The Fresh Loaf

A Community of Amateur Bakers and Artisan Bread Enthusiasts.

Two questions about steam

albacore's picture
albacore

Two questions about steam

I have a couple of somewhat technical questions for all you engineers/physicists/chemists.

We tend to think of steam at atmospheric pressure as having a temperature of 100C/212F (or very nearly).

Inside an oven, the pressure must be atmospheric, but the temperature is maybe 225C/450F. If we steam the oven for bread baking we must be introducing steam at atmospheric pressure, either from boiling water or steam injection; the steam must then heat up to oven temperature.

  • My first question: is this "hot" steam (is this what Engineers call superheated steam?) effective in giving our loaves good spring, crust and ears or is it only the steam that has just been created at 100C? - which may well be called "wet steam".

I'm looking at steam definitions Here.

Of course, in a real oven it will be yet more complex, because although the oven may be at 225C, the just introduced dough piece will be at room temperature.

  • My second question is: how much steam do we need - in terms of volume of water to be evaporated to make steam?

The answer to this one will depend on the answer to the first question, ie do we just need to fill the cavity with steam or do we need to keep generating steam? - assuming the oven is unvented at this time.

As a back of fag packet calculation, if we assume an oven cavity is a 0.5m cube, it will have a volume of 0.125 cu m. Density of steam at 1bar/100C is 0.598 kg/cu m, so weight of steam is 75g or vol of water is 75ml - not very much! This at 100C , I'm not sure about at 225C - you would think it would be less.

On the other hand, if we have to keep generating wet steam, we will need at lot more...

 

Lance

Ford's picture
Ford

I think you have answered your own questions.  Ovens do leak vapors.

Ford

Justanoldguy's picture
Justanoldguy

According to professor Wiki superheated steam and liquid water cannot coexist in the same environment. So it seems to me that if your source of steam is a pan of water in the oven you will not have superheated steam so long as liquid water is in the same environment. In fact, if I understand it correctly (I did doze off toward the end of Prof Wicki's lecture - her monotone delivery is a sure cure for insomnia) if the purpose of the steam is to keep the crust of the loaf flexible enough for oven spring it would need to be saturated steam not superheated steam which would dry the crust. It seems that superheating is a technique for extracting the utmost kinetic energy from the steam when used in either a turbine or piston engine and has no real application for baking.   

Bart Tichelman's picture
Bart Tichelman

Lance,

All steam (aka ":gaseous water, "water vapour") in an oven above ~212F at (sea level) will be dry.  Wet steam is simply where the water vapor is starting to condense into water droplets (like a cloud). You can see wet steam above a boiling pot or kettle. Any oven at bread baking temperatures will have dry steam.

Gaseous water occupies 1,600 times more volume than liquid water. The typical home oven is ~140 liters, so we need <10 ml of liquid water (10 grams, 1 small ice cube) to fill the oven with water vapour. Since an oven leaks, we need more than that to ensure we keep the oven saturated until the end of the steam period.

Bart

B.Sc. Chemistry :-) 

 

DanAyo's picture
DanAyo

Bart, will the dry steam that is created in a pre-heated oven cause the crust to remain flexible in order to produce the rise and ear that we are looking for?

Also, while I have your ear. Is it correct to believe that boiling or very hot water is better than room temp water for steaming an oven. Wouldn’t the oven give up more heat in order to bring room temp water to steam?

Thanks.

Dan

Bart Tichelman's picture
Bart Tichelman

Dan,

Yes, we want an oven saturated in water vapour for the first part of the bake to prevent the crust from setting. "Wet" or "dry" does not matter; as described above, water vapor above 212F (at sea level) is "dry" because it does not contains tiny condensed droplets of liquid water.

Theoretically, using hot or boiling water will minimize heat loss, but my intuition suggests it is not that big a deal (I suspect we lose more heat when we open and close the door).  More importantly, we want to create a lot of water vapor quickly to fill the oven when the dough is loaded, and that is why we should use hot water.  In my setup, I use five cheap bricks on the bottom shelf to increase the thermal mass of the oven (oven temperature returns quickly to the desired temperature), and I have a small baking pan with lava rocks that I pour 4 cups of boiling water into when I load the dough.  I remove the pan after the initial period (usually 12 - 15 minutes) and it still has a little water in the bottom. 

Of course when I use my cast iron combi-cooker, no added water is needed as it is produced from the dough.

albacore's picture
albacore

Thanks Bart, but not sure your figures check out right. You say 140 liters for oven vol. I got 0.125 cu m, which is 125 liters, so we are pretty close there.

Divide your 140 by 1600 and we get 0.0875 liters. Multiply by 1000 to convert to ml and we get 87.5ml, which sounds realistic to me - I think your original calculation was a factor of 10 out.

 

Lance

Bart Tichelman's picture
Bart Tichelman

Lance,

 

Yes, I was out by 10...good catch!

Our Crumb's picture
Our Crumb

I have been been using this method for creating abundant and very effective oven steam since I posted it six years ago and have found no reason to deviate from it, now 300+ bakes later.  Highly recommended.

Happy baking,

Tom

Bart Tichelman's picture
Bart Tichelman

Tom,

Your setup is very impressive, and I am sure you are getting great results.  That being said, it is not necessary to get adequate water vapor in the oven.  A source of liquid water anywhere in the oven will fill the entire oven with water vapour.

Cheers,

Bart

DanAyo's picture
DanAyo

If a preheated pan is positioned directly under the baking stone and 2 cups of boiling water is added, while the steam cool the stone that is located directly above?

If the steam does cool the stone, would it be best to locate the steaming tray above the bread?

I appreciate your input.

Dan

Bart Tichelman's picture
Bart Tichelman

Dan,

While the temperature of the air/water vapour mix coming up from the pan will be lower than the target baking temperature (hard to know how much lower without measuring), it should not have a significant effect for three reasons:

  1. The temperature differential should not be huge (maybe 300 - 400F vs, a preheated target temperature of 475?)
  2. The heat density of the air/water vapour is much less than the heat density of the stone
  3. We'll be slightly cooling the bottom of the stone, and since heat transmission through the stone is slow, it does not affect the top surface much (if at all). My stone is 0.75 in (~2 cm), and when I measure the stone top temperature with a gun, it is at my target temperature.  A baking steel will transit heat energy more quickly, so it will be affected more, but I only use my steel for pizza.  When I first got it I found it scorched the bottom of the dough (because it transmits heat energy more quickly).

My empirical evidence shows no appreciable effect by having the water tray below the stone, and it is much more convenient to place it there in my setup.

DanAyo's picture
DanAyo

I use FireBoard to run thermal data logging. It uses probes (thermo couplings) to gather the data. How would I use the probes to read the top and bottom of the baking stone? I’ve tried to come up with a solution, but as of yet have none. I thought about gel, like used on computer CPUs but I think it would melt away.

Have you an idea? I think the data would be interesting.

Question - you said, “The temperature differential should not be huge (maybe 300 - 400F vs, a preheated target temperature of 475?)“ I don’t understand the 300 - 400F. I understand the stone is 475, but I am thinking that steam is 212F. I am thinking that the 212 steam would cool the 475 stone. Please educate this simpleton :-)

Dan

Bart Tichelman's picture
Bart Tichelman

Dan,

I would not use a probe to measure surface temperatures.  I use an infrared temperature gun like this one.  Once you have one, you'll find lots of uses for it.

212F is the temperature that water boils (more accurately, changes phase from liquid to vapour).  Once the water is in gaseous form, the temperature of the water/air mixture rises to whatever the ambient temperature is. Nitrogen changes phase ("boils") from liquid to water at -320F; if gaseous nitrogen did increase in temperature beyond its boiling point we'd be in trouble as it makes up 78% of air :-).

Bart

albacore's picture
albacore

I've checked my oven cavity more accurately and it has internal dimensions of 480x440x360mm, giving an internal volume of 0.076 cu m.

So this will need 45g of water vaporised to fill it with steam at 100C. (I'm going to ignore the further heating up of the steam to oven temp for the moment).

The latent heat of vaporisation of water is 2260kJ/kg, so it's going to need 102kJ of energy to vaporise the water.

It would be nice to create this steam as quickly as possible when the dough goes in, so ideally it should be generated locally, rather than relying on the oven elements.

We could do this by rapidly dripping freshly boiled water onto a steel plate. Let's assume that plate is already heated up in the oven at a temperature of 225C. We'll let the water cool it down to 125C - below that evaporation is going to be slow.

Given the specific heat of mild steel is 0.511kJ/kg/degC, we are going to need a steel plate weighing 2kg.

This is suprisingly substantial, given that we havn't taken into account any ongoing steam loss through oven vents.

The Roadside Pie King's picture
The Roadside Pi...

Anytime matter changes states (ig. water to steam) There will will be latent heat/ energy involved. I don't know if that is either here nor there, but I do know it is a fact. Smile.