MythBusters - Pizzamaking Edition

[TXCraig1]

Episode 1 - Do pizza stones absorb moisture from a pizza while it bakes?

Like the claim that NYC water makes better pizza, say something enough and people will believe it. Say it a few more times, and they will refuse to listen to anything to the contrary. To this end, there is no shortage of people who believe pizza stones absorb moisture from the pizza as it bakes. It's hardly surprising, a quick Google search turns up dozens, if not hundreds of websites that all say something to the effect: "Pizza stones are made of porous materials to absorb moisture easily." (http://www.foodservicewarehouse.com/education/how-to-use-a-pizza-stone/c31723.aspx#sthash.w7cgmKKE.dpuf) Even Wikipedia, the poster child for 'you can't put it on the internet if it isn't true' gets in on the action. Seven websites promoting the myth are listed here: http://www.pizzamaking.com/forum/index.php?topic=28243.msg379789#msg379789.

Manufacturer,s such Breville for example, use the myth to try to sell product: "The pizza stone's porous surface absorbs moisture from the pizza dough, resulting in a crisp brown pizza crust." (http://www.brevilleusa.com/13-pizza-stone.html). This site that purports to be a pizza stone buying guide and namer of the best pizza stone for 2015 talks about a pizza stone absorbing moisture four times: http://thoroughlyreviewed.com/home-kitchen/pizza-stone-review/

Of everything I read this afternoon however, I found this bit on Serious Eats to be the most interesting:

"The point of using stone or ceramic instead of metal is that the stone absorbs moisture from the dough, resulting in a crisper crust. So, glazing sounds like a bad idea, right? According to packaging description, the glaze is "micro-crazed" which sounds a lot like my mental health some days, but it actually means that the glaze has teeny cracks, so the stone can still absorb moisture." http://slice.seriouseats.com/archives/2011/04/pizza-protips-baking-surfaces-part-6-emile-henry-pizza-stone-review.html

Perhaps at the time, it said this on the Emile Henry Website, but it doesn't today, "The glaze is micro-crazed contributing to crispy well-baked crusts, just like a pizza oven." I don't see anything about absorbing moisture. It would seem that the Serious Eats author is so programmed to believe the myth that she automatically assumed this is what the manufacturer was talking about (maybe they were) and then proceeds to imagine and explain how it works!

So what is the truth? Does a pizza stone absorb moisture as the pizza bakes?

Common sense seems to suggest NO. At one atmosphere, the density of steam is about 0.6kg/m^3 or 0.006g/cm^3. Say you have a typical decent stone (0.5" x 16" x 14"), that would be 1,835cm^3. The mass of steam/water at normal operating temps that could occupy that volume if there was NO stone is 1.1g. Now consider that probably 99+% of the volume is stone and assume there is 1% open space, if it was fully saturated, the stone could hold 0.11g water. That's the MAXIMUM possible; I'd guess that the water absorbed from a pizza by the stone is meaningfully less than 0.01g.

Simple physics also seem to suggest NO. The Ideal Gas law Tells us that PV=nRT (the letters represent Pressure, Volume, amount (in moles), ideal gas constant, and Temperature of the gas, respectively). In simple terms, as temperature increases, if there is nothing to contain the gas, pressure will remain more or less constant and volume will increase - or vice versa at a constant volume. Let's say some steam or water vapor enters the stone from the pizza; unless the stone is cooler than the pizza, the temperature of the steam/water vapor will increase. Since there is nothing containing the steam on the top side of the stone, the volume of steam will increase and pressure will remain more or less constant. The net effect is that most water that enters the stone will be forced OUT.

I also did a couple experiments to test if the theory is supported by real world observations. My pizza stone is to heavy for my scale to weight with 0.1g precision, so I took a solid brick with a pore structure that looked similar to, if not a bit more open than my pizza stone, and broke it in half. I weighed both halves and then put one half in the oven at 475F for an hour and submerged the other in a sink full of water. The piece in the sink gained about 5.7g while the piece in the oven LOST 41.1g. There was some moisture in the brick, and it was forced out as the temperature increased exactly as the theory predicted. If moisture in the stone is being forced out, what are the chances it is going to absorb moisture from a pizza? At this point, I was pretty sure thee myth was dead, but I pushed on.

In the next test, I placed 5ml (5g) water on the stone, closed the oven door, waited 30 seconds, then weighed the stone. It gained 0.1g that dissapeared within the next 30 seconds. I then put another 5ml water on the stone and placed two corn tortillas right on top of the water to try to trap some in the stone. Again I waited 30 seconds and weighed the stone. Like the first test, it gained 0.1g that dissipated in the following 30 seconds.

Lastly, I made a simple 65% dough from flour and water only and baked a small pizza analogue (80g, 5", TF=0.14) for 7 minutes. The stone actually lost 1.4g in the process. Not only did it not absorb any moisture, it forced more previously trapped moisture out just as I would expect from the theory. For the final test, I put 5ml water on the stone and baked another pizza on top of it for 8 minutes. The stone gained 0.1g, however the pizza stuck badly to the wet stone, and some, if not all, the additional weight was crust stuck to the brick.

Bottom line, there is nothing - not common sense, not theory, and not experimental results - supporting this myth being true.

This myth is

[TXCraig1]

I put the wet half of the brick in the oven when I was done, and within the first 10 minutes, it had not only lost the 6.4g it gained after a total 2 hours of soaking but also another 4.9g that was in the brick before soaking.

[TXCraig1]

The data:

[David Esq.]

Oh, but what if the stone is drawing out the moisture of the wet dough only to have it vaporize once it does so? No need for the water to actually occupy the stone for long.

I thought I recalled that most mass is made up of empty space. So maybe that stone is 99% empty space and only 1% stone.

Props to you for running the experiments. The only thing I am truly critical of is the suggestion that atmospheres and common sense being uttered in the same sentence.

[TXCraig1]

Oh, but what if the stone is drawing out the moisture of the wet dough only to have it vaporize once it does so? No need for the water to actually occupy the stone for long.

I saw some websites that seemed to suggest that though I'm not sure most of them perceived a difference when writing. At least one cited "capillary action," however that is a phenomenon involving liquids - not gas. By what mechanism would the water be drawn out, in quantities larger than trivial, that would be any different than baking on a cast iron pan or steel? The moisture in question is a gas - not a liquid - and an expanding gas at that. Moisture is trying to exit the stone, not enter it.

I thought I recalled that most mass is made up of empty space. So maybe that stone is 99% empty space and only 1% stone.

Maybe at the subatomic level or something like that, but water can't flow into that space. In the sense that "empty space" matters here, it's pretty much stone. Besides, it didn't absorb a whole lot of water as you can see in the table above.

The only thing I am truly critical of is the suggestion that atmospheres and common sense being uttered in the same sentence.

I don't understand this comment?

[rparker]

Interesting, Craig.

I wonder if there is a humidity level in the heat radiating from various cooking materials that is different enough from one another to matter?  Something, somewhere gave someone the idea that water was being drawn out.

One day not too long ago, I was doing a bake on my stock BS stone. For whatever reason, I lifted the pie and saw an area perhaps an inch by 2 inches steam for a second or two. The steam at that point was coming from the stone surface. The stone surface even had what looked like a shade darker in that one spot. then as fast as I saw it, it was gone along with the steam. Never seen this before or since. It was not a long time into a bake. Maybe 30-60 seconds.

I'm not going to suggest the stone attracted water. All's I'm saying is that moisture hit the stone and the stone did not want to keep it. My half-witted theory is that the crust would have let the steam from the surface pass the first chance it got. You've seen plenty of pics of my pies. They are rather moist from watery sauce, excessive pepperoni and boiling cheese grease. Not surprising at all moisture got to that spot.

[TXCraig1]

Something, somewhere gave someone the idea that water was being drawn out.

Yup, and it's the same story for about a million other things that turned out to be false. There is no doubt that the stone is porous and absorptive. You put some water, or pretty much any liquid on it for that matter, and it will soak in - so it must absorb moisture from the dough, right?  Wrong. We're not talking about a liquid. We are talking about a gas - an expanding gas. 

My guess about your mystery pie is that it had a small hole in it.

[David Esq.]

If you add water to a hot skillet it dances around a bit before vaporizing.

I wonder, if you do this to a hot stone, will some of the water be absorbed temporarily before it vaporizes?

If you throw a wet blanket on a hot stone (like a pizza dough)  will it for sure go from damp to steam without transitioning any moisture to the stone before being expelled out as steam? Maybe the brief initial period of time when  there is less moisture in contact with the dough means something.

Note: I assume that the success of baking steel pretty much demonstrates that porousness of the material is not a positive characteristic. That is what I would characterize as a common sense conclusion. What you had indicated was more of a scientific approach requiring knowledge that isn't at all common. 

[RobDude]

I'm at a very novice level of pizza making - just following recipes formulated by others without really understanding much.  I also believed the 'drawing moisture' out of the crust (because I'd read it online).

My probably silly question, if you don't mind my asking, is - what's the benefit of a pizza stone?  I have one, and I've seen a few recipes that say to cook on a stone.  If I preheat a regular/cheap aluminium pan in the oven at 500F, what will the difference be to a stone preheated to 500F? 

[Crispy Please]

I'll add a little anecdotal observation to the excellent science provided by TXCraig1.

Most forum members who bake pies in their home ovens report much better results with a baking steel than with a baking stone.

How much moisture do you think steel absorbs?

-Crispy

[TXCraig1]

If you add water to a hot skillet it dances around a bit before vaporizing.

I wonder, if you do this to a hot stone, will some of the water be absorbed temporarily before it vaporizes?

If you throw a wet blanket on a hot stone (like a pizza dough)  will it for sure go from damp to steam without transitioning any moisture to the stone before being expelled out as steam? Maybe the brief initial period of time when  there is less moisture in contact with the dough means something.

No need to wonder, I saw it yesterday. If you put enough water directly on a stone, some gets temporarily absorbed. Initially, water dances on the stone just like on a skillet. Once you have added enough to drop the temp of the very outermost layer below 100C, you see some absorb before it quickly hits higher temps below and also the heat below moves to replace heat at the surface turning the water to steam and forcing it back out.

The difference however is that a wet blanket is not an analogue for pizza dough. This can be demonstrated by a very simple experiment: squeeze a wet blanket and water will come out. I don't care how hard you squeeze pizza dough, you are not going to get a single drop of liquid water to come out.

There is no doubt that a porous surface can draw moisture out of dough given suitable time; we see it happen in wood dough boxes. However this is demonstrated over the course of hours at room temperature - not minutes at hundreds of degrees. I suspect stone would draw moisture out of dough in the same way, but given the slow water transfer rate and rapid heat transfer rate, it would seem that what tiny amount of water there is at the contact surface would turn to steam before it could be absorbed.

Note: I assume that the success of baking steel pretty much demonstrates that porousness of the material is not a positive characteristic. That is what I would characterize as a common sense conclusion. What you had indicated was more of a scientific approach requiring knowledge that isn't at all common.

Maybe; however I'm not sure you can necessarily draw that conclusion. From what I've seen, it would appear that the success of steel as a baking surface at moderate temperatures is largely a result of higher thermal conductivity (the difference in thermal conductivity is the same reason stone is vastly superior at higher temperatures). As far as I know, nobody has tested a porous surface with similar thermal characteristics. While it may not be positive, I don't have any reason to think it's a negative. My gut feeling is that it doesn't matter much one way or the other. One tiny, untested bit of evidence supporting this, despite the glaze of the Emile Henry pizza stone being "micro-crazed" whatever that means, I suspect if you put some water on it, it would act like its non-porous, and from the SeriousEats link above, it appears it performs similarly to unglazed stones. 

[Tscarborough]

That is actually a concrete paver, not a brick.  It has an absorption rate of around 5% so at a weight of 6#, it could under test conditions absorb about 8.4g of water.

None of that has anything to do with your test, it is just provided as background.  I have never heard the stone absorbing water theory, but it makes no sense on the face of it.

[TXCraig1]

My probably silly question, if you don't mind my asking, is - what's the benefit of a pizza stone?  I have one, and I've seen a few recipes that say to cook on a stone.  If I preheat a regular/cheap aluminium pan in the oven at 500F, what will the difference be to a stone preheated to 500F?

As compared to a thin aluminum pan, the main difference is the amount of heat that can be stored. Think of a pizza stone like a heat battery. It functions very much like an electricity battery.  You load it up with heat over a long period of time - we call this pre-heating and it's analagous to charging a typical battery. In both cases, we are loading energy into something.  Once the battery is charged, we can use it to do work. In our case, the work is baking pizza. A stone is much more conductive than air. When baking on a pan, you have a tiny bit of stored energy, but once it has transferred to the pizza, that energy can only be replaced as fast as it can be transferred in from the sorrounding hot air and thermal IR coming from the oven walls. With a stone you have a lot more energy that can be transferred quickly to the pizza. A stone can deliver energy to the pizza faster than the dough can transfer the delivered energy throughout the pizza therefore the temperature quickly increases at the interface creating a nice crispy browned crust.

One of the reasons that steel works well is that it transfers heat >10x faster than a typical stone. This allows you to use intense heat from a broiler to quickly bake the top as the steel quickly bakes the bottom. The benefits of quicker bake times is a much larger discussion.

There are three main factors that matter with respect to the baking surface:

1) Thermal conductivity - how fast can the material discharge (and absorb) heat.
2) Specific heat capacity - per unit of mass, how much heat can the material hold. It can vary a lot. 1kg of brick can hold ~2X as much heat as 1kg of steel. However, steel is denser, so 1kg of steel is smaller than 1kg brick
3) Mass - more mass = more potential stored heat. This is why people often seek out thicker stones or steel. There are deminishing return however. More mass also takes longer to heat and weighs more. At some point, the downside outweighs the added heat storage capacity.

[PrimeRib]

Thanks for testing. I appreciate the science of cooking. I wonder if anyone has tried a Blackstone pizza with no stone, just the metal tray. I know just the stone has been tried. 

[TXCraig1]

I wonder if anyone has tried a Blackstone pizza with no stone, just the metal tray. I know just the stone has been tried.

I doubt it has been tried. If I look in my crystal ball, I see black on the bottom and white on top.

Conductivity can't be considered in isolation. Temperature, location of the heat source, heat balaance, conductivity, mass, insulation, etc. all work in concert.

[Bobino414]

Probably not absorption but..........

To start off I agree there is no absorption of water into the stone.  But there must be an explanation for why the bottom of the pie(using HG flour) bakes so much better on a stone (think even toasted and darkish) compared to metal (blond to light brown). 

A few years ago I did an experiment in my deck oven.  The top shelf had a Cordierite stone while the bottom deck had a Cordierite stone with an aluminum baking sheet on top.  The oven was heated  to 600+ degrees and a pie was launched on each deck.  When the pies were pulled about 4 minutes later the pie in direct contact with the stone was properly baked while the pie on the aluminum surface showed the expected blond crust which I thought was due to trapped steam. 

So my best explanation is that the porous stone allowed the steam to escape laterally so there was no dough tenting thus an even bottom bake.

I guess people that put forward the thought that the stone absorbs moisture are using "stone absorption" as a convenience term but isn't the effect the same?

[TXCraig1]

Bob,

A couple thoughts on those observations;

- with respect to putting a metal pan on top of a stone, you create an air gap in the process which could have a significant effect on heat transfer. Unless you have a way to laminate the two, I'm not sure you can infer much from the test. If it were the case that baking on metal was a problem, why has steel plate been so successful? I still think it's largely a function of heat transfer not surface texture.

-there may be something to the idea that the texture on the stone provides a path for the steam to escape though I've seen plenty of bubbles form under the dough on a stone.

[mitchjg]

Craig:

I have a suggestion for a future episode of MythBusters.  "You cannot maintain a sourdough starter different than that produced in your area"

Here is an example of a random web hit:

"Some places, San Fransisco for instance, are well known for their local starters. You can buy San Fransisco sourdough starter online, but your bread will only taste like San Fransisco Sourdough for a few days or a couple of weeks. If you keep that starter alive, it won't take long before it takes on local yeast and bacteria and creates bread that tastes like the sourdough made by everyone else in the neighborhood."

[TXCraig1]

NY water might be easier. 

[mitchjg]

NY water might be easier. 

I was hoping you would have a quick and easy experimental design in your back pocket that would make it easy.  Sigh...