Occasionally, my wife, Pat, makes bread or dinner rolls, always with great success.  This past weekend, however, her bread turned out flat and cake like, so I set out to figure out what might have happened.

The culprit, it turns out, was likely our new oven.  Pat sometimes leaves her dough to rise in the oven with the light turned on to add a bit more warmth.  The new oven is very energy efficient.  Also, it has a halogen light.  To start, she turned on the oven for a few moments to warm it up a bit.

The next day, I tried warming the oven and then leaving the halogen light on.  To follow what happened, I placed the bare probe from my digital thermometer into the oven and set it to alert me when the temperature reached 82°F (28°C).  About 45 seconds after the start, the alarm went off, so I turned off the oven.  In fifteen minutes, it had reached a temperature of 122°F (50°C), likely because of the oven design.  High efficiency is accomplished in part by better insulation of the oven walls.  Also, the new oven has a hidden bottom burner, so the floor of the oven likely stores heat while it is warming up, contributing to the lag between when the oven was shut off and the maximum temperature was reached.

The optimal temperature for letting bread dough rise is between 70°F (21°C) and 80°F (27°C).  Above that, the yeast grows too rapidly, causing loss of quality and flavor.  Maximum growth occurs at 90°F (32°C).  At a temperature of 122°F, the yeast would have quickly consumed all of the available sugars in the dough and then started to die off.  At the same time, yeast growing at that pace would have caused damage to the gluten structure.  Gases would be able to escape from the dough more easily, and the texture of the bread would be affected.  By the time Pat shaped the loaves, there was likely not enough live yeast or sugar in the dough to give the final rise before baking, resulting in the fallen loaves.

To continue the test, I cooled off the oven and then left it with only the halogen light on.  Even with just the light, the temperature rose to 107°F (42°C); still too high for proofing bread.

A piece of equipment may throw off your methods, as happened in this case, until you get accustomed to how it works.  From now on, the oven light will be turned on by itself for a short period only before the dough is put in the oven to rise.