[creative nickname] wrote:My own theory (which I hold quite tentatively) is that escaping steam first acts to cool the roasting chamber a bit, causing bean temperature to rise more slowly than it otherwise would. This is both because the steam is a bit cooler than the air of the roasting environment, and also because the pressure loss causes cooling directly. Once airflow has moved this steam out of the roasting chamber/heated it up to operating temperature, the temperature rise speeds up again, unless you change the heat inputs to counteract that tendency. This is not, as some have maintained, "measurement error" because what is happening to the probes is also happening to the surface of the beans (which has implications for further roast development).

I have the same theory. And after all that steam is gone and the pressure released, and the beans are now void of all water, the rest of what is left is free to heat up much faster.

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One of my favorite Rob Hoos comments from the roasting presentation I attended several weeks ago was, and I'm paraphrasing:

Coffee roasters spend too much energy and time marketing to other coffee roasters instead of their customers...coffee drinkers. Light roasts are also the rage and very fad-ish, but light roast coffee aficionados are not going to be your regular customers. They try a bag from this roaster or that roaster and are constantly experimenting. But if you win over a dark roast customer, you'll have them for life.

I have found that to be true. All the mumbo-jumbo about the science of coffee and roasting coffee falls on deaf ears when it comes to the vast majority of my customers. All they want is a good cup of coffee...every time.

So it's fun to discuss all this stuff and banter back and forth, but I'm glad I do not need to be an organic chemist, with a minor in Newtonian thermodynamics to roast great coffee that normal people want to drink.

another_jim wrote:So perhaps the problem is one of language.

I think we could say that about most "debate" threads on HB.

"The whole modern conception of the world is founded on the illusion that the so-called laws of nature are the explanations of natural phenomena. Thus people today stop at the laws of nature, treating them as something inviolable, just as God and Fate were treated in past ages. And in fact both were right and both wrong; though the view of the ancients is clearer insofar as they have an acknowledged terminus, while the modern system tries to make it look as if everything were explained." - Wittgenstein, Tractatus, 6.371-2

[creative nickname] wrote:Here is what puzzles me about your explanation Jim. Simple thermodynamics suggests that the bean cores should always be cooler than the bean surfaces, right? So absent falling environmental temperatures at the time of first cracks, how could the steam be heating the roasting chamber beyond its current temperature?

You're probably right, and I may have to retire the idea. My thinking was in terms of kinetic energy -- the beans are at around 30 bar, so the escaping steam is at high pressure and high velocity. My idea was that the vapor pressure stores a significant amount of the thermal energy of the prior roast which then gets converted to heat in the first crack. Probably should have done some calculations first -- if 10% of the roast is moisture converted to steam in the first crack, what is the energy stored charging it up to 30 bar?

Here is another example of endothermic v. Exothermic, which might help other readers of this thread. This example helped me when thinking about roasting beans while reading this thread.

Boiling water is endothermic, despite it giving off steam. It's endothermic because the water requires external heat source to raise it's temperature. It's absorbing energy (heat)

Turning water into ice, freezing water, is exothermic. The water is releasing it's energy (heat)

thuegli wrote:Boiling water is endothermic, despite it giving off steam. It's endothermic because the water requires external heat source to raise it's temperature. It's absorbing energy (heat)

Um, the argument is about time: Water previously heated in a pressure vessel, releasing steam though a valve, is endothermic or exothermic? A bar heated at one end, giving off heat at the other, after the heat source is shut down, is endothermic or exothermic? People who have taken calculus have a different relation to time than people who haven't.

Not sure what you mean by "argument"? I was under the impression it was a discussion about whether roasting beans was an endothermic or exothermic process? And dispelling the myth that at 1C the beans mysteriously become exothermic. As my favorite pop-culture scientist said: "The good thing about science is that it's true whether or not you believe in it."

I was merely providing a basic, and accurate, example that might help people understand the different processes of exothermic v. endothermic.
The boiling water example is a basic and common example given in entry level chemistry classes to describe exothermic v. endothermic reactions.

I'll take a stab at your examples:
Assuming your vessel contains liquid water that would be vapor but for the increased pressure, releasing the pressure would be exothermic, I believe.

Your heated bar example is both, as are all of the examples depending on which direction one reads the equation. Common practice is to read the equation from left to right, and declare the reaction either endothermic or exothermic.


Granted, my two years of college chemistry were nearly 30 years ago....

I was a math major, so I've had my share of calculus. Again, 30 years ago and I haven't used it much since then. Calculus doesn't inform my relation to time, but my physics classes sure did.

Water fluid is endothermic, water being released is exothermic, water being condensed IS exothermic, if it is at all possible for steam to recondense on beans that are what...400F. But considering in the total energy of the system, it would seem that it is (relatively) quickly re-evaporated and so it is taking energy with it, generally becomes endothermic. Also something to contend: the temperature of the water, while in the bean is hotter, as it gets released it is cooling down in temperature (and so is the bean),I believe (not sure) the water is still cooler than the environmental temperature.
What I can say for sure are there are a lot of factors coming into play very close together. Chemically though, the bean does go through exothermic reactions, as I've said previously, but again I do not think those reactions will cause the whole fluid mass (beans) to rise noticeably on these probes (unless you employ Rao's methods), but enough to ruin the taste if not maneuvered well.

I'll leave this here if anybody wants to brush up on their "Calculus".
https://www.khanacademy.org/test-prep/m ... -reactions

LBIespresso wrote:As for the statement above, would it be incorrect to say that a whiskey stone that is 40 degrees F would be endothermic if dropped into a 70 degree glass of whiskey and the same stone would be exothermic if put in a freezer?

There is no reactions taking place, so I don't think either is endothermic or exothermic. In the case where dropped into warm whiskey, heat would flow from the whiskey to the stone. Eventually, they would be at an equilibrium temperature somewhere between the two temps. If stone was put in freezer, heat would flow from the stone to the freezer. In this case, the freezer has a controlled temperature, so at equilibrium the stone would become the freezer temp.

thuegli wrote:Not sure what you mean by "argument"? I was under the impression it was a discussion about whether roasting beans was an endothermic or exothermic process?

This is why I've stopped posting. Thanks for the reminder.

It is not about exothermic or endothermic, since neither concept applies. It is about heat being input at one place and time, and affecting the beans at another place and time. Everything else is blather.

EddyQ wrote:There is no reactions taking place, so I don't think either is endothermic or exothermic. In the case where dropped into warm whiskey, heat would flow from the whiskey to the stone. Eventually, they would be at an equilibrium temperature somewhere between the two temps. If stone was put in freezer, heat would flow from the stone to the freezer. In this case, the freezer has a controlled temperature, so at equilibrium the stone would become the freezer temp.

Yes. I think this is the correct answer for the metal bar example as well. If there isn't a phase change like water to steam or ice or a chemical reaction like when roasting coffee I don't think endothermic/exothermic applies. You can't say a change took energy to happen or released energy while happening if there isn't any change.