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 Yeasts have been fed to animals for more than a hundred years, either in the form of yeast fermented mash produced on the farm, yeast by-products from breweries or distilleries, or commercial yeast products specifically produced for animal feeding. Although yeast feeding has been around a long time, there is confusion throughout the industry concerning what the various yeast products really are.

Yeasts are microscopic fungi — single-cell organisms which are generally about 5-10 microns in size. They are given Latin names which represent their genus and species (e.g., Saccharomyces cerevisiae or Candida utilis). The species differ from each other by: where they are found, their cellular morphology or shape, how they metabolize different substrates, and how they reproduce. While there are nearly 50,000 species of fungi, there are only 60 different genera of yeast representing about 500 different species.

Yeasts are abundant throughout the environment. They can be found on cereal grains, grain by-products, silages, hays and are even present in the soil and water. Our laboratory has found that various feed ingredients contain anywhere from a few thousand (103) live yeast cells per gram to over a million (106) per gram. Several species have proven very beneficial to man, while a few imperfect yeasts are known to be pathogenic. But, most yeast are benign saprophytes and have proven neither useful nor harmful to man or animal.

Very few species of yeast are used commercially. Saccharomyces cerevisiae, also known as “bakers yeast”, is one of the most widely commercialized species. Selected strains of this yeast are used by breweries to make beer and ale, distilleries to make distilled spirits and industrial alcohol, and wineries to make wine. Candida utilis (formerly classified as Torulopsis utilis) is the yeast known as “Torula Yeast”. This yeast is important because it can utilize the pentose sugars from processed wood pulp used in making paper. A third useful yeast is Kluyveromyces marxianus. This is the “Whey Yeast”, which can utilize milk sugar or lactose as a substrate.

Yeasts are “facultative anaerobes” which means that they can survive and grow with or without oxygen. Yeast propagation is an aerobic processes where the yeast converts oxygen and sugar, through oxidative metabolism, into carbon dioxide and usable free energy for efficient yeast cell growth. However, the production of alcoholic beverages (beer, wine, whiskey, etc.) and industrial alcohol are anaerobic processes. Anaerobic fermentation is much less efficient, resulting in considerable “metabolic by-product” in the form of ethyl alcohol. The yeast ferments simple sugars into ethanol and carbon dioxide and the yeast grows very slowly. To optimize ethanol production, the fermentation process is carried out without oxygen being present; but, to maximize yeast cell growth, an abundance of oxygen is provided in the form of air.
In the early days, fermentations were carried out by seeding bread dough, grape must or corn mash with retained portions from a previous fermentation. Our ancestors always retained a portion of a fermenting bread dough for mixing with fresh dough the following day. This yeast dough was called the “starter dough”. The live yeast was carried from dough-to-dough in a perpetual cycle. If the starter was lost or it turned sour due to bacterial contamination, a new seed could be prepared by moistening flour and waiting for a spontaneous fermentation to occur, or by borrowing some starter from a neighbor and initiating a new starter fermentation.

Today, pure cultures of yeast are grown specifically for breweries, wineries, distilleries, bakeries and home use. Commercial or proprietary yeasts are used industrially for the production of all yeast-raised baked goods and alcoholic beverages. Although a few wineries still use the natural yeast found on the grapes to spontaneously ferment their wines, most wineries now depend on pure cultures of specific yeast trains to make consistent proprietary wines.