Many roasters take regular samples during roasting, and use their noses to gauge the progress.
As the roast pushes on beyond first crack, things begin to move faster. Whereas in the early stages of roasting colour development and aroma shifted more deliberately, now the progress must be monitored closely, as important physical and chemical changes are occurring by the second. The plant cell walls of the bean become fractured, denatured and more brittle. The voids left by the quickly departing water begin to expand faster, which in turn, makes the bean more porous. Oils begin to migrate around the structure of the bean, too.
During this time, the subsequent flavour of the coffee will be changing constantly as well. Acidity drops off more or less linearly throughout the roast and bitterness increases exponentially through caramelization effects, while the more elusive attributes of body and sweetness increase, plateau, then drop off again – exactly when that happens is down to the coffee and the whims of the roaster, of course.
If there’s a first crack, it stands to reason that there must be a second crack (otherwise it would be known only as just ‘crack’, I suppose), and this second audible phase normally commences around 2–5 minutes after first crack finishes. Most speciality coffee is dropped into the cooling tray at some time between first crack and second crack, but darker roasts that may be used for espresso brewing will sometimes dip into later stages during, or after, second crack. Second crack is harder to hear than first crack and once again, we are looking at a build-up of carbon dioxide within the bean, but by this time almost all of the water has gone.
After second crack, tensions become high. Things are beginning become reminiscent of a campfire by now, in both sound and smell, at least. At temperatures above 230°C/450°F, the matrix structure of the plant begins to deteriorate. The aromatic lignin (essentially nature’s cement) begins to volatize, which threatens the stability of the bean but also opens up new aromatic potential in the realms of nutty, smoky and charred flavours. As the roast becomes even darker, the coffee starts to burn.
Perhaps only 15 minutes have passed, and the beans have nearly doubled in size, yet lost over 20 per cent of their starting mass, which equates to a drop in density of almost 50 per cent. The character of coffee at this late stage in the roast has been all but obliterated; sooty, astringent and bitter caramel flavours have taken over. Oils begin to bleed onto the surface of the bean and the roast emits a darker, more ominous smoke. Continuing roasting beyond this point would be both pointless – since the coffee is likely already undrinkable – and more importantly, dangerous. Just before the beans reach their ignition point they turn as black as coal. Then the roaster catches on fire.
ROAST PROFILING AND TEMPERATURE GRAPHS
So there is far more to roasting than setting a heat dial and a countdown timer. A coffee’s roast profile can be based on a number of factors, including bean density, coffee variety, origin and type of processing. It may also be based on the intended brewing method of the roasted coffee. Roasting is a highly sensitive process where even the smallest shift in variables can have monumental repercussions in the chemistry of the coffee, ultimately affirming the character and quality of the finished coffee bean.
Thinking of the coffee bean like a checklist of flavour compounds can help to demystify the roasting process. Each flavour, or group of flavours, will only become apparent, or slip away, once a specific bean temperature has been achieved and maintained for a period of time. Some flavour compounds decrease as roasts get darker and others increase. Nicotinic acid, for example, is one contributor to a clean finish in a good cup of coffee, and it is present in higher quantities in dark roasts due to the action of higher temperatures releasing its soluble form. Other flavour groups may remain relatively stable, but their presence might be suppressed or made more noticeable by the increase or decrease in other aromatic compounds.
The best coffees are roasted to specific profiles, where temperature is regularly adjusted to coerce the roast along, or to slow down its progress. Profiles are recorded in graph form, with axes that track the air temperature and bean mass temperature, resulting in a visual record and a useful reference of the roasters approach to the task at hand. Some modern roasters are capable of memorizing the actions of a roaster during a roast, then repeating the exact same steps on subsequent roasts for consistent batchto- batch results.
One of the most important ways that a temperature graph can help a roaster is through its recording of the Rate-of-Rise (RoR). The temperature of the coffee bean during any roast should always be on the increase, but most experts agree that the speed at which the temperature increases should occur at a decreasing rate. That is, the rate of rise should decrease through the course of the roast – think of it like a car that is slowly decelerating, but still constantly moving forward. This has to do with inner bean development. Simply put, it’s more likely that a coffee will taste good when it is the same colour on the inside as the outside. The bean will always be slightly less cooked on the inside than the outside however, due to the nature of conductive heat. To mitigate this fact, roasters aim to increase bean temperature rapidly at the beginning of the roast, then more slowly at the end. The effect is a curious game of catch-up, where the interior temperature of the bean lags behind at first (as the heat has had insufficient time to conduct through) then rapidly increases a minute or two later, thanks to the initial temperature blast. By that time, the RoR on the exterior of the bean has slowed down and the inner and outer temperatures sit in close proximity to one another.
Careful manipulation of the rate of rise, mostly through the control of temperature and airflow, is where the hand of the roaster comes into play.
ROASTING COFFEE AT HOME
Home coffee roasters can be picked up for as little a £200/$315 and what some of them lack in control and stability they make up for in the fact that you have freshly roasted coffee on tap. With a little trial and error these machines can produce surprisingly good results, and more expensive roasters can even match the quality of output that commercial models achieve.
Temperature control is everything in roasting. Most home roasters come with a panel that controls this digitally, and some have the function to programme a temperature curve so that the same roast can be repeated. The controller for this is paired with a temperature probe that usually takes a reading from the metallic surface on the inside of the drum, or from the temperature of the air being blown in. These temperatures are useful because they tell you how the roaster heats up, but this is only truly relevant when they’re paired with a temperature reading from the beans themselves (bean mass). Most roasters do not come fitted with a probe that can do this, so I suggest buying a digital temperature probe and inserting it into the drum, or better still, connecting a USB probe to your computer or smartphone then using one of the excellent data logging apps that will record the progress of your roast. Establishing a set-up like this is surprisingly cheap and easy, but provides a level of control and evaluation that will show in your coffee.
When you’re choosing where to put your roaster ventilation is one of the main concerns. If you’re roasting indoors I would recommend sitting the roaster underneath the extractor hood of your oven, or at least next to an open window. Coffee roasting produces a lot of smoke, even on short roasts, and you’ll not want it hanging around in your house. Better still, do it outside or in an outbuilding, which solves the smoke problem and, for those of you that live in a colder part of the world, also helps a great deal with cooling the beans once they have been dropped into the cooling tray.
Follow the roaster’s instructions and pay particular attention to the recommended batch size. Small home roasters can sometimes be very sensitive to small or large loads. Under-loading leads to inconsistent cooking (like flash-frying, the bean may be underdeveloped on the inside) and overloading slows the roast down, which in extreme cases results in woody/smoky flavours. Also locate where the chaff is collected and how it is emptied from the machine.
If left to build up, chaff can cause problems with airflow and become a fire risk.
On a final note, it’s important never to leave your roaster unattended – you’ll not be able to enjoy the delicious fruits of your labour if your house burned down in the process. Fire risk is something that commercial roasteries take very seriously, and roasting small batches in your home should be subject to the same degree of precaution.
1
Preheat the roaster. It’s important that the bean temperature ramps up quickly at the start of the roast, so sufficient preheating of the roaster’s components is essential. Fire on all cylinders until the air temperature is at least 200°C/400°F.
2
Drop the beans in and set the timer (A). The bean mass probe will show a sharp temperature drop as it comes into contact with the cool beans, but after around a minute it should bottom out and then begin to increase.
3
Keep an eye on the bean mass temperature, which should be increasing rapidly, but beginning to slow its increase as you approach the eight-minute mark (B).
4
First crack will present itself as an audible snapping noise after around 9–12 minutes, depending on the model of the roaster. It’s here that careful temperature manipulation is required to stop the beans from ‘running away’ if too much temperature is applied, or from ‘stalling’ if your roaster has insufficient heat.
5
As the beans develop after first crack the surface will become smoother and the aromas more pleasant. When you decide to finish the roast will depend on how you intend on brewing them as well as the type of coffee you are roasting. Experimenting with this is half of the fun of roasting. Having said that, if you’d like to stick to convention, most experts agree that first crack should occur at around 80 per cent of the total roast time. This means if you hit first crack at 10 minutes you should end the roast after 12 and a half minutes. You may of course wish to roast for longer. Drop the beans into the cooling tray (C).
6
Some home roasters do a much better job of cooling roasted coffee than others. Ideally the beans should be cool enough to handle 5 minutes after the end of the roast (D). Bag them up and allow to outgas for at least 12 hours.
THE IMPORTANCE OF ROAST DATE AND STORAGE
Freshly roasted coffee doesn’t taste nice. No, really. Before you think me mad, I am not talking about coffee freshly bought from the store, but coffee fresh off the cooling tray. That stuff. This should come as no surprise. After all, the coffee has just undergone a wholly unnatural experience; moisture stores and solid matter have been vaporized; sugars and acids have splintered into a diverse mixture of newly formed chemicals; caramelization and browning reactions have ensued, and the overall structure of the bean has been dramatically transformed, resulting in a near doubling of size and changes to porosity, colour, density and weight. Brewing coffee that fresh will produce a drink that tastes flat, ashy and lifeless, with little in the way of complex aroma.
The coffee must be allowed to rest before it is ready to be turned into a tasty beverage. The most important element of resting is the process of ‘outgassing’, or the process of releasing carbon dioxide from the bean. Secondary to that are the minor chemical changes that also take place minutes, hours and even days following the roast.
Freshly roasted coffee is approximately two per cent carbon dioxide by weight, and if left in a nonpressurized environment, it will release this gas more slowly as time goes on. This is all down to the internal pressure of the bean which – like an inflated balloon – forces carbon dioxide and other gases outwardly after roasting. Now, carbon dioxide is flavourless by itself, but when mixed with water – in, say, a French press – the hot liquid acts an effective solvent for the carbon dioxide, which quickly dissolves in and subsequently, out, of the water, creating lots of bubbles. The effect is like pouring a glass of fizzy cola over a sherbet, and as fun as that may sound, it doesn’t make for a great-tasting cup.
There are two main reasons for this. The first is that when carbon dioxide and water mix they produce carbonic acid. The subtle ‘tang’ that you experience from a glass of soda that has been left to go flat is the taste of carbonic acid. The point is that carbonic acid isn’t very delicious, it leaves a ‘licked-battery’ kind of sensation on the tongue, and it can become quite apparent in very fresh brews.
It has also been suggested that carbon dioxide is bad for brewing due to the disruption it causes to the mechanics of brewing, where wet coffee grounds that are rapidly releasing carbon dioxide have the effect of propelling brewing water away from the flavourful solubles that rest within the hallowed inner walls of the coffee cell structure. The result is a less complete extraction (for more on extraction see pages 69–71).
The amount of time that coffee must be allowed to rest and outgas will depend on the approach to roasting and storage. Broadly speaking, darker and hotter roasts will have a higher internal pressure, so will they will outgas slightly quicker and more completely. Lighter and cooler roasts will do the opposite. Faster outgassing points towards a more porous bean structure, which is also more likely to go stale quickly (see below). It would seem that a direct correlation can be made between the rate and extent of outgassing and the volatilization (departure) of aromatic compounds in the first week or so after roasting. A coffee’s aroma tends to be more noticeable in lighter roasts; in darker roasts, the destruction of volatile aromatics (or at least the good ones) eventually outpaces the formation.
FRESHNESS AND STALENESS
The inevitable deterioration in coffee quality after roasting remains an inconvenient truth that, sadly, too many people choose to ignore. While some of the mechanisms responsible for staleness can be limited through packaging and correct technical processing, they cannot be entirely stopped. Typified by a loss of aroma and general muddying of flavour, staleness can be largely attributed to the departure of aromatic molecules and oxidation reactions. We experience this escape of aromatic volatization every time we open a bag or box of coffee beans or even smell a coffee shop on the other side of the street!
Oxidation is a destructive action, responsible for the discolouring of fruit and vegetables and the eventual rancidity of fats and oils. In the case of coffee, oxygen molecules, present within the air, lend electrons to compounds and transform them into new oxidized compounds. The loss of positive flavour molecules and the creation of new, generally inferior (from a taste standpoint) compounds, presents itself as an overall loss of flavour.
The rate of oxidation in coffee is largely proportionate to its rate of outgassing. Very fresh coffee actually oxidizes quite slowly, since the internal bean pressure prevents too much oxygen gaining access. Inevitably, though, as the rate of outgassing drops, oxygen finds its way in. There are other factors at play, too; oxidation is increased by air humidity, for example. Storing coffee in a cool and dry environment still remains one of the best and simplest ways to preserve freshness.
At some point during the exodus of aroma and the effects of oxidation, coffee crosses a threshold of acceptable loss and is eventually deemed to be too old and stale to be enjoyable. As for when exactly the coffee is at its peak level of performance, there are only broad guidelines. Certainly it takes at least 12 hours for the coffee to become drinkable, and in some cases up to a week to reach its full potential. It may be that not all oxidation effects are detrimental to coffee quality and that not all aromatic losses are actually a loss. The variables are too numerous to give even vaguely specific guidelines on when best to use your coffee, but assuming the coffee is stored correctly, I would advise using filter coffee within 1–10 days of roasting and espresso within 7–14 days.
PACKAGING AND PRESERVATION
There are a variety of packaging options, and which one is best for you will be determined by how you intend on using the coffee. Whichever method you use, keeping coffee in an unsealed container is not a clever move as it will leave coffee wide open to staleness. Coffee stored like thi
s should be consumed within 1–2 days.
Valve bags are the most common method of packaging for most commercial roasters. The valve allows the steady release of gases generated during roasting and the containment of aroma. Since carbon dioxide is steadily leaking out, it becomes difficult for oxygen to get in, meaning that the airspace of the bag is effectively flushed with carbon dioxide. I have seen some data that suggests the airspace in a valve bag may consist of as much 50 per cent carbon dioxide in only five minutes after loading freshly roasted coffee. Once opened, oxygen is of course introduced, but assuming the bag is sealed again it is eventually flushed out again. Some roasters take the extra step to flush their bags with nitrogen when filling them, eliminating any oxidation effects until the bag is eventually opened. Vacuumsealing a valve bag achieves the same thing.
Valve bags do not prevent outgassing, however, and since the level of outgassing is relative to the bean’s internal pressure, which is also relative to the menacing potential of oxidation, freshly opened bags of three-week-old coffee, for example, will go stale very quickly.
One workaround would be to store the coffee in a pressurized container. Also, freezing remains a simple and fairly effective method for the long- and short-term preservation of roasted coffee. I have been, for some years now, freezing carefully measured individual portions of roasted coffee to great effect. Be sure to freeze in sealed bags or containers, though, to avoid moisture condensing onto the surface of the beans.
The Curious Barista's Guide to Coffee Page 8