Foundational
Roasting basics: from green bean to roasted coffee
What actually happens to a coffee bean during roasting, from drying through first and second crack.
In short
A green coffee seed is hard, dense and smells of grass. Ten or so minutes of carefully applied heat turn it into something brittle, brown, half as dense and intensely aromatic. Nearly everything recognisable as the flavour of coffee is made in those minutes; the green seed supplies the raw material and the limits.
You do not need to roast to benefit from understanding it. The roast decides how a coffee should be brewed, how long it keeps, and what the words on the bag can and cannot tell you.
The stages of a roast
Widely documented
Drying
The seed takes in heat and loses free water. It stays green, then pales, and smells grassy. Little flavour is formed yet.
Yellowing and browning
The seed turns yellow, then tan, and smells of hay and then bread. Maillard reactions between amino acids and sugars get under way, producing brown pigments and a wide range of aromas; at higher temperatures sugars also break down alone, which is caramelisation.
First crack
The beans begin to pop audibly and swell. The usual explanation is pressure from steam and gas inside the seed, which the research describes more cautiously than roasters do. From here the coffee is drinkable. Light roasts end soon after.
Development
The time after first crack begins. Acidity falls, body and bitterness rise, and the coffee's own aromas give way gradually to ones created by the roast.
Second cracknot always done
The structure fractures again with a faster, sharper crackle and oil migrates to the surface. This is dark-roast territory.
Cooling
The coffee is cooled quickly, usually by air, so that roasting stops where the roaster intended and does not coast onward.
A descriptive model of one roast. Times and temperatures vary widely between machines, batch sizes and coffees, which is why none is given.
What the roaster actually controls
CoffeeHQ explanation
Heat reaches the seed in three ways: by contact with hot metal, by hot air moving past it, and by radiation from hot surfaces. A drum roaster uses a mixture; a fluid-bed roaster, which suspends the coffee on a column of hot air, relies mainly on the air. The operator's controls — how much heat goes in, how much air moves through, and on some machines how fast the drum turns — are all ways of deciding how quickly heat enters the seed at each moment.
Two things define the result. One is where the roast ended, the roast degree. The other is the path it took to get there, the roast profile. Roasters watch how fast the measured temperature is rising, and many hold that a curve which stalls or surges makes flat or harsh coffee. This is craft knowledge passed between roasters. A research group that roasted such curves on purpose noted that it knew of no published data behind one of the commonest of these beliefs.
Light, medium and dark
Tendency, not a guarantee
- Light
- Ended soon after first crack. Keeps the most acidity and the most of what distinguishes one coffee from another. The beans are denser than darker roasts of the same coffee. Many brewers grind them finer or brew them hotter; that is common practice, not a measured rule.
- Medium
- Further into development. Acidity is softer, sweetness and body are fuller, and roast-made caramel and nut aromas sit alongside the coffee's own.
- Dark
- At or beyond second crack. Low in acidity, heavier, more bitter, with oil often visible on the surface. Coffees taste more alike as they darken, because roast flavour replaces origin flavour.
Why two roasters' light roasts are different colours
Sourced
Roast degree is usually judged by eye, sometimes against a set of reference tiles. A research paper on roast colour points out that this is subjective, affected by the lighting, the size of the sample and what surrounds it, and that there is no agreed definition of light or dark: some roasters sell as light a coffee darker than another's dark.
Instruments exist to put a number on it. They measure the light reflected from whole or ground coffee and report it on a scale, and on the best-known scale a smaller number means a darker roast. The same paper notes that the several instruments on the market use different scales, so a reading from one does not correspond to the same value on another. A colour number on a bag is therefore comparable with other bags from the same roaster and not much further.
The paper's own finding is that roasted arabica follows one path of colour whatever the roast profile, which its authors suggest could become the basis for shared definitions. That is a proposal. No such standard is in use.
Espresso roast, filter roast and omni roast
CoffeeHQ explanation
These three labels belong to a different category from light, medium and dark. They describe what the roaster had in mind, not a colour. An espresso roast is usually taken a little further, towards medium or medium-dark, so that it is easier to brew in the short time an espresso shot takes and its acidity is not overwhelming at that concentration. That is the roaster's intention, as roasters describe it. A filter roast is usually lighter, to keep acidity and aroma through a longer, gentler brew. An omni roast is a deliberate middle, meant to give acceptable results either way.
None of the three is a standard. One roaster's filter roast can be darker than another's espresso roast, so the words cannot be compared between companies. And they are advice, not a restriction: any coffee can be brewed by any method. An espresso roast in a French press will be heavier and lower in acid; a filter roast as espresso will be brighter and harder to dial in.
Roast faults you can taste, and cannot brew out
Widely documented
- Grassy, sour, like raw grain
Usually because: Roasters call this underdeveloped. The usual explanation is that the inside of the seed did not roast far enough, even if the outside reached a light-roast colour.
Change first: Nothing in brewing fixes it. Brewing hotter and finer makes it stronger, not better.
- Flat, bready, dull
Usually because: Roasters call this baked and usually blame a roast that spent too long at too little heat. Two studies that made a baked roast on purpose used opposite recipes, so the word describes the taste better than the cause.
Change first: A roasting fault. Fresh beans of the same coffee, properly roasted, will not taste like this.
- Ashy, smoky, charred at the edges
Usually because: Scorching or tipping: burn marks from too much heat too early, or simply a roast taken too far.
Change first: Brewing cooler and coarser softens it slightly. It does not remove it.
- Pale beans among the brown, with a papery, peanut-like taste
Usually because: Quakers: seeds from unripe fruit that will not brown normally. A fault of the green coffee that only shows after roasting.
Change first: Pick the pale beans out before grinding.
After the roast: resting, then staling
Tendency, not a guarantee
Freshly roasted coffee is full of carbon dioxide, which escapes over the following days. Brewed too soon, particularly as espresso, the gas is said to disrupt extraction, so coffee is usually rested first: by custom a few days for filter and often longer for espresso, with lighter roasts taking longest. Those are customs, and no study that tasted coffee at different ages in its first weeks was found. The right window depends on the coffee and is something roasters advise on, not a fixed rule.
Then the clock runs the other way. Aroma escapes, and oxygen reacts with what remains. Whole beans kept airtight and cool hold up for weeks; ground coffee fades in hours to days. A roast date tells you where a bag is in that sequence. A best-before date does not.
Three things people believe about roast
Widely documented
- That dark roast has more caffeine
- Dark roast tastes stronger, but that is bitterness and body, not caffeine. Dark beans are lighter and larger, so a scoop of dark roast holds less coffee than a scoop of light. One study that brewed many roast degrees the same way found the most caffeine in the cup from light-to-medium roasts and less from the darkest, partly because the darkest roasts extracted less. How you measure and how you brew decide the answer more than the roast name does.
- That oily beans are fresh
- Oil on the surface means the roast was dark, or that a medium roast has aged and its oil has worked outward. It is not a sign of freshness and can be a sign of the opposite.
- That dark means strong
- Strength is how much coffee is dissolved in the cup, which is decided by the recipe. A dark roast tastes more bitter and heavier, which people read as strong, but a light roast brewed at the same ratio is just as concentrated.
What to do next
- What happens when coffee roasts — The chemistry of the same stages in more detail.
- Light roast versus dark roast — The roast-degree trade set out side by side, including acidity and caffeine.
- Coffee Expert level 7: roasting — The same ground as a lesson, with an exercise comparing two roasts of one coffee.
- Roasting coffee at home — What it takes to try it, and the safety points that matter.
- Coffee roast defects — What baked, scorched and underdeveloped meant in the one study that made them on purpose.