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Old Dec 4th 2017, 03:40 AM   #1
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Steam saturation in a wood-fired bread oven

Good morning,
We are trying to apply some sound science to the age old problem of steaming a wood-fired bread oven. When baking most breads you need to have as much steam as possible at the initial stage of baking to allow the bread to expand by keeping the crust moist and soft and also to dissolve sugars and give it a shiny look.

There are two main methods used:
1) Pour water in a hot steel container
2) Spray water on the walls with a garden sprayer

Both methods do create steam, but the question is how much water do we need?

Our oven is about 0.28m3 and we bake at 240C. We are 600m above sea level.

When trying to apply proper thermodynamics to this problem, I get really confused. Any help would be greatly appreciated:


1) At 240C, 1m3 could take 16.76kg of vaporized water (steam). This much water would require a huge amount of heat to vaporize, and empirically it seems rather disproportionate. Experience suggests we would probably need 10 times less water, but I can't work out the proper scientific logic to apply.


2) If water expands to 2600 times its volume when it turns into steam, would it mean that with 108gr of water I could fill our 0.28m3 oven with steam?

3) The bread temperature goes from room temperature (24C) to about 98C when baked through. Since we need water saturation in the air around the bread itself, how does this affect our calculation?

4) If we fill the oven with steam, our oven door being not absolutely airtight, there will be a strong vapor pressure to equalize humidity in the much drier air in the kitchen. How can I estimate this pressure and how long it might take for the water vapor to leave the oven? This is quite important as in the last stage of baking one must vent the oven to get air as dry as possible.

We had much hope in the recently released Modernist Bread by Nathan Myrrhvold, Dr of Astrophysics, and a team of fine scientists and bakers. But the book does not go into scientific explanations of the steam.


Thank you for any help
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Old Dec 8th 2017, 12:01 PM   #2
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Points 1 and 2 seem to be at odds with each other
The volume of a gas (under various conditions) is given by the Molar Volume

However this completely fills your oven with steam, displacing all the air,
which, I suspect, is not actually what you want to do.

You want to increase the relative humidity of the air in the oven
to discourage overly rapid evaporation of the water from within the bread
particularly during the early phases of the cooking.

For this purpose, I might guess that introducing a fine mist of hot water into the oven might be best.
The flow of water into this mist could be readily controlled allowing it to be reduced during the cooking.

After that, I would suggest some trial and error.
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Old Dec 8th 2017, 05:26 PM   #3
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Originally Posted by Belphegor View Post

1) At 240C, 1m3 could take 16.76kg of vaporized water (steam). This much water would require a huge amount of heat to vaporize, and empirically it seems rather disproportionate. Experience suggests we would probably need 10 times less water, but I can't work out the proper scientific logic to apply.
I'm not sure how you arrived at that figure ...

Imagine a 1m3 oven at 240C ...lets say the door can remain closed as water is sprayed on the oven wall...

The water will vaporize ...the more you spray the higher the % of water in the hot air inside ... the oven cannot hold pressure so the air will be pushed out ,along with some steam ...as you spray more water , eventually the oven will contain only steam at atmospheric pressure and 240C this will have a density of about 0.4Kg/m3

Your oven is 0.28 m3 so when the inside is 100% steam that is less than 100gms of water ( some of the volume is occupied by the bread ) ...

In practice you will never reach 100%

If the oven were very well sealed , water could be sprayed without opening the door , perhaps 200 or 300 gms would be enough water to get the results you need ...

The steam/air in the oven is very buoyant it will be tying to get out of any gaps near the top , and this will suck cold air in in gaps lower down.. If you open the door , then most of the steam will escape , steam is less dense than air at the same temperature ...

I think this is all about sealing the oven effectively , not much water is required , a tray with 200gms of water should do it , the bread will be giving off a lot of water too.
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Old Dec 9th 2017, 02:06 AM   #4
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These are very sensible considerations, thank you!
So in effect, your physics tell me that
1) Very little water is actually required
2) Keeping steam in is important and difficult

Therefore:
a) Letting steam escape for the last part of baking (crispy crust in dry air) should be easy
b) I need to find a way of retarding vaporization so that steam does not escape while door is being closed

Some people use ice cubes placed in a hot cast iron pot to achieve that. I might try to open my door just a crack to put in my garden mister.

Thanks a lot for this help!
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