Yeasts are living microorganisms that belong in the fungal ecosystem. There are many different species of yeasts, but the one we are most interested in is Saccharomyces cerevisiae. It is pronounced exactly how it is spelled (listen here). This is the type of yeast used in winemaking, brewing and baking. It is the key ingredient in the fermentation process, which produces carbon dioxide that helps bread rises.
I still remember studying “fermentation” briefly as one of the topics in chemical reaction engineering back in college. It is so much fun to break it down in the molecular level, chemistry was my first love! Yeasts will reproduce under certain conditions, a moist environment with temperature around 110°F and with a food source (sugar or starch). This is why we proof active dry yeast in water at 105°F – 110°F with a teaspoon of sugar. When yeasts consume sugar, ethanol and carbon dioxide are produced; the chemical reaction is as follow:
C6H12O6 (sugar) –> 2C2H5OH (ethanol) + 2CO2 (carbon dioxide)
So when you proof yeast, that weird smell comes from ethanol, don’t be grossed out. Yeasts continue to reproduce when mixed with flour, and made into dough. The sugar in the dough provides the food source, and the carbon dioxide produced is then trapped within the gluten network (we discussed that last week, here) and helps the dough rise. Most of the alcohol will evaporate during baking.
Yeast’s growth rate is dependent on temperature. The optimum temperature is at 110°F. At lower temperature, the process will continue but at a slower rate. Dough kept in the refrigerator will rise slower. At higher temperature, 140°F and up, most yeast cell will die. This is important during proofing, if the temperature is too hot, you will kill off all the yeast before you even start making the dough.
The most common type of yeast available to the home baker is active dry yeast, which is what I use all the time. It can be kept in the fridge or freezer for an extended period of time. It must be rehydrated or proofed before use. Another common type of yeast I’ve seen a lot of recipes use is instant yeast. It is similar to active dry yeast but does not require rehydration.
Treat yeasts with care; they are living things after all. Happy Friday!