“Yeast” is a living unicellular microorganism until it is destroyed by heat or other physical or chemical means of about one hundredth of a millimeter in size and can therefore not be observed with a naked eye. Yeasts are a group of unicellular microorganisms most of which belong to the fungi division of Ascomycota and Fungi.

The scientific name of baker’s yeast is “Saccharomyces Cerevisiae”.

Yeast, the most common one being “S. cerevisiae”, is used in baking as a leavening agent, where it converts the food/fermentable sugars present in dough into the gas carbon dioxide. This causes the dough to expand or rise as gas forms pockets or bubbles. When the dough is baked, the yeast dies and the air pockets “set”, giving the baked product a soft and spongy texture. When yeast is used for making bread, it is mixed with flour, salt, and warm water or milk. The dough is kneaded until it is smooth, and then left to rise, sometimes until it has doubled in size. The dough is then shaped into loaves. Some bread doughs are knocked back after one rising and left to rise again (this is called dough proofing) and then baked. A longer rising time gives a better flavor, but the yeast can fail to raise the bread in the final stages if it is left for too long initially.

The industrial production and commercial use of yeasts started at the end of the 19th century after their identification and isolation by Pasteur. Today, the scientific knowledge and technology allow the isolation, construction and industrial production of yeast strains with specific properties to satisfy the demands of the baking and fermentation industry (beer, wine, cider and distillates). Food grade yeasts are also used as sources of high nutritional value proteins, enzymes and vitamins, with applications in the health food industry as nutritional supplements, as food additives, conditioners and flavouring agents, for the production of microbiology media, as well as livestock feeds. Yeasts are included in starter cultures, for the production of specific types of fermented foods like cheese, bread, sourdoughs, fermented meat and vegetable products, vinegar, etc.
The significance of yeasts in food technology as well as in human nutrition, as alternative sources of protein to cover the demands in a world of low agricultural production and rapidly increasing population, makes the production of food grade yeasts extremely important. A large part of the earth’s population is malnourished, due to poverty and inadequate distribution of food. Scientists are concerned whether the food supply can keep up with the pace of the world population increase, with the increasing demands for energy, the ratio of land area required for global food supply or production of bioenergy, the availability of raw materials, as well as maintenance of wild biodiversity. Therefore, the production of microbial biomass for food consumption is a main concern for the industry and the scientific community.