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What happens in the roastery: first crack and second crack

Roasting coffee looks simple: you throw green beans into a drum and wait for them to brown. In reality it is a dramatic process, in which the bean goes through a storm of chemical reactions, and the key landmarks are two sounds: first crack and second crack. They are audible cracks, by which the roaster knows what stage the coffee is at and when to stop the roast to get a light, medium or dark profile. Between them plays out all the magic: the Maillard reaction, the development of aromas and the transformation of the bean. Here is a guide to what happens in the roastery: what the two cracks are, how heat transforms the bean and how these sounds mark the roast level.

Roasting as a drama in heat

Roasting is the transformation of a green, hard, flavourless bean into aromatic, brown coffee - and it is driven solely by heat. In the roasting drum the bean gradually heats up, passing through successive phases. At the start the reactions are endothermic: heat flows from the surroundings into the green bean, which first dries, loses moisture and slowly yellows. It is the phase of preparation. Then, as the temperature rises, the reactions that build flavour and colour set off, and the bean passes through characteristic turning points. The roaster steers this process by regulating the heat and time, and listens for signals that tell them what is happening inside. The most important of these signals are precisely the two cracks. Understanding that roasting is a heat-driven transformation with clear stages is the starting point for the rest. We cover coffee itself more in how coffee is made.

The Maillard reaction in the bean

At the heart of flavour development during roasting is the Maillard reaction - the same that browns bread and sears meat, here happening in the coffee bean. It is a reaction between amino acids and sugars under heat, creating hundreds of flavour and aromatic compounds. In coffee it accelerates when the bean temperature rises above about 150 degrees, generating volatile compounds responsible for the richness of the aroma - from nutty to chocolate notes. It is then that the bean passes from yellow, through light brown, to medium brown. After the Maillard, at higher temperatures, comes the caramelisation of sugars, deepening the sweetness and colour. These reactions build almost all the flavour of roasted coffee. This is why the roaster watches the course of the temperature in this phase so closely - it decides the aroma. Understanding that flavour is born mainly from the Maillard reaction explains why the way of roasting is so important. It is the chemistry of heat turned into aroma.

First crack: the first crack

First crack is the breakthrough moment of roasting, happening at a bean temperature of about 196 degrees. It is an audible crack, resembling popping popcorn, when water vapour and carbon dioxide under pressure rupture the cell walls of the bean. The key is that first crack is an exothermic reaction - the bean starts to give off energy, steam and CO2, instead of only taking it in. It is the moment when the coffee becomes drinkable: after first crack we already have coffee at a light roast level. The roaster treats this crack as the main landmark - from it the development time and the decision about the roast level are counted. Stopping the roast just after first crack gives a light, acidic and fruity coffee. It is the most important sound in the roastery, the boundary between a raw and a finished bean. After it the game for the profile begins. We cover roast levels more in coffee roast levels.

The development phase after first crack

After first crack comes the key, if short, phase called development. It is the time between the first crack and the moment of removing the coffee from the drum, in which flavour development is finished, the sharp acidity fades and the aromas settle. The roaster steers the length of this phase here: a shorter development leaves the coffee lighter, more acidic and fruity, a longer one gives a darker, sweeter and fuller coffee. It is in this phase that the roaster makes the most important decisions about the character of the coffee. Too short a development can leave raw, grassy notes, too long can roast away the subtleties. This is why after first crack the real precision begins: a few or a dozen seconds of difference change the profile. The development phase is the heart of the art of roasting, in which the target flavour is created from the raw bean. It is the moment when the roaster really designs the coffee. The choice of development length is the choice of character.

A table: two cracks

Let us gather the two cracks and the roast levels:

The table shows the axis of roasting: the further in temperature and time, the darker the coffee and the more roasted, less acidic the flavour. The two cracks are audible milestones on this road.

Second crack: the second crack

Second crack happens later, at a bean temperature of about 224 degrees. It is another audible crack, but of a different character - quieter, sharper, when the cell structure of the bean itself breaks, not just the walls. Between first and second crack usually passes from a dozen to thirty-odd seconds (and with slower roasting longer). After second crack the coffee enters the area of dark roasting: oils migrate to the surface of the bean, giving it a shiny, oily look, and the flavour becomes dark, roasted, bitter, with notes of char, and less and less acidic and fruity. Here the subtle notes of origin die in favour of the dominant flavour of roast. A roaster who wants a dark coffee takes it around or beyond second crack. It is the second great landmark, the boundary of entry into dark profiles. After it the coffee quickly becomes very dark.

How the cracks mark the roast level

The two cracks together create a simple map of roast levels, which the roaster reads by ear. Stopping the coffee just after first crack gives a light roast - acidic, fruity, floral, with the character of origin kept. Removing it during the development phase, between the cracks, gives a medium roast - balanced, sweeter, with less acidity and more body. Taking it to second crack or beyond gives a dark roast - roasted, bitter, oily, with the dominant flavour of the roast itself. This is why both cracks are priceless to the roaster: hearing them, they know exactly where they are on the axis from light to dark and when to stop. The roast level is in essence a decision about at which moment relative to the cracks to stop the process. Sound becomes a tool of precision. This shows how hearing is as important to a roaster as sight. We cover freshness after roasting more in coffee freshness.

How to sense it in the cup

The roast level marked by the cracks is easy to sense in the finished coffee. A light coffee (stopped after first crack) is acidic, fruity, light in colour, with a clear character of origin - this is how most speciality coffees are brewed today. A medium coffee is more balanced, sweeter, with notes of nuts and caramel, less acidic. A dark coffee (from second crack) is bitter, roasted, with notes of chocolate, smoke and char, with beans shiny from oil and almost no acidity. If a coffee is shiny and very bitter, that is a trace of roasting past second crack. If light and fruity, it was stopped just after first crack. It is worth comparing a light and a dark roast of the same bean, to feel how the cracks change everything. Over time you will start to associate flavour with the roast level. We cover judging coffee more in home cupping.

The essentials in brief

Let us gather it up. Roasting coffee is a heat-driven transformation whose key points are two audible cracks. First the bean dries (the endothermic phase), then the Maillard reaction above about 150 degrees builds hundreds of compounds of flavour and colour. First crack, at about 196 degrees, is an exothermic crack, after which the coffee becomes drinkable - that is a light roast. The development phase between the cracks lets the roaster perfect the profile. Second crack, at about 224 degrees, brings the dark roast: oils on the surface, a roasted and bitter flavour, less acidity. The two cracks mark the axis from light to dark, which the roaster reads by ear. Now you know what really happens in the roastery and where the roast level of your coffee comes from.

Note every coffee in GustoNote - including the roast level and the profile you sense. Over time you will start to recognise light, medium and dark roast by flavour.