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I was always wondering, why there's no gradient of doneness visible on the cuts of bigger breads. For example:

Bread cut

This one was made in the baking form, it has a thin visible crust, but after that the texture is absolutely uniform. How is that happens that temperature is distributed so evenly in the bread mass? Dough is not circulating like liquids do, so I'm puzzled. Could anybody shed the light on that?

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Sergey Mikhanov
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3 Answers3

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Essentially, the exterior crust and the interior evenness are both side effects of the distribution of water.

The Maillard Reaction - the chemical reaction responsible for the brown crust - happens at about 150° C. Generally you're baking at a much higher temperature than this - say 200° C.

The first question one might ask is, why is the crust only on the outside? And the answer is because only the outside gets dry enough; as the small amount of water on the surface evaporates, the temperature is allowed to rise to ambient oven temperature.

The reason it doesn't happen on the inside is precisely the opposite; water boils at 100° C, so as long as there's still water in liquid form (and bread is, to a large extent, water), the temperature can't get any higher than that. The interior of the bread is always regulated at approximately 100° C, and since the baking temperature is uniform, so is the finished product. It's almost like poaching an egg or a piece of meat; no part of it is able to get any hotter than the liquid around it, so the longer you cook it, the more uniform the temperature gets.

If you continued to bake the bread until all of the interior water evaporated into steam, then it would eventually start to undergo the same process as the steam slowly escaped. But we usually don't bake bread that long, so it doesn't happen. As commenter Ray points out, there are some types of darker bread such as pumpernickel which involve intentional "overbaking", but obviously not to the point of burning it.

Note: There may also be a certain element of the process that is due to steam being trapped inside; the water vapor, like any gas, expands and distributes itself fairly uniformly throughout its container. However, the bread is porous, so this steam eventually has to escape; if it didn't, you'd end up with a very damp interior after cooldown from all the condensation. If you measure, you'll also notice that the weight of a fully-baked bread is about 10% less than the original dough weight. So the steam doesn't stick around indefinitely, but the continuous evaporation coupled with the temperature self-regulation (due to the boiling point) keeps the temperature relatively constant.

Aaronut
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  • Continuing to bake the bread is how pumpernickel is traditionally made, and what gives it its characteristic dark color. – Ray May 25 '11 at 20:49
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    Excellent answer. There is a long discussion of precisely this topic in volume 1 of Modernist Cuisine (http://amzn.to/m1b6lX), by the way. You've summarized it very nicely - in essence there are 3 zones - the evaporation zone (crust), the boiling zone (a thin layer right under the crust) and the conduction zone (the whole remaining interior, at roughly the same temp once it reaches equilibrium). – Michael Natkin May 26 '11 at 15:45
  • @Ray, I doubt pumpernickel gets up to 150C in the crumb, as it would get far too dry, so therefore the browning of the crumb that occurs wouldn't be caused by the Maillard reaction. Can anyone confirm? – Highly Irregular May 10 '12 at 10:13
  • The Maillard reaction can occur at lower temperatures: it just takes longer. So, while the center of a pumpernickel loaf will stay around 100C, over the course of a 12 to 24 hour steam bake, it will gradually brown. 150C is where the Maillard reaction starts to be noticeable (i.e., if you stare at it for a couple minutes, you will actually see it turn brown). Caramelization, on the other hand, only really occurs at temperatures above boiling, so while both Maillard and caramelization can play a role in crust color and flavor, the center of pumpernickel is due to Maillard. – Athanasius Jan 01 '13 at 22:55
  • @Athanasius: [citation needed]. AFAIK, 154° C is in fact the minimum and the only way to achieve it at a lower temperature is to raise the pH (which is well-known for foods like pretzels). Bread gets more than dry enough during baking to reach that minimum - what you're calling a "steam bake" is almost certainly well above 100° C - it is, after all, *steam*, not water. – Aaronut Jan 09 '13 at 03:09
  • @Aaronut: [citation needed]. I have no idea where you got that number. It even occurs at room temperature, one of the reasons soil is brown. Do a Google search for "Maillard reaction room temperature" or something -- you'll come up with hundreds of scientific papers talking about it. It can also happen in long-term storage of foods at room temperature. Also, steam temperatures above 100C require *pressurization*. I'm willing to admit the pressure inside bread is *slightly* above normal, it's nowhere near enough for the temp to rise to 154C, roughly 5.5 atmospheres! Your bread would explode! – Athanasius Jan 09 '13 at 03:30
  • @Athanasius: *Water* temperatures above 100° C require pressurization. *Steam* is *always* above 100° C and can be any temperature above that, right up to plasma temperatures at around 150,000° C. You've got it backwards; high pressure causes condensation, low pressure causes evaporation. – Aaronut Jan 09 '13 at 03:40
  • @Aaronut - by the way, I've actually baked pumpernickel bread for ~18 hours in an oven. You use a covered pan to keep the moisture in (hence, "steam bake"). Since the oven temperature started out at 250F for the beginning and was turned down to 225F after the first hour or so, there's no way it ever reached 154C (309F). Yet, the center was still browned quite a bit. – Athanasius Jan 09 '13 at 03:42
  • @Aaronut: let me try this a different way: can we agree that boiling water can't get above 100C at normal pressure? As long as bread has more than a bit of water (and good pumpernickel bread is certainly not dried out like a cracker), water will be boiling out inside of it. That boiling water will be coming out of the dough at 100C, effectively keeping the interior temp at that point. For water exiting the dough to be at 154C, you'd need pressure of about 5.5 atmospheres. Thus, 154C could only occur in completely dried out conditions, by which point the bread would be crispy, not pumpernickel. – Athanasius Jan 09 '13 at 03:58
  • @Aaronut - To help you see what I mean, I've found some [good photos](http://www.samartha.net/SD/procedures/PPN01/PPN01-4.html) of pumpernickel that was baked 24 hours at 250F. You can see the significant difference in color between the dough before baking vs. the center of the bread after baking. The dough clearly did not reach 154C (309F). – Athanasius Jan 09 '13 at 04:26
  • I'm still not sure why you are still arguing that Maillard can only occur at 154C and above. @Aaronut is trying to say that in order for your assumption to be correct (154C minimum) then you would need 5.5 atmospheres but since this is clearly untrue (the photos that athanasius linked) we have to assume that the maillard is occurring at a lower temp due to other factors such as alkalinity, length of time exposed to heat, etc. – Brendan Jan 30 '13 at 18:00
  • The Maillard reaction, like many reactions, is actually stochastic and time/temperature dependent. It does occur at lower temperatures, but much more slowly. There is not a precise cutoff. – SAJ14SAJ Aug 11 '13 at 14:40
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I may be wrong here as I can't find any definitive answer, but this is probably due to our old friend the Maillard Reaction. This is what causes food to brown - it is often mistakenly called caramelization, which is an entirely different process.

The Maillard Reaction requires surface water to completely vaporize. When you put bread into a hot oven, the water evaporates from the surface rapidly and the Maillard Reaction occurs. This forms a partial 'seal' that prevents too much moisture loss from the rest of the loaf, though this seal isn't perfect - bread still goes stale after a day or two.

Incidentally, it's often thought that having steam in the oven helps form a crust. This is not the case. Steam in the oven in fact prevents a crust from forming, which lets the bread rise more, before the steam evaporates and the crust forms.

ElendilTheTall
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  • Steam doesn't evaporate. Steam, by definition, is water that has already evaporated. However, it does make sense that steam would prevent a crust from forming, which is the opposite of what I've read right up until this moment! – Highly Irregular May 10 '12 at 10:08
  • True - the steam doesn't evaporate, it merely escapes from the oven. – ElendilTheTall May 10 '12 at 10:22
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When the bread is baking the water in the bread turns to steam that, along with the CO2 from the yeast, inflates the gluten network.

The crust is able to brown because it is exposed to much high heat from the oven air than the interior. If you were to take the bread out early before the proteins had set you would also be able to see a gradient- the dough would become progressively more doughy toward the center.

Bread recipes cite 190F-200F as internal temperatures for baked bread. After 212F the bread starts to burn.

My suspicion, therefore, is that the steam regulates the interior temperature until all the protein has set- at that point you take the bread out of the oven. If it continued to bake and the steam all escaped you would start to see a more pronounced gradient but it wouldn't be edible anymore.

In my non-scientific experience, the outer portion of the bread is exposed to more heat than the interior but the difference between the two isn't enough to dry it out or burn it as long as there is steam left.

Sobachatina
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