How Does Yeast Work? Ask Paul

Yeasts are single-celled organisms in the fungus kingdom. The most common culinary species of yeast, Saccharomyces cerevisiae, of which thousands of strains exist, is used in making both bread and beer and as such is generally known as both baker’s yeast and brewer’s yeast. 

The life of a yeast cell is not extraordinarily complicated by our standards. In essence, it consumes sugar and uses the energy from that fuel to create carbon dioxide, alcohol, and more yeast cells. The carbon dioxide is what bakers primarily employ yeast to make: As the gas fills the pores in a dough (which are initially created during mixing and kneading), the dough rises, so when it’s baked it is delectably light and airy.

Yeast also produces enzymes that help it access more of the sugar it needs by breaking complex chains of sugar molecules into smaller sugars that the cell can eat. It has a limited ability to break down starch into sugar as well, but most of that work is done by amylase enzymes that are already in baking flour (if the ingredients list on your bag of flour contains malted barley, that barley is there to contribute amylase).

And, depending on the specific strain as well as its local environmental conditions (temperature, food sources) yeast produces many different types of other compounds that contribute wonderful depth of flavor to breads and brews. Esters, aldehydes, and dozens of types of acids create a wealth of distinctive complexity that sets yeast-leavened baked goods apart from those that use baking powder.

In the wild, when conditions get dry, animals can go wandering in search of water, and plants can extend roots or store moisture in specialized cells. Single-celled organisms don’t have those abilities, but still they’ve evolved ways to survive a dry spell. Some types of yeast—including Saccharomyces cerevisiae—can completely suspend their metabolism in the absence of water. Their cell wall stiffens up, they modify their internal chemistry a little bit to protect against oxidation and heat, and their life functions more or less go on pause, a state called cryptobiosis: “secretly alive.” As soon as they contact water again, they come back to life right where they left off.

The food industry puts this survival method to use. In yeast-growing facilities, yeast lives as a wet, thriving slurry. Or slurries: Different strains of Saccharomyces cerevisiae are bred for highly specific purposes, from pizza to cinnamon rolls, whiskey to Champagne, and kept strictly isolated from each other.

In order to be packaged for sale, yeast can be processed in a few ways.

• Fresh yeast (also known as cake or compressed yeast) is pressed into blocks and refrigerated. In that form, it can only live for a few weeks in the refrigerator because it’s actively alive and needs a food supply. Freezing will keep it alive significantly longer.

There are two common types of dry yeast:

• Active dry yeast is dehydrated to about 8 percent moisture and formed into granules, each of which has a protective coating made from dead yeast cells. Only about three-quarters of the yeast survives the dehydration process, which is why recipes using active dry yeast specify that you need to add more to start with. Active dry yeast also requires several minutes rehydrating before it comes to life. This form of yeast has the longest shelf life.
• Instant dry yeast uses a more recently developed method to dry the cells quickly, with a higher survival rate. The granules are smaller and their coating more permeable, which allows instant dry yeast to be added directly to doughs without a prehydration step. It’s slightly less durable in the fridge or freezer than active dry yeast, but it can survive for a year or more. Osmotolerant strains of instant dry yeast are bred to work well in high-sugar doughs.