Explanation of yesterday’s graph

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.

14 thoughts on “Explanation of yesterday’s graph

  1. The linearity of the curve through minute 5.5 is a result of the fact that water is about 3x more thermally conduc(t)ive than the cell structure (cellulose) of the coffee bean, and it is being eliminated at a steady rate?  As more unbound water is driven off, the sugars are liquifying and wicking through the cellulose transferring heat?  Maybe that curve from 5.5-7min is related to the build-up of  water vapor inside the cell walls causing the bean to almost double in size; aka less dense, aka a better heat conductor than if that expansion had not occurred?  

  2. I think that’s it Kevin.  The ‘free moisture’ is largely gone by the first crack and at that point the cell walls are breaking down releasing the bound water.  This is where beans go from endothermic to exothermic – they’re generating their own heat.  This would be the point where the heat increases get larger over the same time periods and it’s where increasing airflow could make the increases even greater (adding air to a fire).   The beans are reaching the carbonization stage and passing through the caramelization stage. 

    I have a question about airflow at this point that a roaster who has cupped more than I might be able to answer – if I leave airflow through the cooling bin am I imparting any ‘smokey’ flavour to the roast?  I’m thinking Lapsang Souchong here!

  3. > 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

    It’s very hard to speculate on this without seeing the environmental temp and airflow superimposed on the same graph.

    But still, I assumed that the change in slope (ie, decrease in slowdown of uptake of heat) WAS because the coffee had dried ~ minute 5:30.

  4. We’re there any heat application adjustments during the roast? And what percent was the change, 100% being full throttle. And like AndyS asked, do you also have environment / air temp?

  5. Is the word ‘temperature’ missing in your 3rd sentence above, James?… I may be easily confused but was perplexed at first because it says the mass is increasing, not the BMT, and mass of course decreases during drying. Or have I misunderstood?

  6. David, that’s entirely up to your machine, but based on your verbiage I would assume that you are roasting on a Diedrich? If that’s the case, and what is generally the case otherwise, the airflow damper system is maxxed at an 80/20 either way, so you’d never fully trap all of the smoke. You would decrease the amount of smoke that was being eliminated from the roasting chamber, but I doubt that it would produce a “smoking” element that would be noticeable beyond the actual roastiness of the coffee. 

    James, this is a cool graph. 

  7. Thanks Christopher.  I have thought about modifying the damper control for a wider range of airflow but thus far I don’t know if it would be worthwhile for this purpose and a few other ideas.  If you’ve done it (or anyone else) and want to share results send me a note david at stick in the mud dot ca.  No spaces, no caps.  Thanks again.

  8. In mathematical terms, isnt this curve the ‘first derivative’ of the roast profile and hence can be obtained from any roast profile that has been recorded? How is the insight from this curve any different from that obtained from a roast profile.

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