What Are Coffee Roast Defects And How Can Roasters Identify Them?

Peter Lancashire
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January 29, 2021
roasting coffee

Roasting coffee is a complex process and mistakes are easy to make. Without the proper oversight, coffee can come out overdeveloped, underdeveloped, scorched, or baked. These can all ruin the flavour of entire batches of coffee.

Known as “roast defects”, these mistakes typically occur when roasters lack information on what’s going on during key points of the roast. 

Identifying these defects and taking measures to prevent them happening is essential for roasters who want to create consistently high quality coffee for consumers.  

To find out more about roast defects and how specialty coffee roasters can avoid them, I spoke with MTPak Coffee Brand Ambassador Michalis Katsiavos.

See also: Why Should Roasters Create Coffee Blends?

roast defects

What’s A Roast Defect And How Does It Occur?

Roasting coffee is a highly complex process. Not only does it require an understanding of the chemical reactions that take place during the roast, the roaster also needs to know how to control each specific variable.

Even for the most skilled roasters, it can be difficult to achieve a consistent roast every single time. This is particularly true when roasters are trying to perfect the balance of flavour and body with a new batch of beans, which is known as dialling in a roast profile.

The majority of problems occur when too much or too little heat is added at different stages of the roast, resulting in what’s known as a “roast defect”. Unlike defects in the bean, roast defects are caused by human error rather than the quality of the green coffee.

One of the most common roast defects is underdevelopment of the bean. This means that part of the bean is still raw, causing it to have a “grassy” texture and flavours reminiscent of hay, grass, oat, corn, or lentils, among others. This tends to appear when the roast is completed too quickly, and the beans are removed before the sugars within have enough time to fully develop.

The opposite end of the spectrum occurs when beans are overdeveloped. This happens when they are roasted for too long, resulting in a loss of acidity and delicate florals, caramelisation, and the onset of flavours reminiscent of charcoal, burnt toast, and bitter chocolate.

Michalis Katsiavos heads Quality Control at Samba Coffee Roasters in Athens, Greece. In 2018, he was crowned Hellenic Barista Champion, and finished fourth in the World Barista Championship in Amsterdam. 

He tells me that there is a fine line between a dark roast and overdeveloped beans.

“When we are trying to achieve a darker roast, we sometimes find that the coffee has overdeveloped,” he says.

A similar problem to overdeveloped coffee is ‘scorching’. This is when a small area of the roasted bean is burnt. This roast defect can cause similar flavour defects as with overdeveloped coffee, but to a much lesser degree. Scorched coffee will add bitter, ashy, and smokey flavours to a cup.

“Tipping” is a roast defect similar to scorching. It causes the same kind of flavour taint and visual marker, but for a slightly different reason. 

The visual difference between tipping and scorching is the location of the burn marks. While scorching burn marks are located on the flat surfaces of the bean, tipping marks, as the name suggests, are at the tips or edges. The taste is usually similar to that of scorched coffee.

Finally, there is “baking”. This can be a little bit more difficult to identify. This is when a bean’s temperature “stalls” during a roast. There aren’t many physical markers for this defect, but when it becomes more obvious when the coffee is cupped. Baked coffee is usually described as “lifeless”, with flavours reminiscent of oat, grain, or paper.

roasting coffee

How To Identify Roast Defects

The ever-growing demand for high-quality specialty coffee has put roasters under greater pressure to achieve consistency in their roasts. Part of this involves being able to quickly identify any defects and prevent them from reoccurring.

Fortunately, most defects can be identified in a number of ways. According to coffee roasting expert Scott Rao, identifying roast defects, like evaluating green beans, is a skill that one can practise and hone.

The first indicator that a bean is under or overdeveloped is colour. Underdeveloped beans will be a very light brown, whereas overdeveloped beans will be very dark brown or even black and shiny. 

However, while colour is often an early sign of a roast defect, sometimes it’s not until beans are cupped that a defect is properly identified.

Underdeveloped coffee, for instance, contains higher levels of citric acid and lacks sweetness or body. This typically means that it will have a higher acidity in the cup, and, due to a lack of solubility, will appear broken up and watery when brewed in a cupping bowl. The crust of a fully developed roast should sit nicely at the brim during cupping.

In contrast, overdeveloped coffee will often look as if it might overflow during cupping. The additional time in the roaster breaks down the bean and provides easier access to the oils held within, which creates a “bulging” effect during extraction.

roast defects

How Can Specialty Coffee Roasters Avoid Roast Defects?

Once specialty coffee roasters have identified a roast defect, it’s important that they take the necessary steps to stop it reoccurring.

Roasters need to be aware of factors such as the coffee’s moisture content, the altitude at which it was grown, and the way it was processed, as these can all influence the development of roasted coffee. Coffee beans with a high density, for example, will need to be roasted for longer than those with a low density.

Michalis explains that roasters should also keep an eye on the rate of rise (RoR) at all times during the roast. RoR is the rate at which the temperature of the beans increases at any point during a roast.

Roasters should also gather data and use it to control key points during the roast, such as first crack. First crack occurs when the energy in the beans builds up, causing it to rupture and become porous. However, once this happens, its increased porosity means it can easily take on too much heat, further breaking down the bean’s structure and potentially burning the bean throughout.

“After the first crack – the most critical point – we need to be consistent and watch the development time in order to create the right profile for our coffee,” Michalis says.

Roasters can avoid baked coffee as long as they are paying attention to a roast’s RoR figure.

“The trick is to avoid an RoR that’s too low,” Michalis explains. “A low RoR indicates that the coffee beans are taking a longer time to move through the roasting process. When the RoR is too low it can lead to stalling, which results in baked coffee.”

coffee packaging

As a specialty coffee roaster, it’s important to be able to identify and, ultimately, avoid roast defects.

At MTPak Coffee, we offer packaging solutions for perfectly-roasted, defect-free coffee, preserving the balanced and delicate flavours of each bean. You can also include information such as the roast date, roast profile, and recommended brewing methods with our fully customisable coffee pouches.

For more information on MTPak Coffee sustainable packaging, contact our team here.

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What Are Coffee Roast Defects And How Can Roasters Identify Them?
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