Yeast is used in beer brewing and baking. Yeast produces gas from the sugar in your baked or brewed product. Yeast is a perishable product, and should be kept in an airtight container protected from light, moisture and air. To extend the life of your yeast, store it in the refrigerator after it is opened and use it quickly.
Baker’s yeast is a unicellular fungus that comes from Saccharomyces cerevisiae cells. Yeast is a natural product, not a chemical leavening, which is produced through sodium bicarbonate and acid. Active dry yeast must be rehydrated in lukewarm water before it is used, whereas instant yeast does not need rehydration. Yeast is 70 percent water. Of the remainder, 50 percent is protein, 10 percent fat and ash and 40 percent carbohydrate. The more solid the yeast is, the more activity there is in the yeast. If the yeast is grown more rapidly, it will have a higher protein content and lower carbohydrate composition. The opposite is true for yeast grown slowly.
Yeast growth and processing affects its uses, appearance and purity. Molasses is diluted with water and heated. Nitrogen, phosphate and other minerals and vitamins are added before the yeast is fermented. It begins pure and sterile, and fermentation increases the volume of the yeast. Fermentors are used to control the pH, foam, aeration, cooling and feeding of the yeast. After it is fermented, it is either cooled and packaged as cream yeast or passed through a filter to remove water, dried and packaged as dry yeast. After rehydration, the pH should be between 4 and 8, but preferably 6.
Active dry yeast is approximately 0.2 to 3 millimeters in diameter and spherical. Instant yeast is porous and cylindrical. It is about 0.5 millimeters in diameter and a few millimeters long. Yeast can vary in color from dark brown to almost white. It may be gummy or crumbly. The color will be darker if the pH is low during the fermenting process. Yeast that is stored in freezers will take on a mushy texture. A lot of moisture makes darker yeast, but performance is not as good. If yeast is stored for long periods of time, it is also darker. Old yeast is gummy and dark colored. Color and appearance is not an adequate indicator of quality.
Fermentation temperature and anything added to the yeast during its production change the flavor of the yeast. Esters are produced by yeast, and they give products a fruit flavor and smell. You may smell apple, grapefruit, raspberries and bananas in esters. If you are looking for a butterscotch or butter flavor, choose yeast that has bacterial fermentation and produces diacetyl. If dimethyl sulfide is present from improper boiling and high fermentation temperatures, your product may taste like cabbage. Improper production and handling produces undesirable flavors.
References and ResourcesCharacteristic Flavors from Yeast: Jim Graham
Lallemand: Baking Update
COFALEC; General Characteristics of Dry Baker's Yeast; June 2009