Roasting curve design

The biggest innovation in roasting was a roast tracking software. A continuous real-time data collection and roast curves make it much easier.

What is a curve?

A curve is an object similar to a line which does not have to be straight. Intuitively, a curve may be thought as the trace left by a moving point - Wikipedia

The roasting curve is a function of time and temperature. You can draw one yourself if you have a stopwatch and a temperature readout. In fact, before tracking software, that's how it was done. But it was challenging to keep that data accurate, therefore consistency was elusive.

But do the roasting curves make it easier to understand the process?

Yes and no.

The chances are if you're not a roaster, you're not going to understand this:

When analysing the roast curves, you could be looking at 4 or more, plus additional data like gas changes. It's hard to understand it if you don't know what to look for. In fact, even experienced roasters might not know what to look for. 

Let me break down each curve and what it represents.

Bean temperature 

Arguably the most important curve is a Bean Temperature curve (BT). It tracks the temperature of beans during the roast. If you were to have only one probe to measure temperature it would be this one. It forms a "V" shape, like such:

On the Y-axis (vertical) you have a temperature scale. On the X-axis (horizontal) you have Time.

That "V" seems counterintuitive. Putting room temperature green beans into hot roaster is going to make them hotter immediately, right?
But before that, the bean probe is measuring air inside, which at this point could be around 200 degrees.

After charging, the probe is suddenly covered in cold coffee beans. It takes a minute or two for the probe to reach equilibrium with coffee. Since then you can trust the probe as the rough approximation of the bean temperature.

Rate of rise

The most exciting functionality of roasting software is a Rate of Rise (RoR).
RoR is a change of temperature over a period of time. For example, let's take the BT curve from above and calculate RoR using 60s interval:
- at 5min, the temperature is 160 (I'm rounding to 10)
- at 6 min, the temperature is 170

Over the minute, the temperature has changed by 10 degrees. So, your RoR at 6 min is 10.

You can calculate RoR based on any interval you want, frequently roasters choose 30s or 15s, as it gives you a more responsive curve that 60s.

You'll notice, on the right Y-axis is a scale for RoR, here using a 15s interval.

BT RoR shows you the speed at which your roast is progressing at any given point in time. Thus you can notice if your roast is on track before BT curve would indicate that.

Majority of roasters spend most curve-analysis-time on this one. It's a good indicator of a heat application - the addition of heat in particular ways at particular times.
There are many theories created around it, each one deserves a separate article (coming soon!).

Exhaust temperature

It tracks the temperature of the air leaving roaster during the roast. Typically it increases until before the first crack. Why measure it?

ET and BT are using the same Temperature scale. You can notice a sharp increase around 2 min mark - that was the first gas increase.

It gives you an understanding of the flow of heat through your roaster. That's important because BT probe shows you temperature only in one spot. But the roast depends on the thermal energy of the entire machine. If you were to charge the drum with BT: 200 and ET: 180, when usually you use BT: 200 and ET: 210 you risk underdeveloped roast. Without ET probe you can only guess.

Besides, the ET curve provides more reference points and indicators. For example, in some styles of roasting, ET RoR could be used to mark the first crack.

ET RoR peaks at 42.

Why are the curves so important?

Replicating roasts

The biggest challenge in roasting is consistency. Making 2 roasts that are identical takes a lot of skill. 

Roasting software allows you to save previously executed roasts with all details (curves, gas changes, airflow etc.). Using a reference profile for every batch is the standard. 

Tracking curves is the easiest method of replicating roasts. Even if some of your parameters are a bit off or miss a gas change, you still have a good chance of matching the colour and flavour of the coffee. As long as your curves are close enough to the reference.

The next post in the series: Roast Phases Explained

References for the series: Roasting Series References


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