Yeast is a one-celled microorganism growing all around us and on us. It grows when it has food and water and suspends growth when it does not. In suspended animation, it is light enough to be blown by the wind, like a seed. If there is water and food where it lands, it will reproduce and continue the cycle. It is also on human skin and can be transferred to food through contact, with clean or dirty hands. Yeast has been exploited by humans for thousands of years to make bread, beer, and wine. It does so by turning sugar into alcohol and gas to gain energy.
Which Yeast Is Used In Baking
Yeast used in baking is predominately Saccharomyces cerevisiae. Bakers and baking companies have produced numerous strains, meaning clones of yeast with special attributes. This is similar to a gardener breeding a tulip with special colors, height or hardiness. Just as there are hundreds of tulip varieties, there are hundreds of yeast strains. The yeast strains which are popular today are bred for gas production and fermentation speed. Beyond S. cerevisiae there are several yeasts which are useful in sourdoughs, although S. cerevisiae is common there, too.
There are also special strains of yeast which have been created for lean doughs or enriched doughs. Bakers have access to many strains, just like brewers have different yeasts to choose from. The public only has a few strains from which to choose.
What Yeast Eats
Yeast eats sugar, glucose to be specific. If there is no glucose around but there are other sugars, starches or alcohols, yeast creates machines (enzymes) to convert these into glucose. The yeast carries information in its DNA for dozens of machines specific to many food sources.
Flour has a lot of starch in it, which is made of long chains of sugar molecules. Flour carries its own enzymes that work on the starches and chop them into simple sugars. This happens after the flour has been rehydrated with water or other liquids. Then the yeast uses the sugars for energy.
Why Yeast Cells Ferment
Yeast has two ways of releasing energy from sugar molecules to use for their own cell maintenance and reproduction; with or without oxygen.
- With a supply of oxygen, they make carbon dioxide (CO2 - a gas), which is exactly what human cells make, too. They use almost all the energy from the sugar to do this and make a lot of gas. This is called respiration.
- With little or no oxygen, the yeast quickly builds machines that spew out alcohol and carbon dioxide after using some of the energy from sugar. This is called fermentation. Since this is an inefficient way to capture the energy, they have to metabolize more sugar than they do during respiration.
Making bread with yeast uses both respiration and fermentation (mostly the latter). You knead or beat oxygen (and nitrogen) into the dough, which the yeast use up rather quickly, producing gas which is trapped by the dough. Most gas in bread dough is produced within the first hour of fermentation. Then the yeast must switch to making alcohols and acids along with gas and grows more slowly. This gives yeast-risen bread special aromas and tastes. These compounds also affect the structure of the dough, changing the crumb and crust after baking.
How Temperature Affects Yeast
Yeast grows best at 26 C (79 F) and ferments best at 30 - 35 C (86 - 95 F). At lower temperatures yeast slows down both processes and becomes "dormant". At higher temperatures, yeast enzymes do not work well. That is just like a human with a fever.
You can use it to slow down your bread dough if you cannot bake it immediately. This can occur during the first proofing or after shaping. It can be done right after you shape your loaf, or to retard a loaf that expanded before you were ready. Although the latter is not optimal, it usually results in an acceptable product.
Why Refrigerate the Dough
Sometimes it is just to retard the rise so that we can control when we bake the bread. There are discussions about the flavor being created when the dough is refrigerated for several hours or overnight, but it is unclear whether this comes from the enzymes in the flour, yeast metabolites, dying yeast byproducts or other chemical reactions.
There are several bread baking methods that require refrigeration. "Artisan Bread in Five Minutes a Day" (2007, St. Martin's Press) uses it, keeping the dough for up to three weeks in the refrigerator, and Peter Reinhart suggests keeping the primary doughs in the refrigerator for up to three days if you cannot use them right away. Also, the Swiss Wurzelbrot method bakes the loaves directly from the refrigerator and some sweet braided breads can be refrigerated and baked in the morning for breakfast.
Why to Proof Dough Over Warm Water or on the Back of the Stove
Yeast that you buy at the store has optimal fermentation rates at 30 - 35 C (86 - 95 F). Most modern bread recipes call for proofing around those temperatures. If you do not heat or cool your house much, room temperature will fluctuate wildly. At 60 F in our kitchen on a winter day, bread dough rises very slowly. It will affect the end product and results in a tighter, more crumbly bread.
This could be because plant enzymes (found in the flour) work best at colder temperatures and break down more gluten and starch. This would affect the ability of the gluten to hold the gas the yeast produce. Still, a cold kitchen can slow down a dough that is rising too fast or gives a dough more time to develop flavor, which is a good thing, so there is always a trade-off.
How Much Yeast to Use
One gram of yeast contains 20 billion tiny cells. There are about 7 grams in a quarter ounce package that we buy at the store (2 1/2 teaspoons). That's 140 billion cells! When you start making bread, add the amount of yeast called for in the recipe. If it tastes good and has the properties you want, then stick with it. Because yeast does not divide much in bread dough (only 20-30% increase in cell numbers in 4 hours), what you start with is what you end up with in terms of yeast numbers. This can affect the bread by adding a "yeasty" taste if you put too much into the dough. General amounts of yeast are around 1 - 2 % of the flour, by weight. Too much yeast could cause the dough to go flat by releasing gas before the flour is ready to expand.
If you let the dough rise too long, it will start having a yeast or beer smell and taste and ultimately deflate or rise poorly in the oven and have a light crust. This is not because of huge numbers of yeast cells taking over, but due to too little residual sugar and the inability of the gluten to stretch any further.
Some recipes start with a quarter teaspoon of yeast, that is just 10% of a packet of yeast! These recipes are depending on long fermentations to create flavor and mostly start with a very wet dough. This lets the yeast move around and divide while the flour enzymes are doing their thing. A dough like this is usually fermented overnight and often stirred into a final dough with more yeast to aid in the final rise.
The Differences Between Regular Yeast, Instant Yeast, and Bread Machine Yeast
Taste and ease of use. Instant and bread machine yeast is dried in a certain way to allow it to be mixed into flour without being proofed first. It is slightly more expensive than the old-fashioned technology. Regular, active dried yeast results in a slightly different flavor, which some people prefer. We have also found coarse, dried yeast in bulk at health food stores. The way it is manufactured, it takes about twice as long to reconstitute but functions the same as powdered yeast once you proof it.
Cake yeast is compressed fresh yeast and is refrigerated. It has a shorter shelf life than the dried yeast, but we prefer its flavor in many German cakes. It is very expensive and hard to find in the US, so substitute dried yeast; one packet of active dry yeast (2 1/2 teaspoons) or instant yeast (2 teaspoons) for one cake (0.6 ounces in the US) and add a tablespoon or more liquid to the dough. Generally, you can substitute one yeast for the other, although you may want to change the method of delivery. Instant yeast can be proofed if you like, but we do not recommend mixing active dry yeast or cake yeast with the flour directly as it does not dissolve evenly in a stiff dough.
How Salt Affects Yeast
Small amounts of salt can actually help yeast function better (0.5 - 1%), whereas 1.5-2.5% salt (by weight to flour) acts inhibitory. Salt is necessary for bread gluten structure, however, as well as for taste. Many breads are made satisfactorily with 2% salt. Interestingly, sugar concentrations above 6% (by weight to flour) have a negative effect on yeast, as well. There is a special strain of yeast that works well in sweet and sourdough doughs.
What Kneading Does to Yeast
Kneading does very little to yeast since yeast should be evenly distributed after the first mixing. It does stretch and lengthens the gluten so that it can hold the nitrogen and carbon dioxide bubbles. The second knead is important after the dough has risen once, to increase extensibility, even though it may not be a long kneading process.
Beating a dough hard does not hurt yeast either, you cannot break the cells that way. Professional bakers take care when mixing doughs so that the temperature does not exceed what is required by the recipe. Home bakers do not worry about that much because the small amounts of dough used at home do not require as much mixing.