Is it possible to roast the inner bean of a coffee darker than the outside? This is the conversation that I had with Glen from Prodigal Roasters recently. He believes that it is possible and that he has been doing it consistently for quite some time now.
On the other hand, I didn't think it at all possible. After every roast, we would always measure the Agtron color of the coffee as part of our Quality Control (QC) measures. This includes both the readings of the Whole Bean (the exterior) and the Ground Coffee (combination of both the exterior and interior).
Cracking open the coffee
I have yet to come across any coffee, whether roasted by myself or other roasters, where the Agtron color of the ground coffee is darker than the whole bean. This could mean
1. It isn't possible to roast the inner bean darker than the outer bean or
2. I just haven't come across a coffee that has achieved this yet.
At the same time, I can see why Glen would think it possible. I used to believe it was possible too. When you crack open a roasted coffee bean, there are instances where you can clearly see a different color between the exterior of the bean and the inner bean.
Here's an example of such a coffee .
There is a visible difference in colour between the exterior layer of the coffee and the inner bean. Yet, when you put it through the colour meter, you'd still find the Agtron color to be darker with the whole bean compared to the ground coffee.
Does this indicate that the colour meter is flawed and giving false readings? Or does it show that the total percentage of the coffee is still lighter than the exterior of the coffee? That maybe some parts of the inner bean may be darker than the exterior but overall it is still lighter?
So what's the correct answer?
According to Willem Boot, the interior of a coffee bean being darker than the exterior could be a sign of interior scorching, a roasting error/defect. You can read more about it in his 2005 article from Roast Magazine here.
The examples shown in these pictures are clearly much more severe compared to the initial picture I shared. The interior is considerably black and burnt compared to the coffee's exterior. While I can definitely see this as a case of interior scorching, I wouldn't go as far as to say that all coffees with a visually darker interior indicates a roasting defect.
At the end of the day, as Willem suggests in his article, the most important test would be to cup the coffee. The final taste of the coffee is the deciding factor as to whether the cofee has been properly roasted or whether there are the presence of undesirable attributes.
Still, I feel that from a basic physics standpoint, it is not possible to actually roast the interior of the bean darker than the exterior as the heat penetrates from outside-in. Therefore the outside will always experience more heat, and thus be more cooked compared to the inside of the bean. This would also explain why the ground coffee agtron color is always lighter than the whole bean.
But I could be completely wrong here. The bean is not a piece of meat. It is porous and is built in a layered structure with lots of air pockets in between which could possibly allow hot air in to roast the interior of the coffee. It is theoretically possible to cool the exterior of the bean and allow the inside to continue caramelising as the inner coffee reaches significantly higher tenperatures during the roasting process that is high enough to continue the caramelisation.
What do you think? Do you agree or disagree? Comment your thoughts in the comments section below and let's discuss. I would really love to hear your opinion!
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And if you have any coffee questions you'd like us to answer, leave them in the comments section below and we'll make sure to answer them in future posts. P.S. If you are one of those who voted yes on being able to grill the inside of a steak more than the outside, I'd really like to hear why and how you would do it. It's not physically possible, is it?
the green bean inner cell structure is weaker than the outer part ( the inner part is the new cell development )
A very interesting piece, thanks for sharing! While I do not have a definitive answer yet, I can imagine doing this study with numerical simulations and gather further insights (Just briefly, I am from engineering with focus on fluid mechanics and heat-related problems).Will keep in mind and launch the research when time is right. Do let me know if you think the findings can be helpful to your work.
Replied by : Kenneth Ong On 11/29/2019