May 25, 2009

Baking Basics 101 - Yeast & Fermentation

Yeast & FermentationYeast and chemical leaveners are responsible for the beautiful rise in breads and other baked goods. Without them, all our breads & pastries would look like flatbreads. Thus they have one single purpose in baking: to make our baked goods rise.

Yeast-rising is the result of the yeast releasing carbon dioxide into doughs while being exposed to certain levels of heat. Yeast is a living organism, cultured in commercial quantity from the fungus Saccharomyces cerevisiae. The discovery of yeast bread may most likely have been an accident due to the pervasiveness of fungus microorganisms all around us. In many ancient civilizations, the yeast used for liquor and bread were one and the same.

Later on, when Louis Pasteur was able to refine the growth of bacteria, the possibility of commercial yeast became reality. All this commercialization would eventually lead to the packaged yeast that we are familiar with today as well as specialized varieties of yeast for both liquor making and bread baking. Some of these include compressed fresh yeast, active dry yeast, and instant active dry yeast to name a few. More information on available yeast varities will be covered in the next article.

Yeast uses the sugar present in flour and the dough as food, combines them with oxygen, and releases carbon dioxide and small amounts of alcohol, ketones, and aldehydes. Without available oxygen such as the case of being trapped in liquid, the yeast starts producing large quantities of alcohol (hence the reason why yeast is important to liquor making). In the case of bread, as carbon dioxide is released by yeast the molecules of carbon dioxide become trapped in the gluten matrix. This is where we get the first part of the rising action in breads, the proofing stage. As heat is added to the dough (such as when the dough goes into the oven), the heat causes the carbon dioxide to expand further, also causing the gluten matrix to push outwards. This continues until the carbon dioxide dissipates from the dough via steam and heat. The byproduct remnants of yeast such as alcohols, ketones, and aldehydes contribute to the flavor of yeast bread. An overproofed bread causes the yeast to start running out of "food" early and causes the flavor to change (in a bad way) as well as making the gluten matrix too weak to hold up the final product's own weight (which means the bread will collapse easily, have too many holes, or have extra large holes inside).

Yeast breads down flour starch into sugars using invertase while flour has its own enzymes called amylases that also break down carbohydrate in starch to sugars. Thus if we want to improve the amount of food available for yeast and bacteria (and have nice expansion of dough), we need to increase the amount of starch broken down and converted into sugar.

Luckily, there are a couple methods that can help. One method, used commonly in sourdough, is to introduce bacteria into the dough that will work together with yeast and break down and convert the starches quickly. Another method is to incorporate a chef or starter into the dough so as to introduce wild bacteria in addition to the yeast. Chefs and starters are basically leftover dough containing an active yeast population. A third method is to use diastatic malt powder or malt syrup to the mixture (these are sweeteners made from barley or other grains), which contains additional enzymes to break down the starch and maltose to provide food for both bacteria and yeast. Added side benefits include more moisture and sweetness, although the powdered version is preferred in bread dough to avoid sticky dough. Imagine maple syrup or corn syrup and dough. Yes, "icky-sticky" type of sticky.

The next article will focus on different yeast products available in the markets. (But not necessarily in the Philippines)


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