Explanation of yesterday’s graph

January 15th, 2012

So yesterday I posted up a little graph to see if people could identify it.  It was a tricky one, and I was pleased when Kevin Cuddeback, CEO of Gimme! Coffee, chimed in with the correct answer.  What the graph represents is the rate of increase of temperature in the bean mass (calculated to be per minute) measured every 30 seconds in a roast.  I removed the first part because formatting a graph to look nice when you’ve got to drop to -100 on your x-axis is difficult.  Plus, it would have made things too easy!

Here is the graph again, properly labelled:

I posted this for a couple of reasons:  Primarily I thought it was interesting.  Secondly I wondered if other people were looking at roast data like this.  I’ve only produced a few of these curves, as I have to do it manually at the moment, and they are confusing to read.  Bear in mind – if the graph were to flatten out it would simply mean that the bean mass is increasing in temperature at a steady rate.  The roast curve would be point up, it would just be straighter.

Other roasts have produced extremely different profiles.  I need to get more data and start doing comparisons!

This is also interesting because there are changes in the rate of the bean mass absorbing heat that don’t correlate to changes in gas or airflow.  I don’t know whether evaporative cooling has much impact on rate, or why we don’t see a faster uptake of heat once the coffee is dry.  The curve on this roast between 6 and 8 minutes is particularly interesting to me.

I should note that this was a test, rather than production roast.  It was dropped not far past 1st crack, as you may surmise from the graph.  We have a colour meter but it doesn’t produce accurate Agtron numbers.  It cupped pretty well, but was out-cupped by a slightly different profile that I don’t have this kind of data for.  (Annoyingly)

Anyway – I thought it was interesting, and I hope it might generate some discussion – because we are really bad at openly discussing roasting theory online.

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