by Janet Mcleod

Created on: April 07, 2007   Last Updated: September 05, 2010

Bread rises for the same reason that Coke fizzes, champagne bubbles, beer forms a head, and Mentos erupt in carbonated drinks: the presence of carbon dioxide. Although these are completely different processes (fermentation vs. carbonation), their outcome depends on the same compound.

Bread rises through the process of fermentation and is produced by baker's yeast, a living, single-celled fungi and member of the Saccharomyces cerevisiae species. Fermentation is a matter of simple cell-biology, and here's how it works: just as the cells in our body undergo metabolic processes and give off carbon dioxide gas, yeast organisms go through a similar yet simpler life-cycle (below), and yield the same compound: carbon dioxide gas. Carbon dioxide is expelled from our body when we exhale. But when baker's yeast is added to bread dough, the gluten in the dough traps the carbon dioxide; and so, like a balloon, the bread expands as the gas bubbles accumulate. Gluten, which is a combination of protein and starch, becomes stronger as bread is kneaded, more effectively trapping the forming gases. This is the reason it is important to knead bread dough until it is smooth and elastic.

YEAST LIFE-CYCLE
1. Respiration - This is a period in the cell's life cycle during which the yeast collects and stores energy from oxygen and sugars to be used for reproduction and sustaining life.
2. Reproduction - During this phase, the yeast "buds" or reproduces asexually. It grows a bulge, the bulge breaks off, and with an exact copy of the DNA, lives as a clone of the original cell.
3. Fermentation - As the yeast cell goes through the process of converting and storing sugars and creating the distinct flavor that makes home-baked bread so irresistible, it expends energy and leaves behind carbon dioxide gas.
4. Sedimentation - this occurs when yeast runs out of food and energy, causing it to gather and settle, and basically shut down all activity.

YEAST FACTS
Yeast works optimally in warm conditions (around 90 degrees) with plenty of sugar. But it dies at temperatures above 122 degrees. If dissolving yeast, be careful not to overheat the liquid.

There are two common types of baker's yeast: Rapid Rise and Active Dry. Rapid Rise yeast can be used in place of regular Active Dry yeast if you are in a hurry to speed the process of fermentation. It is more finely ground and absorbs liquids faster, making the conversion of starch and sugars to carbon dioxide much quicker. It is also requires only one rising.

Some people add baker's yeast to a "sponge" of flour, water, and a small amount of sugar, and store it as a "starter" for breads, pancakes, and biscuits. Sourdough bread is made from a wild yeast starter. If you keep a starter, you will notice after feeding that bubbles form and escape through the surface. If too much time passes between feeding, a clear liquid accumulates on the surface and should be poured off before stirring fresh flour, sugar, and water into the settled sponge. If you neglect feeding or refreshing your starter for too long, it will go into the sedimentation phase of the life-cycle and may need to be thrown out.

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