Bread has been a staple food for thousands of years, and one of its most fascinating characteristics is its ability to rise, transforming from a dense lump of dough into a light, airy loaf. The secret behind this process lies in the tiny microorganism called yeast. When yeast is added to dough, it triggers a series of biological and chemical reactions that release gas, stretch the dough, and create the soft texture we associate with fresh bread. Understanding why bread rises with yeast involves microbiology, chemistry, and the science of baking.
Yeast is a type of single-celled fungus. The strain most commonly used in bread-making is Saccharomyces cerevisiae. Yeast has been used for centuries not only in baking but also in brewing and winemaking. Its power lies in its ability to consume sugars and convert them into carbon dioxide and alcohol through a process known as fermentation.
When yeast is mixed into bread dough, it finds a ready supply of sugars. These sugars come from the flour itself, where enzymes break down starches into simpler sugars like glucose and maltose. Yeast metabolizes these sugars through fermentation:
Glucose → Carbon dioxide (CO2) + Ethanol (C2H5OH) + Energy
The ethanol evaporates during baking, but the carbon dioxide becomes trapped in the dough, inflating it and causing it to rise.
As yeast produces carbon dioxide, the gas bubbles become trapped within the dough. Wheat flour contains gluten, a protein that forms an elastic network when mixed with water and kneaded. This gluten network acts like a stretchy net that can trap gas, allowing the dough to expand without breaking apart. The more gas produced, the more the dough inflates, leading to a soft and airy bread texture.
Without gluten, the gas produced by yeast would simply escape, and the dough would not rise properly. Gluten provides:
This is why wheat flour is the most common base for bread, while gluten-free baking requires special substitutes to replicate this function.
When dough is left to rest (a process called proofing), the yeast continues to ferment the sugars, steadily producing gas. The dough expands visibly during this stage, often doubling in size. Proofing not only increases volume but also develops flavor, as fermentation produces byproducts that contribute to bread’s aroma and taste.
When the dough is placed in the oven, several processes occur:
Yeast fermentation is not only responsible for the rise but also for much of the bread’s flavor. As yeast breaks down sugars, it produces small amounts of organic acids, alcohols, and esters that contribute to the complex aroma of baked bread. This is why breads made with yeast taste richer and more aromatic than those made with chemical leaveners like baking powder.
Several types of yeast are used in baking, including:
The effectiveness of yeast depends on several conditions: