Oven Spring, Bulk Fermentation, and an Experiment
The motive force for oven spring is often stated to be the expansion of gas cells in the dough, combined with a burst of activity by the dying yeast. However, this is only a part of the story. Gas expansion is unlikely to be sufficient by itself because the expansion of gas from a room temperature of 25° C to 80° C is only about 20%. And a burst of activity from the yeast is likely to be small because gas production declines at temperatures above 40° C. The other important drivers are given by scientific text books as the release of dissolved carbon dioxide as the temperature rises, combined with the boiling off of ethanol, which is also mixed with the water. For example, in “Baking Science and Technology”, Pyler quotes the following approximate contributions, which I have rounded to whole percents:
Ethanol boiling: 50%
Expansion of existing carbon dioxide gas: 29%
Carbon dioxide from solution: 19%
Yeast activity: 2%
That leads to the interesting question of how the carbon dioxide and ethanol come to be dissolved in the water. The answer presumably is that they are already in solution when they are created by the yeast, and remain that way until the loaf is baked. The likely time when this could happen is during the initial, apparently quiescent, part of bulk fermentation before the dough starts to rise. If this is true, the yeast is actively producing carbon dioxide early on, but the gas is not immediately visible because it remains in solution. Eventually, however, the solution becomes saturated and any further carbon dioxide has to be released as gas, causing the dough to start rising.
To explore this idea I conducted an experiment in which I divided a batch of dough into portions and baked them after different lengths of bulk fermentation. I started by mixing 500g of all-purpose flour, 350g of water, and 10g of salt in a food processor for 30 seconds. I removed 100g of the mixture as a control sample, coated it in rice flour to reduce sticking and placed it in a 250ml pyrex beaker. I then added 1.5g of active dried yeast dissolved in 5g of water to the food processor and mixed for 5 more seconds to incorporate the yeast. Then I cut 7 samples from the dough, each weighing 100g. I coated each sample with rice flour and put it in a 250ml pyrex beaker, covered loosely with foil. I immediately put the control sample and the first yeasted sample into a 450° F oven to bake for 20 minutes. The remaining 6 samples were baked at half hour intervals over the next 3 hours.
The picture shows the beakers after baking, lined up in order of fermentation time, with the control sample on the left. The graph below shows the heights before and after baking. The baked height increases steadily for 2 hours, when it reaches a plateau at about twice the unbaked height.
The lower graph shows that the dough height before baking remains unchanged at first, only beginning to increase at 2.5 hours, even though the baked height increases steadily right from the start, consistent with the idea that carbon dioxide and ethanol are present but mostly remain in solution in the dough.
There is no sign of a step increase in baked height which would have been produced by a yeast burst. Indeed, extrapolating back from later samples, the rise in the sample baked immediately corresponds to only about 10 minutes of fermentation at room temperature.
The plateau in baked height after 2 hours most likely indicates a limit to the capacity of this dough for oven spring, since the rise in the unbaked dough after 2 hours indicates that fermentation continues. It would be interesting to run a similar experiment with different kinds of dough handling to see their effect on oven spring.
This experiment demonstrates that the yeast is active right from the beginning of bulk fermentation, even though there is little outward sign. It also shows that oven spring does not require that the dough has risen substantially. By the time that a noticeable rise indicates the end of bulk fermentation, the dough is already loaded with enough carbon dioxide and ethanol to drive a substantial oven spring.