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endothermic reversal point
JETROASTER
I came across this little term and thought someone might help me to understand.....Marshall?
-Scott
Edited by ginny on 11/13/2013 5:46 AM
 
Dan
I've never heard that three-word term before. During coffee roasting the process begins as endothermic (taking in heat), but after second crack it becomes exothermic (giving off heat).

There was a time in the home roasting community that many people believed roasting was endothermic throughout the roast, but this has been proven false.
 
allenb
I've heard many different takes on this but I thought it was endothermic up to 1C, exothermic during 1C and a little after 1C got rolling it reversed and again became endothermic?

I'd love to hear some concrete info on this once and for all.
Edited by allenb on 08/13/2010 8:04 AM
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
seedlings
"Until the start of the first crack, the heat inside the beans is endothermic; the beans are absorbing the supplied heat. Right before the start of the first crack, the heat inside the beans becomes exothermic and the beans start generating heat. At this point the operator has to reduce energy supply in order to gain control of the roast process. After about two minutes of controlling the roast with low energy supply (less BTU), the operator can again increase heat (endothermic heat; the beans are again absorbing heat) to prepare for the finish of the roast."

http://www.bootco...iling.html

Right on, Allen.

CHAD
Roaster: CoffeeAir II 2# DIY air roaster
Grinder: Vintage Grindmaster 500
Brewers: Vintage Cory DCU DCL, Aeropress, Press, Osaka Titanium pourover
 
JETROASTER
Does that suggest an exothermic reversal as well? -Scott
 
seedlings

Quote

freshbeans wrote:
Does that suggest an exothermic reversal as well? -Scott


The diagram shows exothermic during first and second crack, this would mean that after first crack it reverses from exothermic to endothermic, so yes, there are endothermic reversal points and exothermic reversal points. My guess is the endothermic reversal point would be at the first signs of first (or second) crack. The exothermic reversal point would be at the end of first crack (and again, dare I say, at the end of second crack).

www.bootcoffee.com/scurve_v3a.jpg

CHAD
Roaster: CoffeeAir II 2# DIY air roaster
Grinder: Vintage Grindmaster 500
Brewers: Vintage Cory DCU DCL, Aeropress, Press, Osaka Titanium pourover
 
allenb
Very interesting!

Willem Boot is concrete enough for me.ThumbsUp
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
John Despres
If there's a reversal, I'd guess your temperature is dropping... Maybe that's the cooling phase.
John
Edited by John Despres on 08/13/2010 9:13 PM
Respect the bean.
John Despres
Fresh Roast 8, Gene Cafe, JYTT 1k, Quest M3, Mazzer Mini, Technivorm, various size presses and many more brewers.
 
endlesscycles
Until the beans are on fire, they are taking heat, not giving it off. Second crack may qualify as on fire to some extent. First crack is simply giving off moisture, which in a drum with the related increased rate of convection may appear to be exothermic, but isn't.
-Marshall Hance
Asheville, NC
 
seedlings
Without personal knowledge, I defer to Boot.

CHAD
Edited by seedlings on 08/13/2010 10:07 PM
Roaster: CoffeeAir II 2# DIY air roaster
Grinder: Vintage Grindmaster 500
Brewers: Vintage Cory DCU DCL, Aeropress, Press, Osaka Titanium pourover
 
JETROASTER
I've always been under the impression that at the point of pyrolysis, the beans would 'produce' heat thru some alchemy that
I don't understand.
When I bought the Sivetz heat gun roaster, it came with a strange 20 page Manifesto written by M Sivetz and edited by someone worse than me.
I just took that bit of info for granted. The gauges seemed to support it.
I can not say I've seen the first reversal on the gauges. The second one I see everyday.
At the end of a darker roast, gas gets turned way down and bean temp continues to climb.
What's the skinny on this matter?? -Scott

 
seedlings

Quote

freshbeans wrote:
What's the skinny on this matter?? -Scott



Exothermic internal chemical reactions producing heat.

CHAD
Roaster: CoffeeAir II 2# DIY air roaster
Grinder: Vintage Grindmaster 500
Brewers: Vintage Cory DCU DCL, Aeropress, Press, Osaka Titanium pourover
 
allenb
Scott,

When the 1C exothermic reaction occurs, the beans are truly generating heat internally but due to vapor release there isn't a rate of rise increase for some of us. In one of my roasters which is radiant only (due to my convection element burning out), the RoR comes to zero just prior to and during the first seconds of 1C and I have to increase power to compensate.

My guess is that at 2C the exothermic action does not produce as much vapor which causes a RoR increase requiring a power reduction.

The reversal for 1C has a totally different affect on RoR and power input needs versus 2C reversal.

That's my theory.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
JETROASTER
That theory makes some sense. On a different thread I had noted a RoR plateau (at a lower temp range) before 1C. I assumed it was evaporational cooling. Considering the gauge may not be trustworthy, that seems to fit.
So, it sounds like the exothermic is masked by the moisture loss at 1C , not so much at 2C.
Sounds like all this looks different on drum vs air-roasters .
I'm roasting Monday.....we shall see

As always ,Thanks -Scott
 
Dan
What I know about this, and that is very little, is that the roasting reaction is endothermic when the inside of the bean is cooler than the outside. Exothermic is when the interior of the bean is hotter than the exterior.

I'm not sure how Boot determined those points on the curve were exothermic because he doesn't show that he measured the interior temperature of the beans, only the bean mass, which is the exterior of the beans.
 
JETROASTER
So if the bean is hotter inside than outside, is this accumulated heat from roasting, or is it heat that is being chemically created?
-Scott
 
Dan
Chemically created, that's why it is called an endothermic (chemical) reaction. Essentially, the bean is beginning to combust. Not enough that it can sustain the reaction yet, but given a little time it will begin to burn.

 
seedlings
Has to be heat generated from chemical reactions, Scott.

CHAD
Roaster: CoffeeAir II 2# DIY air roaster
Grinder: Vintage Grindmaster 500
Brewers: Vintage Cory DCU DCL, Aeropress, Press, Osaka Titanium pourover
 
Ringo
Just a guess on my part but 1st crack is when the steam breaks open the bean, I always guessed this released steam is the heat.
All you need in life is ignorance and confidence, and then success is sure. Mark Twain
 
Dan
Yes, steam is releasing heat since it is hotter than the environment, and it does tend to remove heat from the bean, making it an exothermic process, but it is not an exothermic CHEMICAL REACTION since the water/steam started and ended as H2O. That is there was no chemical reaction, just simple heating and cooling.
 
JETROASTER

Quote

Dan wrote:
Yes, steam is releasing heat since it is hotter than the environment, and it does tend to remove heat from the bean, making it an exothermic process, but it is not an exothermic CHEMICAL REACTION since the water/steam started and ended as H2O. That is there was no chemical reaction, just simple heating and cooling.



That sounds like a good fit for 1C. Then the exotherm at 2C must be
getting close to combusting on its own? -Scott
 
Dan
Based on my experience, roasts go exothermic right about 2nd crack. On some of my darker roasts (FC+ to Vienna) I turn the heater off when 2nd crack gets rolling. I've noticed that the thermocouple in the bean mass measures a temperature rise even though the heater is off, a sure indication of an exothermic reaction going on.

In commercial roasting circles they talk about the dreaded "3rd crack." A roast that is so violently exothermic that heat builds to where you see flames.
 
JETROASTER
"3rd crack" is followed by "1st fire bottle". Been there.

Although it may not be a true exotherm, the heat/steam
release around 1st C still interests me.
It ties in with the thread about 'monitoring moisture during roast' as well as 'regarding the pre-yellow stage'

If all of this makes no sense, I apologize in advance.

Depending on how pronounced that reversal/release is,(around 1st C) it would seem a strong indicator as to whether or not you entered the roast as you intended.

....But how would you measure it? It seems that the thermometer doesn't tell the whole story during that part of the roast. The moisture release clouds the picture.

Is there a good way to measure humidity within the RC of an air-roaster ? -Scott

 
eric
Nice summary Wallace. I have about as many roasts under the belt as you, all with air roasters. I am more with Ed on the science - though there is plenty of art in getting crude consumer roasters to perform! Coffee roasting definitely qualifies as a chaotic process, which is what makes it so difficult to control. Later stages are very dependent upon earlier for their results. Whole chemical reaction pathways may be cut off by how the roast is set up in its earlier stages: amount of drying affects reaction pathways later in the roast due to residual water availability. RoR will affect vapor pressure within the bean and guide pathways as well. Fiendishly complex. IMHO.

I will take exception to one of your statements - the folklore around exothermic reactions at 1C - I am sure there is no such thing. I have never seen it with my setups that should have shown it: very low thermal mass air roasters with good thermometry can easily detect any exothermic reaction as a rise of bean temp above env temp. Never happens, even on quite low RoR through 1st. There is sometimes a very marked reduction in apparent thermal capacity after what I call the Maillard hump before 1st and if your roaster has any sort of thermal delay between power and BT then you could be fooled into thinking things were getting exo into 1st. The Maillard reactions can be very endothermic (hump usually around BT 180-185C for me). No-one mentions it, but the max RoR through this phase at a given batch size is an important spec for a roaster and where I will hit the power limit first for profiles that I prefer.

On the title of this thread, getting through Maillard as quickly as possible will minimise the sugars consumption of the Maillard reactions and leave them for caramellisation later in the roast. ThumbsUp

My 2c...
Eric
 
Dan
This might help. Raemy and Lambelet measured coffee energies in their study and found that roasting becomes markedly exothermic around 195?F. as seen in this graph.
Dan attached the following image:
raemy_lambelet_1.jpg

1 pound electric sample roaster, 3 pound direct-flame roaster, both handmade; modified Mazzer Mini grinder, LaSpaziale Vivaldi II automatic espresso machine. When the electricity goes out I make vacpot coffee from beans ground on my Zassenhaus hand grinder, and heat the water with a teakettle on the gas range.
 
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