Why do roasters turn heat off at charge? - Page 3

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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Postby lyle » Feb 14, 2016, 7:23 pm

Wow, thanks for the lively discussion, guys. I'm reading with great interest and learning lots.

Based on what I'm learning, this drop-the-heat-on-charge process might be useful for me, because I've settled into a lower charge temp due to problems I was having with underdevelopment. I was trying to apply Rao's "commandments" with a typically high charge temp, but my roasts were going too fast. Lowering my charge temp has allowed me to achieve pretty good results that do match my target profiles (declining ROR, etc.), but I was wondering what I might be missing. Charging into a hotter drum, but with the heat off, might help me start hot but also prevent me from blasting through the first pasrt of the roast too quickly.

Thanks again!

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Postby cimarronEric » Feb 15, 2016, 12:51 am

If you haven't read through this thread, it may provide some insight: What's Your Charge Temperature?

Turning the roaster off or way down helps to avoid a dramatic drop in RoR at the very beginning of the roast.
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Postby 9Sbeans » Feb 16, 2016, 1:31 pm

day wrote:If we preheat to 300 degrees, turn off the gas and just leave it off, then at some point the bean temperature and environmental temp will stabilize and be equal, at first the beans will heat up very quickly But as they get closer to the et the ror will slow down. Thus, let's call that initial ror 60f/min. As the bt reaches 150degrees the drum is still much hotter than the beans but is losing heat and the two are much closer in temp, so now the ror will be lower, let's call it 40f/min

Well said, Brian.
But may I ask a follow-up question, how you reconcile charging different bean mass in this case? Set aside the non-equal contributions of radiation, conduction and convection, is it theoretically possible to achieve similar heating (RoR) patterns when you load 0.5X, 1X and 2X of bean in the case?

Some people use capacitor as the analogy of the drum mass, but I'm no expert. Assuming (drum temperature equals to et) in your above example, and you preheat to 300F and charge 1X of bean. The temperature difference between the drum and the bean reduces overtime.
If you charge 2X of bean at the same 300F, the temperature difference would reduce faster and drop to a lower temperature. If we want to best maintain the BT/RoR pattern and to compensate the effect of greater charge mass, we need to increase the charge temperature and supply additional heat during the process (hence leave the gas on).
If you charge 0.5X of bean at the same temperature as you charge 1X of bean, the rising temperature may go out of control. Gas-off is a method to tame the heat in this low-charge-mass situation.

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Postby happycat » Feb 17, 2016, 9:57 pm

Thanks for this thread! Solved my dry process headache.

With wet process I could do a high charge temp and ride it our with tweaks near end to cut power near FC and restore after FC to avoid flick of death.

But dry process was a headache due to differing bean densities causing both under and over roasting in same batch and confusion of flavours. Low charge temps ran out of steam and high charge temps led to short drying and sometimes out of control FC temps going stratospheric.

However a high charge temp followed by zeroing power then full power around the turning point led to a longer drying time and manageable curve and enough control to tweak a steady ROR decline. Tasted pretty good on brewing... Sweet, acidity, no conflicting extractions.


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Postby farmroast » Feb 20, 2016, 12:34 am

the green bean surface is very delicate. Too high conduction at start will cause some surface pore sealing and if continued scorching. Also vulnerable to convection ET that creates too much early activity of the surface area liquids. Beans are curls with splits/cavities and split skins, more to consider than just getting heat to the center as if solid. Once there is activity throughout they can handle heat much better.
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