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Author Topic: A basic roast profile guide for home coffee roasters  (Read 26056 times)


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A basic roast profile guide for home coffee roasters
« Reply #25 on: 25/10/2016, 06:09 PM »
Since this thread had some recent activity, thought it'd be ok to comment....

The biggest influence on "Turning Point" is actually temperature probe.  Think about it - the beans are at 20*C for example, ambient temp.  The roaster temp probe says it's 200*C in the roaster.  If the probe had no thermal mass and accurately measured the bean's surface temp, as soon as you loaded the beans in the roaster it'd register 20*C.  But the temp probe's thermal mass and the fact that it's not measuring the bean temp, just the temp around the beans, means that the temp shows a decreasing trend - the beans are sucking a bit of heat out of the probe over time.  A low thermal mass probe (eg thin), it sucks heat out faster, a higher thermal mass probe sucks less heat out so takes longer to get down to the same point.   So now we have two probes that show a different "apparent decreasing temperature" rate or slope of the graph.  You have beans that are taking on the heat in the roaster, and are coming up in temperature no matter what the probe reading says.  At some point, you have these two factors coming together, which forms the turning point on the graph - but nothing, besides the reading, is "turning"; the beans have been increasing in temp all the way along. 

This is why comparing my turning point and your turning point is yet another "pointless" thing to do (pun intended).  I do agree that all the other factors mentioned above like energy input and green weight and charge temperature, that influence the apparent TP on my roaster help me understand my roaster and improve my roasts, but the time it takes for me to reach TP is not transferrable to your roaster or your probe location or your probe thermal mass. 

The most important point is that TP is not real - it's just a representation of what is going on when measuring beans through a medium that doesn't transfer heat well.

Some of what you are explaining Brett is correct, but there are important considerations so I would not demote the value or merit of managing TP on any device - particularly in the scheme of a roast batch.

My reference to the importance of turning point remains valid, especially for those wanting to roast larger batch sizes and people with experience on large roasting platforms will attest that it's "all about the turning point". it's the devil in the detail for lining up a roast that most inexperience punters fail to realize or comprehend until they start working backwards in the roast.

In my subsequent post on 1/10/11, it was noted that for some devices the TP does not matter so much and each has a unique characteristic.

The context of turning point is just a stage or phase marker in the lifecycle of a roast. Some engineers call it the inflection point and it is basically signalling the changes in momentum for temperature inside of the heating vessel and a critical control point depending upon the roasting platform - some may choose to ignore, others may act.

TP gives a very real indicator as how the bean (or beans) will respond in the roaster comparative to other similar coffees or batch sizes. There is quite a bit of debate in specialty coffee at the moment on how moisture levels are being incorrectly interpreted or mis-understood by roasters......instead of focusing upon density.

Behavior at TP can give better feedback to the operator than the scientific moisture and density analysis offered by testing samples beforehand. Ever since I can remember, I've not even looked at the mumbo-jumbo stats on a raw coffee because my roaster will tell me more about the bean than the labels for moisture and density.

I was told by a coffee broker that some roasters can't roast coffee without knowing the moisture and density levels.......because they don't know how to treat it......bizarre.

Another point of interest is that thermocouples (probes) are unlikely to affect overall thermal conditions inside of a roaster unless the platform or device itself is very small in relation to the thermocouple. Most of what I discuss relates to roasting platforms that have thermal mass, like metal.

The reason why curves or graphs show a declining value upon charging the roaster with ambient raw coffee is due to the ways in which thermocouples function. They don't actually measure absolute temperature like a thermometer, they determine voltage differentials at the junction, which is a reference value that is calculated via compensation.

The way in which a control or monitoring system arrives at a value for temperature from a thermocouple is quite complex because a large change in temperature causes just a small change in voltage at the thermocouple, e.g. de-amplification.

Accuracy of thermocouples is quite low and they are susceptible to noise and metallurgical variances. People also don't realize things like cable lengths, grounding versus non-grounding, joins/connectors introducing different metals, etc. all affect and influence thermocouple performance. A whole bunch of signal conditioning is needed to translate the sample values into an understandable reading.

In many respects, if the readings were displayed raw it would be showing like a seismograph and illegible  - you would see spikes of hi/lo in a messy array.

Thermocouples in a roaster are still recording the reflective heat from inside the vessel and averaging this with the much cooler raw coffee that is absorbing heat at phenomenal rates in the initial stages of a roast. Implementation specific such as contact time, probe response and the non-linear relationships between voltage and temperature affect what is being shown.

The coffee universe is full of contradictions.

A respected owner and industry expert of a roaster manufacturing company once told me it does not matter what happens until you reach 150 C......then the game begins........whereas others are aghast with such flippancy !

What can you believe other than what you see and taste for yourself.


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A basic roast profile guide for home coffee roasters
« Reply #26 on: 30/10/2016, 02:01 PM »
Very interesting reading. Thanks for the informative posts guys.
I love coffee. It's as simple as that.

Brett H

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A basic roast profile guide for home coffee roasters
« Reply #27 on: 30/10/2016, 10:17 PM »
Just wonderful! Thank you MyCuppa  :thumb:
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A basic roast profile guide for home coffee roasters
« Reply #28 on: 26/12/2017, 11:39 AM »
Is your charge temperature in Celsius and the rest of your temperatures in Fahrenheit? I was a little confused at first.

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A basic roast profile guide for home coffee roasters
« Reply #29 on: 26/12/2017, 11:46 AM »
Is your charge temperature in Celsius and the rest of your temperatures in Fahrenheit? I was a little confused at first.

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In Australia the standard for measures & temp are in metric
So unless otherwise stated always think in metric

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