What is yeast?
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 does yeast eat?
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 do yeast cells ferment?
Yeast have 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 quicky 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 energy, they have to metabolise 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 does temperature affect 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.
Why do we refrigerate dough?
Sometimes it is just to retard the rise, so that we can control when we bake the bread. There are discussions about flavor being created when 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 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.
Can we use low temperature to our advantage in other breads?
Yes, 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 do we 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 my kitchen on a winter day, bread dough rises very slowly. It will affect the end product, I believe it 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 give a dough more time to develop flavor, which is a good thing, so there is always a trade-off.
Lucky for me, I found that the microwave that sits above my stove (the space-saver kind) keeps a temperature of about 85°F when the 25W stove light is kept on all day. So now I use the microwave to proof my doughs, instead of a bowl of warm water or the oven.
- How much yeast should you use?
- What are the different types of baker's yeast?
- Is salt bad for yeast?
- Is kneading good for yeast?