The Curious Barista's Guide to Coffee

by Tristan Stephenson

  Female volunteers roasting coffee at Cadby Hall for Joseph Lyons and Co. in London’s Hammersmith during World War I.

  Today, it’s German-made roasters that continue to lead the way in drum roasting, with Probat, which took over Jabez Burns in 1998 in assuming the role of the most highly revered name in coffee-roasting circles – a hard-earned status that awards the company a large chunk of the speciality coffee pie. Two Turkish roasters, Toper and Garanti (both founded in the 1950s), continue to produce goodquality machines, as well as Dutch roaster manufacturer Giesen. Exciting things are coming out of America, too, from the well-respected names of Diedrich and Loring, the former of which manufactures beautiful traditional-style drums, and the latter, which has become well known for its ‘Smart Roast’ automated system.

  This modern drum roaster is controlled by computer software, capable of producing highly consistent results.

  HOW ROASTERS WORK

  Modern coffee roasters can take anywhere from 7–20 minutes to get the job done and can be conducted in a whole manner of ways, from a humble 250 g/9 oz. counter-top roaster to a computerized million-dollar behemoth. The goal is the same: develop flavour and aroma through the application of heat.

  Pulling apart the process a little more, roasting is broadly a three-stage procedure, with the first two stages overlapping somewhat. First, the beans are loaded into the roaster, where the coffee beans dry, as water migrates from inside the bean to the surface. This stage is not particularly impactful from a visual standpoint, but it is detectable through a grassy, warm-hay aroma that the steam gives off. This leads on to stage two, the proper roasting, where complex caramelization and browning reactions that give us the familiar coffee flavour. The coffee bean becomes less dense, drier, and more brittle during roasting – factors that combine to make it more porous and its components more soluble. Finally, when the roast is deemed to be complete, the beans are dropped out and stage three, cooling and outgassing, ensues.

  TYPES OF HEAT TRANSFER

  Almost all coffee roasters cook beans through a combination of different types of heat transfer: conduction, convection and radiation.

  Heating by conduction is very simple to understand. It is the transfer of energy that takes place when a hot, solid object makes contact with a cooler solid object: for example, when you pick up a cup of hot coffee and your hand warms up. The same thing happens during the roasting of coffee beans when cool beans touch the hot surface of the roaster, and also when a slightly hotter bean touches a slightly cooler bean.

  Conduction shouldn’t be mistaken for radiant heat, however, which is where the hot surface of the roaster, or even a coffee bean, emits infrared radiation and heats things nearby, but not through direct contact with it – the radiator in a house works like this.

  Convection occurs where a liquid or gaseous heat source – in the case of a coffee roaster this is the air – moves as a continuous current, rapidly transferring energy as it goes. A convection (or fan) oven works in this way, which is why it cooks faster even at lower temperatures than a traditional gas oven.

  Heat transfer by convection is the preferred method of cooking for coffee roasters, because it requires lower temperatures and less energy, yet rather conveniently, convection is also quicker. But when all is said and done, coffee beans cook mostly by conduction, since once convective heat rapidly permeates the outer layer of the coffee, it can then only transfer through the static structure of the bean through conduction from one solid layer to another. Convection speeds up this process.

  THE DRUM ROASTER

  The drum roaster is still the roaster of choice for a great deal of the global coffee industry, and in particular it remains the darling of the speciality coffee industry, now taking pride of place in many of the world’s best cafés. Its timeless tumble-dryer design, range of available sizes, and simple, yet highly customizable roasting process, have firmly secured its position in some of the world best roasteries.

  Classic examples of drum roasters comprise a rotating drum made of solid steel or iron, usually encased within a slightly larger drum and mounted on a horizontal axis. Single-drum varieties also exist, but the convection and radiation of heat between the two skins of a double-drum roaster are thought to result in a more even cook. Traditionally, the cylinder is heated from below by a gas flame, and somewhere above the drum is an exhaust that draws up smoke, steam and any other roast by-products. Beans are loaded in by way of a hopper (the part of the machine that stores the beans before they are loaded) on top of the drum and, once finished, are ejected out of the front of the machine into a wide cooling tray. A fan draws air downwards, through the roasted beans, while rotating bristled arms stir them around.

  Indirectly heated drum roasters pump hot air into the roasting drum from an external heat source. This means the drum temperature is much cooler than if it were directly heated and, crucially, the air temperature of the roaster can be that little bit higher to compensate. The natural progression from this is the economically sound ‘recirculating drum roaster,’ which recycles some of the hot exhaust air back into the roasting chamber. This kind of roaster is thought to provide a more stable roasting environment, thanks to higher humidity in the airflow, which in turn curtails the menace of bean surface burning.

  FLUID-BED ROASTERS

  Fluid-bed roasters are a relatively new invention, first trialled by the late Michael Sivetz of Corvallis, in Oregon, who passed away in March 2012. Sivetz patented the first fluid bed roaster in 1976, with the motto ‘keep beans moving’, which it has to be said, is exactly what these contraptions do.

  Because fluid-bed roasters work on the principle of balancing bean mass, temperature and airflow, they are even more sensitive to roast-batch size than classic drum roasters. If the load is too small, the beans tend to bounce around in chaotic waves, interfering with currents and leading to an uneven roast. If the load is too large, the upward air pressure from the blower is insufficient to propel the beans into a fluid motion, and the lower layers cook like the base of a pizza.

  Since these machines come in both manual and automatic flavours, the airflow on manual models needs to be dialled down through the course of the roast as the coffee beans become lighter. The upside of this kind of roasting is that almost all of the heat transfer to the surface of the bean is by convection in the fast-moving air currents, which makes the process faster. Much faster in fact, with some light roasts lasting only 3–4 minutes.

  Fluid-bed roasters also have little or no moving parts, which reduces the risk of malfunction. The downside is that fluid-bed roasters capable of even 5-kg/11-lb batch sizes are scarce, and even then require huge amounts of energy to keep the beans moving around.

  OTHER TYPES OF ROASTER

  Centrifugal coffee roasters are generally only seen in large scale applications, where their gigantic capacity and quick roasting time mean that some models can churn out up to 4,000 kg/8,800 lbs an hour! As the name suggests, they comprise a huge dish that spins on a vertical axis, a bit like the teacup fairground ride. Heat is typically fired down in the centre of the dish as the spinning action flings the mass of the coffee towards the curved outside edge. This centrifugal force shoots the beans up the side of the machine, then numerous small fins split them into uniform streams that direct the flow back into the centre of the roaster to repeat the process. The effect is like that of a doughnutshaped maelstrom of rolling coffee beans, and both the speed they move, along with the rate of airflow, mean that a roast can be complete in only 5–6 minutes.

  Not content with loading and unloading your roaster with 500 kg/1,100 lbs at a time? Well, if money is no object, perhaps a continuous coffee roaster is for you. Like the steam-punk-inspired elongated drum roaster, these monsters can accept a non-stop flow of green coffee beans, which travel along the length of the drum like a journey into the fiery pits of hell, roasting as they go. As you might expect, the roast profile is controlled by a computer, which adjusts airflow, air temperature and the
drum speed to tailor the end product.

  Tangential roasters are, upon first glance of their inner workings, quite similar to drum roasters. However, these massive enclosed boxes are a little more clever when it comes to airflow than your traditional drum. Beans are rotated on a horizontal axis and hot air is channelled downward (at a tangent to the drum) allowing it to whip through the bean mass and exit out of the top of the roaster. The advantage of this kind of roasting is the clean fluidity of hot air currents, which improves the rate of heat conduction from the bean’s surface to the bean’s interior. Technology like this is advantageous to everyone, but especially those roasters buying lower-grade beans of a non-uniform size and density – which of course tends to be those with a need for a higher rate of production.

  Many of these large-scale roasters have fought to overcome the issue of cooling half a ton of hot, roasted coffee. Some have ‘quenching’ systems, where a fine mist of water is sprayed onto the bean immediately after roasting; this doesn’t actually ‘wet’ the bean as such, but causes it to cool as the water draws latent heat from the bean. Studies suggest that it speeds up the outgassing, too. Quenching systems are usually coupled with more traditional (and sometimes not so traditional) air cooling. Cutting-edge designs feature powerful air turbines that blast air at a sufficient speed to levitate hundreds of kilograms of roasted beans in an incessant storm of super-cooled air.

  A drum roaster’s cooling tray, where hundreds of litres of cool air are drawn through the mass of hot beans. Stopping the cooking process quickly is an essential component of a good roast.

  For the remainder of this chapter, all references to roasters in general, unless otherwise specified, will relate to the drum roaster, since it is by far and away the most popular design used in speciality coffee today.

  WHAT HAPPENS WHEN COFFEE IS ROASTED

  Green beans don’t taste of much and they’re tricky to grind down into a powder. If you get a good enough blender (don’t risk breaking your coffee grinder) and brew them into a tea (since it really cannot be called coffee, and you’ll find something thin, faintly acidic, grassy and insipid. There, I’ve spared you the bother.

  It is roasting that unlocks all of the hidden treasures that the green bean guards within its highly organized and densely packed stores of fats, acids and sugars. The coffee oils that will be later converted into flavoursome molecules are at this time tightly packed against the cell walls of the plant, and it is only through roasting that the true soluble character of the coffee bean can be discovered. But ‘true’ is perhaps the wrong word, as there is no set-in-stone destination for a coffee bean on the journey to roasted glory.

  The roaster’s primary goal is to cook the coffee to the required level of doneness, tailored to the specific coffee and its intended use once roasted. Lighter roasts tend to exhibit more of the coffee’s natural character (good or bad) and lend themselves well to more traditional brewing methods. Darker roasts will replace much of the coffee’s natural character traits with the brown roast character that we are all so familiar with – and it’s for this reason that poor-quality coffee is almost exclusively dark-roasted. Darker roasts find themselves at home in espresso, however, where the nature of the brewer dictates that lighter coffees become overwhelmingly acidic.

  The secondary goal of the roaster, which, in truth, is every bit as important as the primary goal, is that of carefully controlled coffee bean development. In a highly generalized sense, if the roast takes too long and/or is too cool, the coffee will exhibit a weaker, slightly ‘baked’ character when brewed; if roasted too quickly and/or too hot, then there’s a risk that the interior of the bean will be underdeveloped and the resulting coffee will tend towards spiky, sour, bitter or smoky characteristics.

  There is no tertiary goal, only the pursuit of deliciousness. We will cover some more roasting techniques in detail later, but let’s first take an indepth look at what happens to the coffee bean during a typical roast.

  THE ROASTING PROCESS

  All drum roasters have an optimum batch size of beans that they can handle. Overload the roaster and you may find that proceedings slow down with detrimental effect; load too few beans in and you risk surface burns on the beans as they slide around, deprived of the tumbling action that larger numbers grant them. The roaster will aim to heat the coffee as quickly as possible in the early stages of the roast, and it is for this reason that the roaster is always pre-heated before the beans are dropped in.

  As roasting rapidly warms the bean, moisture begins to migrate towards the surface of the bean and evaporate away. The rate at which this happens is dependent on temperature, bean mass, bean density and airflow, but under normal roaster conditions the water content of the bean will typically drop from 11 per cent to 2 per cent in around 5–7 minutes. At the earlier stages of the roast, the bean sometimes becomes slightly paler, or more milky-looking, before turning a more orange and eventually cinnamon colour in the later stages. This early sector is known commonly as the ‘drying’ phase.

  A handful of green coffee beans: pretty, patient and, alas, largely flavourless.

  There’s more to this stage than just drying, however, as important steps are also occurring in the development of sweetness, acidity and bitterness (arising from the breakdown of chlorogenic acids and sugar), which will make themselves known in the flavour of the resulting cup. This part of the roast also sees the start of another set of chemical changes: Maillard reactions.

  Maillard reactions are browning effects caused by interactions between amino acids and sugars. They occur at all temperatures, but much more rapidly when heat is applied, and especially above 150ºC/300°F. These reactions are brain-achingly complex in their nature, but the most important thing to understand is that they are largely responsible for the flavours that we associate with such delectable foodstuffs as browned meat, baked bread and toasted cereals. They are also the reason for the brown colour in roasted coffee.

  By this stage, there will be no shortage of aroma emanating from the roaster. The sentiment that the smell of roasting coffee (and even ‘freshly roasted coffee’ for that matter) is to some a sacrosanct intoxicant, triggering olfactory pleasure to the point of physical debilitation, was clearly circulated by someone who had never visited a coffee roastery. During the drying phase, roasting coffee smells like stale popcorn, wet hay and cold toast. Later on, it simply smells like oven-baking a bunch of sticks. To the experienced roaster, it is alleged that aroma can proffer some indication of doneness, but these are not the stirring scents of aromatic transcendence that one might imagine.

  As the bean dries, it also expands, and a fine membrane layer, which is difficult to see on a green bean, emerges and begins to peel away. This is known as the silverskin while it remains part of the bean, and once it becomes detached, it is demoted to the rather more humble-sounding chaff. Chaff is harmless in terms of its ability to affect coffee quality, but it is a concern when it comes to the removal and collection from the roaster, due to its potential as a fire risk should sufficient quantities of it be left to build up.

  As the drying phase comes to a close, the increase in bean temperature further forces the coffee bean to jettison moisture stores. Worse still, the breakdown of sucrose (sugar) and the subsequent caramelization that occurs from 150°C/300°F and upwards, releases more water, as well as carbon dioxide, as by-products. The water turns to steam, which in turn, along with a build-up of carbon dioxide, places increasing amounts of pressure on the physical structure of the bean. Something has to give, and without the plasticity that water once provided, the enlargement becomes more brutal. And like a snail whose shell has become too small to contain it – the consequences are explosive. This stage of the roast is known as ‘first crack’ and it is marked by an audible popping sound reminiscent of snapping pencils and an immediate increase in the size of the bean. First crack typically occurs after 7–9 minutes in a drum roaster (some air roasters can get there in only 2–3 minutes when pushed to it)
and it can last from 30–120 seconds, when the average temperature of the bean sits at around 190°C/375°F.

  It’s no secret that recording and referencing roast data is the pathway to delicious results.

  First crack is more than just an audible checkpoint. It’s also a marker for a significant physical change in the dynamic of the roast as a whole. Up until just before first crack, the coffee has been drawing in energy from the hot roasting air and, if available, the metal surfaces of the roaster itself. First crack, however, is like a firework going off inside the roaster, and the rapid phase change of water into steam creates a temperature hike as the bean itself gives out heat energy (becomes exothermic) for a short time. Anticipating first crack is an important skill for a roaster, as the sudden change in energy dynamics can cause the roast to ‘run away’ and get too hot or, if insufficient energy is provided, it stalls.

  Once first crack has finished, the coffee can be considered roasted and fit for consumption. Coffee this light will usually be bright and fruity, exhibiting far more of the beans natural facets and less in the way of traditional ‘roast’ qualities. Lighter roasts can be risky as they shine a very revealing light on green coffee, readily exposing any off-notes or defects. But when the green coffee is of very high quality and the roaster knows how to handle it, these light roasts gloriously stretch the limits of coffee’s flavour profile into heady heights of fruitiness. Coffee this light is best suited for filter brewing or French press, and almost always too light (and acidic) for espresso brewing.