How Does Yeast Make Dough Rise?
Yeast causes bread dough to rise by forming bubbles of gas in the dough. Yeast is a plant. The whole plant is made up of one cell which is so small that you need a microscope to see it. Great numbers of yeast cells, in the form of a paste, are mixed into bread dough. The yeast cells grow and multiply rapidly. As they grow, they give off carbon dioxide gas.
This gas forms thousands of bubbles in the dough. The bubbles make the dough rise to a nice, fluffy texture. The many small holes you see in a slice of bread are the remains of the bubbles. You can buy different types of yeasts. The first of these is called live or fresh yeast. This is relatively unstable, requiring refrigeration, and it has a very short shelf life.
Dried versions, sold in packages or cubes is essentially cells of Saccharomyces cerivisiae, which are waiting to be activated. Like many fungi types, yeasts for bread dough respond to warm water bringing the yeast cells to life. When exposed to sugars in bread and in flour, it begins to eat, digesting portions of these sugars. This eating process goes on for a short period of time only.
Eventually the yeast will die within a few hours, especially if the dough is allowed to grow cold or exposed to too much air.
As Saccharomyces cerivisiae is feasting, it begins to release gas bubbles of carbon dioxide, and small amounts of ethanol alcohol. These bubbles, trapped in the bread dough, cause the rising action with which we’re familiar.
Once dough has been acted upon by fungi, not all of the cells are quite dead. The heat from the oven makes remaining cells go into overdrive, madly munching away at the sugars and expelling carbon dioxide prior to expiring from the oven heat. This is why bread continues to rise during its early cooking stages, and then may deflate slightly as cooking continues.
There are a few things that inhibit these microbes from their natural function. Too much salt can halt its rising action. Therefore bread dough recipes usually contain a little salt and a little sugar for balance. Shortening and animal fats can also inhibit the fungi, and you’ll note that breads that have butter in them, especially salted butter may not have the same rise due to the butter’s presence.
Sometimes inactivated or dry yeast unfortunately dies before you get a chance to use it. This is why many bakers use a proofing process, adding a package to warm water to make sure the yeast is alive before combining it with flour. Within a few minutes of exposure to warm, not hot water or milk, the fungi begin to bubble up and expand if it is alive.