How roasting transforms coffee beans

James Harper: It's 10:00 AM in Kaffa, Southwest Ethiopia. The sky is covered in clouds, long rolling green hills carpet the countryside. Up in a nearby tree, the white cheeked Turaco is getting hungry. This bird, the white cheeked Turaco, is such a badass bird. It has these piercing red eyes, red lipstick, and a black mohawk.

So this black mohawked red lipstick bird looks down from the branch onto the florist floor. It's hungry. What can it eat? Huh? And it spots a shrub, a spindly tree with thin branches, and along those branches are little clusters of cherries. Some look very green, probably tough, hard. But next to them are some which are very red, soft, sweet. The bird swoops down from the tree. Its beak grips the soft flesh of the coffee cherry, picks it off and then swallows it whole, and then flaps away. Over the next few hours, the coffee cherry makes its way through the digestive tracts of the white cheeked Turaco. The juicy flesh itself is all digested away and what's left is just the pip, like the hard dense coffee cherry seed. And then in the afternoon, as the white cheeked Turaco is sitting on a different tree, it poops out what is left of the coffee cherry, and it drops down onto the forest floor.

This is what the coffee tree wanted the bird to do. The bird was attracted to the cherry, and it pooped it out like a mile away. The plant is spreading its children across the forest floor. So that's how nature, birds and animals enjoy coffee. But for us, of course, coffee is a drink, so what would it taste like if we were to drink coffee the way nature gave it to us?

What if a human came along and picked up all the seeds that those birds had pooped out? Took away all the layers, and just got down to the green bean itself. What would it taste like if we were to turn this handful of green seeds into a drink? And so, one morning here in Berlin, I put the question to the test.

I got my hands on some green coffee seeds that had already been ground up and I made myself a pourover filter coffee. And instead of getting that characteristic brown, black liquid, you know, coffee, it was green, yellow. And I thought, who better to tell me what it tastes like than my flatmate Andrew. He's a former coffee professional. He used to be the roaster for The Barn here in Berlin. He's Q certified, he ran coffee training courses. But I wanted to keep it a surprise from him. So I shot him outta the kitchen when he walked in, told him, "Shut your eyes," and he then took a sip of the green, yellow liquid.

Andrew Johnson: It smells terrible. It smells like wood. Is that even coffee?

James Harper: It's 100% coffee. That's green coffee.

Andrew Johnson: It's not terrible though, it's just not really good.

James Harper: So, do you prefer roasted coffee?

Andrew Johnson: Yes.

James Harper: Why?

Andrew Johnson: Well, I think the fact that millions of people drink roasted coffee, it's like, a good indicator that coffee is better when it's roasted.

James Harper: Indeed. Sorry nature, but, uh, we want to, uh, burn your little seeds because we think it tastes better. And when we do that, when we apply heat to our roast coffee, early on in the journey, it tastes green, vegetal. It smells like wood, or peas. But once you apply the right amount of heat, it turns into something that we love. As we keep applying heat, it begins to turn into something that we detest.

So in this first episode, I'm gonna take you on that flavour journey and show you how coffee's flavour changes as we apply heat. I'll take you into some of the physics and the chemistry, and you can decide for yourself how roasted you like your coffees to be. And then in the second episode, I'm gonna help you learn how to roast coffee, and I'm gonna open up one of the most technologically advanced roasters I have ever seen. I'm gonna gaze into my crystal ball and speculate how these technologies might change the coffee industry forever.

I am James Harper, and this is The Science of Coffee, a spinoff series for my documentary podcast, Filter Stories, and a journey into coffee's hidden microscopic secrets.

So, the start of the roasting journey begins with, well, a bucket of green coffee seeds. I'm in Oslo, Norway, at the offices of Roest. They're the sponsor of this episode and they manufacture fully automatic roasters.

I'm standing in front of a roaster, a lot of wires everywhere. This is not the finished product, I take it?

Callum Gilmour: No, this is a prototype. While there are a lot of wires coming out of it, it is a fully working roaster.

James Harper: This Frankenstein contraption is the P3000, a fully automatic three kilo coffee roaster (a coffee roaster is the thing that turns green beans brown). And to show me how it all works is Callum Gilmour.

Callum Gilmour: I'm Callum, a machine learning engineer here at Roest.

James Harper: Callum gets up on a ladder and pours the bucket of green beans into a chute on the top of this roaster, and when he flicks the switch we begin one of the most mysterious, complicated, but crucial processes in the entire coffee supply chain. The beans tumble into what is basically a fan-assisted hot oven with paddles as well, pushing them around. And because the roost P3000 is fully automatic, there's nothing for us to see or do at the roaster, so we head over to Callum's computer monitor.

Callum Gilmour: So now we're just watching the live view of the roasting.

James Harper: Callum's computer tells me this roast session is gonna last about seven minutes, and I feel a little bit like a pilot: we took off, then I activated autopilot, and then I just sat back and relaxed with an orange juice.

The beans just get steadily hotter. They went into the roaster at around room temperature, you know, 25 degrees Celsius. I watch them get hotter and hotter: 50 degrees, 70 degrees, 90 degrees, 110 degrees. I do notice they change color a little bit, and then another minute goes by the beans, get even hotter, 130 degrees, 150 degrees. And while it just looks like the beans are just getting more yellow, that's actually hiding a dramatic physical change, a change that's gonna set us up for the moment of huge transformation.

To understand this better, I spoke to a man who had literally grabbed a microscope and looked at these beans as they changed.

Mark Al-Shemmeri: So my work was with the University of Birmingham, looking at an engineering doctorate, all on coffee roasting and simulation of coffee roasting, with a chemical engineering perspective.

James Harper: So at this stage, the water inside these green beans, it's agitating and it's moving across the bean. Some of it is also starting to evaporate.

But it's what happens to a small group of carbohydrates that I find most interesting.

Mark Al-Shemmeri: As the temperature increases and the moisture decreases, the cell walls start becoming rubbery and can expand.

James Harper: So, in the cell walls of a green coffee bean, there are some complex sugars. But sugars, you know, in a chemical definition. They don’t taste sweet to us.

Mark Al-Shemmeri: It's the arabinose, galactans and the galactomannans.

James Harper: …and these sugars begin to melt.

Mark Al-Shemmeri: The cell walls start to melt.

James Harper: The coffee bean is now becoming rubbery, stretchy.

Mark Al-Shemmeri: Cell walls transition from the glassy state, to the rubbery state.

James Harper: But at the same time, water is evaporating. This creates pressure inside the bean and also the acids within the green coffee begin to break down, which creates CO2 gas, which adds to the buildup of pressure inside this bean. And then when roasting coffee, you keep adding more heat. The coffee beans hit 170 degrees. 190 degrees. The pressure is building up! We're at the five minute mark — the coffee beans have just hit 200 degrees Celsius. The beans are about to transform.

And let's press pause.

Let's taste this coffee before it transforms. Callum has set up a coffee tasting. He grinds up these beans, pours some hot water on them, lets them brew for a bit, and then he and I, along with Roest's marketing manager, Veronika Bolduc, dip our spoons into the bowl of coffee and take a sip.

Veronika Bolduc: That is grassy, like a watery taste.

James Harper: It's not sweet at all, I find it particularly acidic. So five minutes into this hot spinning oven, the beans are at 200 degrees Celsius and they don't taste a million miles away from what Andrew tasted, my flatmate back at the beginning, when he drank raw green beans. It was still very vegetal. So five minutes into this roast. with the beans at 200 degrees Celsius. we are still a long way from a coffee that either of us would enjoy drinking,

But things are about to change, and change fast. The gases inside the bean are now reaching a crescendo. There is so much pressure that they finally release. And we hear the first crack. The coffee beans start to pop, physically bounce, like popcorn. Back at the coffee tasting, Callum picks up a bean, a very dark roasted bean, and shows me where he thinks the moisture blew out of.

Callum Gilmour: And we can also see the marks from the first crack.

James Harper: Can you?

Callum Gilmour: Yeah, you can see them just in the corner here.

James Harper: Oh yeah, it's like a couple of edges where something's exploded out of it.

Veronika Bolduc: Yeah, it's visible.

James Harper: It's like a cat scratched it right at the tip of the top of the bean. Dug its claws in very quickly.

Veronika Bolduc: Haha, yeah, you could say that.

James Harper: So back in the roaster, we're at five minutes, and we've just hit the first crack. This is the beginning of what is called the development time: How long you want your coffee flavours developed. And what I'm gonna show you now is what this coffee tastes like after first crack, as we develop it further and further. Which of these coffees sound like a coffee you wanna drink? First up, a coffee that's been developed 25 seconds after first crack.

Callum Gilmour: I still don't think we're getting much of the fruitiness or the sweetness yet.

James Harper: We're in the world of quite unripe fruit. And then we let it roast for an extra 15 seconds. With an extra 15 seconds, sweetness was creeping in, and those fruit flavours were tasting riper. The acidity was still very strong, but it was coming down a little bit, and then we let it roast for 10 seconds longer. This coffee was now quite sweet. There was a little sparkle of acidity. There was a good body to it. And those fruit flavours were now quite complex. Riper tropical fruits.

Veronika Bolduc: It's like when you have a pineapple but it's not that acidic.

James Harper: Then we roasted the coffees for another 50 seconds. This is now one minute and 40 seconds after the first crack. The coffee was now tasting a bit thinner. Acidity, nowhere to be found.

The sweetness was still there, and those fruit flavours were much more caramelised and creeping in were flavours you would probably find in a bakery. And then we let it roast for another 40 seconds. This was now two minutes and 23 seconds after the first crack. These beans were a very dark brown, borderline black. And, uh, Veronica and Callum, they live in Norway. They really like the specialty Scandinavian style of coffee, which is lighter roasts. What they tasted a minute ago – they were not looking forward to drinking a coffee this dark.

Veronika Bolduc: Okay, let's brace ourselves.

James Harper: You have to swallow this one, okay?

Veronika Bolduc: Okay, now we have ash. Now we have a leaking ashtray.

James Harper: This coffee now just tasted very roasted.

Callum Gilmour: Like I said before, it just lingers so much in the back of the palate.

James Harper: We were getting strong roasted flavours. The acidity was totally gone, sweetness was quite low as well. And to top it all off, there was a pronounced bitterness.

Callum Gilmour: Unpleasant aftertaste.

James Harper: Look, you've clearly noticed there was a little bit of judgement on our side when we were tasting these coffees. And I wanna address that here. You know, I didn't like that dark roast coffee very much, but it was difficult for me and the Roest team to taste these coffees and not bring our own personal subjective preferences into it. And you know what? It doesn't matter what I like or what they like or what anybody else likes. What matters is what you like. Because as we have seen in this series, we all live in our respective taste universes. We all like different things. You might love those dark chocolate notes, that bite of bitterness you might get in a dark roasted coffee. That's absolutely great. And I encourage you to find coffees that you like.

So let me just recap what was an unbelievable flavour transformation. So for the first five minutes roasting this coffee, the flavours didn't change that much. And then at first crack in the next two minutes, every 10, 20 or 30 seconds or so, the coffee transforms from something quite vegetative to a very acidic unripe fruit, to a tangy slightly sweet fruit, then a very sweet fruit, then a fruit dessert, and then dark roasted flavours. And this flavour transformation was just our experience of this one particular coffee, and every coffee in the world is gonna taste different at different roast levels. Maybe the development of flavours is actually more around chocolate. You go from light chocolate to dark chocolate; with some coffees, you might start with nuttiness and then make your way to dark flavours. Or you could get a combination of all the above. And that's why it's hard for me to describe exactly what the flavour changes are, because it really depends on the coffee. However, almost regardless of the coffee there is something that happens to all coffees as you go from lighter to darker roasts. This was told to me by roasting teacher Morten Münchow.

Morten Münchow: As you roast darker, you'll get more bitterness and less acidity, and as you roast lighter, you get more acidity and less bitterness.

James Harper: So generally speaking, a lighter roasted coffee will be more acidic and less bitter. And as you roast it darker, that acidity decreases and the bitterness increases.

So coffee changes in flavour dramatically after first crack. Now remember how I showed you how just before the first crack if we took a microscope to the bean, we would see that the sugars in the cell walls were melting, the beam was becoming a rubbery. Now I wanna take that same microscope and I wanna show you the chemistry taking place inside the beans to give us this dramatic change. What I found absolutely crazy is how complex, how wild these chemical changes are. So we have the maillard reaction, carbonization, strate degradation, pyrolysis, degassing, acid degradation, lipid oxidation, polymerization, hydrolysis, water loss, amino acid transformation, enzymatic reactions, all taking place at the same time.

Ildi Revi: It's crazy.

James Harper: There's so many things going on.

Ildi Revi: It's so complex. So many things going on.

James Harper: This is another roasting consultant and specialty coffee educator I spoke with.

Ildi Revi: My name is Idi Revi and I've published a chapter in the book, Coffee Production, Quality and Chemistry, a Royal Society of Chemistry textbook.

James Harper: Now remember how when we tasted that Kenyan coffee, when it was roasted lighter, the acidity was very, very high, and then as we developed it further, the acidity disappeared. I asked Ildi to help me understand what's happening chemically to produce that change.

Ildi Revi: I'm gonna look at this chart I have here.

James Harper: And she pulled up some research that was done before my mom was even born.

Ildi Revi: It was so groundbreaking; it was done in 1959.

James Harper: So inside the green coffee bean there are acids, citric acid, malic acid. And we really pick up on citric acid. When we taste it, we’re like ‘wow! That’s acidic’. And at around the first crack, they're at their maximum levels.

Ildi Revi: So you're going to be getting most of the malic acid that is still in the bean.

James Harper: But after the first crack, as we roast the beans for longer, the malic and citric acids begin to break down.

Ildi Revi: Your citric acid, your maleic acid is just at that point starting to degrade.

James Harper: And while they're breaking down, other acids begin forming.

Ildi Revi: Ascetic is forming. Glycolic acid is forming formic acid is forming, lactic acid is forming, through the whole thing,

James Harper: And this is what I found a little bit confusing when I first heard it. It's like, okay, citric and malic acid, they're degrading, but we have other acids which are forming. So you know, on the whole, there's still plenty of acid in this coffee bean. But the thing is, just because something is called an acid doesn't mean it's gonna taste acidic. So what's really happening is that the acids, which taste acidic are degrading and the acids that are forming don't taste particularly acidic. So on the whole, the flavour of the coffee becomes less acidic. Okay, so that's the acids.

But another thing that I was really curious to understand was how we go from a green apple flavour to a ripe pineapple? How does a fruit flavour develop like that? and then eventually even tastes like a baked fruit tart. I wanted to wrap my head around how these different flavour compounds form, and change? And I got my answer speaking with a coffee researcher Anja Rahn.

Anja Rahn: I'm a coffee chemist. So I have a PhD in process chemistry. I've worked at a coffee research lab in Switzerland. And afterwards, I spent two years working as a scientist at JDE Pete's in the Netherlands.

James Harper: Anja is Canadian, and, uh, at the risk of perpetuating a stereotype: she loves the flavour of maple syrup.

Anja Rahn: It's a sweet, nutty type of flavour.

James Harper: And coffee can absolutely sometimes taste like maple syrup. That maple syrup note, it might emerge, let's just say, one minute into the development of the coffee, and it could disappear just seconds later. How is it that in that brief window of time, a flavour is created and destroyed? So here's the chemistry: To get that maple syrup flavour, you need two base ingredients: sugar, and threonine. Now, threonine is an amino acid, it doesn't smell of anything.

Anja Rahn: Most amino acids and sugars are just white, scentless powders.

James Harper: But it's found in unroasted green coffee.

Anja Rahn: So threonine will have a biological function, just like chlorophyll does.

James Harper: And also in green unroasted coffee, there are sugars.

Anja Rahn: Glucose, fructose, sucrose.

James Harper: But when we throw the bean into a coffee roaster, at a certain temperature, those sugar molecules open up in form.

Anja Rahn: And this open form is very reactive.

James Harper: The threonine comes in and they become this new molecule. They combine. You have the threonine and the sugar together. But this new compound they form doesn't smell, yet, of maple syrup. This newly formed compound reacts and creates a different compound.

Anja Rahn: Basically this simple interaction can then splinter off in multiple other reactions.

James Harper: The way Anja Rahn described it to me was like when you're at a fancy party and you have all these champagne glasses stacked in a pyramid and someone pours champagne at the top glass, it overflows and then spills down into the champagne glasses below it.

Anja Rahn: That is sort of the effect of the reaction. You can have one simple reaction and it just sort of flows into all these other ones.

James Harper: And towards the end of this process, as these molecules form, break down, form with something else, break down – at a very specific point that aroma of maple syrup is created. And it's towards the end of this, I'm guessing that we finally get the maple syrup aroma.

Anja Rahn: Yes.

James Harper: And if you keep on going, it is destroyed. And what I find absolutely mind boggling about this entire process, is that this transformation, the cascade of reactions that will eventually give us this aroma of maple syrup, takes place like…

Anja Rahn: It takes place within seconds.

James Harper: Now I explained earlier how after five minutes in Roest's P3000, we came to first crack. And then for the next two minutes, the flavours exploded and changed very, very quickly. But what happens when you just keep on roasting the coffee? Eventually we arrive at the second crack. By now the beans have lost a lot of moisture and the roaster is getting pretty smoky. The walls of the cells of the bean, the thing that's holding the bean together, have become brittle now. The beans continue to create CO2 gas, the pressure builds and builds, and then explodes out of what has become a very brittle bean.

Ildi Revi: It's like twigs breaking.

James Harper: Now around the second crack, the beans have now got very, very black. But we also see another very curious phenomenon. The beans themselves look wet. I saw it with Veronika and Callum when we were around the table looking at these very dark, versed beans.

Veronika Bolduc: Now we have a lot of oils.

James Harper: And to understand where these oils are coming from, I spoke to another coffee scientist.

Samo Smrke: I'm Samo Smrke.

James Harper: Samo works at the Coffee Excellence Center in Zurich.

Samo Smrke: We study coffee from the chemistry point of view.

James Harper: And I asked him, are these oils formed during the roasting process?

Samo Smrke: No, the oils are not formed during coffee roasting. The oils are quite stable.

James Harper: Samo went on to explain that oils, like caffeine as well actually, are both naturally present in the green coffee bean. And they both survive that roast process. All the coffees that we drink have oils. It's just that only when it's been roasted a lot do the oils end up on the outside of the coffee bean. Now Samo quoted to me the traditional explanation for why oil ends up on the outside of a darkly versed bean.

Samo Smrke: The traditional explanation is that the gas that is formed during the roasting process is pushing out.

James Harper: Basically it's a CO2 gas and all that pressure, which pushes the oil to the outside of the bean, but Samo was a bit sceptical of this theory.

Samo Smrke: So I'm not sure if this is true or not.

James Harper: And in his opinion, more research is required. But seeing oils on the outside of a bean, for me, that is a very helpful thing because I personally don't like coffee, which is that darkly roasted and that has saved me many times. When I walk into a coffee shop I look at the grinder next to the espresso machine in the hopper, you know that shoot where the beans are sitting before they fall into the grinder. When I see these dark roasted beans, which are literally stuck to the side of the hopper, I know that they're stuck there because of the oils. And you know what? Coffee that dark, it's not a flavour experience I'm looking for.

That is the second crack. And now, what happens if we keep on roasting? I asked Ildi Revi. Wat happens if you continue roasting after that point? Get a third crack?

Ildi Revi: Uh,that's where you get, your burning reaction.

James Harper: Ah, unfortunately we don't get any more cracks. We now have flaming hot charcoal and the fire brigade is on its way because your house is on fire and so that is the magic of coffee roasting. And maybe this episode has inspired you to try it yourself. You know, give it a crack. Get it?

Roasting coffee yourself. It's an amazing way to understand how many flavours are possible from a single green bean. You have so much control of the flavours you want from your coffee. So in the next episode I am gonna share some advice so you can roast for yourself. And we're gonna be opening up the Roest P3000 prototype roaster to see the incredible technology that allows anybody, even me, to be able to roast coffee consistently, automatically. I found it so impressive that I actually think this sort of technology, it's going to change the coffee industry. And I'll give you my 2 cents as to what my crystal ball tells me about the future of coffee.

Join me next time for all of that.

So thanks so much for listening to this episode. If you enjoyed it, please help spread the word about this podcast. You can do it by writing a post on LinkedIn or going on Instagram. Take a screenshot of yourself listening to this podcast. Post it as a story. Tag me at Filter Stories podcast, and I will repost it and say thank you very much. On Instagram I have photos of brewing green coffee, that green yellow concoction that I served up to Andrew, my unsuspecting flatmate.

Here's what's coming up in the rest of The Science of Coffee. After these roasting episodes, I take a flight to Honduras where I attend Let's Talk Coffee. It's a gathering of farmers and coffee professionals organised by coffee and importer Sustainable Harvest, and it's there that I discover why organic farming is so much better than conventional farming for the environment.

And I go on a journey to answer the question, if organic coffee is so great, why aren't we seeing more of it? And then in the next episode I take you on a very personal journey where I've understood how hard it is to do coffee science well. What good, robust scientific methodology looks like and the journey was made possible by BWT Water and More. They develop water filtration kits for making coffee at home, in the cafe, and when I learned about the science of water, it helped me transform from a naive coffee professional to a much more critical home scientist.

In the final episodes of this series. I explore how you can get the most flavours from your roasted coffee beans. I explore the science of coffee ageing and I get really geeky with the technical experts inside Mahlkonig, one of the specialty coffee industries leading grinder manufacturers. They show me how grinders work and why it is that tiny, tiny changes can lead to such dramatically different flavours. And in the first episodes of this series, I help you become a better coffee taster by showing you how your sense of taste and smell works.

I also get a little bit metaphysical and I ask the question: When you and I drink the same coffee, are we experiencing the same flavours? I put that to the test by brewing up many batches of the same filter coffee using Marco Beverage Systems, SP9. I'm satisfied they extracted the coffee consistently every time, but I was then amazed when people told me how different the same coffee tasted to them.

The Science of Coffee was produced by me, James Harper. I also write and play the piano music. Thank you very much again for listening, and I'll speak to you next time.