Hot vs Cold — The science behind temperature and taste

James Harper:

For the longest time, I never drank cold brew. The only time I would make it was when a family friend would give me a bag of beans. Hey, James, you make a coffee podcast?

Here, here's a bag of beans. And I'm like, oh, thanks. They're really dark roasted, real oily.

Great. There was no way I would brew the coffee with hot water, because the bitterness, the roastiness would be so intense. But with cold water, it would mellow out the intensity.

And so I would make a cold brew. And if you don't know what cold brew is, super simple. Grind the beans, add cold water, let it sit overnight, and voila, cold brew.

And so that was my relationship with cold brew. It made bad coffee I didn't like more drinkable. But everything changed when I went to Guatemala.

I was invited as a speaker at the Producer Roaster Forum. And when I was out and about checking, you know, the cafes in Guatemala City, in one of those cafes, I bought a very pricey tin of geisha coffee from El Inherto, a famous Guatemalan coffee farm. $35 for a small retail bag.

Meanwhile, at the event itself where I was presenting, there was actually quite a lot of cold brew on offer. And the flavors were popping. Made me wonder, maybe I should give cold brew another chance.

So I'm back in Berlin. It's a lazy Saturday morning. And I do something that makes me feel a little bit uncomfortable.

I took these really expensive geisha beans, ground them up, put them in a container, added cold water, and let it sit overnight. And brewing this made me uncomfortable because, OK, look, if I had made a regular hot brew pour over filter coffee, I am confident I would get these wonderful flavors of fruits and a nice florality. But making this as a cold brew?

Like, what if it didn't taste good? I would have wasted a quarter of this bag of beautiful coffee. $9 worth of coffee that I had taken all the way back from Guatemala.

So the next day, Sunday morning, my partner Sophie, she's chilling on the balcony with Leo, the cat, reading a book. And I hand her a glass of the cold brew. She takes a sip.

Her eyes widen. And she's like, wow, this is delicious. I then taste it myself.

And I get this intense breeze of floral notes. Compared to when I brewed these exact same beans as a hot pour over filter coffee, this cold brew felt double, triple as floral. This was the moment I realized I was guilty of one of the oldest prejudices in specialty coffee.

You have to use good beans. If I may speak for specialty coffee people everywhere, I find myself writing off new ways of preparing coffees, new varieties, species of beans because I had a bad experience. Robusta beans or, you know, local Italian espresso.

But of course they tasted bad because they weren't being treated in a specialty way. And I've since learned that specialty Robusta is really interesting. Italian style specialty espressos I have since had in Italy are amazing.

I had written off cold brew because I had never treated it with the same care and attention as with my hot brews. This revelation that cold brew can be delicious set me off on a cold brew journey. A journey where I discover that when you brew with cold water, certain flavors leap out of your cup while others mysteriously disappear.

And if you're still wondering whether cold brew is just like any coffee that you serve cold, don't worry, we're going to answer that question as well. And my partner sponsor for this episode is Toddy. I go all the way to Colorado to learn how to cold brew from the world's biggest manufacturers of specialty cold brew equipment.

Here we go. We're jumping into the science of cold water extraction and the weird, wonderful ways our sense of smell and taste actually works. And this is Filter Stories, the untold stories hidden in it.

And you're listening to series three of the science of coffee. A journey into coffee's hidden microscopic secrets. I'm lying in bed.

I've just taken a very long flight from Berlin to Denver, Colorado. I'm jet lagged. I'm croaky.

Everything's super dry, high altitude. I'm not feeling great. A few hours later, I bumble my way down from the Airbnb to the streets of Denver.

And I'm picked up by Julia Leach and her husband.

Julia Leach's husband:

Howdy, howdy. How you doing? Welcome.

Hi, good to see you. You too.

James Harper:

And Toddy is a family business that develops manufacturers cold brewing equipment for the specialty coffee world. And like I mentioned, Toddy is my partner sponsor for this episode. And very briefly, look, this is what it means.

Every episode of the science of coffee covers one topic and we go deep. And in this case, it's cold brew extraction science. And for every topic I cover, I go out into the specialty coffee community to find companies who are experts in their field, who can support the story I want to tell.

And in this case, it's Toddy. They've been manufacturing cold brewing kits for decades. And they funded a lot of university research into cold brew extraction science.

And that's why I hop in the car with Julia, the owner of Toddy, to begin an hour long drive down the highway in their electric car. And don't worry, it's not a Tesla. Out the left window, I admire the rocky mountains, the listening in the distance.

And eventually, we pull up into an office park with a snow-capped mountain to the background. And emblazoned on the side of this two-story building is the green Toddy logo. We have arrived at Toddy HQ.

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So Julia, the head of Toddy, and I, we sit down at a conference table.

Julia Leach:

Julia, are you- Yeah, I want to go through cold brew myth busting.

James Harper:

And one of the first things I want to clear up is, what is and isn't cold brew? Because the biggest confusion out there are peeps who think cold brew is any coffee you serve cold. But that's not what we're talking about in this episode.

Don't get me wrong. I love the Japanese way of making, you know, an iced V60 pour over. You pour the hot water over the coffee grinds, and then this hot coffee drips down onto ice, which is chilled down.

But it's not a cold brew. It's a hot brew that's been chilled. Although I really enjoyed trolling Julia in front of her team.

Matt Irish:

As we all know, you know, cold brew is just, you know, a V60 over ice, right?

Julia Leach:

No. No, it's not. Cold brew is a brewing method, not a serving method.

James Harper:

So to make it super clear, Toddy's head of coffee, Matt Irish, steps in.

Matt Irish:

You're the coffee guy. Yeah. And you have the mustache to boot.

I do. It's got to go with it, right?

James Harper:

Beneath Matt's mustache, whose twirls defy gravity, he places down in front of us a tray of coffees.

Matt Irish:

Yeah. So we have a flight put together for you today. It's going to be a cold brew, a steamed cold brew, and then the same coffee as a hot preparation.

James Harper:

So Matt brewed two coffees with the exact same beans.

Matt Irish:

A fair trade organic washed Ethiopian.

James Harper:

In two different ways. One is a V60 pour over filter coffee using hot water. And a second coffee as a cold brew.

Basically, the day before, Matt got the beans, ground them up, put them into a container, added cold water, waited overnight, decanted it, and voila, cold brew. And it's the cold brew we taste first.

Julia Leach:

I know, I get citrus, I get some chocolate.

James Harper:

Yeah, I'm getting a nice sweetness to it as well. And so with this cold brew, we were getting lovely flavors. Fruit, bit of chocolate, sweet, intensely aromatic.

Now to really drive home what Julia said earlier, that cold brew is a brewing method, not a serving method. We then tried a hot coffee that was actually first cold brewed, right? So literally that exact same cold brew coffee we just tasted.

Matt Irish:

Nuked this for 45 seconds.

James Harper:

But just nuked it in a microwave. Quick aside, when I finished high school, like almost 20 years ago, I went back to Australia and worked on a sheep farm for a couple of months. And that's where I heard farmers talking about, you know, nuking things in microwaves.

I'd never heard it before. And then it kind of stuck. So anyway, we tasted this coffee that had been cold brewed and then heated up.

And those cold brew flavors were pretty much the same, just... It's hotter. But also more enveloping.

Yeah, it's rounder, it's softer, it's like a warm hug. And Julia tells me that for quite a few American households back in the day, this was how they drank their cold brew.

Julia Leach:

Cold brew was predominantly served hot for decades. People brewed a cold brew concentrate, and then they heated it up with boiling water. And that was their morning coffee.

James Harper:

Cold brew. You can serve it whatever temperature you want. But to call it cold brew, it has to be first brewed with cold water.

Matt Irish:

And then we also did a pour over for you.

James Harper:

And then, finally, I drank the third coffee that was lurking below. That's wonderfully coiffed moustache. These are the exact same beans, but brewed using hot water instead of cold water.

Julia Leach:

We stick to the exact same coffee.

James Harper:

And this is the reason there has been so much debate in the specialty coffee community around cold brew. Because this hot brew, which used the exact same beans, it tasted like a different coffee. I mean, the bitterness is much more pronounced for me.

The bitterness was way up. Yeah, and the body, you know, much more rounded as well. And compared to this hot brew, the cold brew was so much softer, rounder in its body and much more aromatic.

Matt Irish:

When I take a sip of the cold brew and you kind of breathe out as you sip it, I get a much more intense and clear picture of aromas compared to the hot coffee.

James Harper:

So to recap, the cold brew was much more aromatic, sweeter, softer, mellower, compared to the hot brew, which really felt more punchy, more bitter, and it had less body. And this right here has been one of the debates in specialty coffee for many years. Okay, yes, cold brew tastes different, but that's because it's a completely different brewing style.

In the tasting I just did here with Toddy, the brew styles were so different. One coffee brewed in like four minutes, pour over V60 with hot water. And the other is a cold brew, you know, coffee and water have sat overnight in a flask.

Yeah, of course, they're going to taste different. But what would happen if the brewing styles were the same? Does cold brew taste different from a chilled hot brew?

Does brewing coffee cold bring out unique flavors you could not get any other way? And if so, are there coffees out there which taste particularly amazing? But really only when brewed as a cold brew.

So coming up next, I speak with a coffee researcher who did these experiments at a university to a rigorous academic standard. The Toddy team and I do our own experiment, and not only do we find some delicious differences, I'm going to give you explanations for why we might taste those differences. And it's going to be like pulling back the curtain on some of the extraordinary mysteries and games going on inside your mouth every time you taste coffee or eat or drink anything.

So quick recap, Matt Irish had brought me a tray of coffees, and underneath his mustache loops were two coffees made with the same beans, a cold brew and a pour over filter coffee with hot water. And they tasted noticeably different. But if we were to put on some white lab coats, wear some safety glasses, be a bit more scientific about this, of course they're going to taste different because they were completely different brew styles.

I don't know why I went to that accent. So what I wanted to know from the Toddy team when I was with them in Colorado, if we did a carefully controlled experiment where the only thing that changed was the temperature of the water, would cold brew taste different? Does brewing with cold water bring out unique flavors in coffee?

And so I persuaded the Toddy team, hey, let's give this experiment a shot. And I gotta say, it was really challenging.

Matt Irish:

Yeah, yeah, we'll plan our experiments. I'll be here early in the morning.

James Harper:

We prepared the cold brews on day one. We used four completely different coffees roasted by different roasters from around the world.

Julia Leach:

Mexican Chiapas.

James Harper:

Dark roast.

Julia Leach:

Ethiopian, the Robusta. Light roast. And the El Salvador.

James Harper:

And we made some 16-hour cold brews. Ground the beans, put them in a beaker, added cold water. 16 hours later, took out the coffee grinds.

Matt Irish:

Easy. So this was 16 hours. Yeah.

James Harper:

And then on day two, our plan was to make the hot brews.

Matt Irish:

Same vessel, same thing. Everything is the same. Perfect.

James Harper:

And the only difference would be the temperature of the water. So coffee and water sitting together in a beaker, but instead of cold water, it's hot water. But these hot brews were taking so long to extract.

Before we knew it, the Colorado sun had reached midday and was now slowly descending behind the Rocky Mountains.

Matt Irish:

I guess I was expecting it to be more of a kind of rush. Like, oh my God, we got four minutes. You got 30 seconds.

But we are so chill here. We could have a nap.

James Harper:

I wrote the TDS measurements on a whiteboard. TDS is a measure of like the strength of the coffee. How much coffee stuff is floating around in the water.

And I was so optimistic early on. Nice big numbers for comparison. Before the hot brew, we were measuring every five minutes, 10 minutes, 20 minutes.

And my writing had to get smaller and smaller and smaller to fit inside the box. Please be higher. Please be higher.

Please go up. We were doing this for hours.

Matt Irish:

Stagnating here. We're stagnating.

James Harper:

And to make this an even more valid experiment, we also had to dilute the coffee to the same serving strength. But the maths for that was surprisingly complicated.

Matt Irish:

So 0.85 divided by 4.41 equals, that's a 19% difference.

James Harper:

479, 479.6. Kristen Fenwick, Toddy's former education manager, she was the one actually doing the diluting. She was there with a jug of water, waiting for me to tell her exactly how much to dilute each coffee. This is a stressful work, isn't it?

The pressure. It's like you screw it up and it's like you got to do it.

Julia Leach:

We can't go back. You can't suck the water out of it.

James Harper:

We're done. And in the end, oh, we got some results.

Matt Irish:

Oh, 2.29. That's pretty good. That's pretty close.

James Harper:

That's pretty close. Now look, I'm not going to say that the results from this experiment are conclusive. They can't be.

Because what you really need for this sort of experiment is for all the taste testers, like me and the rest of the Toddy team, to be calibrated. But we didn't have time for that because I was only there for two days. And there was only a handful of us doing the tasting, which is just not enough people to have any kind of reliable results.

But I did speak of a scientist who did this experiment with a lot of time and a lot of people.

Prof. Bill Ristenpart:

My name is Bill Ristenpart. I'm a professor of chemical engineering at the University of California, Davis. And I'm the founding director of the UC Davis Coffee Center.

James Harper:

So Professor Ristenpart had the same question around cold brew that I had.

Prof. Bill Ristenpart:

As I'm sure you're very aware, there's a big controversy in the coffee industry. Is cold brew really different from chilled, hot brew?

James Harper:

And Toddy provided the funding for Professor Ristenpart to do this research. And what he and his team did was take three coffees, a Salvadoran, a washed coffee, Ethiopian, a natural, and Sumatran, a wet hold coffee, roasted them all to different degrees, to light, medium, or dark, and then brewed them all with water at three different temperatures.

Prof. Bill Ristenpart:

At 92 degrees Celsius or at room temperature or at four degrees Celsius at refrigerator temperature. And that is the easy part. The hard part was training the taste panelists.

They spent a tremendous amount of time recruiting and then calibrating a so-called expert panel.

James Harper:

They recruited and calibrated well over a dozen panelists, and many had to be dropped actually because they weren't consistent enough.

Prof. Bill Ristenpart:

So that's something I often struggle to convey to people in the coffee industry. Like, you know, we're not just sitting around with, you know, one afternoon tasting a few coffees and, you know, making some conclusions, writing a paper about it. It takes months.

James Harper:

Look, calibrating and training sensory panels to give reliable sensory data. Oh, it's so laborious. And that's why Professor Ristenpart's experiment took almost a year.

Now, going back to my experiment with the toddy team. Yes, we didn't have the resources and time that Professor Ristenpart had, but nevertheless, there were some points of overlap where the flavors we were tasting overlapped with what the UC Davis team found. The flavors that cold water can bring out a coffee that you can't get with hot water.

And this is what I'm going to show you now. It's going to begin our journey into the weirdness of coffee flavors and how we perceive them. The toddy team and I, we all huddled around.

Matt Irish:

Come around, come around so I can put my microphone in your face.

James Harper:

Everyone was given a cup of the cold brew and the chilled hot brew.

Matt Irish:

Cold on the left, hot on the right. Okay.

James Harper:

And the first coffee we tasted was a specialty Mexican chapas that was darker roasted. Okay, that's a difference. Well, that's weird.

And the toddy team began to murmur that the hot brew tasted more bitter.

Prof. Bill Ristenpart:

Bitter, incidentally bitter.

James Harper:

Kristin Fenwick, toddy's former education manager, she articulated it best.

Julia Leach:

It's like a bad grapefruit bitter, like the citrus bitter, but not positive.

James Harper:

And that bitterness in the hot brew, the UC Davis study found the same thing. Averaged out across all the coffees, the coffees made with hot water were about 10 to 15% more bitter. And to me, this is an extraordinary thing, right?

Like here we have data that says on average across all the coffees, cold brew is 10 to 15% less bitter. And this matters a lot to me because when I brew my coffees, I'm trying to find sweetness and minimize bitterness. 10 to 15% on average, meaning some individual coffees could be like perhaps 30% less bitter when cold brewed compared to hot brew.

And I was so curious to know from Professor Barisampat who oversaw the UC Davis study, like why, why is bitterness lower in cold brew compared to hot brews? And he was like, I don't know.

Prof. Bill Ristenpart:

Yeah, it's a good question. And the short answer, the real answer is we don't know for sure. Nobody does, really.

James Harper:

Because this is how it kind of works in academia. You do a practical experiment and you get some findings. Okay, cold brew on average is 10 to 15% less bitter than hot brew.

But trying to then pinpoint the reason why, that's a whole different ballgame, much harder to figure out. And it's also kind of like, who cares? Many scientists say what matters is that there is a difference in flavor.

We shouldn't get too hung up on theories. Focus really on the practical results. And to you, very attentive listeners, I, of course, covered this in a previous episode.

My episode, What is Good Science? Part 2, How to Think Like a Scientist, where I explore how scientists, coffee scientists, think about theories and practical evidence. Link in the show notes.

Okay, but nevertheless, scientists do have theories for why cold brew has less bitterness than hot brew.

Prof. Bill Ristenpart:

The longer answer is we have speculative hypotheses. Sorry, sorry, sorry. Speculative hypotheses.

James Harper:

And I want to share this with you anyway, because they are fascinating. And they pull back the curtain on how our sense of smell and taste actually works. After I learned these things, I couldn't taste coffee or anything without my mind going back to like, ah, so that's what might be going on.

Okay, so here is the theory, keyword theory, for why cold brew tastes less bitter than hot brew. And this comes from a different coffee scientist who works at a different institution from Bill Ristenbach.

Prof. Shahan Yaretzian:

I'm Shahan Yaretian. I'm here, the head of the Coffee Excellence Center in Zurich. First, I think as a fundamental rule, bitter compounds are difficult to extract.

James Harper:

All right, so in a roasted coffee bean, there are many different compounds that could be pulled from the coffee into the water. And the thing is, some of those compounds are much more readily extractable than others. Right?

So some compounds are very easily extracted with room temperature water. But other compounds need a lot more heat energy. The water has to be a lot hotter for those compounds to be pulled into the water.

And it all has to do with the flavor compound's polarity. Kind of like magnetism between the water and the flavor compound. Listen to one of my earlier episodes on coffee extraction to learn more about that.

Link in the show notes. But here's the point.

Prof. Shahan Yaretzian:

And so overall, you need higher temperatures to access these compounds.

James Harper:

And so this theory goes, when you make a cold brew using cold water, you know, those bitter compounds, they're just not going to get pulled into the water because they're not easily extractable. But when you increase the temperature of the water, as the water gets hotter, finally, those bitter compounds, which are harder to extract, do start extracting. And as theories go, this one about, you know, bitter compounds only getting pulled out of the coffee into the water at higher temperatures, it's very neat and tidy.

And we could stop here. But I'm not going to, because there's more to tell you. I also came across a completely different theory.

That's less explainable, more mysterious. Another reason for why hot brews taste more bitter to us compared to cold brews. And it might have to do with how certain aromas play games with our minds.

So this Mexican Chapa that the Totti team and I tasted, it was a darker roasted coffee.

Matt Irish:

For me, there's a more distinct roast note coming through in the hot brew that I'm not getting in the cold brew. Char.

James Harper:

Yeah, yeah. Some aromas change the taste of coffee, especially dark roasted aromas.

Prof. Paul Breslin:

Yeah, change the way it smells and you'll change the perception of how it tastes.

James Harper:

And I learned this when I spoke to another scientist we haven't heard from yet.

Prof. Paul Breslin:

So my name is Paul Bresland. I'm a professor of nutritional sciences at Rutgers University and a researcher at the Monell Chemical Sciences Center. Paul told me a story.

I actually once was talking to someone who had a problem with their coffee where they were complaining that their coffee was extremely bitter. And they wanted to talk to me about bitterness suppression and how do I suppress this taste sensation? And I sipped their coffee and I told them, it's not that bitter.

And they said, of course it is. And I said, pinch your nose. When you pinch your nose closed, you can't smell.

I said, pinch your nose closed and sip your coffee. And they did. And they looked at me with shock and said, wow, this really isn't very bitter.

And I said, yeah. I said, it's a little bit bitter. There is bitterness there.

But your problem is an interaction of the aroma and the taste. And their aroma was that the coffee had sort of a very strongly roasted, almost sort of like a burnt odor. And that was conveying an amplification of the bitterness to make people think that it was really bitter.

But in and of itself, as a taste stimulus, it wasn't very bitter.

James Harper:

So to recap, when you brew coffee with hot water, it tastes more bitter than cold brew. And there might be two explanations. One, hot water is able to extract particular compounds from the coffee which are bitter in and of themselves.

Or theory number two, hot water extracts certain aromas, which when we smell them, it creates this kind of perceptual confusion in our heads. And it makes us think that something tastes more bitter than it really is. And coming up next, volatile florals and additive acidity.

Matt Irish:

OK, let's go.

James Harper:

OK, so coffee tasting weirdness phenomenon number two. The toddy team and I then turned to a different coffee. This was the autumn seasonal blend by the Aussie roaster Cuppie House.

Half anaerobic fermentation El Salvador, half washed Guatemalan. And what really jumped out for me between the hot brew and the cold brew was the intensity of the aromas. Compared to the hot brew coffee, the cold brew was so intensely aromatic.

Matt Irish:

The florality is the big difference here. The cold brew really just unleashes a florality, which I just I'm struggling to pick up in the hot brew.

James Harper:

This was a similar finding from the UC Davis study, where they found the florality of cold brew was about 15 to 20 percent higher than hot brew. Cold brew makes coffee taste more floral, more aromatic compared to hot brew. And I got to say, initially, I was quite confused by this because it didn't fit in my mental model for how coffee extracts.

OK, so florality, aromas. These are smells and smells come from these tiny, tiny volatile aromatics, little compounds that get swept up in the air, tiny little compounds. And before this experiment, I would have thought that it's actually hot brew that is more aromatic than cold brew because hot brew extracts more stuff from coffee, including more aromatic volatile stuff, the smells.

But that's not what was happening here. The findings are saying it's the cold brew that is more floral compared to the hot brew, as in cold water, which extracts less than hot water, is the brew style that gives the most florality. And my mental model for coffee extraction, like, took a quantum leap when I spoke again with Professor Shahan Yaretzian of the Coffee Excellence Center, because he told me that, you know what?

Those small little floral aromatic compounds, they really don't need much heat at all to be extracted.

Prof. Shahan Yaretzian:

The volatile compounds that are small and very good water soluble, they will come, they will be extracted also with colder temperature.

James Harper:

Shahan's saying these small little aroma particles, they are very easily extracted. Cold water extracts almost as much of them as hot water. So why is it then that in cold brew, these aroma molecules are so much more present?

You know, you can perceive so many more of them. Well, a theory is that in cold brew, those light aromatic aromas, they stay in the cup. When you brew with hot water, the hot water has so much energy that the highly volatile compounds, they get lost.

These small delicate aromas, they pick up the heat energy from the water, they whizz up into the air, and they dance their way out of your cup, into the air, out of your life, and they're gone forever. Hot water gives so much heat, energy to these delicate aromas, they fly out of our cups before we even have a chance to appreciate them. Shahan tells me this is a well-known problem in the food industry.

Prof. Shahan Yaretzian:

And that's why a lot of people in the industry, you know, they do that since a long time. They cool down the food immediately, you know, to preserve the aroma.

James Harper:

Because they're light and they're swept up in the air faster.

Prof. Shahan Yaretzian:

Right, they go and you lose them.

James Harper:

But when it comes to cold brew, it's a different story. Because the water is cold, those aromas, they stay in the liquid. And a professional tip here.

If you want to perceive a coffee's aromas at their best, take a sip of the cold brew and breathe out your nose. Because those aromas will dance their way from your tongue, up the back of your throat, into your nasal cavity, and they'll get jiggy with your aroma receptors.

Matt Irish:

Okay, so now we're tasting the Robusta from the Philippines.

James Harper:

And now, the third and final insight. I want to give you on how cold brew tastes different from hot brew. The Toddy team and I tasted a really unique coffee from the Filipino specialty coffee roastery, Curve Coffee Collaborators.

This was a specialty, naturally processed Filipino canephora from the producer, Vilma Lasan. These were big, bold, punchy flavors. But what I want to focus on the most is the acidity.

Okay, so Toddy's head of coffee, Matt Irish, he maneuvered the cup of cold brew under the twirls of his mustache. And he gave me some flavor descriptors.

Matt Irish:

I get a lot of that like rum toffee kind of taste from this.

James Harper:

So in the cold brew, he was getting rum and toffee, which are not particularly acidic flavors.

Matt Irish:

But when he tried the hot brew, and it's almost like a little more red fruit, berry-like in the hot brew.

James Harper:

Red berry, much more acidic flavors in the hot brew. And again, this chimes with the UC Davis study. On average, across all the coffees, the hot brews tasted 15% more acidic compared to the cold brew.

And I wanted to know, why is that? Why does using hot water make coffee taste more acidic? So in the UC Davis study, they measured the acidity of the coffees using scientific instruments.

But they weren't able to explain that difference in acidity between the hot and the cold brew from the acidity measurements they took. And quick aside, there is a really interesting science as to how you even do acidity measurements in the first place. I'm not going to get bogged down into it here, but come to my Instagram, at filterstoriespodcast, and I'll share some of the insights that I got speaking with UC Davis researchers and Professor Paul Breslin.

Link in the show notes. The point is this, the UC Davis team acknowledges, we don't know why the hot brews are tasting more acidic than the cold brews. Because all the tools we have for measuring acidity, their readings are all very similar.

Like Professor Breslin part mentioned earlier.

Prof. Bill Ristenpart:

The short answer, the real answer is we don't know for sure.

James Harper:

And you also now know what the technical scientific term is for when scientists don't know why something is happening. And that word is, ah. But that did not stop the UC Davis team coming up with some really weird, wacky, possible explanations.

Prof. Bill Ristenpart:

The longer answer is we have speculative hypotheses.

James Harper:

Sorry, sorry, I mean, speculative hypotheses, yes. And they speculated its additive effects. When there are two compounds in your coffee, which by themselves you cannot taste, but when they come together, you can perceive one of them.

They add on to each other. Additive effects.

The final team in the artistic swimming.

James Harper:

Additive effects kind of remind me of the artistic swimming routines in the Olympics. It's like you're a spectator in the stands looking down at the pool and you don't see anything. I mean, you know there are swimmers down there under the water, but you can't see them.

These swimmers, they're like flavors in the coffee. They're underneath the water. You can't see them.

You can't taste them. Can't perceive them. But then there is this moment where they come together and one performer just rises majestically out the water.

And you think, whoa, it's like she's floating on the surface. And of course what's happening underneath is this entire team sweating, just pushing this person up, keeping them above the water. And that might be what's happening in the coffee liquid.

So there are these two different compounds, you know, two different performers sloshing around. You don't notice them. You can't taste them.

But there's this moment where they come together, they add together, and the flavor compound that gets pushed up is acidic. And so the UC Davis team's theory is that the hot brews are more acidic than cold brews because there are some compounds that you only extract using hot water. And, you know, these compounds that you get from hot water, they may not have a flavor in and of themselves, but they might play with a different compound that's floating around that is acidic and it pushes it above the surface of the water so we can finally taste it, the additive effect.

And what's mad is that the additive effect is at play every time we brew coffee, drink coffee, it's always at play. It could be enhancing bitterness, sweetness, sourness, so many things. It's a fundamental piece of sensory science.

So in this episode, we have been on a flavor journey. I wanted to know, does cold brew bring out unique flavors that you just couldn't get by brewing using hot water?

Prof. Bill Ristenpart:

So the short answer to the question, is cold brew different from chilled hot brew? The short answer is yes.

James Harper:

And what I tasted with the Totti team overlapped a lot with Professor Borisenpat's UC Davis study.

Prof. Bill Ristenpart:

On average, we found that the cold brew had more floral characteristics and less rubber, less sourness, less bitterness.

James Harper:

So yes, cold brew brings out unique flavors you can't get using hot water. And this was a big aha moment for me because were there coffees that I thought I knew that were actually hidden gems when brewed with cold water? I thought back to all the different coffees I've tried in my life.

We're talking thousands of coffees. I imagine them all in one room, cups stacked on cup all the way to the ceiling, coffee bags lining every inch of wall. You could imagine, it's like some crazy coffee collector's studio.

In all the different cups and all the different bags, you see different coffee varieties, processing methods, countries, roast degrees. But in the center of the room, in a gold box, ah, these are the coffee gems. My favorite, favorite, favorites.

A sweet caramel and nutty wash bourbon from Honduras. Blueberry marmalade from a Kenyan SL28 Peaberry. But here's the thing, I love these coffees drank hot, you know, as a pour over filter coffee.

Because that's how I would normally make my coffees, as a hot brew pour over filter coffee. But after I discovered that cold brew does bring out unique flavors you could not get in a hot brew, it's like I'm standing in the middle of my crazy coffee collector's room. I look around me, I see all the different coffees I've had in my life, so many bags of coffee on the wall.

And I wonder, hold on, could one of you be a coffee gem? Do some of you actually belong in the golden box? Maybe you will become my new favorite, favorite, favorites if I brew you instead with cold water.

I wanted to discover some new cold brew gems. Coffees that I had drunk hot before, which were like good, but nothing amazing. But if I'd use cold water instead, would just blow me away.

And just a week after I left Colorado, I was in Chicago for the SCA Expo. And it's there I met a roaster who had his own story of discovering a cold brew gem.

Chris Pruitt:

I'm Chris Pruitt, I'm a roaster for Akamai Coffee in Maui, Hawaii.

James Harper:

As a roaster, Chris samples his coffees hot and also sometimes cold.

Chris Pruitt:

We just recently had a coffee we got, it's a Obata from Mexico and cupped fine and cold brew is fantastic. Didn't know that until I just made a cold brew with it. What was different about it?

Oh man, just so much more came out of it as a cold brew. Syrupy caramel, really rich chocolate. That's fine as a pour over, it's okay as a drip.

It's good. Yeah, yeah, yeah. But we found as a cold brew, it's phenomenal.

Right.

James Harper:

And once he discovered this cold brew gem on the retail bags of this Obata Mexico.

Chris Pruitt:

We put on the retail that we like this as a cold brew. Just because I feel like it should be on there, that that's our brew method of choice for the coffee.

James Harper:

All right, peeps. Now I want to hear from you. Think back to all the coffees you drank in your life.

Which one of them were absolute gems as cold brew? Tell me, I want to know. I really want to try them for myself and I want to share these tips with everybody.

So come to my Instagram at filterstoriespodcast, link in the show notes and I'll do a post asking for cold brew gems and reply there. Tell me which coffees when brewed cold and really only cold have absolutely blown you away. Quick recap on everything we covered in the episode.

We saw that cold brew tastes different from chilled hot brew. The UC Davis coffee researchers found that cold brew is less bitter than hot brew. And that might be because hot water extracts more bitter compounds or maybe because hot water extracts particular aromas like a roast aroma.

And it's that roast aroma which plays this game in our heads where we think what we're tasting is bitter when it's actually not. The UC Davis team also found cold brews are more floral. And that could be because the tiny little aromatic compounds which are in the cup, you know, in a hot brew, they take on the energy from the hot water and just fly out out of our cups.

But when brewed with cold water, they stay put. And finally, the UC Davis team found that cold brews taste less acidic. And this might maybe have to do with the phenomenon of additive effects where we have some compounds floating around in our coffees which don't taste of anything by themselves.

But when they come together, boom, we're able to taste one of them. Now in the next episode, I'll share with you how I learned to brew cold brew. For years, I have made hot pour over filter coffees.

And every time I got a new bag of beans, I would play with different recipes to tease out different flavors. But with cold brew, I discovered changing the grind size, the brew time didn't make much of a difference. So what does?

And along the way, I uncovered some fascinating coffee extraction science. And I learned about the crazy ways in which our smell, taste, and touch receptors change coffee's flavor dramatically. And I cannot wait to share it with you.

Thank you so much for listening. Now, the entirety of the science of coffee is basically made by just me. Including, of course, writing and playing the piano music.

And if you found this podcast insightful, fun, please, one thing that would mean so much to me is if you could tell your friends about it. One way to do that is to take a screen grab of your podcast player and then post it as a story on Instagram, tag me at fullofstoriespodcast, and I'll be able to thank you personally. If you want to go deeper into the science we explored here, I have some links in the show notes, and you might also consider doing a course with the Coffee Excellence Center.

Now, here's what to expect in the rest of the series. We take a deep dive into roasting science with roaster manufacturers, ProVat. Then I'm in Brazil, looking at how two Brazilian coffee producers find buyers for their coffees.

And one way is through the green coffee trading platform, Augrano. Then we're in Colombia, exploring how coffee goes from the mountain slopes down to the port. And the different journeys coffee takes, depending whether it's a specialty or a commodity coffee.

And for the specialty coffee journey, we see it through the eyes of coffee exporter, The CoffeeQuest. And then to cap the series off, we're taking a sociological, philosophical deep dive into what even is specialty coffee? How do you define it?

Where do you draw the line? And that investigation is supported by the water filtration manufacturer, BWT. Thanks again for listening, and I'll speak to you next time.