March 6, 2017

Roast Defects & How to Recognise Them


Nobody achieves the perfect roast the first time, or the second time, or even the tenth time. It requires expertise and more than a few burnt beans to understand and control your roast profile. But it’s a lot easier to do that when you understand the roast defects you’re seeing.

I visited Tecnocafé in Bogota, Colombia to ask about the most common roast defects and how to recognise them. They’ve been running for 11 years, and in 2016 won 10 medals at the International Contest of Coffees Roasted in their countries of Origin in Paris. It’s safe to say that they know their roasts.

So read on to discover the defects Ricardo Villegas, Tecnocafé’s General Director, said all new – and experienced! – roasters need to know.

Spanish Version: 6 Defectos Comunes en el Tueste y Cómo Reconocerlos

coffee defects

There are many different roasting defects. Credit: Angie Molina Ospina

1. Baked Coffee

This defect occurs when coffee is heated for too long without reaching first crack. You may hear this referred to as “stalling” the roast. Unfortunately, this defect is invisible. It results in a distinctive flat flavour with little sweetness, often described as bread-like or papery.

coffee defects

Baked coffee is only recognisable by taste. Credit: Angie Molina Ospina

2. Underdevelopment

Underdeveloped beans tend to be “grassy”, lacking the caramelised sugars that occur in roasting. Sometimes, but not always, this happens when the roaster has set out to roast light but still needs to adjust their profile a little more.

coffee defects

Underdeveloped coffee beans. Credit: Angie Molina Ospina

3. Overdevelopment

The opposite of underdevelopment is overdevelopment. But it’s a fine line between a darker roast and overdeveloped coffee. In fact, Matt Perger of Barista Hustle takes the stance that there’s no such thing as overdevelopment.

Roasting darker than you intended is still an error, however, and it’s one that a lot of specialty consumers won’t appreciate. The beans will look dark and oily, sometimes even approaching black. The cup will be burnt and bitter, with smoky, coal notes.

coffee defects

Dark beans are just one of the issues with this selection.

4. Quakers

Quakers are unripened beans that are hard to identify during hand sorting and green bean inspection. They’re often, but not always, caused by poor soil conditions which limit sugar and starch development. Technically this isn’t a roast defect, but often you’ll only discover it after roasting.

Quakers will be lighter in colour than the rest of the batch. If they’re not removed, the taste in the cup will be dry, with papery and cereal notes.

coffee defects

Quakers, baked coffee, and more. Credit: Alvin W. Kim

SEE ALSO: 7 Green Bean Defects Roasters & Producers Need to Recognise

5. Scorching

Scorching happens when the “charge temperature”, which is the initial temperature, is overly high and the drum speed isn’t fast enough. It’s easy to recognise. Dark, burnt patches will appear on flat sections of the coffee bean surface – it’s literally been scorched. These beans will taste oily, smoky, and even, Ricardo tells me, like roasted poultry.

coffee defects

Several of these beans are scorched. 

  1. Tipping

This may look similar to scorching, but the main difference is that the burn marks are on the edges of the beans. Ricardo tells me this normally happens during the second crack, although some also say that a too-high charge temperature can also cause it.

coffee defects

Ricardo Villegas of Tecno Café explains the importance of uniformity. Credit: Angie Molina Ospina

How Can You Detect & Prevent Defects?

Ricardo tells me roasters can use specialist equipment, such as coffee roast analysers. These won’t prevent defects, but they will help you to measure consistency in roasting. However, these tools can be pricey.

At Tecnocafé, Ricardo uses a quality control sheet to record the different physical changes the coffee goes through. He also records the Rate of Rise (ROR1, ROR2), which indicates the speed of the roast in degrees per minute. This will help you to understand what is happening to the beans, and from there how to adjust your roasting to prevent defects.

What’s more, recording this information can help you to find the best profile for each coffee. As Ricardo tells me, “The roast profile can never be judged by its colour alone.” Two beans may look the same on the outside, but it’s how they got to that colour that determines much of the final cup profile.

coffee defects

Making notes on a roast. Credit: Angie Molina Ospina

Roasting is an artform as well as a science. The producer may have cultivated and processed those beans with passion, precision, and dedication – but without good roasting, the consumer will never taste that.

Choosing the best profile requires technical know-how and often that intuition that comes with experience. But recognising common defects is an important first step. So keep studying, roasting, and learning until you crack the perfect roast profile.

Perfect Daily Grind is not affiliated with any of the individuals or bodies mentioned in this article, and cannot directly endorse them.

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