The Curious Barista's Guide to Coffee

by Tristan Stephenson

  Water as a solvent is slightly tougher to get your head around, but no less impactful in respect to cup quality. With the exception of water that has been completely demineralized, all water contains some dissolved particles and we broadly refer to this as the water’s hardness; more specifically, it’s calcium carbonate, also known as limescale, that is responsible. Just to give you an idea of the difference in water’s hardness according to the area in which you live, I grew up in Cornwall in the rural South West of the UK, where the tap water was very soft, comprising around 70 parts per million (ppm) of total dissolved solids (TDS). I later moved to London, where the tap water is very hard – containing around 400 ppm TDS. You can buy an inexpensive TDS meter to measure your own, but TDS only tells you how much stuff is lurking in the water, not the specific chemical breakdown. Your local water supply should be able to provide with you that information, however. Common minerals and compounds found in hard water are magnesium, fluoride, sodium, chlorine and a whole array of mineral salts. Some levels of these are acceptable, while others should be tackled with a zero tolerance policy (see the table below).

  The combined efforts of a digital refractometer and smartphone is like having a pocket-sized coffee laboratory.

  Very hard water (high TDS) seems to make a poor solvent and is therefore harder to brew with. This is probably because it already contains more soluble content and is that little bit closer to its saturation point. As a result it extracts less – or less of the desirable stuff at least – from the coffee. This would lead one to think that demineralized water makes the best brewing water, but weirdly (and I have yet to find an explanation for this), it would appear that some level of TDS (above 70 ppm, ideally) is desirable.

  The SCAA (Speciality Coffee Association of America) recommends the following acceptable range for brewing water:

  Total Chlorine 0 mg/litre

  TDS 75–250 mg/litre

  Calcium Hardness 17–85 mg/litre

  Total Alkalinity At or near 40 mg/litre

  pH 6.5–7.5

  Sodium At or near 10 mg/litre

  WATER TEMPERATURE

  The best temperature for brewing coffee is between 90–95°C/195–205°F; the opposite ends of this scale do produce a different style of cup. Water temperature is perhaps an area of coffee geekery that has become overstated. Top-of-the-range espresso machines are now – supposedly at least – capable of adjusting the temperature of water by one-tenth of a degree, which is almost as impressive as it is over-engineered for the purpose.

  It is generally accepted that darker roasts prefer the lower end of this scale, and lighter roasts prefer the higher end, which is largely to do with the higher-density (lighter roasted) bean requiring a little more coercion to give up the goods. But in an instance where a cup is not tasting quite right, I wouldn’t advise anyone to tinker too much with the water temperature until all other avenues – grind size, brew ratio, brew time, water quality and coffee quality – have been explored.

  To get the right temperature at home, I would advise boiling the water and leaving it to sit for two minutes, or simply adding a splash of cold water before pouring. If you have one, it’s worth using a digital thermometer to check exactly if your water sits in the correct range.

  EXTRACTION, STRENGTH AND BREW RATIO

  Getting a handle on extraction is the key to success in every single type of coffee brewing. Badly managed extraction is the most common cause of overly bitter, sour, weak, astringent and insipid coffee. Good extraction is the source of all that is delicious in a cup of coffee; through the clarity of flavour it provides, flavour markers identifying variety, origin, processing and roast-style become evident in the finished cup. The physical extraction of the coffee gives rise to all manner of flavourgiving soluble solids and gases, along with insoluble particles of coffee fibre, proteins and oils that can contribute to the body and texture of coffee. Extraction is, not to put too fine a point on it, the singularly most important goal in coffee brewing and the common cause around which all brewing variables fight towards.

  Secondary to extraction, but still of great significance, is the strength. This can be a confusing term, not least of all because we’re used to seeing strength guides on jars of instant coffee, which are really trying to communicate how well-pronounced the darker elements of the coffee are. Put simply, strength is the percentage of your cup that isn’t water. It might seem obvious, but mixing the appropriate amount of coffee and water (known as the brew ratio) will be the defining factor in how intense or subtle your coffee tastes, and this is one area that comes down to personal taste.

  VARIABLES OF EXTRACTION

  Good extraction can be achieved in a range of different brew ratios. But to do this a myriad of other factors must be considered.

  Without doubt, the two factors with the most clout are the grind size and the contact time. Decrease the grind size and the surface area of the coffee increases, which increases the rate of extraction; drag out the contact time and the extraction will increase, too. Balancing these two variables, whatever the brewing method, will garner you a good footing on the path to a balanced and tasty cup. Playing with grind, dose and contact time is also a big part of the beauty of coffee brewing – discovering the unknown and creating something that has never been tasted before. For example, there is no set-in-stone way to brew an Aeropress; as long as the grind size is appropriate to the brew time, there are an infinite number of doses that will make for delicious results. Within the subject of contact we must discuss the topic of percolated coffee, or coffee made from brew water that is poured through a bed of ground beans and left to drip through a filter, which includes such brewing methods as espresso (see pages 92–95), filter (page 124–133) and to some extent siphon brewing (page 138–141) and the moka pot (page 117–119). In this scenario, we find we find that brew ratio, grind size and brew time are all relative to one another, since the coffee acts as a barrier of resistance against the flow of liquid.

  The next factor that plays an important role is brewing temperature. As with most things in the universe, the higher the temperature, the faster the reaction that takes place. In the case of coffee, this means quicker extraction at higher temperatures. There is also the matter of extraction threshold for certain compounds; caffeine, for example, extracts even at very low temperatures (under 30°C/86°F), but certain astringent compounds will only extract at near 100°C/212°F, which is why we avoid using water above 95°C/205°F.

  There are other factors, too. The degree of agitation or ‘turbulence’ in the brewing water also affects the rate of extraction, which itself is connected to the pressure of the water, which in the case of espresso, forces itself in and out of the porous structure of the fine grounds. If you’ve ever needed to brew a cup of tea quickly you’d have noticed that a quick stir darkens the brew much faster than simply allowing the tea bag to hang around in the hot water. In coffee brewing a quick stir or slow agitation during brewing, or a specific ritual for pouring water and wetting grounds, can yield different results by increasing the efficiency of the brewing process. Some baristas hold their pouring technique – the stages in which water is added and the pattern in which it is poured – in almost supernatural regard. And since water can be poured in an infinite number of ways, this becomes yet another element of coffee brewing that proves difficult to communicate.

  Coffee freshness can also distort an extraction, where exceptionally fresh coffee releases lots of carbon dioxide, which behaves like a force field, preventing proper contact between coffee and water. For this reason, coffee intended for espresso brewing must be rested for at least five days before brewing; otherwise the forces exerted on the coffee inside the filter basket make for a battle between gas and water where coffee is the ultimate loser. Indeed, all coffee must be rested for eight hours before it can be ground and brewed, lest the carbon dioxide upset the brewing process (see pages 60–61).

  BREWING CONTROL CHART

  This chart depicts the
sweet spot between coffee strength and coffee extraction for non-espresso brewing methods. The vertical axis shows the strength or TDS (total dissolved solids) of the drink, expressed as the percentage that is actual coffee. The horizontal axis shows the extraction (yield) expressed as a percentage of the original dry coffee dose that has been removed by the brewing water and landed in the cup. Further down the chart, the strength of the drink decreases and at the top it is higher. For some, it may be a drink that sits closer to one of these areas that is the better one, as strength is, to some extent, a matter of personal taste. The brew ratio directly impacts beverage strength. Perfection is grey box in the centre, where the strength of the drink and the extraction percentage balance correctly (18–22% extraction and 1.1–1.3% strength) and coffee is sweeter, richer and more transparent. The diagonal grey line represents the rate of extraction and illustrates the ease in which a brew can turn out weak and underdeveloped, or strong and bitter.

  In espresso brewing, the colour and persistency of the crema on top of a shot can give some clues about how successfully the coffee has been extracted.

  MEASURING EXTRACTION

  Extraction is best communicated as a yield percentage. What this means is the percentage of coffee (by weight) that has made its way into the brewed cup from its original dry state. It tells us how efficient the extraction process was and gives us an idea of how balanced the cup might be. The rules of extraction apply to all brewing methods, and interestingly, even though an espresso is much stronger than a filter coffee, both brewing methods should achieve about the same yield percentage; an espresso just has less water in it.

  For some 50 years now, studies have been carried out to discover what percentage of the coffee used to brew with is desirable in the finished cup. Approximately one-third of the dry weight of coffee is water-soluble: i.e. it can be extracted. However, not all solubles are desirable and it’s generally agreed upon by coffee professionals that an acceptable level of extraction yield sits somewhere between 18 and 22 per cent, which equates to just over half of all the solubles that are up for grabs. The extremities of this boundary define when a coffee starts to become underextracted or overextracted. In underextraction (below 18 per cent), the coffee is typically sour, thin, weak and flat. In overextraction (above 22 per cent), the coffee becomes bitter, ashy and astringent. I might add that this narrow margin for error was not simply dreamed up, but is based on the sensory analysis of hundreds of brewed coffee samples compared by coffee research panels. The rules can be bent, though, as often it will be the specific coffee and its style of roast that will dictate the best yield in the cup. For example, some coffees can excel around 16 per cent, which brings out bright, clean, fruity characteristics that would otherwise be expressed as jammy, sweetshop notes if brewed around the 20 per cent mark. In the past, it has been suggested that coffee peaks at two distinct yields: first at around 16 per cent, then later around the broader 20 per cent mark, and that 17–18 per cent should be avoided. It’s not understood why tastiness drops off between the two peaks, but I compare it to the Manhattan cocktail, which is delicious both dry and sweet, but tastes bland anywhere in-between. Sometimes we just like things one way or the other.

  You can work the yield percentage out at home by weighing your coffee before brewing (which should be standard operating procedure anyway), then drying the spent coffee after brewing in a low temperature oven and weighing it again. Calculate the difference between the two samples, then divide it by the weight of the original coffee dose and you will have your percentage expressed as a decimal. Of course it’ll take a few hours and it’s not a particularly accurate way of doing things, so if you have a bit of money to spend, invest in a coffee refractometer (the one manufactured by VST Labs is a good option). A refractometer takes a digital reading of the brewed coffee by bouncing light through a 2 ml/1/16 fl. oz sample of cooled coffee. It’s then able to determine the total dissolved solids (TDS) in the cup and, using a conversion chart, determine the solubles’ yield from the coffee. Clever and useful stuff, but it’ll cost you about £500/$782 for the privilege.

  BREW RATIO

  The other important factor to monitor during brewing is brew ratio. This number indicates the likely strength of the final cup by expressing the weight of the coffee compared to the weight of the drink. The brew ratio of an espresso usually sits at around 1.5:1, meaning that the drink is 1.5 times the weight of the coffee used to make it. This could be a 20 g/¾ oz dose of coffee that is used to make a 30 g/1 oz espresso. A typical French press brew is more likely to be 15:1, where 20 g/¾ oz coffee is used to make a 300 ml/10 fl. oz beverage. Remember that these calculations must be made using the beverage weight, not the weight of brew water used, on account of the coffee holding back some of the water through absorption – approximately 2 g/1/16 oz per 1 g/1/32 oz coffee.

  You might be wondering where taste comes into all of this. It’s easy to let graphs and formulas get in the way of the good old human tongue and nose, which constitute not only a far more sensitive system than any digital refractometer, but are also the ultimate deciding factor in whether a cup is deemed good or not. Let your mouth tell you what tastes good, and when it does, record everything you can that got you there. Much of this seems like measuring art by using science, but mastering the variables at the highest level only becomes a realistic proposition when these things are approached with respect for both the art and the science involved.

  CUPPING COFFEE

  Coffee cupping is a tasting practice that is used to systematically evaluate a selection of coffee samples. It is customary for cupping to take place on a long table, decked out with 6–8 cupping bowls, in which various coffee grounds and water will be smelled and tasted in a quiet, ritualistic fashion.

  Cupping is a very useful tool for rapidly comparing coffees from different farms, varieties, processing methods, and for comparing different roast profiles on the same coffee, and is an invaluable practice for coffee roasters. Even though every cupping table has its own method of doing things, it has become more or less standardized across the industry, with the layout of the table and etiquette engineered to remove as many variables from the coffee brewing and tasting process as possible, so that each bowl of coffee is sampled in the same way.

  The only equipment needed to conduct coffee cupping is hot water, a grinder, cupping bowls (or just a wide-rimmed cup, but bear in mind its size as you’ll need to plan to use 1 g/1/32 oz of coffee for every 17 ml/3/5 fl. oz of capacity) and a cupping spoon (or any large round spoon).

  Each cup has the appropriate dose of coarse filter-ground coffee added it to, and guests are invited to smell the dry coffee. Next the water is poured on. I prefer not to use scales for this part, as it is a bit fiddly and the trick is to get the water on top of all the samples as quickly as possible, then get your nose in there. Fill to the top of each cup, start a timer, then take in a good breath through your nose.

  After taking time to nose all of the samples, it is time for the ‘break’. This is usually done around four minutes after the initial pour, mirroring the parameters of a French press brew. Cuppers use the bowl of their spoon to fracture the floating raft of coffee crust that gathers on the surface, releasing aroma from the water below. More smelling ensues, followed by a period of reflection and note-taking, which is then followed by the scooping of the grounds, foam and other crud from the surface of the cup. Then, after nearly ten minutes, the coffee is tasted, an event marked by a cacophony of loud and awkward slurping noises. The purpose of slurping is to introduce as much air in to your mouth as possible, which helps promote proper aroma dispersion. In this practice, cuppers dip their spoons into the cup and scoop up a small sample of the coffee, then move it to their lips and loudly inhale.

  Tasting typically goes on for another ten minutes, allowing cuppers to sample the coffee at a range of temperatures. Tasting cold coffee can also uncover defects in both the bean and the roast that may not have been apparent when the coffee was hot. />
  A typical cupping table, littered with spoons, beans, notes, and, of course, cups of coffee.

  05

  GRINDING

  INTRODUCTION

  In the most basic sense, grinders take coffee beans and break them up into smaller pieces. Coffee cannot be brewed as whole beans, and the increase in surface area provided by smaller particles allows better access to the inner sanctum of the bean’s porous structure. The smaller you go, the more the bean’s surface is exposed, which means flavour is extracted faster. Put simply, if you grind finer, the brewing time is decreased; if you grind coarser, brewing time increases.

  The grinding of coffee also marks a significant point of no return in the brewing process as, once ground, the coffee is more exposed and vulnerable to the effects of oxidation, and so it remains potent only for a brief spell. If you want to make better coffee at home, probably the best piece of advice that I can give you is to grind it fresh; it is no exaggeration to suggest that doing so will produce a dramatically improved drink when compared with a cup made from pre-ground beans.

  Taking this a step further, I would advise you buy the best grinder that you can afford. A good grinder will last years, require less in the way of tweaking and adjustment, and consistently produce better-tasting drinks. You see, chopping up coffee beans might seem like only a minor part of making a tasty beverage, a basic but necessary step before the real skill of brewing comes into play, but shoddy work at this early stage of coffee’s precarious journey has big consequences later down the line.