In defence of coffee science

At Artisan, we’ve all spent hundreds of hours practicing how to make a cup of coffee. It’s not just that we’re obtuse. To understand why it takes us so long to get a decent brew, you need to understand four things about coffee: concentration, extraction, brew ratio and temperature. Looking at these aspects of a drink might seem overly fastidious (verging on wanky) but, for those of us with simple passions, they are highly necessary.

I dislike coffee-wank. It’s probably something about being Kiwi that makes me naturally repel from florid descriptions or empty taglines. I love tongue-in-cheek send ups of people who take themselves too seriously and even enjoy having people do the same to me. Because, I to, wallow in coffee a little too much for it to be cool. Nevertheless, I distinguish between those adherents to the over-technical aspects of coffee and those who use their technical knowledge to further their passion for coffee.

We opened a ‘pop-up’ shop in Bruntsfield, Edinburgh for the month of August and when composing the menu I opted for two black and two white coffees: short black, long black, piccolo, flat white. I would have gone for short white instead of piccolo and long white for flat white, but didn’t want to overly confuse our new customers. As I subsequently found out, the glorious people of Prufrock had already stripped the coffee menu down with the following menu: espresso; espresso with milk 4Oz, 6Oz, 8Oz. Beautiful in its simplicity and describing exactly what each drink is without resorting to romantic, but woolly, names such as cappuccino, macchiato, latte. Names that were invented in another time for a very different coffee culture and drunk by people with very different tastes from my own.

Taste is very important to me. I love the tastes, textures and flavours of the world’s cooking. Our flavour sensing apparatus are amazing and develop the more we use them. Many degenerates don’t. They go through life treating food like a fuel. I feel they must view all the flavours, in which I revel, as superfluous to the catabolic action, the unfortunate necessary in order to power them through the drudgery of their mundane lives. They miss out on the multi-modal sensual experience of taste, texture and flavour. I imagine them looking out at the world and seeing not the vivid shades of green in each tree, the multi-layered depth of the scene with intense movement, rather reduced image size, desaturate, flatten layers, save for dial-up web.

A bit of passion is a good thing. It helps us to know we exist. Truly exist in the moment. To truly kiss the woman, soak up the sauce with the bread, drink the bottle to the last drop, varnish the newly carved wood, to extract the best coffee and pour the best milk possible with the beans I have been given and the equipment that I am using for this customer right here and now.

So, here’s why coffee is not ‘just coffee’, why not all beans are the same and not all baristas are trained equally: Coffee has taste, flavour and texture and the possible variations of these extend to well beyond any other foodstuff that we consume. The majority of coffee prepared in the world lacks subtlty and vitality because the people working with it don’t appreciate the possibilities. And here’s why:


Left, the whole bean. Right, the solutes that taste good.

30% of the coffee is soluble i.e. we can extract up to 30% of the coffee from the coffee beans. However, the resulting brew will taste very bitter. The taste compounds in coffee, between 92℃-96℃[1], dissolve over time in this order: sour, sweet, bittersweet, bitter. What we want is a balance between sour, sweetness and bitterness. In terms of extraction, this equates to around 18-22%. Extraction can be measured using a refractometer. This tool allows us to speed up the process of trial and correction to adapt our recipe. Even with this tool, we need to taste the coffee, decide which taste compounds are lacking and adjust the time of extraction accordingly.


Concentration is a measure of the ratio of coffee to water i.e. the strength. The difference between a weak and a strong coffee is very subtle. A weak cup of coffee is around 98.8% water and 1.2% coffee: a strong cup would be 98.5% water and 1.5% coffee. Another term for strength is Total Dissolved Solids (TDS). This is the dissolved coffee as a percentage of the entire volume of coffee and is the same as the percentage above.

Total Dissolved Solids

Figure 1 Left: coffee beans for one brew. Middle: spent grounds. Right: dissolved solids. To obtain the dissolved solids, I brewed the coffee and then evaporated the water.

Brewing ratio

Don’t confuse the strength ratio with brewing ratio. The strength ratio is the concentration of the resultant coffee: the brewing ratio is the weight of beans to weight of water used to brew the coffee. In general a brewing ratio of 1:17 is a good starting point for brewing coffee at normal pressure. For espresso at 9 bar, a brewing ratio of 1:2 is a good place to start.


Different compound dissolve at different rates. Most organic compounds become more

N.B. These salts are not specific to coffee. However, they indicate the dissimilar solubility curves of inorganic compounds.

soluble at higher temperatures, but not all. Many inorganic compounds have solubility curves that are far from straight (see the graph). The important point, is to note that we can change which compounds we extract by using water at different temperatures and that we need to ensure that the temperature is always the same if we want repeatable results.

Consistent coffee

It is very difficult to get an even extraction from each particle of ground coffee. It is quite common to extract more from some coffee grounds than others. This is especially the case with espresso, because of the pressure involved and v60 preparation. Without careful attention to technique we cannot be sure that our extraction is even, making the resultant flavour uncontrollable. The brewing technique should be perfected before we attempt to control flavour using our understanding of concentration, brewing ratio, temperature and extraction. The website has some excellent methods for brewing coffee. I have my own favourites, which I shall later detail and explain why they are my preferred methods.

To get great consistent coffee you have to prepare the same coffee in the same manner with the same recipe, until some variable changes. Each coffee, roast, humidity level, ambient temperature and day will necessitate a slightly different recipe in order to get a consistent taste. The way we develop that recipe is not magical: it’s through precision testing and adjusting. We start with the standard brew ratio and temperature and taste the coffee. Then change one of the variables according to what flavours you want to change.

Eg. The v60 model

1.       When roasting for v60 we use beans that have the potential to create sweet, juicy, acidic brews. The beans should have some interesting aroma characteristics and these are the delicate volatiles that we want to conserve throughout the roasting and preparation process.
2.       Weigh 11.8g of coffee beans.
3.       Wash out the filter paper and warm the v60 cone thoroughly with hot water.
4.       Grind the coffee quite finely (exactly which grind is dependent on previous tests) around the paper filter mark (4.5-5 on a ditting). We use grinders that give a very uniform particle size and as few fines[2] as possible. The burrs on the blades shear the beans into particles of a narrow range. Each particle comprises of many intact cells as well as ruptured cells. This exposes the oils, which contain most of the potential aromatics. Reweigh the grinds to check that you haven’t lost any in the grinder. Zero the scales.
5.       Start your countdown timer. Take water that has just boiled and pour 2g of water for every gram of coffee quickly into the centre of the grinds. The temperature of the water cools as it falls to the coffee. As it hits the coffee it will be around 91-94ºC. The hot water displaces the gases around the coffee grinds and CO2 gas inside the bean cells rapidly expands as it heats up and escapes through the semi-permeable membranes of the cell walls. This bubbles through the slurry causing turbulence and hoisting some of the grinds out of the slurry on top of the bloom. Make sure that all of the grounds are wet then wait to ensure that all of the CO2 gas has escaped.
6.       While waiting, refill the pourer with boiling water. After around 35 seconds, pour out water to make 200g in slow spiralling circles starting in the middle and working your way out taking care to stay away from the sides aiming to finish pouring at around 2mins. Remove the cup at 2 minutes 30 seconds.
7.      Wait 2 minutes and taste the cup. If you want a more acidic cup, one option is to shorten the time to brew or increase the brew ratio. If you want a more bitter cup an option is to lengthen the brew time.
8.      Change one variable and brew again.

What’s in the cup?

Coffee is a complex chemical mixture reported to contain more than a thousand different compounds including carbohydrates, lipids, nitrogenous compounds, vitamins, minerals, alkaloids, and phenolic compounds[3]. Of these compounds some are soluble and some insoluble. Most of the insoluble particles remain in the coffee bed, however the very finely ground particles can get through the filter and will contribute to the texture of the drink.

The compounds that are soluble may contribute to taste. They can be divided into nonvolatile compounds (caffeine, trigolein, chlorogenic acid, phenolics, amino acids, carbohydrates, and minerals) and volatile compounds (organic acids, aldehydes, ketones, esters, amines)[3]. Hot water extractable polysaccharides are the main high molecular weight components of coffee infusions and play an important role in the viscosity of the brew, in the foam stability of espresso coffee, and hence in the retention of volatile substances[4]. They are soluble at different rates and so how we prepare the coffee will determine which compounds end up in the cup.

The first compounds to come out of the cone are highly aromatic. These are stored predominantly in the lipids that cling to the cell walls. They are the first compounds to diffuse into the water and (along with the undissolved fines) provide texture. Sour compounds then quickly dissolve and come out over the first few seconds of drip. After around ten seconds (water temp and grind dependent) the sweeter compounds dissolve and start to come out. Later, the less soluble bittersweet compounds dissolve into the liquid. After a certain point the remaining liquid is insipid. As with espresso extraction, it is important to stop the pour when the taste is at its best. This can only be done through precise repeatable experimentation.

Feel vs science vs geek

I attended a photoshoot once where a model sat with frozen smile awaiting instruction from us, the people come to practise shooting in a studio situation. What struck me most about the session was the remarkable concentration on f-stops and apertures on the part of the male photographers and the lack of interaction with their subject. For these men it was all about the resolution, the lighting, the lens, the kit. When the first woman came to take a photo, she not only asked the model to sit in a new position, but also asked her about her day, about her other work and where she was from. I never saw the photos. I’m not sure who took the better shots, but I felt that the female photographer was focussing on the most important thing; the subject. That is not to say that the kit isn’t important: it is. The lighting, the lenses and f-stops were the tools she used to get the best out of the subject and she needed a great understanding of what she was using in order to take good photos, but the focus was on the model.

The same needs to be the case with coffee. It’s not possible to get great tasting coffee repeatably by love and intuition. But it’s also no good focussing just on the equipment. The most important thing is the taste of the coffee. The science is the method for how we get there and enables us to be able to repeat the result.

[1] I have only tested the extraction by taste within this temperature range.
[2] Coffee particles ground finer than a coffee cell.
[3] Spiller, M A. The chemical components of coffee. In Caffeine; Spiller, G. S., Ed.; CRC Press: Boca Raton, FL, 1998; pp 97−161.
[4] Oosterveld, A.; Harmsen, J. S.; Voragen, A. G. J.; Schols, H. A. Extraction and characterization of polysaccharides from green and roasted Coffea arabica beans. Carbohydr. Polym. 2002, 52, 285−296.
[5] Nunes, F. M.; Coimbra, M. A. Influence of polysaccharides in foam stability of espresso coffee. Carbohydr. Polym. 1998, 37, 283−285.

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