The Fresh Loaf

A Community of Amateur Bakers and Artisan Bread Enthusiasts.

Heat transfer mechanisms in typical home-oven baking

Doc.Dough's picture
Doc.Dough

Heat transfer mechanisms in typical home-oven baking

This post has no pictures and is not going to interest a lot of readers since I did it to help my own understanding of what is going on in the oven.  Writing it down forced me to explain more when I didn't understand why and fix apparent inconsistencies.  If it is too much technobabble, just jump out and find something interesting. For those who wade through it, I welcome comments, corrections, clarifications, and questions.  Just consider it a work in progress.  When you understand this, you should be able to write the versions that apply to wood-burning ovens and deck ovens with external steam generators.

CONVERTING DOUGH TO BREAD BY BAKING IN A HOME OVEN

The modes of heat transfer from oven to bread include:

  • Conduction (by direct contact with a hot surface)
  • Convection (both natural and forced mechanisms from hot oven air)
  • Radiation (the heat flow between the oven walls and the bread in the oven)
  • Phase change (the evaporation of water from, and condensation of steam onto the dough surface)

For pan bread, the sides and bottom of the loaf are cooked by conduction of heat through the pan while the top is cooked by a combination of radiation, convection, and possibly condensing steam. The relative contribution from each mode is dependent on the oven, the temperatures involved, and whether there is any mechanical stirring of the air to enhance the convective heat transfer.

For freeform loaves baked on a metal pan, the bottom is cooked by conduction of heat through the pan while the remainder of the loaf is baked by other mechanisms.  When the baking surface is tile or stone or firebrick (something other than a thin sheet of typically aluminum or steel), heat stored in the baking surface is transferred by conduction to the loaf which both heats the loaf and cools the baking surface. The rate of heat delivery to the loaf is determined by the mass of the cooking surface, the initial temperature of the material, the thermal conductivity of the cooking surface, and the specific heat (cp) of the cooking surface material as well as the density, thermal conductivity and cp of the dough. The rate at which the energy stored in the baking surface is replaced from the oven primary energy source depends on the geometry, surface temperatures and convective flows, and also on what else is simultaneously in the oven (e.g., other loaves of bread or other pans above or below).

There is always some amount of free convection in any oven, driven by the temperature distribution within the oven which heats or cools air causing it to expand and rise, or contract and fall as its density changes. This results in the top of an oven generally being hotter than the lower shelf positions. Convection ovens have mechanical fans that circulate air within the oven to both increase the heat transfer rate to the food and to achieve a more uniform temperature distribution within the oven (top to bottom, side to side, and front to back). Even the small fans in widely available home ovens deliver very high temperature uniformity and shorten baking times because they increase the heat transfer rate from the oven heat source to the food.  The general guidance for using a convection oven is to reduce the temperature by 25°F and bake for the amount of time that is called for if you were using a conventional oven.

For most non-convection, non-steam injected ovens, radiation from the oven walls is the principle heat transfer mechanism.  The Stefan-Boltzmann law governs radiation energy transfer between the oven surfaces and the bread.  It takes the form of:

Qdot12= s A1 F12(T1^4 – T2^4)

where s is the Stefan-Boltzmann constant, A1 is an increment of oven wall area, F12 is a shape factor that accounts for geometry and surface emissivity, T1 is oven wall temperature and T2 is the bread temperature (both in °K).  Note that the heat transfer rate Qdot is proportional to the difference between the fourth powers of the absolute temperatures.  This is not (T1 - T2)^4, but T1^4 - T2^4 which is a really big number at typical bread baking conditions [T1 might be 250°C (523°K) and T2 might be 15°C (288°K) at oven entry].   At these temperatures, a 30°C reduction in oven wall temperature produces about a 20% reduction in radiant heat transfer rate and about a 13% reduction in convective heat transfer rate.

In steam-injected ovens, condensation of water on the surface of the dough delivers a lot of heat.  The enthalpy of vaporization for water (2250 J/g), is more than five times the energy required to heat the same quantity of water from 0°C to 100°C (418 J/g) and is delivered directly to the surface of the dough when steam condenses. Steam does two things for you; it brings water directly to the dough which helps to fully gelatinize the starch forming a shiny, waterproof, gas tight membrane that prevents CO2 from escaping through the surface (thus forcing dissolved CO2 in the dough just under the skin to form blisters when it comes out of solution as the dough temperature rises to exceed the temperature at which the CO2 can remain dissolved).  The cooked surface is also physically strong and cannot stretch to accommodate expansion of the trapped CO2 (oven spring) and will thus facilitate fracture along the lines defined by your lame when you slashed the dough (or randomly at weak spots if you forgot, or slashed ineffectively).

During the first few minutes in the oven, the dough is cool enough to condense steam on the surface, and the more steam there is in the oven the more effectively and rapidly it cooks what will become the crust.  If there is inadequate steam, the dough will still cook, but the starch will not be fully gelatinized so that the crust is not as shiny or gas tight as you might desire and the coloration will be different and generally dull.

When the surface temperature of the dough gets high enough that it exceeds the local water vapor saturation temperature (oven dew point) steam no longer condenses on the crust.  At this point, while the specific heat (cp) of unsaturated steam is somewhat higher than dry air (by about 2x), the dominant heat transfer mechanism in a non-convection oven switches over from phase change (condensing steam) to radiation (from the oven surfaces). In convection ovens, the size of the fan and the capacity of the heating elements will determine whether radiation or convection will be dominant. In most home ovens, the convection fan is adequate to maintain uniform temperature throughout and does increase heat transfer by about 15% above what it would be with radiation plus free convection, but does not provide sufficient air velocity to raise convective heat transfer to a point where convection dominates radiation as the mechanism for transferring heat to the bread.  In commercial convection or combination ovens, the situation is reversed and since the heating elements and the convection fan are big and powerful, they transfer heat via convection considerably faster than radiation alone.

Gas ovens (with burners that share the bread baking volume) suffer from the absolute need to exhaust combustion gasses when the fire is on and in the process sweep out both the steam that is generated by combustion and any steam that is added to the oven (by both your steam generator and by evaporation from the bread dough itself).  The conventional solution is to preheat the oven to very high temperature, include some additional heat storage capacity in the oven (tile, brick, stone, scrap iron), then turn off the gas and plug the vents after loading the bread until there is no additional value from further steam. At this point you can unplug the vents, re-ignite the flame, and remove your steam generator from the oven.

Crust thickness is determined by the depth to which the baking bread has been depleted of moisture, and is generally a function of both oven temperature and oven cycle time. If the oven is too hot, the bread will over-brown before it develops a thick crust.  If the oven is too cool, the crust will be light in color even though it may be relatively thick.

When generating steam by boiling water inside the oven, some energy that would otherwise go toward raising the oven temperature is used to boil water.  This can be a major factor in small ovens and is important to understand.

Bread loses about 15% of its initial weight to evaporation of water during the bake cycle, thus a 750g loaf will lose ~110g of water.  It takes 2.13 BTU/gm to evaporate the water so you expend about 235 BTU in the process. That 235 BTU is about 68 watt-hours of energy, which you can allocate over the bake cycle and think of as reducing the effective power of the oven.  For a 30 min bake cycle that is like reducing the 2500W heating element by 136w to 2364W except that the effective reduction is bigger at the beginning of the cycle than at the end because there is more water to easily evaporate at oven entry. 

If you consume a pint (pound) of water in a steam generator, you will use 1000 BTU or 0.3 KWH to convert it to steam (plus 1 BTU for every °F that the initial average water temperature is below 212°F).  A 2500W oven will take about 7 minutes to recover the heat lost to the steam generator, and for a 4.5 cu ft oven, it takes about 75g of water to produce enough steam to fill the oven.  You will have to make an assumption about how tight your oven is but it would not be a bad assumption to guess that you lose one oven volume of steam per minute of active steaming. My observation is that after the first five minutes in the oven, the surface of the dough stops looking wet, and for rolls and small diameter loaves, they have completed almost all of their oven spring (note that there is an alternate view that says you should steam until the dough begins to brown – just figure out what works for you).

Seventy five grams of water takes 3.84 KW-minutes to boil, but you need 75g of steam per minute (about a pint for five minutes of steam if you leak at one oven volume per minute), so with a 2.5KW oven, if you don’t want to substantially cool off your oven in the process of making steam, you need some energy storage in the oven.  Lava rock has a cp of around 0.2 so it takes a bit more than a pound of lava rock at 400°F to generate 5 minutes of steam, but that is not unreasonable since you will heat the rock up during your normal pre-heat cycle (I am assuming it takes 1 hr @ 500°F to get the lava rock thermally charged to 400°F in a non-convection oven). And you will want to use boiling water to charge the steam generator so that you don’t use another 20% of additional energy to heat the water up to boiling.

Comments

leslieruf's picture
leslieruf

a lot makes a great deal of sense...  all good and it helps to understand what is going on in the oven.  thanks doc.dough

Leslie

IceDemeter's picture
IceDemeter

Well reasoned, and a grand example of how putting something in to writing, or having to verbalize it, helps to organize the thought process --- and get rid of extraneous ideas that aren't needed for the logical flow.

I've got this bookmarked for reference to the calculations for when I start finalizing oven contents, but even as a generalization it really emphasizes the need for extra heat-holding bulk inside the oven in order to maintain baking temps, especially with the amount of heat loss from door opening, adding / removing steam, and even regular venting.  It will also be very helpful in choosing the most efficient amount of water to add to create steam.

Thanks so much for sharing all of your work on this!

jimbtv's picture
jimbtv

Thanks Doc. Very interesting.

DanAyo's picture
DanAyo

How I wished I had the brain power. When it comes to technical writings of you and Debra I continue to reread hoping to glean new information on each pass. 

So Doc, how much water would my oven (average home convection oven - electric) have to vaporize in order to be most effective? My concern is over working the oven to produce more steam than necessary. It is my understanding that over producing steam will hinder the oven’s ability to recover and/or maintain heat. It seems your slashing video indicates that steam is only necessary for a short time. If I remember correctly, that duration is much shorter than the commonly accepted 20 minutes.

In case it matters, I have a custom cut Fibrement stone (3/4 inch thick) that has 1 & 1/4 inch breathing room on all four sides.

Doc.Dough's picture
Doc.Dough

If it is not dry at the end of 10 min, use less next time.

Steam in the oven after the surface starch is fully gelatinized and the surface temperature is above the dew point in the oven does nothing for you.  Until then, steam condensation on the loaf is contributing to heat transfer to the dough.

DanAyo's picture
DanAyo

It is commonly stated that many bakers steam for 20 minutes. Has your testing shown that 10 minutes is sufficient?

I can calculate the necessary water for a ten minute steam. I will put more than enough water (weighed in grams) in the pan to steam for 10 minutes. After 10 minutes (at a predetermined temp) the pan will be removed and the residual water weighed. By subtracted the residual water from the original, I will know approximately how much water was used. But, because I use lava rocks that calculation won’t tell the whole story. I have noticed that the rocks hold water even when the pan is empty of water. Because of this the empty pan will continue to produce steam. At least the calculated amount of water will make the pan lighter and eliminate spill injuries, since the vessel will have to be removed to stop the steaming of water absorbed in the rocks. The oven will need to be vented at any rate.

You don’t mention steam as an aid to dough expansion and bloom. I think I remember your “In-oven slash dynamics VIDEO” mentioning something about that. Please elaborate about the affects of steam on dough expansion. Does the humidity upon the outer skin facilitate dough expansion?

If I understood you correctly in this POST, the main job of steam is to facilitate more heat to the surface of the dough. And that a ~20% increase is really all the increase in expansion due to steam. It seems a proper score does more for bloom than steam. Is this correct?

According to the post mentioned above, is it feasible to use the gloss of the crust as an indicator of proper steam?

Thanks for taking the time to help a scientifically challenged student...

Danny

albacore's picture
albacore

I thought that 10 minutes was about the norm. Twenty seems like a long time. Also I've come to the conclusion that you've got to keep feeding in the steam - any "one shot" method at the beginning is not so good because the steam just disappears in a domestic oven at a rate of knots. I've even blanked off my oven vent with a temporary blanking plate and it still disappears; the concept of venting the steam out after x minutes is a myth in a home oven - there won't be any left! I think it must just leak out of the door.

So it needs either the lava rocks or Sylvia's Steaming Towels or - The Pressure Cooker!

Lance

Doc.Dough's picture
Doc.Dough

You will continue to make steam if there is water and heat, the only  issue becomes how fast you make steam relative to the rate of escape. Lava rock has a lot of surface area and a pound can store enough heat to do the job if the water is distributed such that the rock can deliver the heat. I have been trying unsuccessfully to come up with a way to deliver the water slowly (preferably after you shut the oven door).  Ice can work but is not thermally efficient.  I don't know whether a convection oven (home type) is better or worse than non convection.  Steam is just a gas so diffusion is quite fast and it will show up where you need it very quickly, but forced convection might deliver heat to the lava rock more rapidly than free convection will.  I do not think that an external pressure cooker is enough to deliver steam fast enough (~75g/minute) to make up for my estimate of loss rate but that is an easy experiment to run.

DanAyo's picture
DanAyo

Doc, if your estimate of 75g per minute for 10 minutes is optimum for a home oven, I’ll have to up my water to 4 cups. At present I know I can vaporize 2 cups over 10 minutes. It may be possible to do more, but I’m not sure I can 4 cups in that amount of time.

Now, your calculations must take into account steam leakage. According to the mirror test, I am able to cut that down to very little. Most of my steam is lost through vents in the oven door. NOTE - I use an electric convection capable oven. But I have not been running convection during steaming. They are located right at the top of the door. To seal the vents I take a wet bath towel anf fold it some to thicken it. Once the water is loaded I take the wet towel and use a dough knife to wedge it in between the top of the door and the bottom over hang of the top. The part of the towell that is left hanging down is used to stop up the other holes in the top of the door. I don’t both to plug the small oven vent that is left cted underneath the control panel.

Here is a video detailing my setup. https://youtu.be/qI3OShEwC8M

DanAyo's picture
DanAyo

Doc, have you considered this?

You wrote, ”I have been trying unsuccessfully to come up with a way to deliver the water slowly (preferably after you shut the oven door).”

I have yet to implement this but I have toyed with the idea of adding a port to the side wall of the oven that would facilitate passage of a food grade high heat silicone tube. That tube could position in the steam vessel and filled from outside the oven with the door closed. My concern is how to protect the metal port from accidently burning someone upon contact. I though about surrounding it in a pvc pipe or something similar, but I’d like a more graceful solution.

I have also wondered if a silicone tube couldn’t be snaked through the control panel vent. I also need to better investigate the heat capabilities of that tubing. I know some go the 500F.

albacore's picture
albacore

Yes I did wonder about energy input to the pressure cooker, from a gas ring in my case; I reckon with my oven I will need about 50g of steam to fill it. If I keep needing 50g every minute then I will need 1700 watts of power. Of course you then have to allow for the inefficiency of the gas ring set up - maybe 70% efficient? However I'm hoping that once the oven is saturated with steam, then it won't need a constant 50g per minute.

One thing you have to think of is that initially the oven will be full of air, then you want to fill it full of steam; if the steam went in and the air stayed in then then the oven would have a pressure of 2 bar absolute, which cannot be, so something has to vent out to maintain 1 bar absolute. It would be naive to assume that all the air vented and the steam stayed; realistically, maybe half the steam we just injected is now lost. Well this is how I think it will work, anyway....

One problem with the lava rock idea is that the rocks don't have much mass and don't act as a heat store for continued steaming; to keep on steaming, the oven element will have to do the work, reducing the temperature of the oven. This is why some (notably German bakers) have turned to a preheated mass of steel; the optimum form of this is steel balls which can give up their heat quickly. Unfortunately they need to be stainless to avoid rusting.

With regards to a steady source of water to make the steam, I recently made a drip feeding apparatus. This comprises a small stainless steel dish with a central outlet hole. A large steel ball blocks the outlet hole. The dish sits above a large piece of 10mm thick steel plate sat in the grill pan. The steel plate is preheated with the oven. The dish is placed on the wire rack above the plate and filled with water via a syringe. There is a thin copper washer under the ball with a fine copper wire attached. The wire is threaded out of the oven and the end wrapped round one of the hob knobs so it doesn't go astray. When ready to steam, a pull on the wire unseats the ball and water drips onto the plate, at a rate specified by the size of the hole in the base of the dish.

Lance

Doc.Dough's picture
Doc.Dough

Very clever design.  Could probably use a silicone O-ring as the seat. I would be concerned about damage to the rim of the drain hole allowing it to leak, though it would not be much and not for very long.

The calculation of how much lava rock you need includes the heat storage required to boil the water, but you still need to get the heat to the water. Small rocks and convective flow to keep them hot was my approach, but your drip method will give an extended delivery period if you can get the steel plate to re-heat fast enough.  I suspect that a thin copper plate might do a better job of capturing and transporting the heat to where the water is though there is probably some optimum thickness and roughness and size for every Cp and k.

albacore's picture
albacore

Thanks Doc. It's a metal to metal seal which seems pretty watertight. I used a square of 6mm stainless plate as an insert to avoid any heat distortion effects. The advantage of the thick steel plate is that it acts as an initial heat store, but I accept the disadvantage that the water will tend to drip on the same area of the plate.

Lance

Doc.Dough's picture
Doc.Dough

I think the differential equation that governs is dA/dS = -V x A/S  where dA is an incremental loss of Air and dS is the simultaneous incremental addition of Steam, V is the volume of the oven and A/S is the ratio of air to steam in the oven.

The air loss rate goes down as the ratio of air/steam goes down so you never get to 100% steam, but after a few exchanges it is good enough.  It is the same case as a pressure cooker - in the beginning it is filled with air but the air is expelled with the escaping steam, but in that case the amount of steam increases as the pressure increases until the vent begins to operate at which point you expel both.

Who ever thought that a baker would need calculus and differential equations to solve a practical problem?  :-)

albacore's picture
albacore

Yes, finding the best solution to problems like this is an interesting blend of the theoretical and empirical! Still, it keeps the mind and hands active!

Lance

albacore's picture
albacore

I found this site of a company that makes steam boilers for deck ovens. It's interesting because there is a nice line diagram of the boiler attached to a deck oven and also because their smallest boiler with a 4kW element produces about 6kg of steam an hour, or 100g/min. So I reckon the max you will get out of a stove top pressure cooker is going to be about 25g/min, assuming 1kw going into the pressure cooker from the hob burner. So, a little bit short of what we want, but not a million miles off. If it came to the worst, I could combine it with my drip steamer for an initial boost.

Next phase: construction!

Lance

Doc.Dough's picture
Doc.Dough

You can generate steam outside the oven with a boiler, or generate it inside the oven with stored heat, or inside the oven by taking heat from the main heating element.  What you do probably depends on the specifics.  If the oven is hot and the heating element needs to be on only 25% of the time to maintian temperature, then you have 3/4 of the available heat to use for makeup if you take the steam energy from the oven proper. Using the preheat time to store energy which you use to make steam when you need it has always won out when I did the analysis.

I see an optimization problem in the making that seeks to configure the best possible shape/material for a steam generator that sits under your dripper and runs off of stored heat plus reheat energy.  It might be a long tube (or tubes) that conduct heat to the bottom side where the water is running. Or a bed-of-nails heat exchanger that collects heat from the air either above or below a plate that guides the water along a spiral path to where the heat is. ...

DanAyo's picture
DanAyo

Lance, you should be thrilled to find out you are wrong about the length and amount of steam capable in a home oven :-D At least I hope your wrong. See the video in this LINK.

I am using an electric home oven with convection capabilities. Although no convection is used during the steam. Now we’ve got to learn from Doc if we are getting enough steam in the oven for optimum bake.

This bread baking hobby can sure get intricate...

Danny

Doc.Dough's picture
Doc.Dough

Danny,

The answer to your questions is here:

Another wives tale worth debunking with data

DanAyo's picture
DanAyo

I just baked a bread in a pan. I set it up exactly like this VIDEO. The only exception is I didn’t include the cover pan shown at the very end of the video.

The instructions called for 20 @ 450F and then reducing the temp for further baking. Because of this I elected to steam for 20 minutes. This time I plugged all oven vents including the top (main) vent located under the control panel. I tested for steam leakage at all exhaust locations and no leakage was found. At the end of 20 minutes I removed the towels and tested the main vent with the mirror. It fogged instantly and the surface of the mirror was very damp.

How can I know that the amount of steam within the oven is adequate?

Danny

Ponchit's picture
Ponchit

Quoting your post, Doc.Dough:

“You will continue to make steam if there is water and heat, the only  issue becomes how fast you make steam relative to the rate of escape. Lava rock has a lot of surface area and a pound can store enough heat to do the job if the water is distributed such that the rock can deliver the heat. I have been trying unsuccessfully to come up with a way to deliver the water slowly (preferably after you shut the oven door).”

I may have found a way. Interested? It’s so simple like a paper clip.

I’ll post pictures next. 

-Ponchit

Doc.Dough's picture
Doc.Dough

Always looking for a better way. 

Doc.Dough's picture
Doc.Dough

What size tubing? What kind of pump? And your sources?

Congratulations! It is a clever approach and can probably deliver water fast enough to do the job, even though it will not work for every oven.

But it looks like you have a gas oven, which means that the combustion gases will go up the flu and carry any steam with it so you need a lot more water (and correspondingly more energy) to assure that there is enough steam in the oven when you need it.

Ponchit's picture
Ponchit

Search Amazon:

  • Silicone 3/16” x 5/16” Silicone Tubing 
  • Electric Spray Bottle

My tube’s ID fits the nozzle of my sprayer. So buy your sprayer first then search for a tubing that fits.

-P