The sensory impact of colour and roasting time

Colour and time are the most important roasting factors in creating coffee flavour. But there has been little research done on that. The opinions of the roasting community vary. Morten Münchow, Jesper Alstrup, Ida Steen and Davide Giacalone published a new paper which can change that.
It's free to access so I encourage everyone to read it:

In this article, I'm going to break it down and look for the practical application of findings.

The Context
It's based on sensory analysis, which makes it very practical. It's also the first scientific study of roast subphases.

The paper is very well structured and easy to comprehend. In case you're new to roasting you need to be aware of a few things to understand it.

Roast Colour is the best indicator of roast level or the degree of chemical reactions that occurred. For example, you can say a darker colour corresponds with more development. It represents a point on a scale from light brown to almost black. The most common scale is Agtron Gourmet. This scale is used in this study. For Agtron, high values (like 110) mean light colour, whereas low values (like 45) mean dark colour. 

Roasting is a combination of temperature and time. Higher end-temperatures result in higher colour readings. That's because increasing the temperature speeds up many browning reactions. But every chemical reaction depends on time as well. If you give them more time, they will develop further. To achieve a balanced result you have to manipulate both variables.

Good roasters think about the whole process not just as the time from start to finish, but also specific roast phases. I wrote a separate article about them here

This paper is analysing 2 subphases:

  • Time to First Crack
  • Development Time

 Model illustrating temperature development over time for the roasting profiles and the major phases (Time to first crack, development time) varied in this research.

What was the goal of the research?

  1.  What is the impact of colour on the cup and individual attributes?
  2.  What are the impacts of overall time and specific roast phases time on the cup and individual attributes?
  3.  How do colour and time compare in their impact?

Those are incredibly practical objectives. Many roasters have had similar questions trying to understand roasting and its impact on the coffee. Certainly, I did.

The paper is based on 8 separate studies. All roasts used drum roasters, but the machines varied. Study no. 6 used a Loring S15, all other used Probat Probatino 1kg. The roasting style was a normal heat application. For simplicity, they used constant airflow and charge temperature (except for study no. 1).

The coffees used for this study were all washed: Kenyan, Ethiopian and Colombian.

The coffee was brewed in a French Press using 50g coffee to 900ml of water at 92 degrees. The brew was stirred, the crust from the top removed and finished at 3:30 min. The brewed coffee was stored in thermoses, before pouring into cupping bowls (200 ml) for evaluation at 60 degrees coffee temperature. 

The coffee was evaluated through SDA - Sensory Descriptive Analysis. Meaning the panellists rated each attribute on a scale, in this case using 0-15. The attributes used:
  • Aroma (Roasted bread, Fruity, Cocoa, Nutty/chocolate)
  • Basic taste (Acidity, Bitterness, Sweetness)
  • Mouthfeel (Body)
  • Aftertaste 
  • Other (Balance, Clean cup)

The data analysis was conducted in the statistical environment R, using a 5% significance level.

The colour has a more significant impact than roast time. The darker the coffee, the more bitterness it had, but less acidity and sweetness. Darker colour indicates more advanced chemical reactions.

Both time to first crack and Development Time have a similar impact on the cup when it comes to bitterness and acidity. But to a smaller degree than colour.

Longer Development Time correlates with lower sweetness, but the time to first crack doesn't affect it much.

When it comes to body, darker colour increased it slightly. However, either of the roasting phases had no impact on that attribute. It's important to note that the body is perceived by everyone a bit differently.

Practical application
Measuring colour and achieving consistent results is super important. It’s the most important factor in modulating flavour of the coffee. Moreover, colour is a better predictor of flavour balance than Development Time.

Development time influences taste similarly to colour, but to a smaller degree. I like to think about manipulating colour and roast phases as different tools to achieve the same goal but on a different scale. 

Imagine that you want to sharpen a blunt knife. It's so blunt that it can't even cut a tomato without smashing it. For that, you need a sharpening stone of coarse grit. Something that will aggressively shape the cutting edge. It won't make it razor-sharp, but it will allow the knife to cut again. This stone would be manipulating the colour.

But the knife is not quite sharp yet. You need to use plenty of pressure to cut anything harder than a cooked carrot. That's when a finer grit stone comes into place. You won't be able to see any difference to the cutting edge with the naked eye. But on a micro-scale, it gets a hundred times thinner. Suddenly the knife cut effortlessly. That finer stone would be manipulating the Development Time.

Finally, all roasting phases have an impact on the final product. Even though modulating the Development Time yields more immediate results, time to first crack changes the taste too. Long Maillard stage increases bitterness through the creation of melanoidins. Some acids already start to decompose, leading to lower acidity if that phase is extended. Manipulating here would be the polishing touch to make the blade razor-sharp.

Roasting Conditions And Coffee Flavor is a fantastic paper. Very much needed. Scientific research will never take away the craft of figuring out how to roast the best coffee. But it can provide a solid ground for further experimentation and confirm some of our existing beliefs. As a roaster, I'm very grateful for the work and the free access. Thank you!

The curves graphic is a figure from the paper -


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