Recently, I decided to measure the volume of the first rise of bread dough containing various combinations of yeast, flour, shortening, etc. in order to see if this volume had any predictable relationship to favorable attributes in the final baked bread. I conducted the test on a small portion and called it an assay. The motivation was to try to gain some reproducibility in the final bread that has plagued my deceptively simple baguette. The volume measured was not predictive of the success of the final baked goods, but the method of measuring the volume of a small dough ball might still be useful.
The expression that titles this post is ubiquitous in bread baking texts. It's a way of telling a baker when to toss the risen dough into the oven, in other words, how long he should proof the final pre-cooked loaf. I don't know why the factor of 2 is significant, I'm guessing it was a more terse way to describe a complicated reaction; proof too long, the bread falls and is dense - under proof and the final loaf is equally ruined. This doubling factor is simply the optimal rise time to get the best oven spring without it flopping upon exposure to high heat.
"Doubled in volume" is not an intuitive end point observation on something shaped like a blob. Using what I learned on the volume measurements, one could take a small fraction of the dough, a small sacrificial piece, and measure its volume in a cylinder over time and use it as a type of indicator for the rest of the dough. In the video, we observe the full range of the lump of dough over time in a narrow cylinder. It actually rises about 3 times its original volume. But, at the end of the rise, we see the curved surface of the dough leveling out, indicating the dough is coming to it's maximum volume. This end of the rise also indicates the point in which the entire mass will collapse with a minor tap. This was a lean dough composed of water 180g, unbleached white flour 300g, yeast Fleischmann's rapid rise 7g, salt 6g, canola oil 6g and from that 100 grams taken for the measurement.
I think it's an interesting diagnostic to run beside a rising loaf. This dough ball is a sample of the bigger loaf; if it rises in the same conditions, it should provide an indicator how long to proof regardless of whether your kitchen is hot or cold, etc. In this case, the loaf should be baked an hour into the rise.
ps Dear @Conagra, can I have a job?