Curve explained: Normal


In the era of tracking software, there is a curve paradigm. "It's gotta be instagrammable". While a bit exaggerated, you get the point. Roasters are obsessed to make their RoRs straight.

For a very good reason, in fact.


This article is the third in the series. If you haven't read previous posts about CURVE DESIGN or ROAST PHASES, click the links now - it will help you understand this one.




This curve is the most common one. The defining trait is a declining BT RoR. It's mainstream because it's the easiest conceptually. Easy to teach, easy to spot mistakes and correct them, easy to execute.

Heat application pattern

There's not a lot of manipulation. Heat is absorbed gradually - as the beans rise in temperature, gas is lowered. Thus, the roast is progressively slower and slower. That results in steadily declining RoR.

This roast has been tracked, but I didn't look at Cropster while roasting. I have roasted it in an "old-school" way, based on smell and colour. The charge was at the standard temperature for that batch size. I used a soak, after about 2min the gas was increased - to the highest setting during this roast. 

I don't change it until coffee turns yellow. On the colour change, I reduce the gas by half. The coffee has gained some heat already (reaching about 150 degrees) and it has less moisture, so it doesn't need as much energy.

About 40 seconds before the first crack, I reduce the gas in half again. That's the setting the coffee went through the first crack. Establishing a correct FC gas setting is crucial in this style of roasting.

Because it was a very light roast I dropped the batch 40s after the first crack. There was no need for another gas reduction.

Some coffees are not going to follow exactly the same pattern. Particularly around the first crack. The heat application will derive from the above. For example, you might reduce the gas a little bit earlier than a colour change. Or reduce it not by 50%, but by 40% or 60%. And most likely have another 5-15 gas changes during the Maillard stage, to make it more gradual.
From top to bottom (right side): ET, BT, BT RoR, ET RoR. You can see where the gas was reduced looking at dips in ET RoR.

Interpretation

The curve is smooth as a result of gas application based on physical and chemical changes of the bean. The initial high setting allows the heat to penetrate the beans. Especially in the very beginning of the drying phase.

Before beans reach 100 degrees, moisture inside acts as a carrier of the energy. After that, it takes much more energy to heat the water as it turns into steam and create internal pressure. That's why the RoR starts decreasing after this point.

As the moisture evaporates and the beans rise in temperature it requires less and less energy to keep them going. Thus from colour change onwards, gas settings steadily decrease.

That allows for the heat to penetrate inside, without overroasting the outside of the beans.

This roasting strategy is not the only one, but certainly the easiest. Usually, my first roast of any coffee would use it. There's plenty you can learn about the coffee this way. 

But it has its limitations.

Crash and flick (baked and roasty) 



The crashes don't get bigger than this. There was little sweetness, acidity or flavour in this coffee.


One of the biggest challenges is avoiding baked flavours. Almost everyone defines baking differently, but it's commonly agreed that BT RoR crash after the FC, could cause it. With this RoR shape, you may find little room for your airflow and gas settings around the FC to avoid crashing.

More developed roasts will be prone to RoR flicking, causing unnecessary roasty flavours.


That's a tremendous flick, after a big crash too. This poor Kenya was absolutely butchered - unfortunately, this roasting style has these traps set up for you with these kinds of coffee.


Modulation


If you're keen on adjusting phases of the roast, you have little wiggle room. While it's easy to adjust the drying phase it gets more challenging further down.

The Development Time (DT) needs a good percentage for even development. Aiming for a specific RoR value could help, but it changes the dynamic of the FC.

It's very tricky with the Maillard phase. Changing its length without affecting drying and DT quickly leads to thinking about the angle of the RoR curve. But that is correlated with the length. What if you don't want to change it?

It is possible, but it's very hard. 

The beautiful curve

Another risk is relying on visual data (shape of the curve) more than the taste of the coffee. I've fallen in that trap myself.

The curve is only one of the tools to help you. You're roasting coffee, not drawing lines. The consumer doesn't care how pretty the curve was, if it doesn't taste good.

The normal curve is great! I roast the majority of my coffees using a form of this curve. It gives you great insight into how the coffee behaves when roasting. Like any other, it has its challenges, but there are solid strategies to mitigate them.

The next article in the series: 

The previous article in the series: Roast Phases Explained


References for the series: Roasting Series References

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