"Nonexistence of Heat Momentum"

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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TomC
Team HB

#1: Post by TomC » Nov 02, 2019, 12:00 am

Using the same title and crediting the author of the original post at CoffeeSnobs.au; Lyrebird, broke down the thermodynamics present in coffee roasting in an impressive yet somewhat impenetrable manner full of equations. Basically shooting down any claim that heat momentum occurs in the bean mass during roasting. He goes on to state that exothermic reactions from the beans essentially doesn't happen unless your beans are on fire. The topic of endothermic vs exothermic roast reactions have been discussed here on HB in the past too.

Very interesting read.

https://coffeesnobs.com.au/home-roastin ... entum.html

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another_jim
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#2: Post by another_jim » Nov 02, 2019, 12:28 am

Thanks for this antidote to ROR blather (the main reason I've stopped posting a lot on home roasting -- I gave a much simplified version of these equations some time ago, and was ignored. I hope people will pay more attention to these)

Here's a bit of backstory.
  • Everyone who ever roasted since the 17th century assumed these equations (they are that old), since they basically describe what happens in an oven -- the oven's heat gradually raises the temperature of what is being roasted, in proportion to the temperature difference between the oven and the roast.
  • People using drum roasters knew from experience that they needed to cut the heat drastically as one reached first crack in order not to have the roast "run away," i.e. finish much too fast
  • Sivetz claimed to have seen an "exothermic" spike at first crack. This spike was never replicated and has been derided in all peer reviewed literature ever since
  • Depite that, it is an actual explanation of why drum roasters see the roast running away on them.
  • Notice that the basic thermodynamics has one expect that the rate of rise must gradually drop as the bean temperature gets closer to the environmental temperature. This happens in air roasters, but not so much during the first crack in drum roasters. Therefore, the idea of thermal momentum and exothermy survive as explanations
.

So the real question is what does happen in drum roasters to create a faster rise in temperature during the first crack. My KISS answer is that the first crack happens. This is a release of superheated steam. In an air roaster, where the volume of airflow per bean is very high, this steam heating is lost. In an drum roaster, where the volume of airflow per bean is low, the super heated steam affects the thermometry.

Does it also affect the actual roast? It's like heating the surface of the bean with the heat contained within it. So a runaway at first crack may create very uneven roasts, much darker on the outside than the inside, rather than accelerating the overall roast. This is easy to test by taking interior and exterior agtron readings of air versus runaway versus stalled drum roasts. It would also explain what drum roasters see without invoking a special case coffee roasting physics.
Jim Schulman
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TomC
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#3: Post by TomC » Nov 02, 2019, 1:13 am

another_jim wrote: ...It would also explain what drum roasters see without invoking a special case coffee roasting physics.
I post much less, since I think it's hard to communicate data in a way that is consistent amongst other roasting platforms. And I don't care to win internet roasting arguments when at the end of the day, what matters most to me is that I enjoy the profiles I establish, separate of dogma about what's shown on my datalogger.

But his comments also explain why drum roasters lag behind fluid bed/air roasters due to the heat absorption difference of the drum as opposed to fluid bed. The heat absorption of a mild steel drum alters heat transfer in a way that isn't seen in fluid bed roasting since it's adding a material not present in air roasting. We should be throttling heat application more aggressively at the tail end of the roast if we're using classic drum roasters when moisture levels are drastically reduced. In my opinion we're flying a tad bit more blind in drum roasting (and that's perfectly ok) as long as we're aware of what's going on and can plan accordingly.

I could never explain with concise scientific terms, but I believed fundamentally that a bed of coffee being tossed violently inside a drum were ever much exothermic, because that would imply that once that state is established, one would have to make corrections to prevent "run-away-heat" which never made sense. Unless your roast started to combust, they're never going to add heat to the system. Is your roast on fire? It's exothermic. If not, it's not exothermic.

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doublehelix

#4: Post by doublehelix » Nov 02, 2019, 8:51 am

This poster on studying exothermic reactions during roasting looks very interesting:
https://www.researchgate.net/publicatio ... n_roasting

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Almico
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#5: Post by Almico » Nov 02, 2019, 1:12 pm

I got into coffee so I don't have to do math any longer...

That said, I have noticed an interesting phenomenon when taking coffees almost to 2C. I can find a profile to illustrate, but it's not necessary.

In a nutshell, as a roast passes 1C, in order to keep the RoR moving nicely down in a consistent slope, I need to reduce heat to almost nil by 1C+ 45s. By 1:00 I turn the heat off completely. If I do not reduce the slope (heat) enough pre-1C, the RoR will begin to rise a full minute after the heat has been turned off.

In other words, I'm watching the RoR trending down in a nice 30* slope, I turn the heat off and let the heat in the drum to finish the roast. The slope continues down for another minute and then for no reason I can think of, begins to rise with no outside heat being added.

So the question is: how is my RoR increasing, even though there is no flame for over a minute?

And when it does increase, the coffee gets a roastiness that it would not have if the slope continued downward, but that's another topic.

crunchybean

#6: Post by crunchybean » Nov 02, 2019, 1:42 pm

In between 1C and 2C the coffee ligins, hemicellulose and cellulose transitions back from a plastic state back to a glass state, no longer fully absorbing most heat applied to it, it starts reflecting heat back in a higher proportion.

I ask: how can people believe a person just because they have fancy equations (as if they could understand them in the first place). Ive been saying "nay" on the exothermic for a while now, wheres my credit?

I still like to use the idea of momentum, even though I have known plant biomass is a) not a good conductor of heat b) carries no/little actual thermal momentum. But is a great way to portray an idea even if it is invalid.

crunchybean

#7: Post by crunchybean » Nov 02, 2019, 1:56 pm

And he is wrong about sugar browning is endothermic. I believe it is the Strecker degradation that does release some heat and not Millard but we umbrella Millard, Strecker and aldol condensation as "Millard/browning".

And since we "I" am nitpicking, heat transference is called diffusion.

OldNuc

#8: Post by OldNuc » Nov 02, 2019, 3:17 pm

The simple answer is the coffee, like every other cellulose material, is endothermic at lower temperatures. At some point determined by the specific material it becomes exothermic. This means the rate of internal oxidation has reached the point of being self sustaining and increasing with temperature. This is why you can have those lovely roaster fires.

Marcelnl

#9: Post by Marcelnl » Nov 03, 2019, 8:46 am

In my mind I've always compared a bean going through FC to putting an egg in shell in the microwave at full blast, partly because I could not come up with a chemical reaction that would create that much energy and partly because like Jim I believe in simple answers.
LMWDP #483

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Almico
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#10: Post by Almico » Nov 03, 2019, 1:03 pm

another_jim wrote:So the real question is what does happen in drum roasters to create a faster rise in temperature during the first crack.
My experience is the opposite happens. On high density coffees, my "uncompensated" RoRs decline rapidly at the onset of 1C and continue downward for 30-45s. Then they rise equally as quickly after about a minute. Less dense Brazils will tend to rise at 1C. I believe this is due to the lower density coffee giving off its water more readily, and earlier, than it denser cousins. There is less pressure held intact at 1C. Fortunately, the resolution for both is to reduce the heat consistently from DE through 1C.

My thinking is that as free water dissipates significantly through DE, there is little to no free water left to mitigate the heat transfer. The same amount of heat pre-DE will cause the RoR to increase too much post-DE. At 1C when all the bound water is released, the coffee is now dry and much less heat is needed to continue to roast. The initial expulsion of bound water is what causes the probe to perceive a drop in RoR. If left unchecked, the now dry coffee will take on heat much faster, resulting in a spiking of RoR and make it appear like an exothermic reaction.