The major microbial products used as feed additives are the two yeast categories approved to maintain their fermentative capacity, "active dry yeast" (96.2) and "yeast culture" (96.8) (Peppler 1983; Official Publication of Association of American Feed Control Officials 2002). Active dry yeasts are used as starters in the baking industry and in other traditional fermentation industries where the leavening activity, and ability to ferment different raw materials, is necessary to improve storability, taste and flavor. At this moment, companies such as Beldem (http://www.beldem. com), subsidiary of the Puratos group, manufactures active dry yeast for use in bakery and Lesaffre group (http://www. lesaffre.com) and Lallemand Inc (http://www.lallemand.com) commercialize a wide range of sourdough starters and yeast products for the baking, brewing and wine industry.
Yeast culture is the only officially defined feed yeast ingredient which is not composed of isolated dried yeast cells only. Yeast culture consists mainly of growth medium with a moderate content in crude protein. It is being used as an aid to the ensiling process (lactic acid formation by naturally present bacteria), which increases feed value and improves the quality and palatability of the resulting silage, but the greater portion is used to fortify feeds and feed concentrates administered to livestock and poultry (Peppler and Stone 1976; Peppler 1983). As an example, Diamond V® produces active yeast cultures for fermenting liquid and cereal grain raw ingredients for animal feeds including dairy, beef, horse, poultry, aquaculture, and pet foods.
Yeast and fungi active cultures are also approved for production of commercial enzymes for the food industry. The FDA accepts the use of, among others: amyloglucosidase derived from Rhizopus niveus; carbohydrase from R. oryzae; lactase enzyme preparation from K. lactis and chymosin preparation from K. lactis or A. niger. New generally recognized as a safe (GRAS) fungi and yeasts are continuously under study with the objective of incorporation of new fungal enzymes into the food industry (Saxena et al. 2001; Wolf 1996).
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