coffee roasting

Thought bubble

inverted Aeropress and coffee stain
A problematic inversion with the Aeropress. This brew method offers plenty of physics connections for those who look.

The Aeropress is not a brewing technique that creates many bubbles on the surface of a coffee. Unlike the crema of an espresso or the iridescent bubbles on top of a black coffee prepared using a cafetiere, the surface of an Aeropress coffee could be thought of as a bit, well, dull. The paper filter within the Aeropress removes many of the oils while this calm brew method generally does not create the turbulence needed to produce bubbles that cling to the side of the resultant cup. Yet it is this brew method that can provide a bubble link to climate change and coffee roasting, and to see why, we need to pay careful attention to our brew.

Although there are many techniques for brewing with the Aeropress (you could try the guide here or here), one step common to most brew guides is that you will need to rinse the paper filter in the basket before you brew. The rinsing step removes a potential paper-y taste from the filter as well as helping it to stay fixed in position (the reason for this could be the subject of another post). Importantly for this particular post though, it also traps air within the holes of the filter, which you can see in the photograph.

The bubble is trapped owing to the strong surface tension of the water dripping from the basket. You could perhaps test this by adding soap to your brewing water in order to reduce the surface tension and watching to see if the number of trapped air bubbles you produce decreases. Or perhaps there are limits to what you are prepared to do with coffee in order to see some physics. Whichever way, the fact that the bubble is there at all can lead us down several thought alleys.

Perhaps we start to think about air that is trapped within water. In a way, this air is characteristic of what is around us now: the pollutants, the oxygen level etc. Which, while it may seem an obvious statement has an immediate consequence. Air that is trapped in water that is then frozen remains as a record of the composition of the air at the exact point of time that it was trapped. So if layers of ice form trapping layers of bubbles of air, and this happens for many years, we can analyse the composition of the trapped air bubble to discover what the atmosphere was like 100, 1000 or 100 000 years ago. This offers a way of understanding how concentrations of carbon dioxide, for example, have varied over the millennia.

An example of air bubbles within the Aeropress filter. In addition to the long bubble caused by incorrect filter placement, you can see two air bubbles in the hollows of the plastic basket under the paper filter (circled with a dotted red line).

But maybe your mind stays with the coffee: what about air bubbles within a coffee bean? In order to turn the green coffee bean into the aromatic substance that we all appreciate, it needs to be roasted. Roasting coffee is a fantastic mix of science and art: using the knowledge of what happens during roasting and applying (and playing with) that knowledge to produce great tasting coffees. At its core, the roasting process involves heating the beans for a certain amount of time in order for the water to come out of the green bean, the sugars to turn in the Maillard reactions and for the various aromatics to develop chemically. The green bean also undergoes physical changes. The colour is altered, the bean expands and the internal gases (first water, then carbon dioxide) build up pressure within the bean and then crack open some of the cell structures during roasting. And while this sounds fairly simple, there are ‘arts’ involved in roasting: how long do you let the beans dry? How fast do you take the bean through the Maillard processes? Do you let the beans cool slowly or cool them really fast to stop any further chemical reactions immediately? Each of these has effects on the final flavour of the bean, some which are fairly similar across the industry, some which rely much more on the creativity and discernment of the roaster.

There are obvious analogues to materials physics and materials chemistry. In order to make the different materials that are studied, raw materials are often heated to a high temperature and left for a significant time before either being cooled slowly or suddenly, by quenching. There is the science: the temperature at which different reactions occur and the way that materials form together in order to produce grains that get larger as they are heated for longer. And then there is the art, how fast to heat, how long to leave it for, whether to cool or quench, even what gas should be used to flow over the forming compounds. Small differences in how the materials are heat treated can have large consequences on the applicability and strength of the final material, with applications from gear cogs to airplane engines.

Kamwangi and Gelana coffee under the microscope
A fluorescence microscope image magnified 20x of two types of coffee after roasting. The microstructure (including pore development) will depend on the type of coffee as well as the style of roasting.

To return to the coffee roasting, the effect of the temperature has a similar marked effect on the microstructure of the resultant bean which will have consequences for how the roast ages. For example, a study about 20 years ago showed the differences between coffee beans roasted to an equivalent level (measured by moisture loss and colour analysis of the roast) at two different temperatures. The physical properties of the final roasted beans were very different. Not only did the higher temperature (260C) roasted beans show a larger volume increase compared to the low temperature (220 ) roasted beans, the pore structures of the beans were also different. For the higher temperature roasts, larger micropores had opened up within the cell walls of the roasted coffee. These pores connected to regions deep within the bean that would otherwise be cut off from the air: trapped bubbles within the bean that, with the higher temperature roasting, now have a way of escaping to the outer surface. Indeed, one day after roasting, the authors of the study saw, under a microscope, many tiny spots of coffee oil seeping from the interior of the higher temperature roasted bean and to the surface.

This has consequences for how the bean will age after roasting and so how we as consumers will appreciate the drink. Roasting is a dark art indeed, and one that I’m grateful for the many skilful practitioners that we now have around. Roasters who help us to appreciate the flavour of our coffee, as well as the directions of thought it takes us on.

An effective medium for coffee roasting?

coffee bowl pour over

How would you measure the moisture content of a coffee bean?

Recently I had the pleasure of a tour of Amoret coffee in Notting Hill. In addition to discussing an upcoming event that Amoret are kindly hosting (an evening of coffee physics, sign up to the events list to find out more), it was great to see the coffee roaster that is installed there. Fascinating, with what looks to be a really interesting series of coffees lined up ready to roast. And in the course of all this, we came upon the moisture meter, which got me thinking.

Measuring the water content of green (and then roasted) coffee beans is quite critical to gaining an understanding of your roasting process apparently. Sitting on the shelf next to the roaster at Amoret, a small box contained an instrument designed for measuring exactly this. Although it looks as if it is a giant ice cream scoop with a central pillar in the middle, it is actually designed to measure the water content of the coffee beans capacitively. How does it work and, knowing how it works, can we make any predictions as to anomalous results that it may occasionally provide?

The simplest style of capacitor consists of two metallic plates with a gap between them. The capacitance changes depending on the size of the metallic plates, the distance between them and, crucially for this subject, the material that fills the space between the plates. When you apply an electric field between the two plates, the electric moments of the material within the capacitor will tend to align with the electric field. Different materials will react differently depending on their “polarisability”. You only have to think about how a stream of water reacts to a statically charged balloon to see why.

Pulp, Papa Palheta KL

Electrical boxes in Pulp by Papa Palheta KL. The moisture meter at Amoret is much smaller than these old boxes at this ex-printing works.

What this means in practise is that a capacitor formed of plates filled with water will have a different capacitance to the same capacitor filled with air. We say that the ‘permittivity’ of the air is different from the ‘permittivity’ of the water. Measuring the capacitance tells us the permittivity of the material between the plates and so whether the capacitor is filled with air or water. Now fairly obviously, it’s not quite as simple as this because a coffee bean is neither air nor fully water and the moisture meter is not two parallel plates. But in terms of the physics of the measurement, the shape doesn’t really matter here while another bit of physics called “effective medium” theory helps us with the fact that the bean is neither fully air nor fully water. Effective medium theory tells us that the relative permittivity of the mixture is simply proportional to the sum of the individual contributions from the polarisability of each set of molecules. So, merely changing the number of water molecules between the plates will change the capacitance. By knowing what the contribution of the dry beans are, we can calculate the moisture content of the coffee beans as a percentage. Or at least, the instrument can do this calculation internally and provide you with a number on the display.

But. This is what got me thinking about the measurements of the coffee at Amoret. Coffee beans come in a range of sizes and shapes, as you can see by taking a look at the online selection at Amoret (here). Some of these coffees are small, tending towards a more spherical shape while some are significantly larger and more conventionally bean shaped. Is it obvious that the moisture content measured for different coffees is directly comparable? This is not to diminish the use of the moisture meter. As a comparative tool to measure before and after roasting for example, it should be a fairly good indicator. But what should we expect for the absolute accuracy of the instrument? Is a 16% moisture content measured in a small bean really equal to a 16% moisture content measured in a big bean?

At first sight it may seem a silly question, after all, the moisture content is expressed as a percentage; why should size matter? But perhaps we could have a little further think about this. The moisture meter will be optimised for a dense packing of coffee beans. So if we filled it with small beans such that there were very few air gaps between the beans, we would expect a fairly accurate moisture content measurement. If on the other hand, the beans were larger such that there were quite a lot of air gaps between the beans, the actual volume fraction of water molecules in the meter would be reduced (16% of 100% full is greater than 16% of 90% full). And as the capacitance is directly related to the number of water molecules in the sample, the water content that was measured would be less than the true value in each individual bean. So this leads to my first question for roasters using capacitive moisture meters:

  • Do your large beans, that don’t pack well into the moisture meter, often show lower moisture contents than your smaller beans?

variables grind size, pour rate, pour vorticity

Coffee roasting is part-science, part-art and requires great skill and attention. But can thinking about a little extra physics help to understand some of what goes on with the process?

A second point is slightly more subtle. Consider that I had two beans of equal moisture content (%). But one of those beans packs more fully into the moisture meter than the other larger, more irregularly shaped bean. On roasting these beans, they both lost the same volume fraction of water so, say, they went from 16% to 12% water content on roasting. Would both beans show that they had lost the same amount of water?

We could start by thinking about packing these beans into the meter. The one that was densely packed would show a moisture content that was close to the real value (in our example 16%).  The one that was less densely packed however would have a lower volume fraction of water and so show a lower water content. If we assume that the beans filled 90% of the space, the percentage that we measure would be 16% of 90% = 14.4%. On roasting, the two beans are again loaded into the meter and again the densely packed one will show a moisture content close to the real value (in our example 12%). The loosely packed one will show a moisture reading of 12% of 90% of the volume which is 10.8%. Crucially, if we are looking at moisture difference, the densely packed bean will appear to have lost more water (16% – 12% = 4%) than the loosely packed bean (14.4% – 10.8% = 3.6%). Which leads to my second question for roasters:

  • Do small beans that pack well into the moisture meter appear to lose more water for an optimised roasting profile than your larger, less densely packed beans?

Clearly, different beans will have different moisture contents anyway and so it may be difficult to discern any pattern between two specific coffees. The moisture readings may genuinely reflect the fact that the smaller beans have higher water content or vice versa. And also obviously, the measured moisture content is only one part of determining a successful roast profile. However the question is one of statistics. On average, do your larger, less well packed beans have a moisture level lower than you expect? And on average, do they seem to lose less water (measured capacitively) on roasting?

I’d be fascinated to hear your thoughts, here, on Twitter or Facebook.

Plastic, coffee and ethical consumerism

“[W]hile 30% of UK consumers claimed to espouse ethical standards only 3% of purchases examined reflected those standards”∗.

Earth from space, South America, coffee

The Blue Marble, Credit, NASA: Image created by Reto Stockli with the help of Alan Nelson, under the leadership of Fritz Hasler

Most of us are aware of the growing number of environmental problems facing our planet and many of us want to do something. The question is what? Take the packaging that we use for freshly-roasted coffee. It often comes in metallised plastic bags with aroma valves on the front. Is this packaging good for the environment, or for our coffee?

Many factors will influence our decisions as consumers. Even our ‘ethical’ decisions can be based on different arguments. One factor though is, hopefully, the insights gained from scientific studies on the environmental effects of different types of packaging. Today’s Daily Grind examines some of this science.

Types of coffee packaging available

When you order coffee from a roaster, or buy it at a supermarket, mostly it will arrive in a metallised plastic bag. Some companies will supply coffee in compostable ‘plastic’ packaging, or paper, but most bags are still made from ordinary plastic. Some, larger, coffee roasters supply their coffee in cans. Although these are 100% recyclable, the increased weight compared to plastic packaging and the limited re-usability of the cans mean that plastic packaging can be more environmentally friendly than canned coffee. This article is therefore only going to consider smaller roasters and the plastic vs paper debate.

The problems of packaging

It is helpful to clarify the environmental concerns with respect to packaging. For the case of paper vs plastic, three major areas of concern are:

  • Depletion of a limited resource, recycling and re-usability.
  • Carbon dioxide emissions – in the manufacture and transportation of packaging.
  • Degradability – in both landfill and as litter.

Recycling and the Limited Resource problem

air valve, plastic, environmental coffee packaging

Disposable products make up about 37% of plastics produced‡. Are we wasting limited supplies by wanting our coffee as fresh as possible?

Paper comes from wood but plastics are generally a by-product of the petroleum industry (5% of petroleum in the US is used to produce plastics). Perhaps you will say that not all plastics are made from petroleum by-products. It is true. “Compostable” plastics are typically manufactured from starch based products (corn etc). However other bio-degradable plastics are petroleum based. “Oxo-biodegradable” plastic is ‘ordinary’ plastic with a small amount of catalyst added to it during manufacture. The catalyst causes the plastic to break down more quickly than the conventional plastic without the additive. Typically oxo-biodegradable plastic will be manufactured to degrade after 18 months compared with many years for ‘ordinary’ plastic.

Both compostable and oxo-biodegradable plastic are sometimes called ‘biodegradable’, but there are crucial differences between the two. For the sake of this article, I’ll be comparing ‘ordinary’ plastic with ‘compostable’ plastic (conforming to EN 13432) and oxo-biodegradable plastic (regulation ASTM D6954).

So the first part of the question would be to ask if the coffee packaging is made from recycled material. Paper can clearly be made from recycled material as can ordinary plastic and oxo-biodegradable plastic. Compostable plastic cannot be recycled and so cannot have been made from recycled material.

The second part of the question is whether you can recycle the packaging after using it. Again, paper packaging can obviously be recycled (provided it is not lined with plastic). Although both ordinary and oxo-biodegradable plastic can, in principle, be recycled, the multilayered and metallised design of the coffee bag means that it is not normally recyclable. Some coffee roasters however have started using specially designed plastic packaging that can be recycled in normal recycling centers. It would be great if more followed suit.

Two questions for your coffee supplier: Are the bags used to package the coffee made from recycled material and are they recyclable?

Greenhouse Gas emissions and energy costs

paper bag roasted coffee

Is a paper bag necessarily better for the environment?

Perhaps it is greenhouse gas emissions that concern you and so want to choose an environmentally sound packaging in terms of its CO2 emissions? Paper or plastic? You may be surprised. The environmental cost of a packaging type as measured by its CO2 emissions depends mostly on the energy that is required to manufacture it and the energy that is required to transport the packaging material to the point at which it is used (ie. the delivery of the bags to the roaster).

A few years ago, the Environment Agency performed a lifecycle analysis of different types of shopping bags (plastic, paper, cloth). Plastic bags are typically significantly lighter than the heavier paper bags. So, in addition to the cost of making the bags, it is going to require more energy to transport paper bags to the point of use. The report calculated that the manufacture and transportation of paper bags consumed so much more energy than plastic bags, that paper bags had to be re-used 4 times in order to have the same CO2 emissions as an ordinary supermarket plastic bag, re-used as a bin liner. The situation for a cloth bag was even worse.

Although the plastic used for coffee packaging is much heavier than a standard supermarket shopping bag, the analysis suggests that if your concern is CO2, paper is not necessarily better than plastic. It depends on how you are going to re-use the bags before you eventually recycle them.

Litter and Degradability

I hope that no one is deliberately discarding their used coffee packets onto the street or onto the beach! But litter and bio-degradability are big issues for plastic based packaging materials, particularly at sea. There are horrific stories about marine animals being starved due to consuming plastic or being drowned because they are entangled in it. Paper will degrade very quickly and so clearly does not suffer from the same problems as the plastic packaging in this topic. However, as mentioned above, not all plastic is the same. As well as ordinary plastic, your coffee could come roasted and packaged in a degradable plastic, either compostable or oxo-biodegradable.

sea no litter

There is a big problem with plastic litter ending up in the oceans

The name ‘compostable plastic’ (EN13432) is, to me, a bit disingenuous. It suggests that it breaks down in a composting facility such as my worm bin. But the standard EN13432 does not refer to such home-composting at all. For a plastic to be deemed compostable it has to break down under industrial composting conditions (ie. it is held at 58 C for the period of its degradation). Not all countries/councils offer such facilities for their waste disposal and so a compostable plastic sent to landfill offers little advantage over ‘ordinary’ plastic. However, in the marine environment it has been shown that the compostable plastic bag did degrade quickly relative to ordinary plastic bags‡.

Oxo-biodegradable plastic on the other hand works very differently. At the time of its manufacture, metal-salt catalysts are added to the plastic that determine how long the plastic survives before it breaks down. As long as it is exposed to light and oxygen, the oxo-biodegradable plastic will break down after, typically, 18 months (though the usable time can be made longer than this). Recent studies have shown that it is safe to recycle oxo-biodegradable plastic together with conventional plastic recycling†. Provided that the bag does not get covered in algae, an oxo-biodegradable plastic will break down after 18 months (if that was the time specified at manufacture) whether it is on land or on sea.

Therefore if litter is what you are worried about, you have to ask where you think that the plastics are going to end up and whether you want to be able to recycle them or just re-use them.

So what should you do?

There’s no point me answering this question for you. Ultimately I do not know your individual circumstances and concerns, nor how you are buying and consuming your coffee. Moreover, these considerations have been solely based on some of the environmental problems associated with different packaging. Coffee consumption has other factors, such as the major issue of how the coffee tastes. Earlier this year, Roasting House conducted an experiment to blind-taste the coffee after it had been stored in different types of packaging. You can find the results of that interesting study here.

a take away cup

The next problem. What should we do about take-away cups?

Personally, my concerns are principally the greenhouse gas emissions and the litter/degradability problem. I also buy coffee that is delivered to me very soon after it has been roasted. So I tend to favour packaging that uses unbleached, recycled paper. There is a caveat though. The CO2 emissions caused by paper manufacture and transportation means that I need to find a way to re-use the bags as often as possible before recycling/composting. Fortunately, I think there is a great use for old paper coffee bags: They are the perfect size for carrying loose vegetables or uncooked fish/meat products in supermarkets (rather than use the plastic bags that can be supplied for these products). Each paper coffee bag can be reused multiple times before it finally becomes unusable.

If I were drinking coffee that wasn’t quite so freshly roasted, I would be in favour of using oxo-biodegradable plastic (preferably from recycled material). I do not currently have an opinion on compostable (EN13432) plastic. The results of the degradation of compostable plastic in a marine environment were encouraging and if it starts to become genuinely compostable (as I understand the word in terms of home composting) it would definitely be a type of packaging to consider.

You may come to different conclusions, if you do so, please do let me know what you think in the comments section below. In the meantime, a map of coffee roasters who are trying to improve the environmental footprint of their packaging in a variety of ways can be found here.

 

I am grateful for discussions with Oh Ying Ying of Miracle Spectrum Sdn Bhd who helped me to navigate the minefield of environmental plastics. There is much more to write about plastics, the environment, litter & the Paris meeting, the whole issue of take-away cups for example!

∗ Yeow et al., “Bags for life: the embedding of ethical consumerism” J. Business Ethics, 125, 87 (2014)

‡ O’Brine et al., “Degradation of plastic carrier bags in the marine environment”, Marine Pollution Bulletin, 60, 2279 (2010)

† A report by the Transfer Centre für Kunststofftecknik GmbH (“TCKT”) dated 12 November 2013 on behalf of European Plastic Converters (EuPC), Roediger Agencies.

ª Plastics and the Environment, Ed. AL Andrady, Wiley-Interscience Publications, 2003