MILAN – The following article is the second part of an edited transcript of a lecture on coffee roasting held by Dr. Chahan Yeretzian, Professor for Analytical Chemistry, Bioanalytical Chemistry and Diagnostics at ZHAW’s School of Life Sciences in Wädenswil (Switzerland), in the occasion of the Coffee Hub organised by Ima at its headquarter in Bologna in May this year.
The first part of the article was published in yesterday’s issue of Comunicaffè international.
by Dr. Chahan Yeretzian
The furan issue
Furan is a compound that developes in big quantities when you roast. But when you store coffee or you extract coffee, the level of furan goes down. Actually, when you drink coffee, there’s only a 10% of furan content left. So when we talk about furan consumption, we don’t have to do that thinking about its presence in coffee. But in how much we’re drinking it.
That’s what we have studied in different papers, to understand how much furan does someone consume related to drinking coffee: through the automatic machines or capsules and soluble coffee? And the conclusion is that, the fully automatic coffee, delivers a big amount of furan. It’s just because when you extract and then you bring it to the cup you have a great chances exported of furan to state. In the fully automatic system you use a lot of coffee. The issue is that many people complain about furan in capsules, but there’s no real reason for that. Because in them you can see so much less coffee per cup than in filter coffee. So in capsules there’s less furan content.
The final subject: freshness and packaging
We want to talk about freshness not in relation to green coffee but to the roasted coffee. We have to make this distinction, it’s not the same. Once the coffee is roasted, that’s the point 0. From there we start to talk about freshness.
But how can we measure it actually?
And how can we measure changes in the freshness of coffee during storage. Because a lot of people talk about freshness but they don’t have the knowledge to make rational argument about why coffee lost it. There was no way to measure this element since a few years ago. So this is important for many things: from the cup quality but also, for the specialty coffee movement. And that’s where I started. Because in this sector we always talk about the freshness of coffee. And everybody claims that their specialties are fresh, but then they don’t know how to measure it.
So we wanted to create a rational approach
Freshness in the past was basically measured by high roast, high grinding, high extraction: if I do this and that, then my coffee will be fresh.
But how can I actually measure it? According to me, freshness is maintaining the original qualities of freshy coffee: in time, it stays as it was fresh. And it is a relative concept because each variety has a different starting point, each roasting has a different starting point. And so it’s all relative to this particular coffee.
How can we measure?
We have two thermometers to do that: one is the aroma. Once you roast coffee you have a collection of aroma compounds and they change all the time. Somehow you have to look to these compounds in a simple way and you have to observe if they have lost freshness during the time of roasting.
The other thermometer is the CO2 content. When you roast coffee, you have a lot of CO2: looking at the remaining of CO2 is a good element to measure freshness. Infact, the more you have left CO2, the more the coffee is fresh.
In the aroma sight: when you roast coffee, the concentration of aroma compounds will decrease. So the first thing is: coffee is a instable product, and it will degrade. The only thing you can do is to slow down, trying to keep the pression. And then you have to care about the way you store and what packaging you use.
The question is: can I measure that?
I focused on 4 compounds and here, I want to underline only two of them: Methanthiol and Dimethyldisulfid. You can find the first one only in fresh coffee: if it’s gone, then also the freshness has disappeared. The second one, it simply doesn’t exist in the fresh coffee. If it evolves, then freshness goes away.
The storage temperature is directly expressed in the coffee freshness. An element that goes away very fast around the range of 42 degrees- 52. And the cooling too, slow the loss of freshness. If you reduce the temperature by ten degrees the shelf life extends to 5half point2.
The example of different kind of packaging
We analyzed 3 types of packaging: plastic, paper, aluminum bag. In them, you can trace different kind of compounds. The movement inside the packaging takes around a year, whereas is paper, plastic or aluminum.
What about capsule, both in plastic and aluminum?
We took two capsules, one of full aluminum one is not. If we look for freshness and how it changes. What we can see in a Nespresso capsule full of aluminum, over fifty weeks the same ration hasn’t move on. The most damage has happened during the packaging.
The second shelf life
How can I conserve coffee, once I opened it? You can measure it. We have studied different way to close packaging after the first opening: the scotch, or to put coffee into a tin. A lot of people think that is the best way to preserve coffee. If you put the coffee in to a canister, that’s really bad. It ages faster. The reason is actually very simple: as you know there’s CO2 into this canister and when you pull over it, you loose his primary protective atmosphere, CO2, and the coffee is no more protected. If you leave coffee in his original package but then you close it, you are actually pushing out the gas. They’re the best way to conserve coffee, but only if we are closing them properly.
The CO2 aspect
It’s a real good indicator of freshness. In a roasted coffee bean: it has become porous. It has a porous structure and all in these pours there’s CO2. This CO2 has a distribution that you can measure from 43 to 45 micro meters pour sizes. And all of these pours are full of CO2, which will be released during the time.
What want to measure the gas that is inside the coffee. And how is it released over time.
We put coffee into small reservoirs on a high scale. The coffee is sitting on a scale and it’s degassing. When it degasses It restrains, because CO2 gives weight. When we roast coffee, 2% of the coffee weight is given by CO2. And that in time, gets lost.
So you put the coffee on the balance and you can measure the loss of weight and CO2. We have to prevent this degassing during the packaging. You can control it through temperature monitoring and other methods.
You could measure degassing also with a vessel where to put the coffee, and during degassing the pressure inside of it increases. Measuring this pressure is a way to understand how much gas is coming out.
We analyzed different speed of roasting
If you are able to measure the amount of gas that’s coming out, you can for example understand how much you have to degas the capsule if you do it from a ground coffee before package the capsule. You can know how much pressure it can support, how much gas will be released.
You can apply it to a different roasting degrees, from dark roasting to slow roasting or medium one. Of course, if choose a specific kind of roasting, you change the process of degassing.
We can measure it also for the ground coffee in capsule. When you grind coffee you loose around the 60% of CO2. It depends on how you grind it. And then the dynamics are very different: if you want to degas a ground coffee you should pack in the first jar because in packaging you can make a big difference on the remaining of CO2.
There are also differences between roasting Arabica and Robusta. In the Robusta you can see more CO2. With Robusta, you can get much more gas.
The implication on extraction
The CO2 has an implication on the crema. It’s he direct expression of the CO2. If you have a fresh coffee, you will have more crema. And if you have Robusta, the crema will be more present. CO2 has an impact also into the sensory aspects: infact, CO2 generates an acid that influence differently the roasted coffee.