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Note: the text below was copied from the Inquisitive Cook website.
It's a mystery.
It happens every year, in regular cycles, between the late spring and the early fall, but never in the winter. It's the strange case of the recurring question: "What makes homemade bread slump?"
In searching for the answer, we'll discover the first clue when we look at what happens as a yeast dough rises. Once all the ingredients are mixed and kneaded, rising time provides an opportunity for the yeast cells to become active and grow. Yeast is a living fungus and, as is the case with all living things, it requires sources of energy to drive the inner workings of each of its tiny cells.
While sugar is the preferred food of yeast, it can also break down starch into forms of sugar it can use. As it converts sugar into energy, yeast gives off carbon dioxide, which rises in its warm surroundings, stretching the dough, as small bubbles of gas push their way to the top. Thus, the process of rising results from the collective action of millions of individual yeast cells.
Punching down the dough after its first rising redistributes sugar, oxygen and warmth, so the needs of the cells are re-supplied. After the dough has been shaped and slipped into baking pans, yeast cells are working so sufficiently, that a second rising always takes less time than the first.
Rising takes place more quickly when the surrounding conditions are just right. Temperatures of 77F to 86F (25C to 30C) degrees are considered ideal for rising dough, just about the temperatures of a warm summer day. And that means that bread made from spring to fall on warm days can easily be overproofed, that is, allowed to rise for too long a period of time. It may, indeed, be the same length of time as usual, but under ideal, warm conditions, rising takes place more efficiently.
Overproofing also means more carbon dioxide than necessary has been generated. Then, for the first few minutes the loaf is in the oven, its volume increases even more, a phenomenon known as "oven spring."
At this point, the dough has reached heights greater than its inner structure can support. So part of it slumps, collapses or balloons over the sides of the baking pan. This leaves a very wrinkled crust, or a lopsided loaf. Just as an elastic snaps when stretched too far, the dough has been stretched beyond its limits.
It's perhaps a comfort to know that when slumping happens, often the conditions have been nearly ideal for rising bread. On a good warm day, simply a little less rising time is usually the answer. It seems that too much of a good thing is perhaps unwise for bread, just as it sometimes is for people.