The science of yeast rising

Chemistry - Rising Yeast

CHEMISTRY: THE SCIENCE of YEAST as a LEAVENER in BREAD

Visual Examination of the Yeast Organism:

Yeast is a living microscopic fungal organism that exists in 160 known species.

One species, Saccharomyces cerevisiae, is called "baker's yeast" because it is used to make bread rise. Students can examine yeast by placing a few yeast granules under a microscope with a few drops of water and some sugar. Each yeast plant appears as an individual oval-shaped cell. Some of those cells also have a tiny bud on it; that is how the yeast plant reproduces (UNESCO 1962).

Explaining the Use of Yeast in the Bread Leavening Process:

Yeast remains in a dormant state when it is dry but becomes active and begins to consume the complex sugars in flour as soon as it is reactivated by warm water. The yeast organism consumes sugars and excretes carbon dioxide gas as a byproduct of that digestive process. The carbon dioxide gas takes up space and the resulting pressure increase inside the flour causes the bread to rise and take on the fluffiness in bread that makes it different from flat breads like matzo. The chemical reaction of yeast digesting sugar produces a mechanical force in the form of gaseous pressure; it is that pressure from the yeast that causes the individual cells in bread to expand making bread rise into the form of loaves and other baked products.

Demonstrating the Mechanics of Yeast Causing Bread to Rise:

The simplest way of demonstrating the mechanical force produced by yeast digestion involves a bottle with some yeast, water, and some sugar for the yeast to digest covered by a deflated balloon on top. As the yeast digests the sugar, it releases carbon dioxide gas that increases the pressure inside the bottle. When the pressure increases sufficiently, the balloon on top of the bottle fills up and expands. This is the same principle that causes the bread cells to expand inside the bread and the bread rises exactly the same way that the balloon on top of the bottle fills up with carbon dioxide gas excreted by the yeast digesting sugar.

Demonstrating the Carbon Dioxide Content of Gaseous Yeast Digestive Byproducts:

To demonstrate that the gas produced by yeast is carbon dioxide, the instructor can let students exhale through a straw into a test tube filled with lime juice. The lime juice will turn a milky white color, which is the test for carbon dioxide. To show that the gas produced by yeast also contains carbon dioxide, fill one test tube with lime juice and another test tube with warm water, yeast, and sugar. Then use a rubber stopper with a hole in it to seal the test tube with the yeast. Place a glass pipette through the hole in the rubber stopper (but not all the way into the yeast solution). Place the other end of the glass pipette into the lime juice in the other test tube. When the gas produced by the yeast begins to bubble into the lime juice, the lime juice will turn milky white again, exactly the way it did when the student exhaled into it, demonstrating that the gas from the yeast contains the same carbon dioxide as the breath that we exhale (UNESCO 1962). Demonstrating the Catalytic Effect of Heat on the Gaseous Production from Yeast: