Coffee Extraction

James Harper

So here's the question.

How different can the same coffee taste?

What is the range of flavors a single coffee can offer?

And I've actually made a podcast on this topic before.

Scott Bentley

Welcome to Adventures in Coffee, a podcast.

James Harper

So I make a number of podcasts.

In addition to photo stories, a history of coffee, and this one, I also make a podcast called Adventures in Coffee.

Scott Bentley

I mean, it sounds like you've got a great coffee butler and just get him to make you coffee whenever you need one.

James Harper

The show's upbeat, easy going, and we try to make coffee fun.

And on one of the episodes, Jules, who is the co-host and a self-proclaimed everyday coffee lover.

Jools Walker

I have never, and I repeat never, used an AeroPress before in my life.

James Harper

She had never made an AeroPress before.

Jools Walker

What's that there?

Scott Bentley

It's an AeroPress.

James Harper

And if you don't know what an AeroPress is, just Google it, guarantee you've seen one.

And on the show, she went on a journey to see just how different one coffee can taste if you adjust different brewing parameters using an AeroPress.

So she began with a standard AeroPress recipe that comes on the side of the box.

One scoop of coffee, put water up to the number one line, so not that much water.

The water has to be 80 degrees Celsius, stir 10 times, and then plunge.

And then she took a sip.

Jools Walker

Is it weird to say that it tastes a little bit like some kind of liqueur, but it's fruity at the same time.

James Harper

And then she made the brew a lot weaker.

By putting in more water, she filled her AeroPress all the way up to the number three line.

And that liqueur taste she tasted earlier transformed.

Jools Walker

I'm tasting nuts with this one.

So almost like hazelnuts, like a sort of Christmassy hazelnut taste is what I'm getting from this one.

James Harper

But then, instead of using 80 degree waters like before, she jacked it up to a hundred degrees.

Jools Walker

Okay, that's different.

That's really different.

It's made me sort of scrunch my nose up a little bit as I've tasted it.

Scott Bentley

Wow, okay.

James Harper

And the thing that I loved about making this episode was this moment of revelation for Jules.

It's a revelation that most of us coffee professionals have experienced a long, long time ago, and maybe even forgotten where or when we had this revelation.

But for Jules, we were able to capture it there and then in her kitchen.

Jools Walker

This is a wild ride.

This is the same coffee, and this is just in the nicest way.

This is very bizarre how much this is changing.

James Harper

So what exactly was going on inside that AeroPress?

Why does adding more water create certain flavor notes?

And why does temperature have such a profound effect?

And so what I wanna do in this episode is explore some of the science of coffee extraction.

And to be frank, there is still so much we do not know.

But as you spend time in the coffee world, you're gonna come across a lot of theories, but only a number of them are actually established.

And those are the ones I'm gonna present to you.

And we're gonna look in some detail at the work conducted by the University of California Davis Coffee Center in partnership with the Specialty Coffee Association, where they have been showing how changing different parameters affect what we taste in coffee.

And we're also gonna see why what we taste when we drink coffee is not simply the flavor molecules in a cup of coffee.

Our tasting experience is so much more complex than that.

And at the end of the episode, I'm gonna tell you a story about some of the challenges that coffee shop owners had, you know, about 10, 15 years ago, trying to offer consistently extracted filter coffee to their customers.

And how Marco Beverage Systems, the sponsor for this episode, developed the SP9 to essentially eliminate many of those variables that were making filter coffee so inconsistent.

Because the thing is, when it comes to making coffee, there is so much, scientifically speaking, that can change how our coffee tastes.

I'm James Harper, and this is the Science of Coffee, a spin-off series from my documentary coffee podcast, Filter Stories, and a journey into coffee's hidden microscopic secrets.

So let's kick things off with what we know when we add hot water to ground coffee.

And the first thing to understand is that water itself is a solvent.

And I want to explain to you how water pulls flavor compounds out of coffee.

We're gonna get a bit technical, but a very simplistic way to understand it, it's kind of like magnetism.

All right, let's get technical.

Here's Samo Smrke from the University of Applied Sciences in Zurich.

Samo Smrke

I am a scientist at the Coffee Excellence Center.

James Harper

What makes water so great as a solvent?

Samo Smrke

Water is a very special molecule.

It forms what is called hydrogen bonding, which also means that the other compounds that are polar, that can form hydrogen bonds, these molecules are very eager to come to bind with water.

Water is polar because it has a certain geometry.

James Harper

It has a slight positive charge on one side and a slight negative charge on the other side.

Let me help you visualize this.

So water, as we know, is H2O, two hydrogen atoms and an oxygen atom.

And when teaching chemistry, there are these very simplified diagrams of molecules.

And you know, like one way you can represent a molecule is by having differently sized balls stuck to each other.

Now, I want you to picture Mickey Mouse.

You know the Mickey Mouse silhouette, big round ball, two smaller balls on top.

That is also how a water molecule is often depicted in chemistry textbooks.

So Mickey Mouse's face is the big ball, oxygen, and the two hydrogen atoms, you know, H2O, they are Mickey Mouse's ears.

And his ears are positively charged.

But Mickey Mouse's face, the oxygen atom, is negatively charged.

And because of the shape, which is actually somewhat bent, I won't go into details here, but the point is because of that shape, H2O, water, is quite polar.

And a very simplistic way to think about polarity is to think of water a bit like a magnet.

You have a positive end and a negative end.

Now here's the thing.

Polar molecules, like water, attract other things very quickly and easily that are also polar.

And in a tablespoon of ground coffee, it's full of flavor compounds.

Some of them are very polar, some of them less polar.

And so when water hits the ground coffee, the easiest, fastest things that are going to extract are the polar flavor compounds.

They pull from the ground coffee and dissolve into the solution, our coffee brew.

So water has hit the coffee grounds and coffee flavor molecules have been attracted into the water through polarity.

But the journey doesn't end there because now this little droplet of water with coffee flavor compounds in it has to move from the coffee grinds into your cup.

And so I introduce to you a second very established concept in coffee brewing science, diffusion.

Samo Smrke

Extraction is basically a diffusion process where particles from inside of the coffee particle need to move to the outside of the particle to go into our solvent, the water.

So we have to imagine that these molecules need to be soluble in water that we make the coffee and they need to diffuse from the inside of the coffee particle into the solution.

James Harper

So I wanna get a bit more technical again, but at a high level, diffusion is quite simple.

You experience it countless times a day.

For example, if you spray some perfume in the air, it proceeds to fill the room with those smells.

And it's all because of diffusion.

When something wants to go from a place of high concentration to lower concentration.

Okay, but how does it work in the context of coffee brewing?

Okay, so imagine you're about to make a drip filter coffee.

You know, your filter paper is in the Chemex, Kalita, the V60, you know, whatever it is.

And in the middle of the paper is sitting dry ground coffee waiting for your water.

And then you pour your first round of water into your drip filter brewer.

The water sloshes its way through the dry coffee.

And let's imagine like a single little piece of ground coffee sitting there in that paper.

Water wraps around it.

The water worms its way through the little cracks and nooks of that little piece of ground coffee.

It dissolves some of the flavor compounds and emulsifies the oils and is now quite concentrated.

And concentrated liquids want to move to places where there's less concentration, where there's less stuff in it.

And that happens to be quite nearby.

Because all around this ground up piece of coffee, fresher, less concentrated water is flowing past, pulled down by gravity into your carafe.

That is diffusion.

So let's explore the flavors you're gonna get over the course of brewing a coffee.

What flavors are extracted first and what are extracted later?

And to answer that, I spoke with another coffee scientist, Mackenzie Batali.

Mackenzie Batali

I have a PhD in food science from UC Davis.

And then I worked as a postdoctoral scholar.

James Harper

And right now she's working at Compound Foods, a company trying to create coffee without coffee itself.

Fascinating topic and I'll probably cover it in a future series.

But I want to focus on the work she did while at UC Davis because she was a lead researcher in a study that looked at that question I just asked.

What gets extracted first and what gets extracted later when making a pour over filter coffee?

And some parts of her experiment are so simple, you could do it yourself in your own kitchen.

Just do what we've just visualized, making a pour over filter coffee.

But instead of letting all the coffee drip down into one carafe.

Mackenzie Batali

I basically switched the carafe under the stream of the brew every 30 seconds.

So I'd start, hold the carafe under that and then 30 second timer switch to the next carafe.

So then I'm collecting the second 30 second chunk.

And then I did that throughout the entire brew.

So I had eight fractions that were enough material to taste.

Bill Ristenpart

The fancy term for what Mackenzie did is fractionation.

And so you're getting the different fractions of coffee that come out earlier in the middle or towards the end of the brew.

James Harper

And that by the way is Professor Boris Risenpart who oversaw Mackenzie's work and he runs the coffee center at UC Davis.

So Mackenzie brewed a pour over filter coffee and in these different carafes, we have the flavor compounds that were extracted first and the flavor compounds that were extracted later on.

Mackenzie then gave those different fractions of coffee to a panel of taste testers who had been trained for months and calibrated quite carefully.

I mean, the calibration is so specific that for example, they all calibrate to even what rubber tastes like.

Bill Ristenpart

But rubber can mean different things to different people.

And so what they do is they'll have everybody agree.

They're like, okay, the smell of a rubber band or whatever the reference is, everybody smells it, everybody agrees that this is rubber.

And then they'll do things like dope the product.

In this case, coffee with different amounts of different things like soak it in rubber and then they'll agree on that.

And then they do their best to have everybody agree.

They're like, okay, this is what a 50% rubber aroma is like in the sample.

But then they do that for many different attributes.

So sweetness, sour, bitter, those obvious ones, but then roasty and citrus and vegetable.

So we ended up testing about 30 or so different attributes.

James Harper

So what did these trained taste testers think of that first fraction of coffee?

The fraction which had the most polar flavor compounds in it, the flavors which were extracted first.

Bill Ristenpart

What you'll find is that the first fractions are very intensely, very intensely bitter and sour.

Mackenzie Batali

It did not taste good.

My panelists called that the evil coffee.

James Harper

Intensely bitter and sour.

So that's the first fraction of coffee.

And the interesting thing is this trained panel of taste testers described the later fractions as much more pleasant.

Mackenzie Batali

The last couple of fractions were significantly more sweet, had more perceived floral flavors, fruity.

But it was a really interesting thing to see that after you flush out all of the acids and all the bitterants, that's when you are starting to get some of the sweetness and some of the more delicate volatiles.

James Harper

The later fractions were floral and sweet.

So why is that?

We've just been speaking about polarity.

And it is the case that those first fractions of coffee had flavor compounds in them which were more polar than the later fractions.

And when I first started diving into this research, the first thing I couldn't help but do was create a very simple model for understanding how coffee extraction works.

And it is totally wrong, but it was this idea that the first fraction doesn't taste good because it has a lot of more polar compounds in it and more polar compounds don't taste good.

But the later fractions tasted good because the flavor compounds in them were less polar.

And because less polar compounds taste great, that's why these fractions tasted great.

And like I said, completely wrong.

And when I started to understand how many layers there are, it's my goodness, coffee is really, really complicated, but fascinating.

And a great example comes from a very peculiar finding that McKenzie discovered when doing this experiment.

So these panelists, they said they liked the later fractions of coffee and they described these later fractions using a specific word.

Our expert panelists are saying, yeah, these are sweeter.

The later fractions tasted sweeter.

But here's the thing.

After these taste testers had given their verdicts, the researchers used some instruments to figure out, chemically speaking, what was actually in each fraction of coffee.

Bill Ristenpart

So we did some mass spectroscopy and analyzed all the different fractions for a whole bunch of different sugars.

And not just sucrose, but like things like fructose and glucose and mannose and galactose and all these sugars most people have never heard of.

And the punchline is that there were no concentrations of any monosaccharides of any sugars above the human threshold for perception.

So in other words, we found trace amounts, but they're in such small amounts that like basically it's very difficult to say that they're responsible for any of the perceived sweetness.

James Harper

The panelists were saying these later fractions tasted sweeter.

But there was very little in the coffee brew, chemically speaking, that was made of sugar.

Mackenzie Batali

In all of the fractions and in a full unfractionated regular drip brew, sugar content is well below any sort of human sensory detection threshold.

I found that very fascinating.

I never realized that the sweetness that can sometimes be perceived in black coffee, I assumed that it would be from sugar and it's not.

James Harper

Now, we do not know why we are perceiving sweetness when sugars are not perceptibly present.

But many of the people I spoke to had different theories and listen, they're just that, theories, hypotheses.

But I thought they were quite interesting.

Here's Professor Bill Risenpart's theory as to what's going on.

Bill Ristenpart

And so what we think is going on is that there are either masking or associative effects taking place.

James Harper

Masking or associative effects.

Now, a masking effect is, yes, there is something that is sweet, but we're not able to taste it because it is hidden, masked by other flavors.

Bill Ristenpart

And so masking means that like, yeah, there's like kind of naturally perceptible sweetness there, but it's so bitter and so sour early on that like your nerves are just overwhelmed by that.

So you can't taste anything else.

James Harper

Another of Bill Risenpart's theories is that our brains are tricking us into thinking this tastes sweet.

These are associative effects.

Bill Ristenpart

Associative means that there are other molecules present that your brain associates with sweetness.

So even though you're not actually doing the chemical perception of sweetness of a sugar on your taste buds, you are smelling other things that your brain associates with sweetness.

And so it basically tricks you to think, yeah, there's something sweet here.

James Harper

And here's an experiment that shows associative effects in action that you can try out this weekend.

Bill Ristenpart

Have you ever taken a bite of a potato while blindfolded, while somebody holds an apple under your nose?

They will think they're eating an apple, even though there's a potato in their mouth because the aroma, the apple aroma, really dominates their perception.

And so we think something similar is happening on the coffee.

There are other molecules present.

We don't know which ones your brain associates with sweetness and you can pick them up much more easily at lower total dissolved solids.

James Harper

Again, interesting theories, but I want to pick up on something that Professor Bill Risenpart mentioned right at the end.

You can pick them up much more easily at lower total dissolved solids.

What he's saying is that a coffee brew that is weaker is perceived to be sweeter.

And right now, we don't know why this is the case, but I'm told research is being done at the moment to try and figure it out.

And in the meantime, as far as you and I are concerned, when it comes to brewing coffee, the easiest way to counteract bitterness and amplify sweetness in the research that we've done is just to make it weaker.

And that, by the way, is Peter Giuliano, Chief Research Officer at the Specialty Coffee Association.

Now, this links to some other research from the UC Davis Coffee Center that I want to share with you because it holds the key to how you can adjust your coffee flavors at home to your taste.

But first, you need to understand two concepts, total dissolved solids and extraction yield.

Let's start with total dissolved solids.

The number of total dissolved solids in your coffee brew, also known as the TDS, is a quantitative way of saying how strong or weak your coffee is.

But total dissolved solids is quite a clunky phrase and what does it mean?

So it is a number of coffee flavor compounds that are dissolved in your brew.

Bill Ristenpart

Total dissolved solids is actually really, it means dissolved molecules that are completely hydrated by water.

It'd be incorrect to imagine like little, little, tiny, fine coffee grounds floating around.

We're actually talking about really dissolved stuff.

James Harper

Now, another term for this is brew strength, how concentrated your beverage is with these dissolved coffee flavor compounds.

And if you go back to that fractionation experiment that McKenzie performed, those later fractions of coffee were less concentrated.

They had fewer total dissolved solids in them relative to those first fractions, which were intensely bitter and sour.

And if you really push it to an extreme, a coffee with a very high TDS, as in with a very high number of total dissolved solids would be an espresso.

And a coffee with a very low TDS, as in very few total dissolved solids in a given volume of liquid would be a filter coffee.

And now I want to introduce a second concept, which you can use to measure a coffee quantitatively.

And this is the idea of how extracted coffee is, which is related, but quite separate from the strength of the brew, you know, the amount of total dissolved solids we've just been speaking about.

So let's rewind to McKenzie's fractionation study and let's look at that dry bed of coffee itself sitting there in the filter paper, literally waiting for water.

It's staring up in the sky saying, hey water, where are you?

Before water hits those coffee grounds, it is full of flavor compounds in the dry powder itself.

But when that first flush of water came through in that first fraction, many flavor compounds moved from the ground up coffee into the water.

And so a percentage of all the flavor compounds in that coffee has now been extracted.

Let's just throw a number out there and say it's 8%.

That first flush of water has dissolved 8% of the coffee.

Now in that second round of water, yet more flavor compounds move from the coffee powder into the water.

So if you were to look at that ground coffee, now totally soaked after two flushes of water, maybe 12% of what was originally there has now been dissolved into the water.

This coffee powder is 12% extracted.

This is the concept of extraction yield.

How much of a coffee's flavor compounds are actually in the water and no longer in the coffee grinds themselves?

And if you're curious to know, how much can you extract from ground coffee?

What's the upper limit?

So Professor Chahan Yeretzian at the University of Applied Sciences in Zurich told me, listen, if you have an espresso machine or you're making a filter coffee, you extract about 20, you can go up to 30%, you know, in general, and then you start to hit a wall.

But Chahan himself spent many years at Nestle researching coffee specifically.

And of course, one of Nestle's biggest products is Nescafe.

Chahan Yeretzian

And depending on your techniques.

You can actually extract up to 50% if you do instant coffee.

There are techniques to get 50, 55%.

James Harper

And I think that fact alone explains why there isn't a jar of Nescafe in my cupboard.

So we've now just explored two of the fundamental concepts that specialty coffee professionals use when measuring their brews.

The TDS, the concentration of the brew, and also the extraction yield.

What percentage of the coffee has been dissolved in the water?

Brew strength and extraction yield.

These form the basis of a tool in the coffee industry called the brewing control chart.

If you Google it, you'll probably find the older version of it.

It's a very busy chart where you have the brew strength on one axis and the extraction yield on another axis.

And bang in the middle is a square which says ideal.

As in, if your cup of coffee is in this square, it is the ideal brew strength and extraction yield.

But this chart is problematic.

And actually, Professor Bill Rissenpart at the UC Davis Coffee Center has been doing a lot of work to modernize it.

I'm not gonna get into the details here, but it is a really interesting story.

And maybe I'll cover it in a future series.

But let's just cut straight to the chase.

Bill Ristenpart

One of the most exciting things about it is that like what we're showing in this new updated chart, and not just from this fractionation experiment, but from lots of different experiments, what we're developing is a chart that shows that like if you wanna maximize perceptible sweetness, it's gonna be in the bottom left corner of the classic coffee brewing control charts.

James Harper

So let me explain.

And for this, you might actually now want to Google the brewing control chart to help you visualize this.

So the chart is made up of two axes.

The vertical axis is total dissolved solids.

The brew strength.

Horizontal axis shows what percentage of the coffee has been dissolved into the water, also known as the extraction yield.

Now, here's the cool thing.

Let's say you're making a pour over coffee, like the one we visualized earlier in that fractionation experiment, and you want to make it a very sweet brew.

Bill Ristenpart

So if you wanna maximize perceptible sweetness in your black coffee, then you want to brew it to a lower total dissolved solids and a lower extraction yield.

James Harper

So if you want a sweeter cup, make it more diluted.

You know, add more water relative to the coffee.

So now we have a lower brew strength, a lower TDS, but to get a lower extraction yield, lower the temperature of your brew water.

Now, quick caveat.

Anytime you change one parameter, like how much water you use, depending on how much you change it and what your brew method is, you could affect both the brew strength and the extraction yield at the same time.

Coffee is messy like that.

So that's what you have to do if you wanna maximize sweetness.

But maybe a light sweet cup is not what you're looking for in your morning coffee.

Bill Ristenpart

If you wanna make it maximally sour, if you want a sour bomb, then you wanna do high total dissolved solids and low extraction yield.

James Harper

So to make it more sour, use less water.

So now your brew is more concentrated and brew with cooler water.

Bill Ristenpart

If you wanted to make it super kind of bitter and roasty, kind of the classic coffee notes, then it's in that top right corner.

James Harper

So use less water, so it's a more concentrated brew, but also use hotter water.

Bill Ristenpart

And then if you want tea-like or kind of like floral notes, then you wanna go in the bottom right.

And so that's really high extraction yields and low TDS.

James Harper

So make your coffee more diluted, add more water relative to the coffee so it's less concentrated.

And also use hotter water.

Now, of course, there are many other things you can do to get your coffee into different parts of that brewing control chart.

So in addition to the temperature and the amount of water, you could change the grind size.

You could agitate it, you know, get a stick in there, swirl it around.

Pressure, if we're talking about espresso.

And let's not forget the amount of coffee you use relative to the water.

But I really now wanna focus on temperature because it really does have a dramatic effect on the flavor of coffee.

Bill Ristenpart

So in general, the hotter the water, the faster everything happens.

So that's just the rule of thumb.

And so if you really wanna geek out, there's a phrase, Arrhenius temperature dependence, named after a famous chemist from a couple centuries ago.

And basically what it means is that things like the rate of mass transfer, the rate of extraction, vary exponentially with the temperature.

And so that's a fancy way of saying that a very minor change in temperature can have a big impact on the rate of extraction.

Temperature affects extraction exponentially.

But just imagine a curve that continuously gets steeper and steeper and steeper.

It's like a hill that you're climbing and every foot you climb, it gets steeper, right?

And so that's an exponential curve.

And it's only kind of stops when you get to the physical stopping point of the boiling temperature.

So you can't get past 100 degrees Celsius.

James Harper

And so the closer we get to 100 degrees, the faster, and exponentially faster, a flavor compound will be extracted out of the coffee.

Let me demonstrate this effect by hopping on the piano.

Which, by the way, all the piano music you've heard so far, I write and play that too.

So let's say we have citric acid.

It is somewhat polar.

So even at lower temperatures, you know, 40, 60 degrees, it can be extracted faster than less polar compounds.

And this is how it works.

As you increase the temperature, you know, from 20 to 30 to 40 to 50 to 60, it is able to be extracted faster and faster all the way up to 100 degrees.

But what about a compound which is relatively less polar?

Caffeine.

Samo Smrke

Now, for example, with 90 degree water, caffeine is easily soluble in water.

Whereas at room temperature, it's soluble, but it's not great.

It's not so easy to dissolve it.

So at lower temperatures, it dissolves very slowly.

James Harper

And in fact, it takes a much, much higher temperature until it starts to be extracted very quickly.

And it really skyrockets from 80 degree onwards.

Now, here's a mystery that I was very curious to shed some light on from my own coffee brewing experience.

I will occasionally make an AeroPress.

And I have found when I brew it with, let's say, water broadly around 85 degrees, it is fragrant, sweet, I like it.

But when I brew an AeroPress with water close to 100 degrees, you know, straight off the boil, it is super sharp, very bitter, which is, by the way, the same reaction that Jules had at the top of the show.

Jools Walker

That's really different.

It's made me sort of scrunch my nose up a little bit as I've tasted it.

James Harper

And I really wanted to know, is it that at the higher temperatures, flavor compounds are being extracted, which are just not being extracted when I use cooler water, and these are the ones which are responsible for making my AeroPress taste so sharp?

Or is it the case that the brew is just overall so much stronger?

And it turns out the coffee center at UC Davis has done a number of experiments broadly looking at this question and was able to give me a few clues.

Mackenzie Batali

So we did another study that hasn't actually been published yet, looking at cold brew versus hot brew.

James Harper

At a very low temperature and a very high temperature, but they kept the strength the same.

They wanted to get a clue as to what are the different flavors being extracted at different temperatures.

But of course, hot water extracts much faster than colder water.

So to keep the brew strength the same, they adjusted time.

Bill Ristenpart

And we're not just talking a couple of minutes What takes a hot coffee to do, you know, maybe 10 minutes, it can take 36 to 48 hours.

So it's much, much slower.

James Harper

So they had their hot brewed coffee and their very slow cold brewed coffee, and then gave them to a trained panel of taste testers.

Mackenzie Batali

And we did see differences, even when they're consumed at the same temperature.

So there's same extraction, same TDS, all chilled.

And compared to the filter coffee made using hot water, the trained sensory panel said that the cold brew was less bitter, less sour, kind of less rubbery tasting and more floral.

James Harper

The study suggests that some flavor compounds at low temperatures are just not being extracted very much at all into the cup.

So when I learned this, I was like, aha.

So maybe what's going on in my AeroPress is that with 100 degrees boiling water, I'm just extracting certain flavor compounds, which are by their very nature, very bitter.

It may not be because it's a stronger brew overall.

But then Bill Risenpart told me about a very similar study they had done, but this time looking at hot coffees, specifically with a temperature range between 87 and 93 degrees Celsius.

Now, of course, a coffee brewed at 93 degrees is going to be extracting a lot more than a coffee brewed at 87 degrees.

This is our friend, the Arrhenius equation kicking in.

So in this experiment, they kept the brew strength the same, this time by adjusting the grind size.

Bill Ristenpart

And here the outcome was the temperature doesn't affect the sensory properties of coffee provided with a huge caveat, provided that you keep the total dissolved solids and the extraction yield fixed.

James Harper

So this trained panel of taste testers said there was no perceptible difference in flavor

with the coffees brewed at different temperatures, you know, from 87 to 93 degrees,

if you keep the brew strength and extraction yield the same, which suggests the reason I don't like

my AeroPress brews when I use 100 degree water, it's not because 100 degree water extracts this

one particular flavor compound, which I find particularly bitter.

It could be because the brew itself is just much stronger.

And if you, dear listener, make your AeroPress with a 100 degree water, share me a recipe.

I'll give it a try.

Now, when I began my research into the world of extraction, I had another rather naive concept in mind.

And that is, if a flavor compound is in the coffee brew, you will experience that flavor.

As in, if swimming around in your cup of coffee is citric acid, you will therefore experience the flavor of citric acid.

But that's not true.

There is so much more to the coffee flavor experience.

A lot of it is shaped even before you start making your coffee in the first place.

And the same exact coffee can actually have many different types of flavor experiences, depending what you do after you've actually brewed it.

I plan to unpack many of these things in a future series and in later episodes, but for now I'll give you like a little snippet.

Okay, so let's imagine we're brewing a filter coffee like we did in the fractionation experiment earlier.

There is some ground coffee sitting in the middle of the filter paper, staring up, waiting for water to be poured on top.

Now, if you zoom into that ground coffee, in amongst all the flavor compounds in that dry ground powder is one in particular, a floral volatile aromatic compound.

It's relatively small and relatively light.

And now we add hot water.

We add energy.

Peter Giuliano

So basically the smaller a molecule is, the lighter it is.

And the lighter it is, the easier it gets swept up in the air.

James Harper

And when we brew coffee, that floral volatile aromatic compound could be swept up in the air too.

Peter Giuliano

That's one way to think about volatility.

If it's a heavy molecule, it's going to stay put and can't be swept up in the air.

Whereas a very light molecule get volatilized very, very quickly.

And those are the kinds of things when you open a bottle of vinegar, for example, that's acetic acid.

Within a few minutes, you open a bottle of vinegar and you can smell it all over your kitchen.

James Harper

And so the theory is, let me repeat that.

The theory is we perceive more florals in our coffees at lower temperatures because at higher temperatures, they get picked up and taken into the air.

And this links to the experiment I explained earlier between the cold brew versus the hot brew coffees.

Mackenzie Batali

Remember how the trained taste tester said the cold brew was less bitter, less sour, kind of less rubbery tasting and more floral.

James Harper

Cold brew was perceived as more floral.

And the theory is because some floral aromatic compounds are relatively small and light in the hot brew, that compound was taken away into the air.

But in the cold brew, it stayed in the actual liquid.

And because the two coffees weren't drank hot, they were drank at low temperatures, the floral stayed in the cup as well.

If, for example, the cold brew was heated up to say, I don't know, 70 degrees, there's a chance that floral flavor would have disappeared because it would have been taken up in the air.

All this to say, the temperature of the coffee when you brew it, but also when you drink it, could possibly affect what aromatic compounds are in the liquid for you to perceive in the first place.

But it doesn't stop there either, because the human body adds its own layers to the coffee tasting journey.

For example, the receptors in our nose and our mouth eventually do become saturated.

And that has a whole bunch of effects.

And all the while, your brain is computing all this information and changing your sensory experience on the fly.

Peter Giuliano

And so this is why you may be cleaning your apartment or something.

And it's at first the cleaning product, you smell it a lot, but then you stop smelling it.

Then you go outside, get the mail and you come back in and wham, you smell it again.

And you've been habituated to that smell.

Your brain has actually screened it out.

These are very powerful effects.

And they make a difference in coffee.

If you're at the cupping table, right?

A professional coffee tasting.

You've got five coffees and you care about floral.

So you, oh, the first coffee is super floral.

Second coffee, not as floral.

Third coffee, still not as floral.

Now, the thing is, that second coffee, is it really not as floral?

Or did you habituate yourself to floralness on the first coffee?

And so you can't recognize it in your second.

And this is why multiple tasting, changing the order is really important when you're doing important coffee tastings.

James Harper

Okay.

Let's press pause on the theory and the academic studies of extraction.

And let's just get a bit more practical.

I want to tell you a short story about how the specialty coffee community was grappling with the problem of getting consistently extracted filter coffees and how this episode's sponsor, Marco Beverage Systems, how they developed the SP9.

Because here's the thing, getting the same flavors out of the same coffee beans one day to the next is not easy.

So cast your mind back about 10 years ago or so, and you're going into one of your favorite specialty coffee shops and you're in the mood for something a little special, like a high-end filter coffee.

David Walsh

You know, this Panama Esmeralda Geisha, or you can have this Costa Rican Hermosa fully washed something.

James Harper

This is David Walsh, who today is Marco's R&D manager, but at the time, 10 years ago or so, he had this coffee blog.

And he was hopping from cafe to cafe, writing about this new growing movement called specialty coffee.

And at the time, these cafes were brewing filter coffees all in the same way.

They were using kettles and standing over, you know, a V60 or a Chemex.

Now, these are all wonderful ways to brew coffee at home, but the problem in a cafe setting is that it was very labor intensive because you had to stand with a kettle for three minutes at a time.

David Walsh

And it was kind of inconsistent, like disappointing a lot of the times.

James Harper

Inconsistent and disappointing.

And I'm going to raise up my hand here and say I have also felt this way when I've gone to a new city, got my phone out, searched specialty coffee, went there, paid $7.50, and the filter coffee I bought just tasted lame.

But I knew the beans themselves were roasted well.

The problem lay in the brewing.

Now, of course, if these cafes had been making a batch brew, you know, one very large pot of coffee that you serve to 10 people or so, this would be a much more consistent flavor experience.

And listen, batch brews absolutely have their place.

But one of the challenges of offering a batch brew of a very high scoring specialty coffee, it just comes down to the economics.

Here's Danny Pang, Makro's Asia-Pacific Technical Sales Manager.

Danny Pang

So let's say you do a two liter coffee.

Guess how much coffee you'd be using?

Oh, 150 grams or something.

Okay.

So let's say you're 150 grams.

Let's say a pound of coffee costs you about $50 US.

So that's like how much per, just for the grounds itself, you're talking about like, what, $12?

James Harper

So a cafe could offer two liters of batch brew, but it's going to cost them $12.

And the thing about batch brew is that you have to drink it in a relatively short amount of time, an hour, maybe, two at most.

Cafes are a relatively low margin business.

You can't afford to be pouring $12 down the drain every couple of hours.

And the thing about those high priced, very special coffees is that it's rare for a cafe to sell two liters of these sorts of coffees every hour or so.

Which is why if you go into a specialty coffee shop today, they're going to be made to order individually.

Okay.

So let's get back to our problem.

Why were individually made filter coffees often so disappointing?

So what often happens in a cafe is that the head barista set the recipe for the hand brew, the made to order filter coffee.

And the way they make it is top notch.

The challenge is having other staff members make it the same way the head barista did.

So let's take a seat in a cafe and just observe the bar over the course of a couple of hours.

We'll see different baristas hand pouring coffees over and over.

Here are some of the inconsistencies you're going to see.

Danny Pang

It really gets affected by how tall, how short you are over the counter bar, how high or how low you raise your kettle over the coffee bed, how fast and how slow you pour, how many turns you make, how long you take to do it and what kind of temperature water do you have.

James Harper

So to me, the first obvious problem is that the water temperature is going to be quite different between the baristas.

Imagine this, you get an order for a filter coffee, you fill up your kettle, you put it to boil.

Now one barista might start pouring immediately when that kettle is finished and the next barista might wait a couple of minutes.

And as we've explored, when we're hot water, we're in the grips of the Arrhenius equation.

Bill Ristenpart

And so that's a fancy way of saying that a very minor change in temperature can have a big impact on the rate of extraction.

James Harper

So around 15 years ago, Marco recognized this problem and came out with the Uberboiler.

David Walsh

Which was, you know, this super precise temperature water delivery station.

So what it could do was deliver water temperatures controlled to 0.1 degrees Celsius.

And everyone got really excited and James Hoffman was, you know, kind of the lead customer.

James Harper

Now if you've been to a few specialty coffee shops, I guarantee you have seen the Uberboiler.

It's essentially an undercounter boiler and on top there's a stainless steel dispenser that's shaped a bit like a crested wave.

And the engineering trick to get the water temperature to 0.1 degree of accuracy, at the point at which it leaves that crested wave dispenser.

We're recirculating from an undercounter to the head.

Was to essentially circulate hot water throughout the brewer at all times.

And the specialty coffee cafe community sees this, they're like yes this is great.

But unfortunately it didn't solve the problem of temperature inconsistency.

Because what was happening is that barista staff were pouring this hot water delivered to 0.1 degree accuracy into a cold gooseneck pouring kettle.

David Walsh

And the problem there is you're losing all the heesh immediately from whatever you've dispensed the kettle from.

James Harper

So Marker put their minds to developing a new system.

How could they standardize the temperature of the water that actually hit the bed of coffee?

And David, who joined the team around this time as a kind of interface between the specialty coffee community and the research and development department, told me about the octopus.

It was actually called the pillar but I prefer octopus.

David Walsh

Imagine a central stainless steel column coming up out of the counter and attached to that are four braided hoses and at the end of each of them is a wand.

James Harper

Basically a hot water spray gun.

And they engineered a recirculation chamber which was the same principle of hot water recirculation as the uber boiler but this time it also enabled consistent pressure as well.

And what that meant is that the temperature coming out of that spray gun could be delivered at 1.5 degrees of accuracy.

But guess what?

Even here with a barista literally spraying the right temperature water onto a bed of coffee, there was still so much scope for inconsistency.

David Walsh

It certainly made the temperature more stable but it didn't solve kind of one of the major problems.

You're still relying on a barista to stand over and to press the button when he thought water should pour and stop when he thought it shouldn't.

James Harper

How long water is in contact with the coffee grounds as it's making its way down also affects the rate of extraction.

And while we're at it, here's another inconsistency.

How long that hot water is in the air before it hits the ground coffee, that too will change its temperature.

Depending on how cold the room is and for how long that hot water travels through the cold air, the water could cool by a couple of degrees.

And that is the point at which a customer could look up at the coffee menu, see the flavor notes that were written by the head barista and think to themselves, I'm not tasting that.

And it's such a well-known problem that, you know, for example, the Specialty Coffee Association, they accredit automatic coffee brewers.

They go through a very onerous testing process.

And here's Peter Giuliano again, the chief research officer at the Specialty Coffee Association.

Peter Giuliano

But we actually, when we're testing brewers, we don't even measure the temperature of the water itself in the tank.

We have a little sensor that we bury in the coffee bed and measure the temperature of the coffee bed itself, because that's all that matters.

It doesn't matter how hot you heat the water in the first place.

It matters how hot it is when it's actually touching the, what we call the slurry at the top of the filter bed.

Youtube Kid

Hey guys, welcome back to our channel.

James Harper

And if you want a really extreme example of how quickly hot water can cool in air, there are loads of videos of people living in ridiculously cold places, throwing pans of boiling hot water into freezing air, and watching it freeze almost instantaneously.

Now, of course, no respecting cafe owner will put the air conditioning to minus 30 degrees.

But so many of the inconsistencies we've discussed now were pretty much fixed when Marco came out with the SP9.

You've probably seen one of these before.

It's a minimalist tower that comes up on the counter.

And it's a super simple concept.

It's a drip coffee maker, but like much, much more accurate.

It has these two knobs on the side where the head barista can decide how much and how often to add water.

And all the barista staff have to do is put the brewer under the shower screen, press start, and voila.

Coffee shop owners could finally get a grip on some of the inconsistencies affecting their filter coffee brewers.

Danny Pang

It gives you a very consistent water temperature exiting from the top.

It gives you a very specific amount of water, and it gives you a very controllable time.

James Harper

And Marco incorporated many of the engineering principles that they had figured out with the uber boiler and the octopus.

Essentially, hot water circulating across the system at all times, which means the temperature when brewing coffee for Danny.

Danny Pang

My experience, within plus minus one degree Celsius.

James Harper

And just for the record here, SP9 by itself won't deliver an amazing cup of coffee.

You still need staff training, a good recipe set by the head barista, and good coffee.

So in this episode, we've explored some of the theories around how extraction works, the really important role of temperature, and there are many things that you can control, which if you control them right, will give you a great cup of coffee.

And if you don't know what you're doing, it might be making your coffee quite lame.

So, there's a huge variety of coffee out there, more coffee brewers than you can shake a stick at, and there's your palate.

So it'd be crazy for me to give you specific recommendations for how to brew your coffee.

But what I can recommend are some tools that you can use in your kitchen to make better coffee.

First up, invest in a thermometer.

Know the temperature of the water you use to make your coffee and play around with that to see what tastes best.

Measure how much to dry and how coffee powder you're putting in and measure how much coffee liquid you're getting out.

Make sure you're using good water for coffee, listen to the episode before this one, and then my recommendation is just like go wild.

Experiment with different temperatures, experiment with more or less coffee in, experiment with a different grind size, just experiment with all the variables.

But most importantly, note down what you're doing and how it tastes.

And also be a little bit scientific about it and only adjust one thing at a time.

From my experience, the effects can be dramatic and delicious.

Thank you for listening to the second episode of the Coffee Science series.

Did you enjoy it?

I'm actually genuinely quite interested to know what you think.

And I'd also like to know what else in the world of coffee science and extraction you'd like me to cover.

So tell me, find me on Instagram at filterstoriespodcast.

And also another way you can show what you thought of the episode is by going on Apple Podcasts and Spotify and you can give ratings.

I've got links in the show notes to both.

Every time I read a nice review, it really motivates me to keep going.

The next episode comes out in two weeks, but if you don't want to wait, you can head over to my new channel, The Science of Coffee, and hear it right now.

Now for this episode, I really want to thank Bill Rissenpart and Mackenzie Battaglia.

By the way, Bill heads up the UC Davis Coffee Center, which was only opened a couple of years ago.

And if you are a young budding scientist and you like coffee, consider applying to UC Davis and get in on their coffee science courses.

And also if you're a coffee organization and you want to further the research that Professor Rissenpart is doing, they are super keen to work for the coffee industry and uncover the mysteries of coffee extraction.

I would also like to thank Peter Giuliano, Samos Marquet, Danny Pang and David Walsh and the team at Marco.

Fun fact, I'd actually planned to visit them in Dublin, but you didn't hear any of that because I got COVID and had to cancel everything.

C'est la vie.

So let me tell you about the other episodes in this series.

The one before this is on water, and water has a noticeable impact on the flavor of your coffee.

And in that episode, I explore how BWT filtration kits, you know, remove and add certain minerals to help your coffee taste more vibrant.

Now, the next episode is about coffee genetics.

And I take you on a journey I had with Trabocca as we visit Kenyan coffee farmers to understand how they are adapting to climate change.

Now, Kenyan coffees can be quite acidic.

And if you were to brew them in the top left of the coffee brewing control chart, you're going to get a really big sour bomb.

However, if you dial back the strength and increase the yield, instead, you'll get these wonderfully citric berry notes all balanced out.

The fourth episode is all about espresso technology and how it's culminated in super automatic espresso machines.

And many of the principles we discussed in this episode also apply to espresso preparation.

And I show you how Ephesus, a super automatic espresso machine, has managed to control many of these variables.

The penultimate episode is on the science of good latte foam.

And when I was in Sweden, I learned that Oatly's barista edition was specifically designed to have a mild, subtle taste.

Because if you put all this energy into extracting amazing coffee flavors, their thinking was, we don't want Oatly to overpower those flavors.

And the final episode of the series explores sonic seasoning and the science and R&D landscape more generally.

I mentioned in this episode how the grind size is an extremely important variable to help you land in different places on the brewing control chart.

And I show you how Firenzato developed their all-ground grinder to enable you to go as fine as Turkish coffee and coarse enough for a V60 as well.

Now, I'm deeply indebted to Britta Fulmer and Peter Giuliano for their editorial guidance.

Raymond Detweiler reviewed some of the early drafts.

Cole Dachalker provided sound and editing assistance.

The series was produced by me, James Harper, and I also write and play the piano music.

Take care, experiment and go wild in your kitchen, and I'll speak to you next time.