Roast defects

What counts as a roast defect#

A roast defect is a flaw that comes from the roasting process. It does not come from how the coffee was grown, processed or stored as green coffee.¹ The main sensory study on the subject focused on defects of the roasting process. It left flaws from production, processing and storage of the green coffee for future work.¹

In that study, six experts from the Specialty Coffee Association of Europe (SCAE) designed one normal roast and five defect roasts.¹ All six roasts used the same washed Kenyan arabica coffee.¹ The defects – light, scorched, dark, baked and underdeveloped – were meant to represent faults commonly found on the market.¹ The normal roast had average levels of the key aroma compounds and a balanced sensory profile.¹ Consumers liked it best, and it was significantly preferred over every defect roast except the baked one.¹

Defects fall into two groups. Some come from the time–temperature path of the whole batch: underdeveloped, baked and over-roasted. These show mainly in the cup and in the roast log, the record of temperature and time for each batch. Others are marks on individual beans that roasters see when they inspect a batch: scorching, tipping, facing and craters. Uneven roasting can be either. The second group is described almost only by practitioners; no peer-reviewed study of these marks was found (see Open questions).^12,13,14,15

Profile defects: underdeveloped, baked, over-roasted#

Underdeveloped

What it is. The word is used in two ways. A practitioner paper notes that under-development has been blamed on too short a development time and on too low an end temperature.¹² Development time is the time from first crack, when the beans start to crack audibly, to the end of the roast. The paper keeps “under-developed” for the first and calls the second “under-roasted”.¹²

In the defect study, the “Light” roast came closest to this idea: it followed the normal curve but was stopped 10 s after first crack.¹ The study’s “Underdeveloped” sample was something else. It was charged (put into the roaster) into cooler air, and its temperature curve stalled at the start.¹

How it looks. The short-developed Light roast ended much lighter than the normal roast; the slow-start roast ended at almost the same colour.¹

In the cup. Trained assessors rated the Light roast lower than all other samples in most attributes, and higher in citrus, sweet and roasted rye bread.¹ Consumers called it thin, mild and bland, linked it with hay or straw, and liked it least of the six.¹ The slow-start roast was expected to give a flat, slightly sour cup, but its aroma composition was close to the normal roast.¹

Short development is not a defect in itself. In a 2020 study that varied only development time at one colour, the shortest development (90 s) scored highest for acidity, fruitiness and clean cup.³ The defect lies at the extreme, as in the 10 s Light roast, not in short development as such.^1,3

Check and avoid. Roast colour and the time from first crack both reveal the Light roast: it differed from the normal roast in both.¹ The slow-start roast differed in neither colour nor development time.¹ It differed only in its start and total time, so it is found in the roast log and the cup.¹

Baked

What it is. A 2020 development-time study defines a baked roast by a drastically stretched time after first crack, with little or no temperature rise.³ A multi-study paper reports that the Specialty Coffee Association (SCA) roaster certification includes it as a defect of overly extended development time.⁴ The word “stalled” is used for both underdeveloped and baked roasts, at different moments: the slow-start roast of the defect study stalled at the beginning.¹ A baked roast stalls after first crack.³

How it looks. There is no distinct sign. In the defect study, the baked roast ended close to the normal roast’s colour.¹ In the development-time study, all roasts – baked included – were brought to the same colour.³

In the cup. In the development-time study, the baked roast had the lowest sweetness and acidity and the highest bitterness, roasted and nutty–chocolate notes.³ The authors note that it gained characteristics akin to darker roasts.³

Check and avoid. Colour did not reveal the baked roasts in either study.^1,3 The roast log does: a long development with little temperature rise after first crack is the definition of the defect.³ In the cup, according to the multi-study paper, the SCA roaster certification characterises it by strong roasted-bread notes.⁴

Over-roasted

What it is. A roast taken too far. In the defect study, the dark roast followed the normal curve but ran longer.¹ According to a roasting textbook chapter, the reactions late in the roast turn exothermic – they release heat of their own – and speed up.⁵ The chapter notes that a few seconds can separate the desired roast degree from an over-roasted product.⁵ It adds that an uncontrolled roast can catch fire.⁵

How it looks. The dark roast of the defect study was much darker than the normal roast.¹ According to the textbook chapter, oil moves to the surface faster in dark roasts.⁵ In extremely dark roasts, oil can appear during the last stages of roasting.⁵

In the cup. Trained assessors rated the dark roast, together with the scorched one, highest for burnt, bitter, tobacco and liquorice, and gave it the weakest acidic aftertaste.¹ Consumers rated it the strongest and most intense, and called it unpleasant more often than any other sample.¹ A chemistry chapter describes over-roasting as giving unbalanced, burnt, harsh off-notes.⁶ According to the roasting textbook chapter, over-roasted coffee may show a strong “roasty” note but often a thin mouthfeel.⁵

Check and avoid. The checks are a colour measurement and a look for surface oil.^1,5 According to the textbook chapter, the roast should be stopped abruptly at the desired degree, with efficient pre-cooling or cooling.⁵

Marks on the bean: scorching, tipping, facing, craters#

These four are practitioners’ names. The sources that describe them are a roaster manufacturer’s technical paper, a retailer’s glossary and a trade magazine.^12,13,14,15 They do not agree on every definition (see Contested).^12,13,14,15 A roasting textbook chapter supports the mechanism behind one of them.⁵ In its account, a drum that turns too fast lets the beans “hug” its hot wall longer.⁵ This over-develops the outside of the bean and, at worst, scorches the flat sides.⁵ A doctoral thesis on roasting describes a roaster’s main task as mixing the beans well enough for a homogeneous roast and preventing scorching.⁷

Scorching

Look and cause. A roaster manufacturer’s paper describes small brownish-black or black specks on the surface.¹² It sees them usually before the bean turns brown, and puts them down to contact with metal that is too hot.¹² Another practitioner source describes a larger dark spot, typically on the flat side, from a surface too hot for the bean.¹⁵

In the cup. The defect study’s “Scorched” sample was defined by its roast profile, not by marks on the beans.¹ It was charged into hotter air and finished sooner than the normal roast.¹ Trained assessors rated it as intense, burnt and bitter as the dark roast, and found little difference between the two.¹ Yet the aroma compositions of the two roasts differed strongly.¹ Consumers rated it less intense than the dark roast.¹

Prevention. The doctoral thesis names good mixing.⁷ The textbook chapter names a drum speed that does not press beans against the wall.⁵ The manufacturer’s paper names charging too hot, too little airflow and too much burner early in the roast as causes.¹² It also argues that a drum not heated directly is hard to overheat – a claim about its own design.¹²

Tipping

Look and cause. In roasting practice, tipping is the name for ends of the elongated bean that look burnt.¹³ One practitioner sees this charred spot most often early in the roast, before browning, and puts it down mainly to roasting too rapidly.¹² A retailer’s glossary names an overheated roaster at the start, too much coffee in the drum, or possibly too little air movement.¹³

In the cup. A retailer’s glossary describes burnt or smoky flavours and a lack of sweetness.¹³ A trade-magazine article says a tipped roast can still give a good cup, and that the cup should decide.¹⁵ No sensory study opened for this entry has isolated tipping.

Facing

Look and cause. In roasting practice, facing is the name for scorch marks on the flat side, the “face”, of the bean.¹⁴ A practitioner paper reserves the name for scorching late in the roast.¹² It gives two causes: the swollen batch forces the beans against the drum wall, or the drum metal overheats towards the end.¹² The textbook’s drum-speed mechanism fits this pattern.⁵

In the cup. Not established; practitioners treat facing as a sign of scorching.¹⁴

Craters

Look and cause. In roasting practice, a crater is a small round or oblong piece blown out of the bean during roasting, sometimes called chipping.¹² One practitioner sees craters only in dark roasts, at or past the start of second crack.¹² The same practitioner knows of no researched reason and suggests pressure trapped in the brittle bean.¹² A trade-magazine article places craters near or into second crack, when built-up gas is released so fast that it blows a piece off.¹⁵

In the cup. Practitioners disagree. One practitioner paper says craters do “not seem to have any perceived impact on flavor”.¹² A trade-magazine article treats them as a possible cause of unwanted flavours.¹⁵

Uneven roasts#

Within one bean. According to a roasting textbook chapter, rapid heating can create a steep temperature gradient from surface to core.⁵ In extreme cases the outside is over-roasted while the core stays under-roasted, which can give “burnt” and “greenish” notes in the same cup.⁵ A doctoral thesis adds that colour then becomes a less reliable guide, because the interior is less roasted than the outside.⁷

Between beans. According to the textbook chapter, when a blend is roasted in one batch, its components can end with visibly different roast degrees.⁵ The chapter describes split roasting – roasting the components separately and blending afterwards – as letting each be roasted to its own optimum.⁵ It notes that some roasters accept or even want the variation from a green blend, and that the decision rests on the cup.⁵

Check and avoid. The sources name three measures: looking at the whole beans for colour differences⁵, mixing the batch well⁷ and avoiding extreme heating rates.⁵

Quakers: not a roast defect#

According to a cupping and grading chapter, a quaker is a bean that stays light, pale brown after roasting.⁸ It looks distinctly different from the fully roasted beans around it.⁸ The chapter traces it to a cherry picked before it was fully ripe, so the seed did not develop fully.⁸ A 2021 study confirmed that the colour and sensory characteristics of quakers follow from the chemical composition of the immature seed.⁹ It found that their aroma compounds came from precursors already present in the immature green coffee.⁹ According to the chapter, the remedy lies before roasting: high-quality coffees are hand-picked by pickers trained to take only ripe, red cherries.⁸