Originally posted by Javaphile
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Java"correct"phile is quite right. I've read so much about endo/exothermic reactions over the past year that I glossed right over it and failed to pick up the error.
Sorry about that.
I know what Staub was alluding to but his use of terminology is a bit too relaxed, I agree.
None of the dictionaries say that to 'exotherm' is to lose heat.
An exothermic reaction is one that produces more heat than what is required to initiate the reaction.
An everyday example is a burning candle, where the total heat and light energy produced is greater than the energy required to light it.
A more interesting example is the freezing of ice. This exothermic reaction is used in viticulture to prevent frost killing new shoots in spring.
By spraying water on the shoots, when the frost forms at 0°C ,the 'latent heat' produced by the freezing of the applied water
stops the shoots from freezing! See pic 1&2
But I digress, from all my reading it seems that the science of roasting is a relatively new pursuit and is lagging behind the sudden
growth in home and boutique roasting that has occurred over the last 10 years. None of the material that I have read gives a definitive
account of the complexity of endo/exothermic reactions which occur in the second half of a roast. The most promising looked to be in a book which I had to purchase…..
It seems that beans start going exothermic when pyrolysis of sugar commences after the Maillard Reaction. In my roaster it seems to start around 180°C.
Yesterday I roasted some Brazil beans and left the heat and air settings the same, from 130°C onwards, to see the difference to a normal roast
where the heat is turned down a couple of times and the airflow is manipulated during part of 1st crack.
From the photo there is a gradual acceleration of °RoR from about 180°C which continues until the second half of first crack.
At approx 205°C there is a marked slowing of °RoR until approx 212°C, at which point it starts to accelerate again.
It would seem that it is this phase (of slowing) where the roast is most likely to bake by going into a prolonged negative °RoR.
i.e. flatlining from this point.
What this all means in terms of when the reactions are actually happening I don't know, it's not that clear cut.
If exothermic beans are releasing heat but the reaction slows there must be a point where the airflow removes more heat
than what the beans are producing. Where does the energy required for bean expansion come from and how does that
impact on exothermic reaction?
It doesn't need to be understood but…………..
I've got too much time on my hands. ;-D


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