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Roasting Theory
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Perhaps. I will leave it to someone more knowledgeable and have the right equipment to find out.
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Yes and No.
Yes it considers only the purely chemical changes rather than the physico chemical changes. No it isn't likely that this is enough to cause the result: the physico-chemical changes are highly endothermic due to the enthalpy of vaporisation of water.
I do not buy the idea that internal pressure has any notable effect on the chemical reactions: the typical maximal pressure inside the bean is about 20 kPa, not enough to affect any reaction much (obviously gas phase reactions will increase in rate but my understanding is that there aren't any important gas phase reactions in roasting.
The idea that the bean changes are primarily driven by steam pressure is similarly frequently overstated. It looks more likely that they are primarily driven by changes in the structural carbohydrates in the cell walls
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Lyrebird, thank you for providing the reference. It was an interesting read. I note that in this study they first grind the green bean into a powder and then dried it. The powder is then heated in a tube with probes to record readings. By my interpretation, this study is about the chemistry of the coffee in isolation (i.e. independent of the physical structure of the coffee bean).
I understand that when a whole bean is roasted, the moisture inside the bean turns into steam and becomes pressurised, i.e. each bean is like a miniature pressure cooker. The pressure gets released when the structure of the bean changes during the roast. The study does not consider the contribution of the bean pressure to the chemistry and heat transfer during roasting. Perhaps that explains the difference between this study and others.
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Don't knw how to quote from a comment.
Lancruiser asked "would be interested if you could cite any study that shows that coffee roasting is an entirely endothermic process."
Vladimir Strezov & Tim J. Evans (2005) Thermal Analysis of the Reactions and
Kinetics of Green Coffee During Roasting, International Journal of Food Properties, 8:1, 101-111,
DOI: 10.1081/JFP-200048060
Fig 4 shows the heats of reactions of roasting coffee vs temperature. It is initially neutral then becomes endothermic at about 100oC and remains so until 260oC. Above this it becomes exothermic. If you just glossed the paper you would come to the conclusion that coffee roasting was both endothermic and exothermic.
My statement was that a normal roast is entirely endothermic, "a vigorous exotherm at greater than 260oC" is a chemist's description of a roaster fire.
I read this paper in great detail because I wanted to use their data on the Arrhenius kinetics of the roast reactions to construct a predictive colour algorithm.Last edited by Lyrebird; 24 February 2021, 08:52 PM.
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I actually do this with my setup because the "Big Loaf" uses pans that are relatively shallow compared to most, and I have observed scorching/tipping if I allowed the HG point downwards directly at the bean bed. Using an insulated lid and insulating the bread-pan does mitigate this somewhat as lower HG output is then possible.
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I reckon your spot on.Originally posted by flynnaus View Post
That's the first time I've heard that and not sure how you would achieve it. Certainly position the heatgun so it isn't too close to the bean mass to avoid scorched beans.
I think all the wisdom needed for home roasters is here on CS...and your own taste
We can theorise all we want, in the end it's what we finish up with that is important, that's easily done by experimenting with a few variables, very easy to manage with a Coretto set up.
Obviously keeping notes, particularly early in your amateur roasting career will be of great benefit. ?
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Lyrebird . But how does the description (correct or otherwise) of stages of the roast being exothermic / endothermic help address the specific questions asked by the OP (i.e. the effect in the cup of extending various stages of the roast)? I realise that the issue is relevant to the broader topic of 'Roasting Theory'.
FWIW, Bonnlander et al. in Chapter 4 (pp.179-181) of Illy and Viani (2005, 2nd Edn) describe the process thus:
"From a chemical-engineering point of view the roasting process consists of a combined heat and mass transport superposedby endothermic and exothermic reactions. Thus the application of heat to the coffee beans not only generates a temperature field , it also causes inner pressures and a re-distribution of moisture depending on time and location...[a paragraph describing the endothermic part related to the evaporation of moisture in the bean as temperature rises]...Roasting reaction...begin at higher temperatures...starting at the bean surface and moving toward the inner dry pre-expanded sturcture of the bean. This second front of moving latent heat is exothermic. Gaseous reaction products - mainly CO2 - are generated and entrapped within the cell structure, increasing inner pressure until they permeate through the wallls that are weakened and partly destructed by the high temperatures as well..."
The four authors hold PhDs in food chemistry and related disciplines.
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True but nothing wrong with conjectures. Rob Hoos was up-front that the book is not scientifically rigorous. It is based on his experience and observations roasting and tasting coffee over the years. It is still a good guide to how changing roast profiles can change the taste of coffee. it would be an interesting read if he comes out with a revised edition. Such is the evolution of knowledge.
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Lyrebird, would be interested if you could cite any study that shows that coffee roasting is an entirely endothermic process. My understanding is that when coffee is roasting both endothermic and exothermic reactions are involved. When the heat released by the exothermic reactions exceed the heat absorbed by endothermic reactions, the bean is said to be exothermic. [Raemy, A., Lambelet, P., 1982. A calorimetric study of self-heating in coffee and chicory. Journal of Food Technology 17, 451e460].
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Ah yes science. The idea that knowledge is advanced by forming a theory then testing it. Not really relevant in a thread titled "Roasting Theory".
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We've been through the science lesson before. You are probably right (though many sources will disagree). But it's not really relevant to the issue at hand...and it doesn't mean that Anne's advice on roasting profiles is any less useful.
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Except that it is demonstrably wrong in a number of places. An example: there are no endothermic and exothermic phases in a normal roast: the reaction kinetics show that the beans are always endothermic.
If your roast is exothermic, it is on fire
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