Roasting Theory

[zuroy]

Hopefully these aren’t silly questions. Obviously asking this “in theory” as I understand there’s a lot of variables.

- let’s say I reach first crack at 10mins. In my next batch if I lower the temperature so it hits first crack at 11mins (or even 9mins) then how would this affect the taste?

- let’s say first crack was 10mins and second crack was 14mins. Again what would happen if next batch I aim for second crack at 13mins or 15mins?

As you can tell I’m trying to understand better how slowing or speeding up my roasts at those intervals will affect flavour. Let me know if there’s other things I need to watch out for.

I’m using a Coretto set up incase it matters.

[Rami]

I have no idea. But I think it depends on the beans

[lancruiser]

Rob Hoos has an excellent book on this subject. It's a bit pricey for a short book. Try and borrow one if you can or convince your local library to stock it.

https://www.jetblackespresso.com.au/...file-of-coffee

Having said that, nothings beats testing it out yourself.

[Lyrebird]

There is a lot of conjecture in that book. Rob says he is preparing a new edition with the help of someone with a science background.

I'd wait for that one

[lancruiser]

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.

[Barry O'Speedwagon]

Oops....wrong comment list.

[flynnaus]

In general, faster roasts tend to brighter and possibly more flavoursome while slower roasts tend to be smoother and flatter.

Best to try it out yourself

[Barry O'Speedwagon]

This is pretty good as well, someone linked it in another thread last year (couldn't find that thread):

https://www.cafeculture.com/news/the...-roast-profile

[Lyrebird]

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".

[lancruiser]

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].

[Barry O'Speedwagon]

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.

[Steve82]

Rob Hoos and the Cafe Culture article from Anne
( which is pretty helpful for a standard commercial drum roaster starting out) have ZERO relevance to heat gun in a bread machine and best not to confuse yourself with it.

The common wisdom here is to position the heat gun in such a way that its not directly hitting the beans. Then slowly bring your mass of beans up to your preferred end temperature over 15 to 25 mins.
15 mins for corretto to start of 2nd crack would be quite quick...

Come up with your own hypothesis and then seek to disprove it / blind cup your results until you have your own working "theory" for your setup.

[Ronin]

Rob and Anne are both great with massive amounts of commercial roasting experience. Both love Loring roasters (Rob is or at lease was, ambassador for Loring)

Some relevant info but also differences to home roasting

[lancruiser]

Rob Hoos' book does not attempt to teach you how to roast. Its content will apply to any roaster. It's worth a read regardless.

[flynnaus]

The common wisdom here is to position the heat gun in such a way that its not directly hitting the beans.

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

[Dimal]

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.

[flynnaus]

Fair enough. Mine's a single loaf and deep enough to not have scorching.

[Yelta]

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

I reckon your spot on.

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. 🙂

[Lyrebird]

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 100^oC and remains so until 260^oC. 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 260^oC" 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.

[lancruiser]

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.

[Lyrebird]

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

[lancruiser]

Perhaps. I will leave it to someone more knowledgeable and have the right equipment to find out.