Roasting

What you will be able to do

• Describe the stages of a roast in order and what marks each
• Explain what first crack and second crack are physically
• Explain the trade between a coffee's own character and roast character
• Name three roast defects and their causes
• Explain why roasted coffee needs to rest and why it then stales

Assumes: Level 6: Green coffee.

Heat, and how it gets into the bean

CoffeeHQ explanation of the mechanism

Roasting is the controlled application of heat to a green seed for somewhere around ten minutes, give or take several, at air temperatures that end above 200 °C. The seed loses water, expands to nearly twice its volume, turns from green to yellow to brown, and undergoes hundreds of chemical reactions that create nearly all the aroma we recognise as coffee.

Heat reaches the bean in three ways. Conduction is contact with hot metal. Convection is hot air moving past the bean. Radiation comes from hot surfaces. A traditional drum roaster uses a mix dominated by convection with some conduction; a fluid-bed roaster, which suspends beans on a column of hot air, is almost entirely convective. The roaster's controls — heat input, airflow and, on some machines, drum speed — are ways of shaping how fast heat goes in at each moment.

In more depth: Roasting basics · What happens when coffee roasts

The stages of a roast

From published standards or research

Drying. For the first few minutes the beans absorb heat and lose free water. They stay green, then pale, and smell grassy.

Yellowing and browning. The beans turn yellow and then tan, and the smell moves to hay, bread and toast. This is where Maillard reactions get under way: reactions between amino acids and sugars that produce brown pigments and a huge range of aroma compounds. At higher temperatures sugars also break down on their own, which is caramelisation. The two overlap, and 'caramelisation' is often used loosely for both.

First crack. Steam and gas pressure inside the bean builds until the structure gives with an audible pop, like popcorn but quieter. The beans expand sharply. From here the coffee is drinkable, and light roasts are ended shortly after.

Development. The time after first crack begins. Acidity falls, body and bitterness rise, and the coffee's own distinguishing aromas give way progressively to flavours created by roasting.

Second crack. If the roast continues, the bean's structure begins to fracture again with a faster, sharper crackle, and oils migrate to the surface. This is dark-roast territory. Beyond it lies charcoal.

In more depth: Light roast · Medium roast · Dark roast

What the roaster is deciding

A typical tendency, not a guarantee

Two things define a roast: where it ends and how it got there. The end point — roast degree — is the trade described above. Lighter roasts keep more acidity and more of what makes one coffee different from another, and are harder to extract. Darker roasts are less acidic, more bitter, heavier and more soluble, and coffees converge in flavour as they darken.

The path is the roast profile: how quickly temperature rose at each stage. Roasters watch the rate of rise, the speed at which bean temperature is climbing, and generally aim for it to decline smoothly. A profile that stalls or surges is associated with flat or harsh results. Much profile advice is craft knowledge rather than established science, and experienced roasters disagree about it.

'Espresso roast' and 'filter roast' are intentions, not standards: espresso roasts are usually taken a little further to make extraction easier and tame acidity under pressure. An 'omni roast' aims to suit both. Any coffee can be brewed any way.

In more depth: Espresso roast · Filter roast · Omni roast · Logging a roast

Roast defects, resting and staling

From published standards or research

Roasting has its own faults. Underdeveloped coffee was not roasted enough inside the bean and tastes grassy, sour or like raw grain. Baked coffee spent too long at too little heat and tastes flat and bready. Scorching and tipping are burn marks from too much heat too early. Quakers, the pale beans from unripe fruit, are a green coffee defect that only becomes visible after roasting.

Freshly roasted coffee is full of carbon dioxide, which escapes over days. This is degassing. Brewed too soon, especially as espresso, the gas disrupts extraction, so coffee is usually rested first: a few days for filter, often longer for espresso, and longer still for light roasts.

After that the clock runs the other way. Aromatic compounds escape and oxygen reacts with what is left. Whole beans stored airtight and cool hold up for weeks; ground coffee fades within hours to days. Bags with one-way valves let gas out without letting air in.

In more depth: Resting roasted coffee · Why coffee goes stale · Home roast tastes grassy or flat

Practise with a tool

Exercise: Light beside dark

See all practical exercises

Work it through: Sour, but not under-extracted

A home roaster's coffee tastes sour and grassy however it is brewed. Grinding finer and using hotter water makes it stronger and no less grassy. The beans are light brown and very hard to grind.

Reasoning it out

1. A brewing fault should respond to brewing changes. This one does not.
2. Grassy and grain-like flavours, with beans that are hard and dense, describe coffee that did not develop far enough inside the bean.
3. The outside reached a light-roast colour while the inside lagged, or the roast was simply ended too soon after first crack.
4. No brewing recipe can create aromas the roast never formed.

What to do

The fix is at the roaster: allow more development time after first crack, or apply heat more gently early on so the inside keeps up with the outside. Logging time and temperature makes the next attempt a comparison rather than a guess.

Home roast tastes grassy or flat

Check yourself

Answer in your head first. Nothing is scored and nothing you open here is recorded.

1. 1. What causes first crack?

   Show the answer

   Steam and gas pressure building inside the bean until its structure gives way.

   It is audible, and marks the point from which coffee is drinkable.

   Back to “The stages of a roast”

2. 2. Which reactions need both amino acids and sugars?

   • Maillard reactions
   • Caramelisation
   Show the answer

   Maillard reactions.

   Caramelisation is sugar breaking down on its own at higher temperature.

   Back to “The stages of a roast”

3. 3. As a roast gets darker, do different coffees taste more alike or less alike?

   Show the answer

   More alike.

   Roast-created flavours progressively replace the coffee's own distinguishing aromas.

   Back to “What the roaster is deciding”

4. 4. Why is very fresh coffee difficult to brew as espresso?

   Show the answer

   It is still releasing a lot of carbon dioxide, which disrupts even extraction.

   Resting for several days lets the gas escape.

   Back to “Roast defects, resting and staling”

Comparisons worth making now

• Light roast or dark roastThe roast-degree trade, with caffeine and acidity myths addressed.
• Airtight or vacuum storageWhat staling is, and what a container can and cannot do about it.

Reference for this level

• Roasting coffee at home
• Home coffee roaster
• One-way valve bag
• Roasted and smoky flavours

Where the facts in this level come from

• Technical writing on coffee roasting and extraction, Scott Rao
• SCA brewing and espresso standards — Specialty Coffee Association (SCA)

Sections are labelled with the kind of claim they make. How CoffeeHQ uses sources, and what each one is relied on for, is set out in the source and evidence policy.