The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

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Since the facility to accept text from Word documents is not yet implemented here, the text of this post can be found at this link.  The figure below is notional but while it does not reflect lab measurements, it should be thought of as an aid to thinking about the underlying issues.

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A friend asked for some help with a recipe for a Portuguese corn bread known as broa, and the formula she had been given was not working for her and was producing hockey pucks.  So, having not made any corn bread for a long time I decided to try and figure out what it should be.

It turned out to be one of those on-line formulas that was probably never tested as written and was destined to produce pretty good hockey pucks if you followed it.  But it did give some history and I found some photos that were helpful but nothing that seemed to be really authoritative.

The recipe called for about equal weights of corn flour and AP flour but called for almost all of the water to be used to hydrate the corn and almost none allocated to the wheat flour component so there was never going to be any significant gluten development and thus it was going to become a hockey puck.  And there was no fat in it so it was going to quickly become a stale hockey puck.

But after a couple of iterations I got it to work pretty well.  The first issue was what to use for flour.  That was solved by using instant masa for the corn which is quite fine and does not leave you with a sandy/granular mouth feel, though if you don't add a fair amount of water and fat it will still be a hockey puck, and I went to a strong bread flour in place of AP so that I would have the potential capacity to carry the large load of corn without getting the hockey puck effect.

The second issue was process.  How to develop the gluten before incorporating the masa?  I was not sure it would be possible because of the large fraction of corn flour that was called for.  The solution was to use enough boiling water to saturate the masa and to include all of the salt with the masa.  This made the saltiness of the final bread just fine without inhibiting the yeast very much (since the corn does not get mixed in until the yeast is well established). The bread flour then gets some sugar, the IDY, and enough very warm water to yield a 100°F dough after it begins to mix.  I let it sit for 15 min to autolyse then mixed it at high speed long enough to develop the gluten but not to the point where it would not accept the hydrated masa, incorporating 6% solid fat in the process.  The fat would further stabilize the gluten and should improve crumb texture, mouth feel, and staling qualities of the bread.

Now the corn was added and mixed until it was fully incorporated.  The dough got sticky enough to stay on the side of the mixing bowl, but was easy to scrape off with a silicone scraper.  And it had a good degree of extensibility so I thought it would tolerate a short bulk fermentation to get some gas into the dough.

A 30 min BF followed by very gentle shaping into a boule and about 40 min of final proof (about 50% volume increase) was as much as I thought it could take.  So into the combi oven it went:  500°F for 15 min at high fan speed, followed by 15 min at low fan speed while it cooled down to 350°F.  At this point the core temperature was 205°F and I pulled it out to cool. I think that if you bake it in a conventional oven it will certainly take longer but getting it to brown first is important since the corn does not help much with crust color.

After 2 hrs to cool, it was time to test.  Contrary to the first round, this loaf sliced easily and the crumb texture is much more like a conventional loaf of wheat bread than a loaf of corn bread, even though by weight of ingredients there is about 50% corn masa in the mix. The flavor of the corn is totally dominant though you may want to personalize it by adjusting the amount of sugar. Photos below of the crust and the crumb illustrate the reality of iteration two.



202g  instant masa (corn flour)

12g    salt

260g  boiling water (for the masa)


207g  bread flour

28g    sugar

8g      instant dry yeast

168g  130°F water (for the wheat flour dough that contains the yeast)

24g    solid fat

In a bowl, mix the corn flour and salt; add boiling water and stir/fold until evenly moistened; cover and let sit about 30 minutes to fully hydrate. 

Stir together bread flour, sugar, and yeast. Add the 130°F water and mix until it forms a dough; let sit covered for 15 minutes. Mix until gluten is beginning to develop (~4-5 min), add solid fat and continue to mix until fully incorporated.  Add warm wet masa and continue to mix until the wheat flour dough and the corn flour dough are thoroughly combined (~5 min) [when it was fully combined the dough began to stick to the sides of the bowl but could be easily scraped off with a silicone spatula.  Using masa instead of corn meal and extra water make the dough soft and developing the gluten in the wheat flour dough before incorporating the masa assures that the gluten is strong enough to support a 50% corn fraction].

Turn dough out onto the counter and using a bench knife and a little flour to keep it from sticking, shape the dough into a ball about 5 inches in diameter and place in a lightly oiled bowl to bulk ferment.  Leave it covered for ~30 min, then turn it out and fold a few times to tighten up the boule and place it on a Teflon or non-stick pan that has been lightly greased or sprayed with Pam/oil.

Dust the top with rice flour and cover with a kitchen towel. 

Let rise in a warm, draft-free spot until the volume increases by about 50 percent, ~30 -40 min. 

Meanwhile, heat the oven to 500°F with a rack in the middle.  Slash just before oven entry.  There is a lot of yeast in this formulation, but there is also a lot of corn so don't expect a huge oven spring.

Bake the bread for 15 minutes at 500°F.  Reduce to 325°F and continue to bake until deep golden brown, another 15 (with convection) to 30 minutes (without).  Crumb temperature should be 202-210°F when it comes out of the oven.  It will rise another few degrees before it begins to cool just from thermal soak back.  Transfer the bread from the baking sheet to a wire rack and let cool completely, about 2 hours. 


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Below is a proofed demi-baguette that was marked with lines spaced 1.25" apart.  It is about to be baked without any steam as a baseline for testing the hypothesis that steam facilitates the stretching of dough. Since this loaf is not scored, we should expect it to blow out along the side.  But still, if the surface stretches in response to internal pressure generated by the expanding CO2, then we will be able to observe and measure how much stretch there is.

Raw marked no steam


Here is the resulting baked demi-baguette. The spacing between the lines is still very close to 1.25" indicating that there is little or no stretching when baked in a dry oven (this was baked in a combi oven set to hold the box humidity below 20% which effectively removes even the steam that escapes from the bread itself).

Cooked marked no steam

The photo below is another demi-baguette from the same batch that was baked with steam.  It too was marked with lines spaced 1.25" apart before it was baked.  This loaf had a defect on the top that allowed it to open slightly (actually the side-to-side dimension of the slit it almost exactly 0.25"), and the post-bake line spacing is very close to 1.375" except where the defect increases it to 1.625".  So there is some small amount of surface stretching that seems to be facilitated by steam in the oven.

Cooked with steam marked

This loaf was baked in the same combi oven but with the steam generator and humidity controls set to maintain 100% humidity in the oven for the first 7 minutes of the bake (when it was just beginning to brown).

The photo below is another demi-baguette from the same batch that was slashed and baked with steam, illustrating the surface expansion that occurs when a well proofed loaf is slashed to allow the oven spring to open the loaf where you want it to split.

So the data indicates that the difference between having no steam and maximum steam is the difference between no surface area increase, and perhaps ~20% area increase even when there is no steam in the oven. It is a measurable but not significant effect.  However, you can see the difference in color between the steamed and un-steamed loaves, with the steamed loaves having a more yellowish tone and a shiny surface (as opposed to a dull brown surface for the loaf baked without steam.

It is worth noting that this experiment has a sample size of 1 which does not imbue it with great weight in a statistical sense. But it does set expectations and will guide further experimentation.  This particular batch of dough was mixed at 70% hydration, which is a bit higher than the 67% at which I would normally make baguettes. The objective was to build a fairly stretchy dough that I thought might be more amenable to surface stretch than a lower hydration mix.  The next step up would have been 75%, but at that level it is ciabatta and I was not sure that I could put marks on the surface without deflating it.

Slashed unmarked w/ steam


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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.


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.

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For about the last year I have been working to understand exactly what is going on when a properly proofed and slashed loaf is baked with steam. What is the role of the steam?  What is the role of the yeast?  How does hydration and proofing impact the results?  Deep slash or shallow slash? What are the differences between large and small loaves? ...

After a number of false starts, I have produced a short video showing what is going on. It is annotated but not narrated. I offer it for critique.  What is missing?  And what questions are not addressed?

You can find it at:



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