How does yeast work?
Yeast is a singled-celled fungus, there are 160 species of yeast and its preferred conditions are an oxygen-rich, warm and moist environment. Many people know of yeast as the beige granules you find in packets at the supermarket. This is dormant until it is reactivated with water.
Yeast undergoes anaerobic fermentation where sugars such as glucose are converted to ethanol and carbon dioxide.
Word Equation: glucose –> ethanol + carbon dioxide +ATP
Chemical Equation: C6H12O6 (aq) —> 2C2H5OH (aq) + 2CO2(g) + 2ATP
Once reactivated, yeast feeds on the sugars present in the flour and carbon dioxide is released to make the bread rise. This is a slower raising method than using baking powder. Using yeast also gives breads many of the flavours and scents that are often associated with it. To work yeast requires flour to contain it (otherwise it would just be a big bubbly mess), the most common type of flour used for breads is wheat flour as it contains glutenin and gliadin which when combined with water form gluten. Kneading bread dough helps to develop the gluten and it becomes more stretchy as it fills with gas bubbles.
The carbohydrate starch makes up around 70% of flours weight. The starch granules are attacked by enzymes in the flour and release the sugars that the yeast feeds on. The starch also reinforces the gluten structure and absorbs water during the baking process which aids the gluten in containing the pockets of gas that are produced by the yeast.