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Factors effecting acidity in the cup

There is an often repeated statement that lighter roasted coffee is more acidic than darker roasts. This is a misleading generalisation that becomes conflated into: acidic coffee is lightly roasted and coffee with little acidity is darkly roasted. There are many factors impacting the degree of acidity in the cup, and here I want to identify them.

Brewing Parameters
The brewing parameters can have a major impact on the perceived acidity of the final cup. This is even more true in espresso drinks, as everything is magnified. I won’t go into this in detail – that’s a job for a barista trainer – but suffice to say at this point that water chemistry, water temperature, extraction strength and extraction percentage all commonly have huge impacts on acidity.

Immediately after roasting, coffee beans begin to de-gas. The primary gas given off from the beans during this is CO2 – which forms a partially ionized solution in water – Carbonic Acid. Very fresh coffee will therefore tend to be more acidic than more rested coffee, which has lost this CO2.

Lighter roasts (those taken to around mid/late first crack approximately) will not tend to be very acidic. In these roasts, the acidic flavours (as well as other flavours) will not have been developed.

Beyond this point, acidity will begin to be developed. The extent to which it is developed depends upon the roast profile used – how much energy, and when, it is supplied to the beans during the roasting process, determines what compounds are created and what are broken down. Two lots of coffee, of apparently identical degree of roast, will possess different degrees of acidity depending on the how the profile used has caused acidic compounds to be developed and broken down.

At darker roasts, acidic (and other bean specific) flavour compounds will have been broken down, reducing perceived acidity. Additionally, caramelised & roast flavour compounds will have been created which will tend to mask acidity in the cup.

Green Bean Freshness
As a fresh product, green beans will undergo changes as they age. The changes are not really predictable, but it’s generally true that the intensity of flavours (including acidity) start to noticeably decrease as beans age beyond 12 – 15 months after picking/processing.

How the picked coffee beans are processed impacts acidity significantly. In general, washed coffees are the most acidic, pulped natural coffees less so, and natural processed coffees the least. This is a very broad generalisation, but processing certainly impacts acidity and other flavours to a high degree.

Green bean & terroir
There is a vast range of factors impacting the flavour of coffee even before processing – coffee plant varietal, farming practices, altitude, soil type, weather – to name just a few. Some farms produce very consistent cup profiles from year to year – other vary widely. Whilst it is possible to spot trends within these factors, generalisations should be handled very carefully – these factors are too interlinked for easy analysis.

This list of factors is not intended to be exhaustive or complete – although please feel free to comment if there’s something major I’ve missed! Rather, it’s to point out the wide variety of factors which influence acidity. Most people – whether working in the coffee industry or as customers – will rarely get to see the impact of just one of these variables.


In defence of the subjective: a response to Colonnaandsmalls

This post is a response to a strong argument by Maxwell Colonna-Dashwood that quality in coffee requires an objective framework, or risks collapse into self-defeating relativism at the hands of subjectivity. I agree that Maxwell raises an important issue, and accept some of his points – however I wanted to offer some responses to those I disagree with 🙂

All taste is subjective. Taste is a person’s experience of a thing (their sense-data), so it cannot, by definition, be objective. Coffee has objective qualities that might be used to judge quality – e.g. relative balance of component  compounds. The subjective/objective gap is between what an individual’s experience when tasting a coffee and the objective, quantifiable qualities of the coffee that are responsible for that experience. It is not simply that what tastes we each enjoy are different – rather, a person’s taste experience (sense data) is unique to them and may be different, even radically so, to our own.

Avoiding the relativist pit
There are two avenues available to avoid the yawning pit of relativism, which dismisses all issue of quality as a personal value judgement. The first is an appeal to expertise – roughly, this person is an expert (a Q Grader, Cup of Excellence Judge, whatever), which means their subjective judgement holds greater value than that of a non-expert. The second is an appeal to popularism – coffees which either appeal moderately to a large portion of consumers, of appeal strongly to a subset of those, are good. I don’t wish to express an opinion on these at the moment – suffice to say, I think we can adequately avoid extreme relativism without an objective framework.

For customers
As I have argued in a previous blog post, our taste descriptors are not clinical. They are ladened with both explicit and implicit emotional and personal content. Giving customers likely taste descriptors for them is helpful. However, communicating our subjective experience is even more valuable. I’d much rather receive a good coffee from someone who loves it, than a great coffee from someone who doesn’t. If the barista’s personal enjoyment of a coffee can be communicated to receptive customers, in the right way, it can give license to those customers to take equal joy in their coffee.

Art, Craft or Science?

There’s an ongoing discussion in coffee about the role of a barista or a roaster (or, for that matter, a farmer) – about whether what they do is a science or an art. In my view, the difference between art, science and a third class of activity – craft – is important and can lead us to better understanding and valuing of the different roles of coffee professionals.

There is very very little speciality coffee science. Science is an experimental testing of a hypothesis – use the current thinking to posit a theory, design a test which would disprove the theory, carry out the test. If you disprove your theory, change it – if not, expand it and test again. Science is looking for understanding and nearly all coffee professionals are predominantly involved in creating a product – understanding is a byproduct and is not done is a scientific pattern. One of the big issues in what speciality coffee science there is, is pier review. One of the key features of main stream science is that you have enough experts that work by one is scrutinised my the others. This ensures the work isn’t just plausible, but is thoroughly, rigorously tested – if it holds up, we know it’s very strong evidence. An established issue with more obscure areas of science is that the group of experts, with relevant knowledge and interest, is too small to properly critique research. This is very true in speciality coffee research.

Much of the work of coffee professionals falls under this heading. The goal of a craft is the production of a product, using manual skills, meeting a defined criteria. That criteria can be broadly defined (make a shelf, make coffee to fit in this cup) or tightly defined (make a shelf of 1000mm x 30mm x 150mm to within 5mm, or pull a shot of espresso using a dose of 17g, in 27 seconds, of mass 31g). Craft requires manual skills (including sensory skills) and some theoretical skills.

Art is imaginative. It may or may not make use of physical skills & techniques to express itself, but the distinguishing feature against a craft is the lack of strict criteria to achieve. Certain roles in coffee are an art in this sense – and we should value these all the more for the artistic talent they require. As an example, consider the person in a shop who decides what the coffee should taste like (be they head barista/owner/whatever). They make an imaginative leap – to see potential in their coffee beans and decide how that should be experienced by the customer. It’s the craft of the other baristas to recreate that flavour consistently for customers.

Appropriate Accuracy

A quick example of the importance of appropriate accuracy with measurements.

I’ve been brewing a syphon at home on a pretty frequent basis since christmas (wonderful present 🙂 ), and have got my method down and had some very tasty results. I use a standard food probe type thermometer to measure temperatures, and have found this very helpful in getting the best from coffees.

In the last couple of weeks I have brewed a syphon at work for the first time. This has some new challenges – a different grinder, for example – but one of the most interesting has been to do with measuring temperature. At work, I have used a Fluke temperature probe. This is a more accurate measuring device than my home thermometer, yet less useful when making a syphon. This is because the Fluke is too accurate.

As with any heated mass of liquid, the heat distribution is not homogenous and changes throughout the process. The fluke is sensitive enough to give very frequent and misleading (though accurate) readings. It’s difficult to look at it and judge the average temperature of the water. In contrast, the home thermometer gives an ideal indication of this. There are still visible variations and misleading readings if it is moved around, but generally the less frequent and specific information provided is more simple to understand.

It’s an important issue if you want to measure your coffee making, that you use measurement methods and precision which are appropriate to your needs – more isn’t always best.

Not Black & White

This is a fairly to the point blog post, for a change – we should stop talking about light and dark roasts so much.

My issue is that I have heard many people – home enthusiasts, baristas, trainers, roasters – talk publicly about roasting dark or roasting light as if this is an absolute that sums up all you need to know about how the coffee will taste. The simple truth is that lots of different things impact on how coffee from a roaster will taste – what coffees they buy, how they store them, how fresh the crop is, the machines they use, the time they take to roast, the temperature profile they use during the roast, among many others. Lots of that isn’t reflected in “light” or “dark terms. Add to that, there is not a direct link between how “dark” the roast is and how dark the beans appear – not all coffees change their appearance in the same way during roasting (for example, Yemeni coffees I have come across have looked very darkly coloured even very early on during a roast).

As with any simplification, talking just about light and dark can make it easier for people to understand that there are differences between roasts. However, aren’t we now at a point where most of us interested in coffee know this? Encouraging all parts of the community to use these sweeping simplifications tacitly dismisses the other variables. That propagates untrue assumptions, myths and false beliefs about roasting and how it effects the beans. Don’t judge a roaster by what colour their beans are – judge them by what they taste like.

No roots

A big focus of my thoughts recently has been coffee education and training. That’s been fueled by conversations with Mike Haggerton and others, and particularly the Tamper Tantrum Live presentation by Ellie Matuszak.

One of the big concerns for me is that we (meaning the quality focused coffee obsessives) are starting to talk a lot about professional development, but not a lot about educating the public. Whilst I agree on the value of more skilled baristas/roasters/trainers/etc. I am dubious as to the value of formal education and certification in achieving this. However, what worries me most is that this distracts us from an easily achievable, extremely beneficial and worthwhile pursuit – communicating about coffee to the larger population.

I suspect a large portion of us are burying our heads in the sand about the reality of most people’s coffee knowledge. I believe most people in the UK have only experienced instant coffee and the highstreet espresso based beverages. The portion of people who have ever had freshly roasted coffee is much smaller again. Most people have no idea that coffee is part of a fruit, that it can have any non-roasty flavours, that different coffees taste different without adding artificial flavourings… And so on. And I would guess I’m being optimistic.

Don’t see this as a negative however. As with much of life, this challenge presents an opportunity. Good coffee tastes a lot better than bad coffee, and even people who don’t mind bad coffee tend to prefer good coffee. Good coffee isn’t expensive in comparison to bad coffee – well at least not by much. There is therefore lots of scope for good coffee to grow and expand it’s public awareness. That is achieved by good shops both in delivering great coffee and a great experience to customers and by running home brewing classes – but we must not sit back and become complacent just because some people are doing a great job. There needs to be more of this happening. We need to be finding new venues and ways to communicate how easy great coffee at home can be. We have the chance to build roots to the coffee community, to engage more people in the wider population, to expand public awareness.

I think there are challenges about how we do this and I hope we start having more discussions on this soon – now is the right time to establish common ground and consensus on how we, as a speciality coffee community, relate and connect with the wider world.

Tea vs Coffee: brewing comparison

I’ve always enjoyed tea as well as coffee, but I never took the step up to “TeaGeek” like I did with coffee. That’s something I’m in the process of fixing.

I’m still very early on in this, but I’ve already developed my tea brewing a long way from where I was a few months ago. As I was thinking through what I’ve learnt, it occured to me that it made an interesting comparison to coffee brewing technique – so here are my thoughts!

Extraction Strength & Extraction Quality
For both tea and coffee, the amount of flavour extracted into the brew is important. For both of them, there is some variation between what tastes best for different coffees/teas, between different palettes and between different brewing methods. At the same time, for both, the range of tasty strengths is relatively small. For coffee there are guidelines (60g/l, <4 mins) that provide good starting points. For tea that is not so true – the guidelines for a black tea are very different to a rolled oolong, for example, because the rate of extraction depends on the surface area available. In coffee, that is controlled by the grind used, but in tea it depends on the leaf size and processing style, taking it out of our control.

Over and under (and bad) extraction is a problem in both tea and coffee brewing as well. The perception of these differs between the two. For me,

Over extracted coffee is chiefly bitter, under extracted is chiefly sour.
Over extracted tea is chiefly astringent, under extracted is chiefly bland.

The general consensus is that coffee is best brewed between 85 and 95 degrees Celcius. In contrast, teas vary much more widely, from about 60 degrees Celcius for some green and white teas up to a rolling boil for Pu-erh.

For both tea and coffee, brewing at the wrong temperature can cause some major off flavours. Possibly the most common mistake I have found (and been guilty of) in tea brewing is using too hot water for green teas. I have found that over 70 degrees Celcius tends to bring out a harsh, vegetal flavour that overwhelms and which I find very unpleasant. In contrast, the same teas brewed at about 65 degrees Celcius are mellow, floral and complex.

Coffee is very dependent upon freshness. Green coffee tends to degrade in less than 18 months (at most), roasted coffee see a big drop within a couple of weeks of roast. In contrast, tea lasts much longer. That said, it is still a seasonal product and many types of tea degrade within a year of pluck.

Taste that extraction

A while back now, I had a Twitter conversation about under and over extraction, and tasting these. I ended up saying that you could taste under/over extraction and recommending an experiment (although a distinctly unscientific one). I realised I had never actually done the experiment myself, and I felt a bit daft. This post is about that experiment (which I have now done) and my new thinking from it. Thanks to the tweeters who prompted this 🙂


I used Scott Rao’s “Everything But Espresso” Gold Cup Brewing Control Charts, but a similar chart is available on the SCAE website.

The experiment:
I used a V60 and two very balanced (and tasty) coffees. I brewed 3 cups of each coffee, to 3 different brew recipes (using scales) – 60 g/l, 70 g/l & 45 g/l approximately (I recorded the various errors, but for the sake of simplicity I’ll just list the results here 🙂 )

I timed the extractions, measured the brewing water temperature and the resulting brew TDS. I tasted the brews (making note of the temperature when tasting) and made notes on my impressions – whatever words came to mind. I also tried them in comparison to the other brew recipes for that coffee.

I used the brew recipes and TDS measurements to identify each brew on the Control Chart and where it fitted in the Over Extracted(Bitter)/Under Extracted/Weak/Strong headings. I then compared the lists for the different brews, looking for patterns. From this I concluded that I tend to use the following descriptors for different extractions:

Strong Extraction: Cloying, Heavy

Weak Extraction: Watery, Cardboardy, Washed Out, Insipid

Underdeveloped Extraction: Sour, Simplistic, Brief, Simple

Overdeveloped Extraction/”Bitter” : Bitter, Not Clean

One of my concerns, and a motivator in doing this pseudo-experiment myself, was the suspicion that we often collect a whole load of different brewing problems (uneven extraction, too strong, unwanted grind distribution, etc.) under the heading “over-extracted”. I still believe this is an easy trap to fall into and one which over-simplifies what is happening, thus setting us back in trying to correct the problems.

I have also concluded that I tend to prefer coffee brewed in the lower half of the gold cup standard box, and am more tolerant of weak extractions than of strong extractions . It’s interesting to see, and it has effected the way I brew coffees – I’m now more likely to try lower dose (55 g/l -ish) and greater extraction % on some coffees (those with low acidity and “heavy” flavours).

I had previously tended to describe over-extracted coffee (i.e. A brew that progresses too far along it’s extraction, leaving it above and right of the desired point on the gold cup chart) as “sour”. My psuedo-experiment suggests that what I am tasting (the element of over-extraction I am sensitive to) is “too strong”, rather than coffee with too great an extraction %

So I’ll finish by saying – I was wrong, you can’t taste over-extraction. You can taste high extraction %, and high extraction strength – but you can only call these over-extracted if you know the intended brew ratio and extraction.

What’s in a name?

Once upon a time, I was a philosophy student. It’s one of those subjects that gets under the skin – once a philosopher, always a philosopher. Because philosophy isn’t about what you think about, it’d about how you think about things.

If there is one philosophical lesson I learnt it’s this – most disagreements turn around two people having different definitions of the same word/concept. This is a lesson that I wish more of the coffee world would learn. I think this problem is common throughout the coffee world, but lets start with one that everyone will have come across – what is a cappucino?

This debate can be seen across the internet and in countless cafes, but I’m not interested in taking part. I’m not interested in why your recipe is right – whether it be the history, or the romance, or just tastiness – what I’m interested in is why do so few people have a recipe?

If you can’t define what your version of a cappuccino is, then you can’t discuss it in comparisons to other people’s. How can a customer know whether they’ll like your version of a cappuccino if you can’t tell them what it is? What makes it different to your other drinks?

I asked one of my favorite shops (North Tea Power in Manchester) about what constituted their Cappuccino, and got a great answer – there was a clear definition of the espresso (beans, dose, shot weight, time, flavour description, etc.), a cup size, a microfoam thickness (relative to their latte), presentation info (no chocolate sprinkles, appropriate latte art) and also descriptors it should invoke (velvety, delicious looking, etc.).

You might well think that sounds simple – I always assumed that any serious coffee shop would do this – but my experience is that a surprising number of places and people don’t. How many coffee shops will tell you they use the rule of thirds for cappuccinos? A lot. How many of those serve 6oz cups, with 2oz shots and milk that is stretched to a 50/50 foam/milk? None I’ve ever come across. I’m not saying whether that recipe would be good or not, or even whether the so called rule of thirds is useful for customers or not – but it’s certainly not an accurate recipe to what these shops serve.

Now the point of this post was that this is just one obvious example of a common problem. There are plenty more out there; what constitutes “a shot”? how much water do you put in to make it an Americano? what is your “Medium” roast? We have disagreements, debates… customers feel disappointed, mislead, mis-sold… and we could fix it by just spending a few seconds to think about what we actually mean before we say something. So my request for everyone – consider all these words you use within your coffee making and think “how would I explain it to someone else?”

Talking about flavour

I was reminded that I haven’t blogged anything in a while. So here’s one that I’ve been mulling over for nearly a year now – ever since In My Mug 68 actually.

This In My Mug episode was filmed at a small coffee shop in Norwich – The Window – and features the owner, Hayley, along with the inimitable Steve Leighton. At 12minutes in, Steve and Hayley are discussing the two coffees for that week – have a watch, and listen to the descriptors they use.

What’s really interesting to me is how they use different types of words to describe the coffees. Steve uses flavour names (lychee, grape, thick mouthfeel, etc), whereas Hayley uses emotive descriptors (summer, sitting by the fire, etc.).

Flavour is an emotive thing as much as a sensory thing – Steve points out in a recent Audioboo on tasting that he isn’t just associating a coffee flavour with an abstract notion of another flavour, but a specific, personal sense of that flavour.

I’m from a fairly scientific background, so it’s not surprising I find it easiest to describe coffee in flavour language – but that can allow me to overlook the emotional framing of the language I use. When I say a coffee is bright, I don’t just mean it has an acidic taste – I’m also talking about an emotional response of being refreshed.

At the same time, the language we use impacts our tasting experience. If Steve describes a coffee as like caramel, a particular caramel jumps to my mind – that used for “caramel cup” chocolates actually. When I taste the coffee, that’s what I look for (not always consciously of course). I might find it, I might not, or I might find something else – toffee, or a different “caramel”, or burnt sugar. But the language gives me cues that can point me in a particular direction before I’ve even smelt the coffee, let alone tasted it.

So I’ve been mulling this over, and it’s been very interesting, so now I’ll see if I can convert this into something practical. My focus when I’m next tasting coffees will be trying out emotive descriptors. This should have two uses – to help expand my tasting experience and ability, and also to improve how I can communicate my enthusiasm for coffee to others.