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Fermented bread is bread made from dough that has been allowed to rise slowly using a starter of naturally occurring bacteria and yeast carried in the air. Unlike quick-rising breads that use a single strain of specially cultured yeast, usually sold in packets and called fast-rising or instant yeast, fermented bread contains a variety of yeasts and bacteria that help to convert the important nutrients and sugars in the dough into a form that is more readily digestible than instant yeast breads. The taste, texture and overall character of fermented bread are often markedly different than that of non-fermented breads.
One of the largest differences between fermented bread and bread made with quick-rising yeast is how the yeast actually works. Quick-rising yeast is specially chosen for its ability to multiply very quickly. It also is only a single, specific type of yeast that is isolated from other bacteria with which it might occur naturally. When it is added to dough and allowed to rise, the instant yeast will very quickly consume the nutrients it needs from the bread and create carbon dioxide gas that causes the dough to grow in volume. The process happens so quickly that not all of the elements within the flour can be properly converted.
A slow fermentation with natural yeasts and beneficial bacteria from the air creates a different result. The yeast is able to process the ingredients in the bread at a natural rate while also interacting with bacteria and enzymes in the dough, so more nutrients are converted into other forms or are consumed. This results in the gluten in the dough developing into a softer form than would happen with quick-rising yeast. The softer glutens help the dough to retain more of the gas that the yeast is releasing, resulting in a final product with a crumb that is not overly spongy and holds moisture for a longer time.
There are several health benefits that come from cooking and eating fermented bread. For people who have blood sugar disorders, the glycemic index of fermented bread is considerably less than that of white flour breads made with instant yeast. The digestibility of slow-risen fermented bread also is greater, making it easier on the stomach for people who have slight intolerances to the grains. Finally, fermented bread is exposed to good bacteria called probiotics that can help promote the health of the digestive system in the body.