Yeast
Leaveners are ingredients that fill batters and dough with bubbles of gas.
The work leaven comes from a root meaning “light, having little weight” and refers to how they cause dough to rise. Humans have been leavening bread for 6000 years; but we did not understand how the process truly worked until around 150 years ago through Luis Pasteur’s work with yeast fermentation.
Yeasts are single celled microscopic fungus, relatives of mushrooms, that metabolize sugars for energy. Some species can be harmful to humans, but one in particular—Saccharomyces cerevisiae—is responsible for converting sugar into carbon dioxide and used for both baking and brewing.
Originally yeast was obtained from the surface of grains or a dough starter, but today it is grown on molasses in fermentation tanks to be cultivated and resold.
How yeast works
The reaction that takes place in yeast converts one molecule of a simple sugar (called glucose) into 2 molecules of alcohol and 2 molecules of carbon dioxide.
In brewing, the alcohol remains in the liquid but the carbon dioxide gas escapes. In baking, both the alcohol and carbon dioxide gas are trapped in the dough causing it to expand but are later expelled through the heat from baking. In addition to causing the dough to rise, yeasts release chemicals that affect doughs consistency, strengthening gluten and improving elasticity.
Active Dry Yeast
Introduced in the 1920s, active dry yeast is removed from the fermentation tanks and dried into granules, rendering it inert.
It can be stored at room temperature for several months. It must be reactivated by soaking in warm water (105F-110F) before mixing the dough.
Types of Baker’s Yeast
Cake Yeast
Also called “compressed yeast.”
A moist block of fresh yeast cells directly taken from the fermentation vat, the cells are alive and produce a stronger leavening effect than the other types of yeast. Cake yeast must be refrigerated and has a short shelf life of 1-2 weeks.
Instant Dry Yeast
Also known as “bread machine yeast” and “RapidRise” yeast.
Instant yeast was introduced in the 1970s and is more shelf stable than Active Dry. Instant yeast is dried more quickly in the form of rods that take on water more easily than active dry so it does not have to be pre-hydrated prior to mixing into the other dough ingredients. It can be stored frozen for up to 2 years.
Other Leaveners
Chemical leaveners use acidic compounds mixed with an alkaline compound to produce a reaction that will result in carbon dioxide gas much faster than yeast.
Yeast produces carbon dioxide slowly, over the course of an hour or more, so the dough around it must be elastic enough to contain the gas for most of that time. Weaker doughs and runny batters will not hold gas bubbles for that extended period of time, so they must be leavened with a faster acting source.
Baking Soda
Sodium Bicarbonate
Baking soda is commonly used as a leavener and provides the alkaline component when mixed with a dough or batter that has an acidic ingredient to react with it. These ingredients are commonly sourdough cultures, buttermilk, yogurt, sugar, molasses, chocolate, fruit juices, and vinegar.
Baking Powder
Sodium Hydrogencarbonate
Baking powders are complete leavening systems that contain baking soda and an active acid in the form of crystals. The ingredients are mixed with ground dry starch to prevent them from reacting prematurely. The reaction begins when baking powder is mixed with a liquid. The baking soda dissolves immediately and begins to react with the acidic components. Most baking powders are “double acting” which means they react when they are initially mixed with liquid and then again when introduced to heat during the baking process.
Sourdough Starters and Preferments
Starters are a combination of yeast and bacteria made with a mixture of flour and water that has begun to ferment.
Yeast and bacterial spores naturally occur in flours and in the air around us, so each starter will be a slightly different mix of microorganisms and will provide additional sourness and earthy flavors to the dough.