Thank you for the links Peter. Reading through his comments, it appears that Marco list 2 reasons for easy digestibility of pizza dough. 1) protein content and 2) long fermentation times.
1) Protein content makes a lot of sense to me.
2) length of fermentation doesn't make sense to me.
One thing that helps me understand the connection between fermentation and digestibility is the self destructive nature of wheat (and grains in general). As wheat reaches maturity, it begins to generate enzymes that will convert almost all of it's components to sugar as food for the budding germ. Basically, Momma wheat sacrifices herself for baby germ. Farmers go to great lengths to harvest wheat before too much of these enzymes are produced (hence the importance of the 'falling value') so that the components don't degrade too quickly when a dough is formed. Some flour producers then take another self destructive grain, barley, carefully take that past
maturity in order to generate an abundance of enzymes and then add that to wheat. These plants basically generate the necessary enzyme cocktail to break themselves down, so the enzymes in protein deficient barley will spare the valuable gluten forming protein in wheat flour.
Every type of enzyme attacks one type of target. They only break down the specific constituents that they're made to break down. It's like puzzle pieces. Water activity carries them around until they find the type of molecule that they 'fit' into and then they split it apart. There's a host of different enzymes slicing and dicing in pizza dough, but the biggest player, by far, is the starch degrading amylase. Malted barley flour supplements this amylase even further. The amylase attacks the available starch in the dough and converts it to sugar. There's also, as Peter mentioned, protease. There's not a lot of protease in flour, but there's enough that if you make a dough and let it sit for a very long time, the gluten will eventually be consumed.
One important concept to grasp is that fermentation involves two major processes- yeast activity and enzyme activity. The yeast are generating some enzymes themselves, but, for the most part, yeast activity is about CO2 and alcohol, while enzyme activity is about entropy. All the constituents in the wheat are on the slow train to becoming simple sugar (glucose). The entropy from enzyme activity represents a pre-digestion of the dough. The further the enzymes break down the dough, the less work your stomach has to do to digest it.
Enzymes are favored by colder temps. As you lower the temperature of dough with cold fermentation, the yeast become dormant, and, although the enzymes slow down as well, the enzyme train doesn't slow down quite as much. A huge part of the 'additional' flavor/popularity of cold fermented dough is simply the extra sugar generated from the relatively faster acting enzymes (and other byproducts that add complexity, but, imo, I think the additional sugar-without physically adding sugar, is a big part of it's appeal).
Enzymes are favored by time. As you dial back the yeast to produce dough that doubles in a longer amount of time, you're giving the enzymes more time to do their thing.
With unmalted/enzyme deficient Italian flour, time, as Marco points out, is critical. You want enough enzyme activity to break down the flour so that it sits well in the stomach, but not so much that the gluten structure is compromised. Kneading/gluten formation, hydration (water activity), dough temp, and the characteristics of the wheat from which the flour is milled are all going to impact the manner in which enzymes perform their tasks, but the bottom line is that an emergency dough with minimal fermentation (less than 6 hours) may contain a ton of added yeast/generate lots of CO2 and blow up quickly, but the enzymes will never have enough time to produce a crust with good digestibility.
The protease is, imo, the main player when it comes to digestibility, so even protease deficient, amylase rich, malted American flours still need plenty of time to reach peak digestibility.
A baker has a tremendous amount of control over yeast activity. You can add more, add less, increase the temp, decrease the temp, and make sure the yeast train reaches it's destination exactly when you want it to. The enzyme train, though, is not so easy to manipulate. On the commmercial level, huge bakeries add pure enzymes to dough and manipulate enzyme activity with relative ease, but, for the home cook with a set amount of enzyme in their flour, you're pretty much stuck with extending the fermentation clock if you're looking for an ideal dough.
Now, just a quick aside, I have been researching some of the pure enzymes that commercial bakeries are using and I think there's ways for the home baker to utilize these ingredients and shorten the fermentation clock (imagine a 3 hour dough that has all the properties of a 3 day cold fermented dough), but I'm not there yet.
Summing up, fermentation is not just about yeast. It's easy for the beginning baker to look at a dough that's doubled and assume it's ready. It's also a common pitfall to look at a dough that's deflated and incorrectly assume it's overfermented. Volume/CO2 production paints a portion of the fermentation picture, but not all. If the yeast quantity was high or the dough was warm, it might have doubled in a short amount of time and will not have had the necessary enzyme activity to be properly fermented. The only way to really know your dough is to learn to recognize the signs of enzyme activity. Clear plastic proofing containers will reveal the enzyme degradation if you learn how to recognize it. There will be plenty of bubbles, but the slivers of dough that make up the bubbles will start to look wet and a little bit gooey. That's when the dough is properly fermented. The finished crust tells the story as well. If you end up with a toothy/knobby crust, you took the dough too far.