Dry yeast is related to brewer’s yeast, which is used to ferment alcoholic liquids, such as beer. In contrast to fresh yeast, it doesn’t need to be hydrated before it is added to a dough mixture. Dry yeast is produced by extracting a large percentage of water so that the yeast cells are inactivated, but not killed.
In baking: S. cerevisiae is used in baking; the carbon dioxide generated by the fermentation s used as a leavening agent in bread and other baked goods. Historically, this use was closely linked to the brewing industry's use of yeast, as bakers took or bought the barm or yeast-filled foam from brewing ale from the brewers (producing the barm cake); today, brewing and baking yeast strains are somewhat different.1
You generally have the choice between fresh yeast (cubes), dry yeast (shelf life: about 1 year), and liquid yeast. Inactivated baker’s yeast in yeast doughs is usually used at a rate of about 3 to 6 % of the flour weight. Doughs with a high fat content can contain up to 8 % yeast since the low water content causes slower yeast activity. A standard 7 g package of dry yeast sold in the supermarket is roughly equivalent to half of a 42.5 g cube of fresh yeast.
In brewing: Saccharomyces cerevisiae is used in brewing beer, when it is sometimes called a top-fermenting or top-cropping yeast. It is so called because during the fermentation process its hydrophobic surface causes the flocs to adhere to CO2 and rise to the top of the fermentation vessel. Top-fermenting yeasts are fermented at higher temperatures than the lager yeast Saccharomyces pastorianus, and the resulting beers have a different flavor than the same beverage fermented with a lager yeast. "Fruity esters" may be formed if the yeast undergoes temperatures near 21 °C (70 °F), or if the fermentation temperature of the beverage fluctuates during the process. Lager yeast normally ferments at a temperature of approximately 5 °C (41 °F), where Saccharomyces cerevisiae becomes dormant.1
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At room temperature, yeast can be stored for quite a long time. However, you should not use the yeast if the expiration date has passed since the ability of the yeast cells to reactivate decreases with time.
Saccharomyces cerevisiae is a species of yeast. It has been instrumental to winemaking, baking, and brewing since ancient times. It is believed to have been originally isolated from the skin of grapes (one can see the yeast as a component of the thin white film on the skins of some dark-colored fruits such as plums; it exists among the waxes of the cuticle). It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium. It is the microorganism behind the most common type of fermentation. S. cerevisiae cells are round to ovoid, 5–10 μm in diameter. It reproduces by a division process known as budding.1
Baker’s yeast is produced industrially using pure yeast cultures that multiply until the desired quantity of baker’s yeast is achieved. If you start with about 8 kg of material and let it double around 33 times, almost ten billion times the original amount is produced in about 11 days.
Pure yeast cultures are obtained from selecting and cultivating sourdough cultures or brewer’s yeast. Baker’s yeast ferments more quickly than the wild yeast in sourdough, but in contrast, it doesn’t tolerate acids, salts, and fats as well. Dry yeast is produced by extracting a large amount of water from the mash which inactivates the yeast cells, but doesn’t kill them.
The emulsifier citrem (citric acid esters of fatty acids) is usually added in order to prevent the yeast cells from drying out to much. This way, the cells are only inactivated and don’t die.
Many proteins important in human biology were first discovered by studying their homologs in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes. S. cerevisiae is currently the only yeast cell known to have Berkeley bodies present, which are involved in particular secretory pathways. Antibodies against S. cerevisiae are found in 60–70% of patients with Crohn's disease and 10–15% of patients with ulcerative colitis (and 8% of healthy controls).1