Pizza, Bread, Cake, Souffle, Popovers...
Let me clarify some of the logic on this whole issue of whether spring is caused by steam vs. yeast continuing to eat. This has been referenced on several threads and I'm really addressing all of these threads here. Pete, as I've said before, the bread making guys are looking for an explanation that fits what they are doing and they found it with the yeast rise theory. A 45 minute bread bake leaves their bread yeast in the warm zone for a good 20 minutes so there's enough time to do some real metabolic activity and create some CO2 based rise.
Some of these pizzas are cooking in 1 minute, leaving the yeast in the warm zone for all of 30 seconds and there's just no time for any real eating or metabolic activity to occur.
Yeast is not necessary for foods to rise. Souffles & Popovers have no yeast at all. Trapped Steam is the primary leavening agent in their rise.
A tiny amount of water, turned to water vapor, is a huge volume. At standard temperature and pressure 1 gram of water has a volume of 1 cc. Converted to vapor the volume is 22,414cc (Mole volume) / 18 (molecular weight of H20 = 1+1+16) = 1245 cc. Now account for the temp difference. The baking institute says rise stops at 153. That is 340 Kelvin which is 24.6% higher than standard temperature and pressure (273 Kelvin). So 1245cc*(340/273) = 1551 cc. So one gram of water is equal to over a liter and a half of water vapor at 153F.
Someone on another thread complained that steam could not be the answer because the dough did not reach boiling temp therefore there was no steam. This is faulty logic. Water does not begin to turn to water vapor at the boiling point. Water will turn to water vapor at any temperature. Even ice evaporates.
What causes water to convert to water vapor and at what temperature?
Water molecules on a surface have varying temperatures. Temperature is a measure of AVERAGE velocity of the atoms. At any given moment, almost all molecules are either above or below that average. The temp is like the peak of a bell curve. Few molecules are exactly on the peak line, they are all over the curve. Pool balls make a good analogy. During a break the average velocity of the balls is high, but at any moment, if you looked, some balls are almost stopped and others are moving very fast. An individual surface molecule evaporates when, during the pool-table like jostling, it happens to get up enough speed to pass a threshold - It's moving so fast that it can break away from some electric forces which are holding it to the other water molecules. In other words, the items at the top of the bell curve escape. This lowers the average temperature of the remaining molecules - they keep losing their best players. This is why water is always colder than the surrounding area. Oil, which does not evaporate, does not feel cold on your skin. Here's an example: you have 10 molecules with an average speed of 50 - a total momentum of 10x50 = 500 . But this energy is not distrubuted evenly. As molecules bang in to each other the transfer the energy around from one to another. The total remains the same, bu the distribution is always changing. Let's say that one has a speed of 200 and the remaning 9 split up the remaining 300 units of momentum. If the fast one is moving inwards towards the others, it will hit one of them and slow down. But if it's moving out from a surface, it will have enough speed to break away. So let's say that the 200 molecule breaks out. Now you have 9 remaining molecules with a total momentum of just 300, so they average just 33 per molecule. They are now colder, by definition. Get it?
Continuing our example, the escapee pushes off the other 9 at a speed of 200. This means that the water is exerting an outward pressure. Molecules are literally jumping ship, pushing out. This is 'vapor pressure'. It occurs at all temperatures. It does not start at 212. If it did, then room temp water would not evaporate or feel cold on your skin. Even ice evaporates and has vapor pressure. The pressure is the sum of the momentum of all of the escapees. Air also has pressure. At sea level Air has a pressure of something like 760 millibars of mercury. If you had a pool of mercury, (a heavy liquid metal), and it was 760 millimeters deep, it would push down on the bottom of the pool with a certain amount of force. It's heavy. Regular air at sea level exerts the same amount of force. It's the heavy weight of all the air up to the top of the atmosphere pushing down. Water at room temp exerts wapor pressure. But at room temp the pressue something like 22 millibars. So the peak of the temp bell curve is so low then only a few molecules all the way to the right of the curve are energetic enough to escape. Evaporation is slow. But as the sample rises in temp, the whole bell curve moves over and more and more of the right side of the curve is over the threshold. As the more vertical part of the curve moves over the threshold, small increases in temp result in larger and larger increases in vapor pressure. At 212, the vapor pressure is 760. This is the boiling point because now the vapor pressure is more than the air pressue and a majority of molecules have enough energy to escape. The water stops rising in temp at that point because as energy is pumped in, water molecules use it to escape, leaving the cool ones behind, until they've all escaped.
The point is that the increase in pressure does not begin at the boiling point of 212. Rather, the pressure rises along a curve up to that point. At lower pressures (higher elevations) the boiling point is lower because the threshold is not 760 mb but lower.
Only 1 gram of water turning to vapor is over a liter and a half in a risen dough. This is plenty of volume to account for the rise of the dough.
As I said on my site, yeast is needed to start the bubbles. The yeast creates what amounts to a foam - a body with lots of small air pockets and LOTS of surface area. This surface area is where the water evaporates into. Water in a cup evaporates slowly. Spread on the counter, it evaportates quickly. Both samples are at room temp. The water in the cup evaporates slowly because the energetic molecules in the center bump into other molecules in the center, transfer their energy and don't escape. Only molecules at the surface escape. The water spread on the counter has lots of surface area. The vapor pressure is the same because pressure is measured per area of surface. But the amount of surface is greater, so the amount of evaporation is greater. Risen dough is essentially a foam. It has lots and lots and lots of surface area, mostly on the inside. It's like your lungs. Look how much water vapor comes out of your lungs with each and every breath. The volume of air you exhale is greater than the volume of air you inhale, because the air you exhale has more water vapor in it. The lungs have a huge surface area inside, just like the dough. Dough without yeast would have no bubbles inside. It would heat like the cup of water. Only the surface would evaporate. Interior molecules would simply bump into other molecules an not evaporate. As it heated, it would eventually heat enough that the vapor would press out and create escape routes, but it would not be anything like the even expansion that you would see in an airy foam.
A souffle is a similar type of foam with beaten egg whites forming a foam of air and proteins. Both Souffle's and popovers are good examples of steam-only rises. No CO2 gas producing leavening agent such as yeast or baking powder is needed. Only steam and a protein is needed. Unlike breads, souffle's have no geletanized starches to hold their shape so they are fragile and fall when cooled because the vapor pressure stops. Popovers, sit somewhere in the middle. They have starch, but since popovers do not have the benefit of being foamed before their rise, the distribution of the expanding gas is not spreadout through the batter. Instead the expansion simply occurs in a few large pockets. No latticework of geletanized starches is created. Therefore when cooled and the vapor pressure is reduce, popovers will also collapse. This is why it is best to poke the popover before it cools, to let the steam out before it condenses (condensation reverses the evaporative pressure).
Beating the hell out of a dough is like disturbing a souffle or popover. You are breaking the structure - weakening it's ability to capture whatever gas is trying to expand it. The amount of gas produced might be similar, but the structure will not hold it in the same way. November is correct about overrisen dough. In fact overrisen dough has plenty of live yeast in it. If you culture it, you will see that. But overrised dough has weakened it's gluten structure in at least two ways. It may have expanded too much creating thin and fragile bubble walls, and it has chemically broken down some of the gluten by producing too much acid. When the steam is created during baking, it just leaks out.
Look at the video that someone posted (I think it was Bill) of the brick oven bake. There's a bubble that you can actually see expanding. Most of the expansion is done in under a minute. This is a straight physical process.
