water soluble, but if you don't do a great job brewing the coffee, you won't actually get all the caffeine out. If you do an incredible job brewing the coffee, you'll get a little bit more. All of these things come together into pretty large variations for the kind of consumer experience.

Harry - Historically the coffee industry has been really opposed to putting caffeine measurements on their products. In part, it must be somewhat down to the difficulty when trying to extract caffeine. It also must have something to do with the massive variation between each batch of coffee beans. I think it's pretty fair to say that we haven't got a clue how much caffeine is actually in there and maybe we should know. It does seem particularly important for some members of the population, like pregnant women.


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