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FOOD SCIENCE

The Science of Baking with Yeast

By Fine Cooking Editors, editor

December 13th, 2013

By David Joachim and Andrew Schloss
from Fine Cooking #127, pp. 24-25

Before domesticating cattle, pigs, chickens, and other animals, human beings harnessed a much smaller living organism: yeast. Without it, some of our earliest foods and beverages, such as bread, beer, and wine, wouldn't exist. Here's a closer look at how yeast works its magic so that you can make better breads, rolls, waffles, and more.

What exactly is yeast?

Yeast is a single-celled microorganism related to mushrooms. About 1,500 species exist, but in the kitchen, we use mostly just one, Saccharomyces cerevisiae (which means "sugar-eating fungi"). Used to make bread rise, it's available in various forms, which differ mostly by moisture content.

Cake yeast (aka fresh yeast or compressed yeast) is made from a slurry of yeast and water with enough of its moisture removed so that the yeast can be compressed into blocks. Experienced bakers swear by its superior leavening and the nuanced, slightly sweet flavor it gives baked goods. Cake yeast is highly perishable and lasts only about two weeks in the refrigerator.

Active dry yeast was developed by the Fleischmann's company during World War II so that the U.S. Army could make bread without keeping yeast refrigerated. Partially dehydrated and formed into granules, it contains dormant yeast cells that keep at room temperature for several months. To use active dry yeast, rehydrate it first in warm water (about 105°F) along with a pinch of sugar to feed the yeast. The resulting foam is confirmation that the yeast is still alive.

Instant yeast (aka quick-rise yeast) was first manufactured in the 1970s. It's a smaller form of dry yeast that rehydrates faster and can be added directly to the dry ingredients without being soaked first. Some types of instant yeast, such as RapidRise yeast and bread machine yeast, dissolve faster than others and may include ascorbic acid or other dough conditioners (ingredients that help to strengthen the gluten or soften the crumb).

How does yeast make bread rise?

As bread dough is mixed and kneaded, millions of air bubbles are trapped and dispersed throughout the dough. Meanwhile, the yeast in the dough metabolizes the starches and sugars in the flour, turning them into alcohol and carbon dioxide gas. This gas inflates the network of air bubbles, causing the bread to rise. During rising, the yeast divides and multiplies, producing more carbon dioxide. As long as there is ample air and food (carbohydrates) in the dough, the yeast will multiply until its activity is stopped by the oven's heat.

Most homemade bread recipes call for an hour or two of rising. This will produce perfectly fine bread, but if you want more artisanal results, give your dough a long, slow rise by putting it in a cool spot, such as the refrigerator. This allows more time for fermentation, which creates desirable secondary flavors that counterbalance the yeast's earthiness. Along with the yeast, bacteria are growing in the dough as it rises. The bacteria often include some of the same lactic-acid-producing bacteria that turn milk into yogurt, which gives slow-risen breads a delicious tang.

In most bread recipes, the dough rises twice, once before the loaf is formed, and once after. During the first rise, heat from fermentation builds up in the center of the dough ball, the multiplying yeast gets packed into clusters, and alcohol builds along with the carbon dioxide that is rising the dough. Punching down or stirring a dough at this point before forming it into a loaf refreshes the yeast's environment, evening out the hot and cold spots in the dough, breaking up overcrowded yeast clusters, and releasing the buildup of alcohol, which would result in off flavors and could create a toxic environment that kills the yeast. With a fresh start, the yeast is better able to aerate the loaf during the second rise, just before baking.

What can go wrong?

When bread doesn't rise, it can be for one or more of several reasons.

The yeast was dead before you used it. When you open a package of yeast, it should smell earthy and "yeasty." If it doesn't, you can test or "proof" the yeast's liveliness by combining it with some of the warm water from the recipe and a pinch of sugar. If the yeast is active, it will produce a bubbly mass within 10 minutes.

The water used was too cold or too hot. Water below 70°F may not be warm enough to activate the yeast, but rising the dough in a warm room will activate it-it just might take several hours. Water that's too hot can damage or kill yeast. The damage threshold is 100°F for cake yeast, 120°F for active dry, and 130°F for instant. All yeasts die at 138°F.

Too much salt was added or added too early. Adding salt before the yeast has had a chance to multiply can dehydrate it, starving it of the water it needs to survive.

The dough was not punched down or stirred enough. This allows alcohol to build up and poison the yeast.

Beyond Baking

Yeast is used for more than rising bread. It's essential for brewing beer and making wine, as well as other food products, such as soy sauce and vinegar. Regardless of what it's used for, all commercial yeasts are select strains of the same yeast used for bread. Here's a look at what makes each strain different.

Brewer's yeast comes in two basic types, top-fermenting and bottom-fermenting. Saccharomyces cerevisiae rises to the top of the brew during fermentation and is used for pale ales, stouts, and other top-fermented ales. Saccharomyces pastorianus settles at the bottom during fermentation and is preferred for lagers and pilsners.

Winemaker's yeast contains strains of S. cerevisiae selected for their vigorous fermentation and tolerance of the 10% to 14% alcohol in most wine.

Yeast extract is a flavoring made from a salted slurry of S. cerevisiae and water. The salt encourages enzymes in the yeast to break down its own protein into its constituent amino acids. One of these is glutamic acid, which has a deep umami (savory) flavor and accounts for the primary taste of products like Vegemite and Marmite. Nutritional yeast is a mild-tasting strain of S. cerevisiae that's been deactivated. The yeast is then rinsed, dried, and packaged as yellow flakes or powder. Popular among vegans, nutritional yeast has an umami flavor, is often fortified with vitamins, and is a good source of complete protein because it contains all nine essential amino acids.

How much yeast do you really need?

Yeast has a fruity fragrance and an eggy hint of sulfur that's pleasant in low concentration, but too much can lend a harsh, mushroomy aroma and unpleasant alcohol aftertaste to finished bread. For the best flavor, use a minimal amount of yeast and a long rising time in fairly low temperatures (below 70°F).

The exact amount of yeast needed to rise bread dough depends on three things:

The type of yeast used. You need about twice as much cake yeast as active dry or instant to rise the same weight of dough.

The temperature of the dough. A higher temperature makes the yeast more active, so you don't need to use as much yeast in a warm environment. You also don't need to use as much yeast in a cold environment if you're doing a long, slow rise; the only time you'd need more yeast would be for a quick rise in a cold environment.

The length of rising time. The slower the rise, the less yeast you need. You can control rising times to fit your schedule by varying the amount of yeast and the temperature of the rise. For example, a recipe may call for 2 teaspoons of yeast and 2 hours of rising, but if you're going to be out for the day, you can reduce the amount of yeast to ½teaspoon, rise the dough in the refrigerator overnight, and finish the bread the next day. The lower temperature and longer rising time will allow the yeast to multiply more gradually and create a more complex flavor.

posted in: Blogs, baking, bread, food science, yeast
Comments (1)

classicknitter writes: very informative, been baking bread for a while and learn the difference bet the types of yeast Posted: 8:45 pm on January 21st

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