#81 | The Science of Coffee Freshness | Samo Smrke, Expo Lectures 2019
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Coffee freshness is one of the core values of specialty coffee. But why is preserving the freshness so important? We might strive to maximize coffee’s potential to keep its vibrancy as fresh as the day when roasted or we keep coffee fresh to ensure quality and consistency. Regardless of why we may want to keep coffee fresh, understanding the fundamentals of freshness and applying them in our daily routine will help to improve our cup of coffee.
In this lecture, Samo Smrke explores the topics of roasted coffee freshness as seen by a scientist’s perspective. Two particular fields will be looked into detail: chemical freshness or loss of coffee aroma during coffee aging, and physical freshness or degassing (also called outgassing) of coffee, a process of gradual gas release after coffee roasting. If you’re already familiar with Samo’s work, you’ll be excited to learn that today’s lecture includes his newest findings that haven’t yet been presented.
Samo Smrke is a scientific associate at the Zurich University of Applied Sciences in the group of Professor Chahan Yeretzian. He is involved in research projects in collaboration with industry partners and in fundamental research on various topics of coffee chemistry, research of coffee aroma using mass spectrometry, on-line monitoring coffee roasting processes, linking instrumental analysis of coffee aroma to sensory analysis, studying coffee freshness and degassing of coffee. Samo is actively participating at coffee conferences, is one of the co-authors of the SCA Freshness Handbook and Water Handbook, and has contributed to scientific papers and book chapters about coffee science.
Special Thanks to Softengine Coffee One, Powered by SAP
This episode of the Expo 2019 Lectures podcast is supported by Softengine Coffee One, Powered by SAP. Built upon SAP's business-leading Enterprise Resource Planning solution, Softengine Coffee One is designed specifically to quickly and easily take your small-to-medium coffee company working at any point along the coffee chain to the next level of success. Learn more about Softengine Coffee One atsoftengine.com, with special pricing available for SCA Members. Softengine: the most intelligent way to grow your business.
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Episode Table of Contents
3:15 The Main Causes for Coffee Losing Freshness
9:45 Scientific Approaches to Measuring Coffee’s Physical Freshness
31:45 Scientific approaches to measuring coffee’s chemical freshness
44:40 Linking physical and chemical freshness and the impact on the sensory experience
54:30 Audience questions
1:02:30 Outro
Full Episode Transcript
0:00 Introduction
Heather Ward: Hello everyone! I’m Heather Ward, the SCA’s Senior Director of Content Strategy, and you’re listening to the SCA Podcast. Today’s episode is part of our Expo Lecture Series, dedicated to showcasing a curated selection of the extensive live lectures offered at our Specialty Coffee Expo. Check out the show notes for relevant links and a full transcript of today’s lecture.
This episode of the Expo 2019 Lectures podcast is supported by Softengine Coffee One, Powered by SAP. Built upon SAP’s business-leading Enterprise Resource Planning solution, Softengine Coffee One is designed to quickly and easily take your small-to-medium coffee company working at any point along the coffee chain to the next level of success. Learn more about Softengine Coffee One at softengine.com, with special pricing available for SCA Members. Softengine: the most intelligent way to grow your business.
The episode you’re about to hear was recorded live at the 2019 Specialty Coffee Expo in Boston. Don’t miss next year’s lecture series in Portland – find us on social media or sign up for our monthly newsletter to keep up-to-date with all our announcements, including ways to get involved in next year’s Expo and early-bird ticket release!
Coffee freshness is one of the core values of specialty coffee. But why is preserving the freshness so important? We might strive to maximize coffee’s potential to keep its vibrancy as fresh as the day when roasted or we keep coffee fresh to ensure quality and consistency. Regardless of why we want to keep coffee fresh, understanding the fundamentals of freshness and applying them in our daily routine will help to improve our cup of coffee.
In this lecture, Samo Smrke explores the topics of roasted coffee freshness as seen by a scientist’s perspective. Two particular fields will be looked at in detail: chemical freshness or loss of coffee aroma during coffee aging, and physical freshness or degassing (also called outgassing) of coffee, a process of gradual gas release after coffee roasting. If you’re already familiar with Samo’s work, you’ll be excited to learn that today’s lecture includes his newest findings that haven’t yet been released.
Samo Smrke is a scientific associate at the Zurich University of Applied Sciences in the group of Professor Chahan Yeretzian. He is involved in research projects in collaboration with industry partners and in fundamental research on various topics of coffee, including coffee chemistry, research of coffee aroma using mass spectrometry, on-line monitoring coffee roasting processes, linking instrumental analysis of coffee aroma to sensory analysis, studying coffee freshness and degassing of coffee. Samo is actively participating at coffee conferences, is one of the co-authors of the SCA Freshness Handbook and Water Handbook, and has contributed to scientific papers and book chapters about coffee science.
Also, I will jump in occasionally to help you follow along.
Okay, let’s dive in!
3:15 The Main Causes for Coffee Losing Freshness
Samo Smrke: Hello, everybody. Welcome to my talk. My name is Samo and today I’m going to be talking about coffee freshness. So, I’m a scientist so this talk will be from a scientific perspective and I’ll try to make it as clear as possible. So, I work at Zurich University for Applied Sciences in the group of Professor Chahan Yeretzian. I will tell you about the group a bit later but let’s go first to the talk.
So, what I’m going to be talking today. So, when I say coffee freshness, of course, first we need to define what coffee freshness is. What are we actually talking about? Then I’m going to take a look at two different types of coffee freshness. First, it’s what we call physical coffee freshness, which is related to coffee de-gassing and second is the chemical coffee freshness which is related to aroma degradation and oxidation. Of course. In the end, I would like to kind of link physical coffee freshness, chemical coffee freshness and sensory, and conclude the talk. So, to start, to ask the first question. What is coffee freshness? So, today I’m talking only about the roasted coffee freshness. So, why is this so? So, we can look at the process when we go from green coffee bean to a cup of coffee. We can look at this processes and decide okay, where does the strongest change in the material happen? So, the strongest change happens during the roasting process. So, this is the step where aroma is generated, where the bean structure changes drastically, and this is also where the bean becomes a lot more unstable. So, we’ll lose the stability, we change the structure, we form the aroma. So, when I talk about freshness, today I’m talking about roasted coffee freshness. So, there’s a lot of things happening actually during roasting. Like I said, we produce aroma, but not only aroma we drive off moisture off the bean. We produce carbon dioxide, and this all is trapped within the bean after roasting. So, during storage, these molecules are slowly released from the whole beans, and if we grind the coffee, they’re released much more faster. We’ll take a look a bit to this process a bit in detail later. But other reasoning why coffee freshness? I’m talking about from roasted coffee freshness. The time scale that this happened. So, if we look at the freshness of different coffee, a different steps, we can say that green coffee… So, you can argue about this but let’s say that the green coffee is good for a year. When we talk about roasted whole beans we’re talking about weeks to months of freshness. We’re talking about ground coffee; we’re talking about the timescale of from a few minutes to two months and then for the brew we’re talking about minutes. Of course, this time scale that I’m giving here. There’s a large span because, of course, it depends from application to application and how strict we are with our freshness.
So, as we see with time coffee, is an unstable product and will change so it will lose its freshness. So, what are then the actual drivers? Why does the coffee lose its freshness? We have some perimeters. Here, the most important perimeters, which are the drivers that cause the coffee to those freshness. So, like temperature, time, space, oxygen, humidity and light. So, let’s take a look at the four most important to these parameters that cause the coffee to lose freshness. So, the first is the oxygen. So, as probably, you know, oxygen produces staling reactions with the coffee basically any kind of food, and these reactions produce unpleasant tasting compounds. So, mainly the problem is that it reacts with coffee oil, and this produces unpleasant tasting compounds that costs the coffee to become rancid, to have a rancid aroma. So, we want to avoid this because even if we have a very small amount of the rancid aroma in our coffee beans, it already is reflected to have an unpleasant taste.
The next parameter is space, so this is a concept where we are talking about free space where aroma can be lost from the coffee bean. So, imagine if we have a perfectly hermetically sealed small package and we have inside coffee beans. The aroma cannot escape from this. But, if we have open coffee bag, the aroma released from the beans will go out in the air and diffuse out and then we will lose this aroma. So, the more space we give to aroma, the more loss of aroma from coffee beans will have, and the more aroma loss we have from the beans, the more freshness we’re losing from the coffee. Then it’s the temperature. So, temperature is the main driver of all these reactions that cause loss of coffee freshness. Basically, with temperature if we were able to cool down the beans, to a very, very low temperature, to absolute zero, we would stop all the other reactions. All the chemical reactions, all the physical changes, and we would preserve the freshness of for coffee being indefinitely. But, of course this it’s not practical to do it very hard to do so we always have a constant aging of the coffee bean regardless of all the storage conditions. Any kind of storage conditions will cause the coffee to age to a certain extent. So, if we talk about temperature. It’s a driver of the reactions because it causes the molecules, the atoms to move around and this causes reactions to happen and then we’re losing freshness with these reactions. Then, of course, time. This is fairly obvious. So, the longer we are waiting, the more freshness we’re losing. So, the perfect freshness is at time zero This is our definition of freshness. So, just after roasting in this case.
9:45 Scientific Approaches to Measuring Coffee’s Physical Freshness
Samo Smrke: So, if we want to have a scientific approach to coffee freshness, we want to have some kind of way to measure it, so we should be able to measure it as objectively as possible. And we were doing studies on this field and trying to evaluate how can we measure freshness?
Heather Ward: Samo is showing a video of espresso coffee pouring out of a group head into a transparent espresso cup. We see swirls of brown and black in the espresso cup and a layer of thick crema forming on top.
Samo Smrke: When we ask this, we can take a look to decide is this coffee fresh or not. So, those of you who are familiar with espresso extraction you probably recognize that the very thick crema flowing out from the spout here and a lot of crema in the in the cup. This is a sign that the coffee is fresh. So, okay, we can say this was a fresh coffee, but just based on this kind of observation we’re not able to quantify how fresh this coffee was. So, my goal as a scientist is to try to explore coffee and try to come up with a way to quantify, to say how fresh coffee is, to find measures to measure coffee freshness.
Heather Ward: Samo now has a photo of aged, stale roasted coffee beans with oil visible on the outside of the beans.
Samo Smrke: And coffee freshness, it might not be only perceived as perfectly fresh coffee, but for me as a scientist, this kind of ugly looking rancid coffee it’s also related to freshness. It’s coffee that lost all the freshness. So, this is also useful for me. I can look at it from another perspective. Coffee that is completely not fresh can be also something to start our measurements with or to end our measurements with but start an explanation how this freshness behaved from the point of the most fresh coffee to this point, where it’s completely stale and has no freshness left at all.
So, like I said, I’m going to talk about two types of freshness. So, there’s different reasons why we want to divide freshness into two parts. So, one is the way how we measure these two freshness and the second is what kind of impact these types of freshness have. So, when I talk about chemical freshness, I’m talking about coffee aroma because coffee aroma are basically chemicals that are present in the coffee beans. So, the loss of chemical freshness means lots of aroma, oxidation of aroma or the change of aroma. So, in this case, we want to measure how much of these different aroma compounds are in the coffee bean and try to evaluate then the chemical freshness from it.
On the other hand, physical freshness is what we relate with de-gassing of coffee. We say physical although it’s carbon dioxide which is the driver of the de-gassing. It’s by itself a chemical but physical because it impacts the physical properties of coffee. For example, we have coffee that we pack, and we have de-gassing. The coffee bag will inflate, so the impact of this freshness, it’s in the physical world, not in the chemical world.
All right, so let’s start first with what was in the right side, the physical freshness or we can also call it the gassing. So, what is de-gassing for you that you don’t know. When we roast coffee, we produce a lot of aroma compounds, but with these aroma compounds we produce also a lot of carbon dioxide. The carbon dioxide is trapped in the coffee beans after roasting. Remains in these coffee beans, and it’s slowly released during storage.
Heather Ward: Samo has two de-gassing graphs up on screen. Both show that CO2 is slowly released from roasted coffee both after roasting and after grinding. The coffee’s roast level also affects the rate of de-gassing.
Samo Smrke: So, we can have here an example of a de-guessing curve. So, how coffee is losing CO2 with storage. So, here we have de-gassing time. These are ours. This is about one month in total and you would have specific mess loss in grams per kilogram of mass. So, 10 would be 1%. So, as you can see, this is the case for dark coffee, dark roast coffee. We lost about 1% of mass of coffee during storage of one month. So, there was a lot of gas trapped in this coffee which was gradually released during storage. On the other hand, when we grind coffee, we increase the surface area of the particles a lot and release the CO2 much faster. So, we have shorter de-gassing times but also we release much faster the CO2. To be able to understand how coffee is able to trap so much gas inside its structure, in one coffee bean. Let us take a look at this video. This is a computer tomography 3D model of a coffee bean. So, it’s kind of like the medical computer tomography just on a much smaller scale and we’re looking at the coffee bean and we want to see its structure.
Heather Ward: Samo is showing a video of a magnified coffee bean. As it slowly rotates we see many tiny holes on the surface. And then we see coffee bean cut in half with a clear view of the coffee’s internal structure. It’s similar to the texture of honeycomb.
Samo Smrke: And as you can see here, the roasted bean is not solid. So, the bean has very high porosity, about 50% or more of the bean volume is basically not solid. It is empty space or basically filled with air, with gas. So, after roasting, the CO2 remains trapped in all those small pores and remains trapped under pressure and so much of this gas that as we saw in the previous slide. We had about 1% of coffee weight as gas after roasting. So, we can take a look at this too as another way to just look at thin slices of this coffee bean.
Heather Ward: Samo is showing different slices of coffee beans and their internal structures. The green bean has a very dense structure with a scattering of small pinholes. But, once the coffee is roasted, the holes expand so that the coffee’s texture resembles honeycomb.
Samo Smrke: We see one slice of a green bean as you can see here. The structure is solid. We don’t have any space where we can trap gas in the coffee bean. Then once we come to a light coffee roast, after the first crack, this is just after first crack we see that this porous structure forms and remains porous until a very dark roast. So, this structure enables gas to be trapped in the bean structure. But this is not only CO2. It’s also the aroma that remains trapped in here and there’s a lot of volume that is able to trap gas there. That’s why we have really a lot of CO2 present after roasting. The size of these pores is about 50 micrometers. This is about the width of a human hair, so they’re pretty small.
So, just to explain how this actually works. How this CO2 is released from coffee; so, if I just go back, if you see the structure, it kind of looks like this. It’s all in here and the CO2 would need to go from the inside of the bean out through many pores. So, there has to be a mechanism when the whole beans are stored to the CO2 is released out from the coffee bean. So how does this work? So, we have two fundamentally different mechanisms of this release that causes the de-gassing of coffee bean. So, we can have CO2 trapped in these pores as gas.
Heather Ward: Samo has zoomed into one of the pores that look like honeycomb.
Samo Smrke: So, this is one of the electron microscope images of one of those pores. So, CO2 can be trapped here as gas and if we have a crack here, for example, or this pore is open, the CO2 can flow out as hydrodynamic flow. So, this is basically a stream of gas. So, in the physical world, so this is driven by pressure. So, if we have high pressure inside the bean and low pressure outside then the pressure would drive the gas outside. So, this is a fast process and it’s a physical process, but we don’t have only physical processes going on here. We also have what we call physical-chemical processes, and this is called diffusion. So, these individual CO2 molecules or also aroma molecules will be in here or stuck on the surface of the material, trapped inside the surface of the solid material or dissolved in coffee oil that is here on the surfaces and the molecules will after roasting, slowly diffuse from inside of the bean to outside. So, there’s different modes of this diffusion. I won’t go into details here, but basically what is important is that if we have diffusion which is a physical-chemical process, the pressure that it’s outside won’t impact how fast the molecules are moving outside. So, if you have high air pressure outside, the diffusion of individual aroma molecules will not be impacted by this pressure outside. They will just look at this perimeter which I introduced earlier. This is the empty space, so the aroma molecules would go out until the individual aroma molecules fill the empty space around the coffee beans. So, if it’s a bag, they will move from the bean until they filled the space in the coffee bag. And once they’re in what we call equilibrium between the gas phase and the what is trapped inside the material, they will stop defusing.
So, if you want to take a look at how then this de-gassing process is impacted by how we roast coffee. So, this is quite interesting because, as we saw earlier, the structure or the size of these pores does not really change for when we go from a light roast or dark roast. But we’ve measured de-gassing so, how much of the gas is trapped in the beans and how fast it’s released?
Heather Ward: Samo has a few graphs on screen that show the rates of de-gassing for light roasted, medium roasted and dark roasted coffee. The graphs also shows the speed of roasting and how that translates to different rates of de-gassing. For example, fast roasted dark roast coffee de-gasses more quickly than slow roasted dark roasted coffee.
Samo Smrke: We see in this curve, if we have a dark roast, we have about 1% of the coffee weight as gas after roasting and if we roast with a slow profile, medium profile or fast profile we have gradually more and more gas trapped in the coffee. So, how do we explain this? So, this is a very simple explanation. If we roast slow, we produce during roasting more CO2 but because we roast longer there’s also more CO2 that is already escaped from the coffee bean during roasting. Because of really high temperature of the roasting, the CO2 can escape during roasting but then again, if we roast fast, the pores will be able to retain more CO2. Because during roasting we may be created less if we roast it faster, but then everything happens much more faster, so more is retained in the bean. So, that’s why we also see here that a slow roast has much less gas than the fast roast.
And then it’s another explanation why a slow roast degasses slower than a fast roast because as you see in a time like about 100 hours, which is about four days, nearly double the amount of gas is released from a fast roast and from a slow roast. This is because if we roast faster the processes that occur in the bean damage the bean structure more and it becomes more porous. So, the gas and consequently also the aroma from the beans is able to escape faster from a fast roasted coffee bean compared to a slow-roasted coffee bean which remains more dense in its structure.
If you could see what kind of impact the roast level has. So, if we roast to medium roast level, to a light roast level you see that we have the strongest effects on the amount of the gas that is trapped in the coffee bean. So, this is explainable by the fact that if we roast longer, we produce much more CO2 which remains in the bean but if we roast a shorter amount of time, then less CO2 is produced in the bean.
So, to summarize, these values are quite interesting. We have up to 1% of coffee mass as gas after roasting in dark roast bean but only up to 0.2 to 0.3% in light roasted beans so this will be like a light filter roast and this will be a dark Italian espresso roast style.
So, then next question is okay. Now, we have this de-gassing occurring from whole beings but what happens if we grind coffee? So, when we grind coffee, we damage a lot of the porous structure of bean. So, because of this damage, while we grind coffee we lose a lot of CO2.
Heather Ward: Samo’s graph shows the de-gassing of two types of roasted coffee: whole beans and ground beans.
Samo Smrke: So, here in this graph, I have comparison of remaining gas in the whole beans. So, compared to completely non fresh coffee, So, I’m starting with 10 grams per kilogram or 10 milligrams per gram of gas in the coffee. So, this is gradually released and here, I have comparable measurement for ground coffee. So, for ground coffee, we see that we start with much less compared to the whole beans. Why is this so? For once we ground coffee, we damage a lot of these pores by the grinding process. We opened a lot, of pores because we have small particles. So, a lot of gas that was present in the pores was immediately, during grinding released in the environment and then what is remaining here is just the gas that remained inside each particle, inside the pores of the particles. This is much less than in the whole bean. So, we have much less gas being released from ground coffee than from whole beans.
So, what we did here is this was both done from freshly roasted coffee, but this is not what we would do practically. So, practically we would roast the beans and then we would store the whole beans for a certain amount of time and then grind them at a later stage in time.
Heather Ward: From this point onwards, Samo’s slides get too technical to describe.
Samo Smrke: So, we did this kind of experiment for two roast levels for a dark roast degree and a light roast degree and we have our de-gassing profile for whole beans and then we compare this to what happens if we have whole beans and we grant them fresh. We have whole beans, we grind them after a week, we grind them after two weeks, after four weeks and after six weeks. What we see is what ends up in the ground coffee after grinding is also gradually going down and down compared to whole beans but at some point, we get to a point where we’re actually still getting some gas out from the coffee. Although if we would compare to the whole beans, we would think okay, we fully de-gassed the coffee but actually by the fact that we’re opening these pores and damaging the structure during grinding. We are able to liberate some more gas from the coffee bean. We still can get some gas out from the ground coffee, although we kind of thought that from whole beans, we fully de-gassed our coffee and comparing dark roast level or a light roast level this effect is pretty similar as you see here. From both sides, the curves look comparable and this has an effect that if we observe espresso extraction and we have espresso coffee roasted to the same. So, we have the same coffee that was stored for a year, that was stored for two months, for ten days, and a fresh roast. So, it was the same coffee, same roast profile, just a different age and we did the extraction and we set up the extraction so that we got the same brew ratio, the same beverage volume and the same extraction time. So, we had to adjust the grind size a bit to achieve this and I’ll show you now, what these extractions look like.
Heather Ward: Samo has a video of four espresso coffees being simultaneously extracted into clear glass cups. The difference between the extractions is the age of the roasted coffee – there’s a fresh roast, ten-day-old, two-month-old and one year past roast. The oldest coffee produces the lightest, thinnest crema.
Samo Smrke: So, as you can see they will start simultaneously and they will end simultaneously and notice the difference between the flow going out from a fresh roast from a 10 days old coffee, two months old coffee and one-year-old coffee. So, as you can see, it’s clearly visible that the amount of gas that it’s in the coffee is related to the thickness of the flow going out to the amount of crema that is produced and although we would say that after two months the whole beans were fully de-gassed like we saw in the previous slide. There’s still some gas remaining in the beans that we liberate when we grind the coffee. So, there’s still some crema that it’s formed from a coffee that was two months old and even after coffee that was stored for a year, we still get some crema from this coffee.
So, I said earlier that we had to just tweak a bit the grind size to be able to extract these profiles that they ended up with the same crew ratio and to the same beverage volume. So, this is what we did here. So, we have extraction profiles. We have the extraction time and the beverage weight here and we have used just for three of those a fresh coffee, a two months old coffee and a days old coffee. As you see here, we the Mahlkonig EK43 and these are the settings we need to use on our device, our grinder. So, we had to adjust them a bit different for each of the coffees to be able to end up with approximately the same beverage weight. So, because we needed to change the grind size, you can see also that the amount of gas that is in the coffee not only impacts the crema formation but also the flow dynamics. So what we observed is that if we have all the coffee’s ground to at the same grind size is that fresh coffee was flowing really fast through the portafilter and then on old coffee was also flowing faster but then a coffee that this about 10 days old and this is roughly saying on our experience for coffees about from a week old to two weeks, three weeks old they flow much slower. So, for the same grind size, we got much less beverage volume. So, that means that the resistance caused by the gas released during the extraction caused more back pressure on the espresso flow. This is quite surprising. We did this experiment quite a few times and we always got this kind of result that very fresh coffee was flowing fast. A coffee that it’s aged a bit flows slower but then once it’s getting really old, it starts to flow again faster.
31:45 Scientific approaches to measuring coffee’s chemical freshness
Samo Smrke: Okay, so this was the first part of the story about the physical freshness and the de-gassing. Now, I would like to talk about the other aspect, which is more related also to sensory, of course, because aroma, chemistry and sensory are all working together. It’s about chemical freshness. So, we want to see what happens with the aroma after the coffee’s roasted. So, how the aroma develops during storage. A lot of volatile compounds that escape from coffee after roasting is about 1000 compounds identified in coffee and out of those, 50 compounds are important for the aroma of coffee. Here we have an example how four coffee aroma compounds develop after roasting and I’m plotting them relative to one another. So, we start with 100% from the fresh coffee, and then we look at what percentage compared to fresh coffee remains after certain time. So, when we look after one month some of the compounds like this one Methanethiol ends up at less than 10% of what we had in the fresh coffee, but also others they behave very differently. This is means that the relative composition of the coffee changes with time. So, it means that also, the aroma will not be the same for a fresh coffee compared to an older coffee and indeed these compounds they have very different smells, So, Methanethiol can be sulphery or sharp sulphery aroma. Butanedione is a very buttery aroma. Propanal has a flowery aroma in the low concentrations and Methylpyrazine has an earthy or a nutty aroma. So, these are very, very different aroma characteristics and they behave very differently with time. So, it means that the coffee that is fresh might have completely different characteristics than a coffee that is just a bit aged and actually nothing was formed during this storage of coffee, just different aromas were lost at a different rate during storage.
How this relates to freshness and to measurements of freshness is because we have these compounds that are behaving very differently in the coffee. We can use them as markers of freshness. Like I said, there’s very little compounds or nearly no compounds sitter for but still some of them are formed during storage because they’re oxidation products or degradation products of molecules. So, this one I mentioned before, Methanethiol, is a very liberal molecule. It oxidizes very quickly and releases from coffee very quickly and it forms this one Dimethyldisulfid in the coffee. So, we expected this one will decrease in concentration rapidly, and this one will increase in concentration during storage. Indeed, this is the case. If we measure the amounts of these molecules in coffee with time, we see that this one, the Methanethiol will decrease with time and this one will increase because it’s formed from the degradation of this one. This is actually a very useful for me as a scientist. I have a molecule which is decreasing with time and a molecule which is increasing on equal ratio of the one that you think, rethink and divided by the other one that is decreasing, and this ratio, what we call a freshness index, helps me evaluate how fresh the coffer is or how the freshness of the coffee is changing with time. Also, in other words, it helps me to say how much of the aroma was lost during time or how fast it was lost and this tool that helps to do all sorts of experiments related to coffee freshness to packaging which we’ll see in a few slides.
Heather Ward: We are now on slide 22, titled “Evolution of Ratios – Impact of Temperature.”
Samo Smrke: So, we did some experiments to check how these freshness indexes can be applied, and one, for example, is the impact of temperature. So, I’m plotting this ratio that was measured from a coffee that was stored in a bag that was laminated with thick aluminum where we don’t expect any oxygen going in the bag during storage. So, we should be as little oxidation as possible in this bag, and I’m putting this freshness index. These are four roast batches and four sets of bags, which were measured at each point. So, fresh coffee start after one week’s storage, after two weeks’ storage, after three weeks’ storage, and after four weeks at room temperature, and then the same experiment for coffee that was stored at 50 degrees Celsius.
So, as you can see here. that we have in both cases a gradual increase in this freshness index. So, we’re having less and less fresh coffee. So, higher value of index means less freshness but take a look at these values. So, here we have around five the ratio, and here we have around 005. So, an increase in temperature of 25 degrees Celsius has caused a loss of freshness by hundredfold. So, this is a massive difference as you can see. The temperature has a really strong impact on how freshness is lost. Of course, we’re here looking at the most labile coffee aromas. Most labile compounds, so they are most sensitive to freshness lost, but nevertheless, this shows us that temperature is something we have to be careful with if you want to keep the coffee fresh.
The other thing is, of course, what we can look at, the atmosphere in the coffee bag. So, we wanted to compare how the oxygen that is present in the coffee bag compared to coffee bags which are flushed with nitrogen. How this would impact freshness. So, we did this again, using freshness indexes of these two compounds and you can see here we looked at three different temperatures four degrees Celsius, 15 degrees Celsius and 30 degrees Celsius and we have coffee bags which were flushed with nitrogen, with 0% oxygen and then we had coffee bags which were enriched with the oxygen so that the oxygen concentration was 50% just to be able to see the oxidation reactions much faster in a shorter time. As you can see here at low temperatures, the atmosphere in the coffee bag did not have an impact on the freshness. Also, at 15 degrees this is just below room temperature. We don’t see really a strong impact of the atmosphere in the coffee bag. But if we have a bit higher temperature 30 degrees Celsius, then we start to see a really strong impact of the oxidation. So, it seems that at lower temperatures, the oxidation is not as important as when we go to a bit higher temperatures where the temperature is driving these reactions much faster, and then oxidation starts to play a major role in the loss of coffee freshness.
Also, next time what we can do when we start to talk about oxidation, oxygen percentage of course, we can then also use this kind of method of measuring freshness index to compare different packaging material. So, here we compared coffee that was stored as whole beans in different coffee bags. So, with paper coffee bag, simple paper, plastic composite film, plastic composite film with a thin aluminum layer, just paper deposit and a thick aluminum layer.
So, what we expect here that these two are very porous. So, basically the oxygen from outside readily diffused in the package, was easy to go in and we lost freshness during storage. So, the higher index means that we lost more freshness. Then the other two cases when you have a thin aluminum layer, it’s not completely tight, so some oxygen was able to go into the packages. We had a bit of increase of the freshness index, but when we have a thick aluminum layer, we don’t see any the oxidation occurring. This is a bit of a different freshness index that we used before. This one is more sensitive to oxidation reactions. So, here we were looking at oxidation and here we didn’t see any oxidation when we have a package that it’s packed with aluminum layer. Then, of course we can go to the next step.
When I was talking here about freshness and storage, I was always talking about what is called primary shelf life. So, we have packages which are stored and once we do measurements, we say, okay, after two weeks of storage, we open the package, we take the coffee out, we measure the aroma in the coffee, and we have the results and then we discard the last of the coffee. But also, what is important is the consumer behavior or the secondary shelf life. So, what happens after a package is opened for the first time and what has happened in with the coffee after this point? So, this method of measuring freshness is also suitable to study this kind of, so I would say consumer behavior or secondary shelf life studies. What we see here is we have measured freshness indexes on a series of samples. So, here we have a reference. This is the primary shelf life.
So, this is a coffee that was short in a coffee bag which was closed until we opened it and then we used to coffee, and then we didn’t use this bag anymore. So, it’s the same index as before. We’re trying to look at the oxidation of the coffee and with primary shelf life, we don’t have any increase. So okay, our coffee was stable in these coffee bags but what we wanted to see next is what happens if we opened the coffee bag, we take some beans to measure and then we close the coffee bags and then we use the different approaches to how to cause the coffee bag.
So, first was to transfer all the beans in the can and then for each step here, take beans from the can and then close the can again. We use scotch tape to close the bag. We use a clip to close the bag and the actual study was to evaluate how this kind of new packaging system would behave compared to these three approaches and what we saw here is that transferring the bean into a can has the worst results. So, we have the most oxidation. This is obvious because even if the beans are packaged in an inert atmosphere in the package, we take them all out. We mix everything with air, we have a lot of oxygen in the can. We open a big part of the can so each time we open we add more oxygen in, so we have a lot of oxidation. Then the tape was quite similar to the can. Of course, closing a coffee bag with tape, it’s not really tight, so we had oxygen always slowly going in and also, if we cut the bag all the way, we open it, take it out and so we always add oxygen.
So, the clip is actually much better than tape because the clip closes. We can always close down where the beans are so every time we close, we push out as much oxygen as possible and then even if while we’ve taken up the beans, if we introduce some oxygen, we can always close the bag as tight as possible So, we don’t have a lot of oxygen in so it was performing quite well and then this packaging was performing the best. The reason is because the opening is very small. So, each time we take out the beans we didn’t introduce a lot of air in. So, what we take home from this is when we have a bag open and we want to reclose it, try to introduce as little oxygen is possible or have the smallest possible opening of the bag so that we don’t introduce oxygen to the coffee beans to avoid oxidation.
44:40 Linking physical and chemical freshness and the impact on the sensory experience
Samo Smrke: So, we can do a bit of science on both these datasets to try to link chemical and physical freshness so we can take a look at, for example, how different molecules behave and how long does it take that they’re released from the coffee. So, we can measure characteristic time constants and we see that if we compare fast roast to a slow roast. What you see here is that for the CO2 we have different characteristic times. This is from this slide that I showed earlier that a fast roast releases CO2 much faster than a slow roast. It’s very interesting that when we compare aroma compounds there not really much different between a fast roast and a slow roast. We come back to the slide when I was talking about different mechanisms of release.
So, because CO2, is impacted by the pressures and it’s more hydrodynamic release, a physical release we have higher pressure after a fast roast inside the beans than the slow roast, so it’s pushed out faster whereas aroma molecules follow different rules. They follow the diffusion, and this is not really impacted by the pressure inside the bean after roasting. Therefore, there won’t be a difference between the fast roast and the slow roast. Although we might have produced more of this compound to the fast roast the time, how fast it’s released, is not really impacted. There’s also one thing that I want to talk about. I already talked about temperature a bit. So, we have effective temperature on the aroma oxidation, the aroma degradation but also the de-gassing is something where temperature is impacting, impacting our de-gassing rates.
So, we did an experiment just to try to see how much the temperature impacts de-gassing. We degas beans at 35 degrees Celsius. It’s about a bit higher than room temperature and there was this curve from freshly roasted coffee then in parallel, we put beans into a freezer and store them into the freezer and then after this measurement was finished here after 70 days, we took the beans out from the freezer and then measured de-gassing from these beans and this is this one, the blue line. So, what you see here is that already from the start, we start to get a bit less gas out from the coffee. So, although the beans were stored in the freezer, we still get less gas. Also, there was some degassing occurring in the freezer. Actually, if we compare those two, we can evaluate that what happened in 70 days in a freezer, it’s comparable to what happens two days at 35 degrees Celsius and by knowing a bit of chemistry and the physical chemistry processes that are happening here, we can evaluate what the temperature impact is.
These kind of reactions follow what is called an Arrhenius equation. It’s this kind of equation, it’s Arrhenius law and this is relating to temperature, to the rate of which chemical reactions occur and once we put these number’s in here we have two different temperatures, two different times and what we can calculate is that every 10 degrees Celsius, we increase the degassing by 1.8 times. So, to put this into more numbers that are a bit easier to understand, we can say okay, we have an ambient room temperature and a freezer temperature of 25 degrees and 18 degrees. We had a bit of colder freezer here, this domestic freezer. So, we calculate this, and we find that by putting coffee in the freezer we decrease the degassing rate by 12.5 times. So, what does this mean? Practically if we say our optimal freshness is wanted for weeks if we put coffee in the freezer. If we look at the degassing, I think the optimal fresh insists. three months to one year.
Of course, we’re making a tiny bit of assumptions. We’re assuming that this Arrhenius equation that is coefficient is the same for aroma molecules as for CO2 and that they will behave the same, which is not necessarily the case, but just by looking at this physical freshness, we can say that we do roughly this extension of the coffee freshness by putting coffee in the freezer. In the end, there’s also another thing which it’s important. Of course, all this doesn’t make any sense if we don’t look at the sensory impact. This is, in the end, what is the most important. So, how much is the impact of loss of freshness to the sensory. This is actually much harder to do than all these measurements we did before, because once we have instruments, they’re very reliable. We can get very reproducible results. But once we have to do sensory studies it’s much harder to do so and our data is much more noisy. But nevertheless, it’s just one example of a study of how cupping score is decreased by coffee storage. You can see that in this case there was for two different coffees, one from Brazil and one from Rwanda. We could see a decrease in the cupping score days after roast. But from one coffee from Columbia, the result was, for example, not that clear. So. this is just a to show you that, as a scientist, I tried to make instrumental measurements to have reproducible results, but in the end, this is what matters. But to be honest this is really hard to measure compared to scientific experiments because it’s really hard to get really good, reliable data.
So, to conclude this talk. What are my take home messages from what I learned while studying coffee freshness? So, we can objectively quantify freshness in coffee by measuring degassing or by measuring aroma and using freshness indexes. This is important and why coffee is losing freshness with time is because it has an excess of gas trapped in the bean after roasting and it’s highly porous structure of coffee is causing this gas to release as well as the aroma is released through the porous structures of coffee after roasting. It’s a very unstable material. So, roast profile strongly impacts how fast we’re losing gas from the coffee when we talk about degassing and then overall freshness is strongly impacted by temperature. It’s very important, like I showed earlier, just an increase of 25 degrees Celsius. Increase the freshness lost by a hundredfold. So, it means having coffee store for a very short time in high temperatures can promote a lot of loss of freshness and also the oxidation reactions, they’re also much faster once we have high temperatures. The studies that we did after we developed these tools helped us to understand packaging, staleness of coffee, how it affects coffee extraction, coffee of different freshness and the formation of crema.
In the answers before I finish with this talk, I would like to say a bit more about where I come from. I work in Switzerland at Zurich University of Applied Sciences in the group of Professor Yeretzian. He just came in there in a red jacket and we do research on coffee. We’re focusing on five what we call pillars of coffee. So, each of the associates working there is focusing on one of the pillars. So, Sebastian, he’s involved in origin projects, green coffee, but more botanics of coffee, biology and the transformation of coffee. So, talking about roasting, packaging, grinding. Then Marco, he’s an expert in extraction. Impact of water to extraction and Anja, she’s an aroma scientist. She’s an expert in aroma formation. So, the formation of good smelling compost, but then also what we call process contaminants. So, furans, acrylamides, studying these kind of things, not only the good parts, sometimes the bad parts and we also have some coffee courses and Sabine’s taking care of this continuing education on coffee. With this, I would like to thank you for your attention and I’m open for any kind of questions.
54:30 Audience questions
Heather Ward: An audience member is asking how financially accessible are the instruments for you need to conduct these experiments.
Samo Smrke: This is what is called, we use a gas chromatograph mass-spectrometer to do these measurements. The cost of the instrument is about US$100,000 in this range. So, it’s very inaccessible to a small roaster but could be accessible if you partner with the university that has this kind of equipment.
Heather Ward: An audience member is asking what would be the impact of putting an antioxidant in bags of roasted coffee?
Samo Smrke: So, we’re actually working with another group at our university who is a packaging group. So, they’re packaging experts and what they do is they have oxygen scavengers in the packaging. So, they put a piece like this on the inside of the packaging, which is the catalyst which reduces the oxygen in the packaging and then basically they can lower the amount of oxygen in the packaging during storage. So, actually we study more different kinds of foods, not really coffee, but we see a strong impact in those foods that have antioxidants in the packaging to reduce further the oxygen content. Even if we flush the packaging with nitrogen, it has an impact to help preserve freshness.
Heather Ward: An audience member is saying they read that you shouldn’t put coffee in the freezer because of humidity.
Samo Smrke: Oh, there’s one thing I didn’t mention. So, you have to be very careful, because there’s also one parameter which is humidity that might interfere with freshness. So, you have to be careful. If you put coffee in the freezer is that once you take the coffee out you should not open the bag until it’s at room temperature because if you open the bag before the environment and ambient humidity, the water will condense on the coffee and then it will get wet and then you will do worse than you saved by throwing in the freezer. But overall, just storing in the freezer is not the problem. We did many experiments and we try to see if there’s an impact effect that putting coffee in the freezer and out and we couldn’t see any effects.
Heather Ward: The same audience member is asking how long the coffee should stay at room temperature before opening it after freezing.
Samo Smrke: Actually, it’s more than you think because the coffee beans are very of luminous. They don’t have a lot of heat conductivity. So, even if the package outside already feels to room temperature, the coffee inside still might still be cold and will condense. So, I would recommend at least one hour to wait before taking out for the freezer for a packaging of 200 grams. So, a kilogram packaging, to be on the safe side maybe. So, two pound packaging, two hours. Let’s say in this range.
Heather Ward: An audience member is asking Samo to elaborate at what temperature the staling process stops happening once coffee has been roasted.
Samo Smrke: Yes, this is what I was saying. This is kind of surprising results we got. So, we’re still looking into this to really reproduce these results. These are quite preliminary results that I showed but this is what we observed it up to a certain temperature. We didn’t see staling effects. We just saw the regular loss of aroma through diffusive processes but no staling effects. So, there was no impact of oxygen.
Heather Ward: The same audience member is asking whether that would mean it’s okay to put coffee in an oxygen-rich environment, such as a container, and then freezing it?
Samo Smrke: Yeah, yes, yes, exactly. Because the chemical reactions, they would stop at certain temperatures. So, when you go to -50 Celsius, you really stop all the chemical reactions and freezer -20 Celsius. You’re stopping by far nearly all chemical reactions. Yes.
Heather Ward: An audience member is asking whether it’s possible to freeze coffee, take out only the beans you need, and put the rest in the freezer and what effect this will have on the coffee’s freshness and flavor.
Samo Smrke: Yeah. You feel transferring too much. I’m not sure how much because coffee’s hydroscopic. So, it will also take up moisture from the room where you’re handling the coffee. So, transferring to container in and out. I’m not sure. I can’t say for sure. Yes, because you don’t know if you’re opening in the freezer and you open the freezer, you’re getting warm air from outside and you still might get condensation or freezing on the surface of the beans and it’s getting too complicated. It’s possible to test all this kind of practices of handling, but right now I can’t say how the impact would be.
Heather Ward: a member of the audience is referring back to the thin and thick aluminum layers for coffee packaging Samo spoke about when talking what types of packaging prolong roasted coffee’s freshness. She’s asking whether the thin aluminum layer is a metalized plastic and what type of packaging uses the thick aluminum layer.
Samo Smrke: I’m not sure what the thickness of this layer is, but it’s a vapor deposit, metalized plastic. Yes, but this is not consistently deposited so there’s still places where the oxygen can diffuse through. Yeah, that’s the 10 years and the thick is actual metal sheet that it’s laminated between plastics yes.
Heather Ward: A member of the audience is asking if light affects coffee freshness and aroma.
Samo Smrke: The light can also cause some degradation of aroma. But to be honest, I don’t know so much. I don’t have so much experience. How much is it in case of coffee. Probably what would be critical in terms of light, if you have, like a hopper of the grinder and you have oil on the surface of the hopper and it’s transparent, and then you promote a lot more the oxidation of this oil by having light. But overall, inside the bean, the light shouldn’t have a big impact, because the bean is absorbing the light. So, maybe it’s because it’s only some effects on the surface, but definitely not within the bean. So, probably makes a big difference if the beans are oily or if they’re really dry but we haven’t done any studies yet, so that’s just my hypothesis.
Heather Ward: A member of the audience is asking whether the heat applied to the coffee beans during grinding affects the amount of CO2 that eventually turns into crema when brewing espresso.
Samo Smrke: So, the loss during grinding that I had done one slide that you had so much gas lost during grinding. This is not because of heat. This is because when you grind you open these pores and the gas is just lost. But really the heat, how much impact. So, I’m not sure, really. But I can imagine that if a coffee comes out of the grinder very hot compared to when it comes out colder that what is lost from grinding until the coffees in the portafilter and being extracted if you do espresso can have a difference. We just had some students a few weeks ago, and I wanted to show these experiments and they were a bit clumsy by doing espresso extraction and then I had a really fresh coffee in the coffee that was one week old, and they were like oh, it’s not really such a big difference and I was a bit surprised but then I say that they were actually taking too long before they extracted the coffee. So, already from grinding to the espresso extraction, a lot of gas was lost. So, they didn’t see really this difference and then I did the really fresh coffee, directly grinder tamping extraction and then suddenly there was double the amount of crema. When they did it where they took me five minutes until they were doing the extraction because they didn’t know how to handle the expresso extraction that well. So, it could be within this time from grinding to extraction that you have some differences.
Okay and thank you again so much for coming.
1:02:30 Outro
Heather Ward: That was Samo Smrke at Specialty Coffee Expo in April 2019. Remember to check our show notes for a full episode transcript of this lecture and a link to coffeeexpo.org for more information about this year’s event.
This has been an episode of the SCA Podcast’s Expo Lecture Series, brought to you by the members of the Specialty Coffee Association, and supported by SAP’s Softengine Coffee One. Thanks for listening!