Thermodynamics of first crack - Page 6

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
Tonefish
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#51: Post by Tonefish »

Almico wrote:But while it's under pressure there is relatively little steam present in relation to liquid water.

It's like a small propane bottle. There is liquid propane in there because it is under pressure. There is only a small amount of propane gas present relative to the space allowed by the container. As the container is opened, gas escapes and more liquid propane converts to gas. As soon as the valve is shut, the conversion continues only the the point that the container allows. In the case of a coffee seed, the container is somewhat expandable, but not that much. Once the temp inside the seed reaches the temperature required for the pressure to overcome the seed structure, it blows its stack.
This is where the biggest issue is in my opinion. Without the steam, there is no pressure. It is the steam that causes the pressure. The bean is expanding and the water as a liquid or as an evaporated (versus boiling) vapor can't be causing the expansion to the extent that the volume increases.

Propane or CO2 or many of the compressed gases we know of don't exist as a liquid at standard atmospheric conditions, like water does. These compressed gas cylinders have a lot of work going into them to compress the gas back to a liquid at room temperature inside the tanks. Then as the gas is released that energy storage is also released.

WIth the water in the bean, it exists as a liquid at standard atmospheric conditions, so it is not like the gas cylinders mentioned until a lot of work goes into it in the form of getting the water to "boiling" point, which is pressure dependent, and then the pressure generated by the steam does the work of compressing the fluid by the means of water vapor. Only then are there analogies to the gas cylinder because the temperature environment is much higher than standard atmosphere, actually both the temperature and pressure are "non-standard" at that point.
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Almico
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#52: Post by Almico »

Tonefish wrote: so it is not like the gas cylinders mentioned
I believe it is exactly the same. It's just the liquid/gas state temperature is shifted.

The volume of steam is 1700 times the volume of water. There cannot be that much steam inside a bean. When you compress steam enough, like propane, it becomes liquid. It is in liquid form until the pressure is released.

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EddyQ
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#53: Post by EddyQ »

Tonefish wrote:All true, but how is there pressure if there is not steam?
Every time I read this I think "the chicken or the egg come first?" :)

Jim, I think I misunderstood what you meant by "No Pressure, No Steam" and thought you was thinking there was no steam at FC.
I completely agree the initial pressure is likely be due to water partially boiling to steam. But I also believe there is remaining steam/water at FCs unless you run long and bake it out. But is it vapor (tiny water droplets), saturated (wet) steam or superheated dry gas? The answer I am suggesting relies on how pressure builds and exhausts with time. Illy calls it a "front". I think this does aligned with Rao's declining RoR theory in that if RoR flat lines, the pressure declines even though temperatures are rising because there is always some exhausting of gases and the build/exhaust are not balanced. If we race into FC with a fast front of rising pressure, we get a crash due to too much pressure getting released quickly.
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DaveC
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#54: Post by DaveC »

I keep reading about steam and 1st crack and pressure. A few things have always puzzled me over the years.

1. During the time to 1st, the beans are at a high enough temperature for long enough that all water will have gone before 1st
2. beans with more moisture don't crack louder than beans with less...often it's the reverse
3. temperature always increases during 1st and I can't see the miniscule amount of water doing anything to the massive drum full of heat

So why steam and first crack are discussed together I don't know?

Tonefish
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#55: Post by Tonefish »

Almico wrote:I believe it is exactly the same. It's just the liquid/gas state temperature is shifted.

The volume of steam is 1700 times the volume of water. There cannot be that much steam inside a bean. When you compress steam enough, like propane, it becomes liquid. It is in liquid form until the pressure is released.
I agree that it is like a shift, but the difference is that at the starting point (standard atmospheric conditions) water exists as a liquid already, whereas the CO2 or propane exist as gas and must be worked into a liquid in the tank. That is a big difference.
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!

Tonefish
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#56: Post by Tonefish »

EddyQ wrote:... But I also believe there is remaining steam/water at FCs unless you run long and bake it out. But is it vapor (tiny water droplets), saturated (wet) steam or superheated dry gas? The answer I am suggesting relies on how pressure builds and exhausts with time. Illy calls it a "front". I think this does aligned with Rao's declining RoR theory in that if RoR flat lines, the pressure declines even though temperatures are rising because there is always some exhausting of gases and the build/exhaust are not balanced. If we race into FC with a fast front of rising pressure, we get a crash due to too much pressure getting released quickly.
I agree. :)
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!

Tonefish
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#57: Post by Tonefish »

DaveC wrote:I keep reading about steam and 1st crack and pressure. A few things have always puzzled me over the years.

1. During the time to 1st, the beans are at a high enough temperature for long enough that all water will have gone before 1st
2. beans with more moisture don't crack louder than beans with less...often it's the reverse
3. temperature always increases during 1st and I can't see the miniscule amount of water doing anything to the massive drum full of heat

So why steam and first crack are discussed together I don't know?
Not all of the water is gone by 1C and since the steam occupies a volume hundreds of times the equivalent amount of water, it doesn't take much to help in the expansion and pressurization. 1C has a big CO2 release (I think .. if I'm grasping the publications correctly) too. Maybe that is what is dominant then.
LMWDP #581 .......... May your roasts, grinds, and pulls be the best!

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Tonefish
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#58: Post by Tonefish »

AssafL wrote:I am a bit perplexed by the popping sound of coffee. What is it that we hear? We think it is water. In popcorn the popping is caused by water (14-20%): the steam is trapped in the hull of the corn. Internal temp goes up to 180C or so, hull breaks, the steam expands inside the pliable (hot) starch and popcorn is formed.
Assaf, I thought back to this as I was just looking at one of the Schenker (Pore Structure) articles talking about shifts between glass transition and rubbery phases during the roast. They say the bean starts as a glassy structure and as the heat is applied and things start to change, the bean structure becomes rubbery, allowing for expansion. Then even later the bean becomes glassy again due to the loss of moisture. I would extend that this brittle (glassy) structure may well assist in the pop.
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Tonefish
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#59: Post by Tonefish »

Has anyone found the 2002 Schwartzberg article, Modelling bean heating during batch roasting of coffee beans, in its entirety?
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Almico
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#60: Post by Almico »

Tonefish wrote:I agree that it is like a shift, but the difference is that at the starting point (standard atmospheric conditions) water exists as a liquid already, whereas the CO2 or propane exist as gas and must be worked into a liquid in the tank. That is a big difference.
I disagree. I believe it is a distinction without a difference. The starting point is irrelevant. Various matter exists as solid/liquid/gas at various temperatures/pressures regardless of our random atmospheric temp and pressure.

At least that's the way I remember it.