Using Chemistry To Get The Perfect Cup Of Coffee

16:31 minutes

an espresso machine dripping espresso into a cup
Credit: Shutterstock

A cup of coffee first thing in the morning is a ritual—from grinding the beans to boiling the water and brewing your cup. But following those steps won’t always get you a consistent pour. Researchers developed a mathematical model to determine how the size of grind affects water flow and the amount of coffee that gets into the final liquid. Their results were published in the journal Matter

Computational chemist Christopher Hendon, who was an author on that study, talks about how understanding atomic vibration, particle size distribution, and water chemistry can help you brew the perfect cup of coffee. 

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Segment Guests

Christopher Hendon

Christopher Hendon is an assistant professor of Chemistry at the University of Oregon in Eugene, Oregon.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Is making your morning cup of coffee like a sacred ritual, carefully grinding the beans just a certain way depending on if you’re having an espresso or a drip? Next, you boil the water to the exact temperature, not too hot. And finally, you brew.

Are you going to steep it, pour it over, or leave it up to the coffee maker? There’s a lot of details that go into coffee, but what is this science going on behind those steps? Because as we know, a dose of science can make most things better and your coffee more consistent.

A team of researchers used math models to figure out the optimal grind size to get the most out of your espresso pull. The results were published in the journal Matter. Christopher Hendon is one of the authors on that study and Assistant Professor of Chemistry at the University of Oregon in Eugene. He’s here to talk about these results and to talk coffee science, because we know the best part of waking up is having chemistry in your cup. Welcome to Science Friday.

CHRISTOPHER HENDON: Hi, Ira. Thanks for having me.

IRA FLATOW: I know you spend a– I’ve read all these papers you’ve written, and you spend a lot of time on coffee chemistry don’t you?

CHRISTOPHER HENDON: That’s right. You know, I like to think of it as one of my more sophisticated scientific outreach programs.

IRA FLATOW: So I know you’re working on building new kinds of materials, right? What does that have to do with coffee?

CHRISTOPHER HENDON: Well, a lot of the chemical principles that I use in my day-to-day lab with my graduate students and undergraduates here at the University of Oregon are based on this or an application of the same principles that we’re using in the coffee science. What I study here at UO is related to energy materials. And more often than not, in energy, there is transfer of mass moving from one place in space to another. And you can actually think of that problem as the same problem when you’re modeling or thinking about how you extract coffee.

IRA FLATOW: Interesting. I want to get into the chemistry of coffee, so I’m going to give out the phone number– 844-724-8255. You can also tweet us @scifri. Let’s get into the grind. Ha. Your study looked at getting a consistent shot out of your espresso pull, and espresso is usually brewed with a very fine grind size. But you found something different.

CHRISTOPHER HENDON: Right. So currently, the average way of producing espresso is to grind very, very fine and use something like 20 grams if you’re in the United States, Australia, Germany, et cetera, and move about 40 to 50 milliliters of water through this 20-gram puck of coffee. On average, you’re going to extract something like about 20% of the mass. So 80% of the mass is going to be wasteful, and that 20% of the mass has ended up solvated in the cup and hopefully, tastes good to you as the end user.

When we were examining building a model, we thought well, OK, the extraction must be related to the surface area available from grinding the coffee. And so as we developed our model, we realized that as you grind finer, you expose more surface area. And so one would then expect for the same amount of coffee and the same amount of water that you should be able to extract more, you know, proportional to the surface area.

But in reality, we found that actually, grinders produce very small particles and very relatively large ones as well, and these two particle sizes co-exist. And when you produce a critical number of these small particulates, they tend to clump. Now, at certain grind settings sufficiently fine, you’ll find that the clumping actually causes the water to percolate through that bed in an inhomogeneous or uneven way.

And it is at these grind settings that we also happen to find are widely used in the production of espresso. And so the conclusion from or at least when examining the status quo is that conventional espresso today is used with an uneven exposure of the coffee to water resulting in variation in flavors that are not attributed to the human but rather attributed to the method in which we produce espresso right now.

IRA FLATOW: All right, let’s say I want to make my own espresso, or I just want to do it in the coffee maker. How do I decide what’s the best grind for me? ‘Cause I can go to the department– the supermarket and grind it to different consistencies.


IRA FLATOW: What can I do at home to make sure I’m getting the best grind? And how do I experiment with that?

CHRISTOPHER HENDON: That’s right. So this is sort of touching on a flavor perception problem as well. So typically, we would advise to start with coarsely ground coffee. And now, by and large, when I mean coarse here, I don’t mean French-press coarse. I still mean an espresso grind. It’s still quite fine to the human eye.

You start grinding with that dose, and you’re going to brew some of the coffee. And if it tastes a little thin and a little weak and perhaps a little too acidic and not enough chocolate, then you want to grind a little finer. And now, you’re going to progress doing this to a point. But at some point, you’re going to notice that the concentration of the coffee, in other words, the strength, is beginning to decrease, and the flavor intensity is beginning to increase in a negative way.

And at this point, you know you’ve gone over the hill, and you’re no longer evenly extracting from your coffee. You’re in the side of uneven or inhomogeneous extraction, and you need to back off from there. Once you’re at that point, that’s where the real fun begins.

IRA FLATOW: Wow, that’s great. I can experiment with that because I’m a geek, and I can become a coffee geek, I think. One thing I’ve heard controversially over the years, and we’ve talked coffee for almost 30 years here on Science Friday, is people have different opinions about how to best store their coffee– in the freezer, in the refrigerator, on the counter. What have you found?

CHRISTOPHER HENDON: So actually, in 2016, I actually studied this, and we published in an open-access journal. One of the key things with coffee is that it begins its life as a seed from a plant, so it has about 11 and 1/2% moisture contained within. And once you roast it, you’re going to drive off almost all of that water if not entirely all of it, make it completely anhydrous.

And so obviously, this material is going to be much more dry than the atmosphere that it’s surrounded by, and so it’s going to want to condense water within. So the first and most important thing is to make sure that you keep the coffee air free, but only because you’re trying to keep away water or moisture. However, we’re then put in a bit of a pickle because the Arrhenius equation tells me that if I cool things down, most rates of reactions go slower.

So if I want to preserve coffee for a long time, I want to keep it as cool as possible, perhaps in the freezer. But the problem with the freezer, of course, is it’s pretty wet. So you’re stuck in this sort of in the middle here of push-pull. And so we actually end up recommending people do is to keep it in the freezer but in a vacuum-sealed type vessel.

You see this occurring more and more. And at the moment, we don’t have a polymer, the type of plastic bag, perhaps, that you would be able to reuse, so it’s somewhat wasteful at the moment. And so therefore, you only see this in very special cases where you’re trying to preserve very high value types of coffee.

IRA FLATOW: Why couldn’t I put it in a jar and put the lid on tight?

CHRISTOPHER HENDON: That works perfectly fine. But if you open up the jar while the coffee is still cold–


CHRISTOPHER HENDON: –in the atmosphere, then you’re going to condense water from the atmosphere onto the surface and inside those beans. And so the more times you do that, you know, progressively, you’re going to have more and more water being condensed within that vessel. So–

IRA FLATOW: That’s freezer burn.

–it’s a tricky problem. Indeed, it is freezer burn, yeah. And you don’t want your coffee smelling like fish sticks. That’s the [INAUDIBLE].

IRA FLATOW: That’s true. Now, when I take the coffee out to grind it, do I grind it frozen? I want to grind it frozen that way?

CHRISTOPHER HENDON: So we’ve actually found that temperature does directly affect the size of the smallest particles in the grind, and it happens to be favorable. So actually, as you cool down, cracks propagate through materials more quickly and result in more unified particle sizes.

And so these small particulates, actually, are formed and are very reproducible when we cool this coffee or cool any material, for that matter. And so we do recommend, actually, grinding it frozen. And then once, you know, once you expose the frozen grounds to hot water, they very quickly equilibriate to the temperature of the water, and brewing is as per normal.

IRA FLATOW: All right, let me get to the water ’cause I found something amazing in your research, and that is we should not be using distilled water, reverse osmosis, whatever. We should be using hard water when making coffee.

CHRISTOPHER HENDON: Yeah. So this language of soft and hard is complicated because it’s very easy to define soft, you know, water that doesn’t have very many minerals dissolved within. But hard water is simply defined as water that contains lots of minerals, but it depends on their identity. So instead of talking about calcium, magnesium, and they are important, the main mineral we’re trying to avoid using is bicarbonate, because bicarbonate creates a buffer to try and stabilize the pH.

And coffee, itself, is acidic. In fact, we like the acids in coffee. Even if we don’t perceive them, necessarily, as the peel of a lemon or whatever, we still enjoy the acids because they taste like sweetness and acidity. So we simply don’t want to use water that contains high levels of bicarbonate. But if we could have other minerals there that did not buffer acids, then that would be ideal.

IRA FLATOW: Doesn’t water softener have bicarbonate in it?

CHRISTOPHER HENDON: So typically, a water softener actually is exchanging out calcium and putting in sodium. And so why it’s softening the water is it’s preventing calcium and carbonate to co-exist forming limescale. But it’s not actually killing the bicarbonate. It’s simply, actually, removing the thing that we actually want for coffee extraction which is calcium.

IRA FLATOW: Wow. I’m just– you know, ’cause I always thought the purer the water is– so well, let me go back to that. Does that mean, you know, we here in New York, we talk about– we take our coffee seriously. We take our bagels seriously.


IRA FLATOW: I mean, could that be the same kind of watery thing going on in what makes a bagel better than what makes one coffee better?

CHRISTOPHER HENDON: Interestingly, there is a company that has recreated the chemistry of New York water for making bagels. So people have caught onto this idea already. The reason that we were talking and you first mentioned the idea of using soft or distilled water as being a positive thing for coffee is not because it’s somehow, you know, that’s what’s taught. It’s not because it’s somehow better or worse. It’s because it doesn’t have bicarbonate in it. That’s just a– it’s a guarantee.

So at least you know you’re going to get some of the positive flavors out. But when you’re looking at New York tap water or a municipal water provided wherever in the state of New York, you’re right. It does impart some sort of terroir, some chemistry related to New York’s specific water. And that has enabled the production of things like bagels and exceptional coffee in the city.

IRA FLATOW: Ah, see. You’ve just justified what we like to say here in New York. Thank you for that.




IRA FLATOW: Yeah, OK. Let’s go back to brewing methods now. I’m going to move on through ’cause a lot of interesting things. There’s a trend now at coffee shops where you can choose your brew method, right? Your pour over, your steeping. How do these methods extract the coffee molecules out differently, and how do you choose which one is right?

CHRISTOPHER HENDON: Yeah, that’s extremely complicated. So shops that offer a wide variety of brew methods are really working hard to present that as an option for the customer. More often than not, you’ll find maybe one or two choices for a given coffee. Typically, baristas, shop owners, roasters, home enthusiasts are making decisions about how they’re going to extract the coffee based on primarily three things.

First of all is the roast degree. So it is typical in the coffee industry to find darker-roasted coffee being used in espresso. And the reason is is that you’re going to sort of moderate some of the acids that exist in the green bean so that you’re not going to get something that’s extremely acidic and very hard to enjoy. And so you find that dark-roasted coffee are more prevalent, and that pretty much summarizes the Seattle coffee scene at places like Italy and so forth.

The next thing that practitioners typically want to consider is the concentration in which they want to consume the coffee. So if you make a filter coffee, you’re going to use 20 grams of coffee, say, but you might then pour over 250 milliliters of water on it. And that makes a much more dilute beverage than if you were to use 20 grams of coffee and use only 40 milliliters in an espresso machine. And so indeed, it is coffee-specific as to whether you want to taste the concentrate or the stretched out version.

IRA FLATOW: Let me just remind everybody while we’re having our next cup of coffee. I’m Ira Flatow. This is Science Friday from WNYC Studios. Talking with Christopher Hendon, Assistant Professor of Chemistry at the University of Oregon in Eugene. Would you be a coffee geek? Would that be the correct classification or coffee enthusiast?



CHRISTOPHER HENDON: I think now, I’m a coffee researcher, I guess.



IRA FLATOW: Do you have your favorite method of brewing, brewing method that you use?

CHRISTOPHER HENDON: Yes. So in my office when we have academic visitors, I love to present them a coffee that has unusual flavors and so forth. But I typically find the most reproducible way for me to do that in my hands is a pour-over method, something like a V60 or a Kalita Wave or something like this.

But if I were to go out and enjoy a coffee, by far my favorite format is to try the coffee both as an espresso and with espresso with a little bit of milk. And I call that a one and one where you have the same shot split in half. That way, you know you’re getting the same espresso, but you can see how the flavors translate when you add milk.

IRA FLATOW: Is that called café au lait in France?

CHRISTOPHER HENDON: The café au lait is indeed. Yeah, you’re right. It’s a coffee plus milk but probably a bit bigger than the old espresso and cortado combo I’m talking about.

IRA FLATOW: I remember growing up with a percolator.


IRA FLATOW: How does that work differently than the way we brew coffee now?

CHRISTOPHER HENDON: So it’s– interestingly, it’s becoming more popular once again. There’s been a resurgence in the percolator.

IRA FLATOW: No kidding.

CHRISTOPHER HENDON: Yeah, it’s back.

IRA FLATOW: I’m going into my basement.

IRA FLATOW: Yeah, well, yeah. It’s probably worth a lot of money now. And the inventor actually recently passed away. And there was a big ceremony in Italy for this. It was essentially a party because, indeed, the percolator was sort of one of the earliest embodiments of the portable espresso machine in the sense that you start with water at the bottom, you boil it, you create a critical pressure of steam, and that steam pressure is then forced through the puck of coffee.

That gives you an extraction that is only enabled by having that addition of pressure, and then you can get a very concentrated beverage out of it. And so actually, a percolator is essentially a portable espresso machine without all the bells and whistles.

IRA FLATOW: Yeah, but it also recycles the coffee over and over again, doesn’t it? Goes through the basket, goes back up again.

CHRISTOPHER HENDON: Yeah, there is a little– so it depends on the design. Of course, there’s ones that look like a fractional distillation column where once the water is sufficiently boiled and steam passes through the puck, it then goes through the sort of cap and can no longer recirculate. So it depends on the embodiment. But you’re right. You could–

IRA FLATOW: That’s the vacuum one, is it, with the big bowl on the top and the pot on the bottom goes up through.

CHRISTOPHER HENDON: Yeah, so I think the one I’m referring to specifically is sometimes called the moka pot. And the percolator coffee does have this. It’s more or less a moka pot with a recycling feature. But either of them are having a resurgence because of its portability.

IRA FLATOW: So it really is– so it’s not the machine we think it is. It’s something a little more complex.


IRA FLATOW: And, you know, just for someone who grew up with a percolator as a teenager, it’s something also– I don’t know, it helps your body. You feel good when a big pot of coffee comes over, you know? It’s the experience that you don’t get from a little pour-over and a cone.

CHRISTOPHER HENDON: Yeah, so in the percolator sense, you’re thinking– in this case, we’re referring to like a Mr. Coffee-type product, one of these big pots that you find at the– oh, I see. So when I think of the percolator, I was referring more of the moka pot. I’m sorry.

IRA FLATOW: No, no. I’m talking about the percolator that was the stainless steel thing you plug into the wall.


IRA FLATOW: That’s the one I’m referring to.

CHRISTOPHER HENDON: Yeah, yeah. So you’re right. There is something that still, you know, it still resonates with me as well. And I go into a diner, for example, and I see this big pot of coffee sitting on top of the brewer. And, you know, it means it’s going to be a good day.

IRA FLATOW: I’m going to end it there ’cause that’s my cup of coffee also. That’s the kind of coffee I like. Thank you very much, Christopher.


IRA FLATOW: Absolutely enlightening us about coffee and everything we needed to know. Christopher Hendon is Assistant Professor of Chemistry at the University of Oregon in Eugene.

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