As any living form, this one-cell organism is sensitive to temperature and moisture. It is most comfortable between 75 – 95 degrees Fahrenheit (24 – 35 degrees Celsius). It dies when temperature is above 138 degrees Fahrenheit (59 degrees Celsius), and becomes dormant below 34 degrees Fahrenheit (2 degrees Celsius).
Bakers yeast feeds primarily on glucose (a monosaccharide) that is available from the breakdown of sucrose (a disaccharide better known as table sugar) and, to some extent, damaged starch in flour. During this feeding process, alcohol (ethanol), carbon dioxide, and heat are released.
There are numerous wild strains of this fungus in the air, but three commercially common types are:
Compressed – Fresh compressed yeast is a mixture of about 30% yeast and starch. The rest is moisture. It comes in moist, creamy, light grayish cakes or blocks, and have a fresh, yeasty smell. It’s also available in a crumbled form.
Compressed yeast can maintain optimum activity for 3 or 4 weeks when refrigerated. When frozen, its shelf life raises up to 3 or 4 months.
Active Dry Yeast (ADY) – Without moisture, this type of bakers yeast is dormant and has a longer shelf life than fresh compressed yeast. When properly packaged and stored, it can stay active for up to a year.
Each part of fresh compressed yeast can be replaced by ½ part of active dry yeast.
For optimal action, active dry yeast should be rehydrated in lukewarm water of 4 or 5 times its own weight before adding to flour or other ingredients. Cold water will rupture the yeast’s cell wall and release glutathione. In fact, some of the yeast cells is already damaged during the drying process.
Instant Dry Yeast (IDY), on the other hand, is popular because it can be added directly to flour or other ingredients without rehydrating. Developed before World War II, it usually contains an emulsifier or ascorbic acid that helps remove water during drying, and absorb moisture during mixing.
IDY and ADY have very different drying procedures that separate them from each other. The former is dried by a constant moving stream of warm dry air. This method enables all of the yeast cells to survive.
When substituting for active dry yeast, use ¾ of instant for every part of active dry called for in a recipe. Like active dry yeast, instant dry yeast should be kept in a cool, dry place, refrigerated, or frozen.
FIGONI, P. How Baking Works – Exploring The Fundamentals of Baking Science. First Edition, 2004.