Baker's Yeast

Baker's yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and bakery products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. Baker's yeast is of the species Saccharomyces cerevisiae, which is the same species commonly used in alcoholic fermentation, and so is also called brewer's yeast. Baker's yeast is also a one celled microorganism growing around us and on us.

The use of steamed or boiled potatoes, water from potato boiling, or sugar in a bread dough provides food for the growth of yeasts, however, too much sugar will dehydrate them. Yeast growth is inhibited by both salt and sugar, but more so with salt than sugar. Fats such as butter or eggs slow down yeast growth, however others say the effect of fat on dough remains unclear, presenting evidence that small amounts of fat are beneficial for baked bread volume.

Saccharomyces exiguus (also known as S. minor) is a wild yeast found on plants, fruits, and grains that is occasionally used for baking; it is not, however, generally used in a pure form, but comes from being propagated in a sourdough starter.

Types of baker's yeast

Baker's yeast is available in a number of different forms, the main differences being the moisture contents. Though each version has certain advantages over the others, the choice of which form to use is largely a question of the requirements of the recipe at hand and the training of the cook preparing it. Dry yeast forms are good choices for longer-term storage, often lasting several months at room temperatures without significant loss of viability. With occasional allowances for liquid content and temperature, the different forms of commercial yeast are generally considered interchangeable.

• Cream yeast is the closest form to the yeast slurries of the 19th century, being essentially a suspension of yeast cells in liquid, siphoned off from the growth medium. Its primary use is in industrial bakeries with special high-volume dispensing and mixing equipment, and it is not readily available to small bakeries or home cooks.
• Compressed yeast is essentially cream yeast with most of the liquid removed. It is a soft solid, beige in color, and arguably best known in the consumer form as small, foil-wrapped cubes of cake yeast. It is also available in larger-block form for bulk usage. It is highly perishable; though formerly widely available for the consumer market, it has become less common in supermarkets in some countries due to its poor keeping properties, having been superseded in some such markets by active dry and instant yeast. It is still widely available for commercial use, and is somewhat more tolerant of low temperatures than other forms of commercial yeast; however, even there, instant yeast has made significant market inroads.
• Active dry yeast is the form of yeast most commonly available to noncommercial bakers in the United States. It consists of coarse oblong granules of yeast, with live yeast cells encapsulated in a thick jacket of dry, dead cells with some growth medium. Under most conditions, active dry yeast must first be proofed or rehydrated. It can be stored at room temperature for a year, or frozen for more than a decade, which means that it has better keeping qualities than other forms, but it is generally considered more sensitive than other forms to thermal shock when actually used in recipes.
• Instant yeast appears similar to active dry yeast, but has smaller granules with substantially higher percentages of live cells per comparable unit volumes. It is more perishable than active dry yeast, but also does not require rehydration, and can usually be added directly to all but the driest doughs. Instant yeast generally has a small amount of ascorbic acid added as a preservative. Some producers provide two or more forms of instant yeast in their product portfolio; for example, LeSaffre's "SAF Instant Gold" is designed specifically for doughs with high sugar contents. These are more generally known as osmotolerant yeasts.
• Rapid-rise yeast is a variety of dried yeast (usually a form of instant yeast) that is of a smaller granular size, thus it dissolves faster in dough, and it provides greater carbon dioxide output to allow faster rising. There is considerable debate as to the value of such a product like this; while most baking experts believe it reduces the flavor potential of the finished product, Cook's Illustrated magazine, among others, feels that at least for direct-rise recipes, it makes little difference. Rapid-rise yeast is often marketed specifically for use in bread machines.

For most commercial uses, yeast of any form is packaged in bulk (blocks or freezer bags for fresh yeast; vacuum-packed brick bags for dry or instant); however, yeast for home use is often packaged in pre-measured doses, either small squares for compressed yeast or sealed packets for dry or instant. For active dry and instant yeast, a single dose (reckoned for the average bread recipe of between 500 g and 1000 g of dough) is generally about 2.5 tsp (~12 mL) or about 7 g (1/4 ounce), though comparatively lesser amounts are used when the yeast is used in a pre-ferment. A yeast flavor in the baked bread is generally not noticeable when the bakers' percent of added yeast is less than 2.5.

Use in research

Because it is readily available and easy to culture, baker's yeast has long been used in chemical, biological, and genetic research. In 1996, after 6 years of work, S. cerevisiae became the first eukaryote to have its entire genome sequenced. It has over 12 million base pairs and around 6000 genes. Since then it has remained on the forefront of genetic research. For example, most of our knowledge of the cell division cycle was worked out from experiments with yeast.

Baker's yeast contains enzymes which can reduce a carbonyl group into a hydroxyl group in fairly high yield, thus making it a useful bio-reagent in chemical syntheses. It is known to reduce organometallic carbonyl compounds in very high yield.

Baker's yeast can also be used to produce ethanol via fermentation for use in chemical synthesis, although doing so in some places requires permits.