The purpose of this booklet is to provide both a semi-technical and non-technical understanding of the various yeast products offered to, and used by, the feed industry.
It does not deal with animal research and product efficacy, but merely describes what the products are and the benefits they are purported to provide. Efficacy in the animal is too complex a subject to deal with in this short booklet. The hope is that the reader will get a better understanding of what the various yeast products are, so that he or she can make educated decisions about the various products, based on the knowledge of what they are and the research data presented by the supplier of the product. Feed industry professionals can not afford to take the attitude that "yeast-is-yeast", because there are significant differences in the various yeast products. There are both biochemical differences, as well as functionality differences, and each product should be used for its own unique properties. Not all yeast products are the same.Yeast -- An Overview
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 yeasts 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 strains to make consistent proprietary wines.Viable Yeast Products
Active dry yeast (95% dry matter) is the predominant viable yeast available to the feed industry. Although wet yeast cake (30% solids), and to a lesser extent yeast cream (18-20% solids), are used extensively by the bakery trade, active dry yeast is the form of viable yeast used in most animal feeds. All three forms of live yeast, however, can be used as inoculum to manufacture Yeast Culture, which is a yeast fermented product that will be discussed later.
The production of active dry yeast is accomplished using submerged culture bioreactors or aerobic fermentors. The objective is to feed the yeast nutrients (oxygen, nitrogen, carbohydrate) and allow them to reproduce and form new generations of yeast cells. They reproduce using "budding", where a daughter cell evolves from the mother cell, forming a bud, and when the daughter is mature, they will both reproduce. Thus, their growth is exponential.
For example, one cell becomes two, two becomes four, four becomes eight, and so on. After the effective concentration of yeast is achieved, the growth-broth is centrifuged to form a yeast cream, filtered through filter presses to make yeast cake, formed into wet yeast-noodles, and then dried at temperatures which will not destroy the yeast’s fermentative activity.
Active dry yeast consists of pure, dried yeast cells with viability counts ranging from 15-25 billion live yeast cells or colony forming units (cfu) per gram. It is marketed in three physical forms, depending on the process used to dry the yeast: tunnel dried yeast which is a granular powder, fluid-bed dried yeast (also known as Instant or Quick Rise yeast) which looks like small torpedoes, and rotolouver dried yeast which looks like small spheres or balls. In the United States, tunnel dried yeast and fluid-bed dried yeast are most common, while rotolouver dried yeast is most prevalent in Europe and Latin America. The fluid-bed drying process is becoming more popular, because it causes less damage to the yeast cells, resulting in better leavening properties in the yeast.
Active dry yeast is showing up more frequently in the feed industry, not as the pure product, but in the form of diluted yeast products having a wide range of yeast viability counts. This is one reason there is so much confusion about live yeast products. For example, a live yeast product guaranteeing 5 billion cfu per gram of live yeast count contains only 20-25% active dry yeast, which generally has about 20 billion cfu per gram. The remainder of the product consists of cheaper diluents like rice hulls or distillers solubles. Active dry yeast generally costs less than 7.5 cents per billion cfu per gram of yeast viability, while the diluted products currently on the market sell for 10-15 times as much per billion. Therefore, it is important that the nutritionist and purchasing agent know what they are buying and how their cost per billion cfu's compares to commercial active dry yeast.
The stability of active dry yeast is dependent upon how the product is packaged … whether or not oxygen is present. Yeast packaged in vacuum or inert gas has much greater stability than yeast packaged in air. Commercially, most active dry bakers yeast is packaged in either a vacuum pack or purged with nitrogen gas. The presence of air or oxygen has a dramatic effect on fermentative activity.Effects of Pelleting on Yeast Viability
Significant confusion exists in the feed compounding industry concerning the effects of pelleting live-cell yeast products and yeast cultures. This confusion stems from the lack of differentiation between the two products and a misrepresentation of what pelleting does to the live yeast cell.
Yeast cultures have been manufactured in the United States for more than 50 years, but the last decade has witnessed a proliferation of commercial live-cell yeast products being touted as concentrated, high-cell count "yeast cultures". These products are not, however, fermented yeast cultures, but rather mixtures of dried viable yeast (active dry yeast) diluted with a carrier. Unfortunately, this has caused a misunderstanding of what true yeast cultures are and how they differ from live-cell yeast products.
True yeast cultures are complex fermented products containing the yeast and the metabolic by-products produced by the yeast during fermentation. Yeast cultures are not fed as a source of live or viable yeast cells, but as a nutrition supplement to provide undefined fermentation factors, which are recognized to stimulate bacterial growth in the digestive tract. These fermentation factors, sometimes referred to as "nutrilites", appear to be heat stable and are not significantly affected by high temperatures or pelleting. This heat stability is illustrated in the figure above, demonstrating that even autoclaving has minimal affect on yeast culture’s ability to enhance bacterial growth.
Live-cell yeast products, on the other hand, consist of viable active dry yeast blended with a diluent to provide a specified number of live yeast cells. Active dry yeast is defined as pure dried yeast (without fillers or diluents) containing not less than 15 billion (1.5 x 1010) live yeast cells per gram. Thus, a live-cell product claiming 5 billion cells per gram would consist of 20-25% active dry yeast, with the remainder being carrier ingredients like distillers solubles or rice hulls. These definitions help differentiate active dry yeast and true yeast cultures; the first being a source of viable yeast cells and the second being a fermented culture. Since the live-cell yeast products are made with active dry yeast and marketed as a source of viable yeast cells, the question of whether or not pelleting destroys their activity is a major issue to the feed manufacturer or nutritionist using them in pelleted products.
The normal pelleting process utilizes steam to condition the feed for proper extrusion through the pellet die. It is generally accepted that steam is one of the most effective methods for killing microorganisms (steam is used in autoclaving for sterilization). Moist heat denatures the enzyme systems within the organism, destroying their metabolic activity and inhibiting life processes. The higher the temperature, the more complete and irreversible the process. It is also well documented that dry heat is much less destructive to yeast viability and enzyme activity than moist heat. Therefore, heating dry yeast without moisture present is much less detrimental to the yeast cell than when steam is used.
To determine whether or not pelleting adversely affects yeast viability and/or fermentative activity, there are three possible approaches:Yeast Plate Counts:
The plate count technique is the most accurate method, because it determines the number of actual live, viable yeast cells in the product, before and after pelleting. The method is performed by serially diluting a suspension of the yeast-containing product in water and plating the different dilutions on nutrient media to allow the yeast cells to grow and multiply. After about three days, each place on the nutrient gel where a single yeast cell was deposited will grow up into a colony consisting of thousands of yeast cells and the colonies become visible to the naked eye. The number of colonies are counted on each plate and the live-yeast count of the product tested is computed and reported as colony forming units per gram (CFUs per gram).
The pelleting process exerts a "heat denaturization" effect on the metabolic enzymes of the yeast cell and is the basic principle behind pasteurization and sterilization with heat. Heat can denature the metabolic enzyme systems of the yeast cell, which not only prevent metabolic activity, but reproduction and life itself. Because of this relationship, if pelleting-heat prevents the yeast cell from reproducing in a petri dish, it most likely also destroys the yeast cell’s ability to carry out other metabolic functions as well. This is why the plate count method is the method of choice to test the effects of pelleting on yeast activity -- it is a well established technique and is quite accurate, plus it allows the use of selective growth media to prevent interference from other non-yeast microorganisms in the feed mixture.
In evaluating losses in yeast viability, it is important not to use logarithms, but actual numbers, to express viability counts, because logarithms can be misleading. For example, if a feed contains 5 million (5.0 x 106) cfu per gram before pelleting and only 50 thousand (5.0 x 104) after pelleting, that represents an actual 99% loss in viability. However, if the cfu numbers are converted to log10 logarithms, the numbers would be represented as 6.699 before pelleting and 4.699 after pelleting which makes it look like only a 30% loss. This technique has been used in the past to imply that pelleting has less effect on yeast viability than it really does. So, make sure you understand how the cfu counts from an experiment are being presented so you can make an accurate interpretation.Metabolic Activity:
This method might be an alternative to the plate count method, since it deals directly with measuring the end-products of metabolic activity or the disappearance of substrate. However, it has some important draw-backs. There might be considerable interference from other microorganisms, especially bacteria. Many thermophilic bacteria could survive pelleting and their metabolic by-products could interfere with the interpretation. Also, the feed being pelleted would probably need to have higher than normal inclusion of yeast so that their metabolites could be detected following a short fermentation period. A simple manometric apparatus could be used to measure carbon dioxide volume, while more complex analytical procedures would be necessary for monitoring glucose disappearance or ethanol production.Methylene Blue Staining:
This method is not recommended, even though it is a very quick and simple method. Our laboratory has tried to correlate this quick method with actual yeast viability and fermentative activity many times, with little success. This technique seems to correlate well when the yeast populations are healthy and yeast viability is reduced by natural aging and autolytic death. However, it is not a reliable method when yeast cells have been subjected to heat, because it tends to give an over-estimation of viable yeast counts. Methylene blue staining depends on destruction of the yeast cell wall, allowing the stain to enter the cell; thus, cells with damaged cell-walls stain blue while undamaged cells do not. The problem is that if the yeast cell dies due to heat denaturization of enzyme systems and the cell wall is not affected, the dead cell will not receive the stain and the yeast cell will appear to be viable when it is not.Yeast Culture
Yeast Culture is unique and different from the other yeast products. Yeast Culture is the only defined feed-yeast product which does not consist solely of yeast cells or yeast biomass, but, rather, is a yeast-fermented product designed to provide fermentation metabolites resulting from a specific fermentation process. By definition, Yeast Cultures contain residual yeast viability, but, they are not considered a significant source of viable yeast cells or yeast biomass. The figure on the left provides a schematic representation of a commercial "yeast culture process", which illustrates how the fermentation proceeds and how the product is made.Fermentation Metabolites:
Production of fermentative metabolites by the yeast cells, sometimes referred to as "nutrilites", is the principle behind the yeast culture fermentation. A specific culture media is inoculated with live yeast cells, allowed to ferment under a specific set of conditions, and then the entire fermented media is dried … it contains both the residual live yeast cells used in the fermentation, as well as the metabolites or metabolic by-products the yeast produced. Yeast Culture is a very complex product and contains both the "intracellular" yeast cell nutrients and the "extracellular" metabolites of fermentation. Simply feeding yeast cells will not duplicate the total "nutrient" range found in yeast culture.
When yeast cells ferment sugars, they secrete metabolic by-products … alcohol is a good example of an extracellular metabolite resulting from yeast fermentation under brewing conditions. Variations in metabolite profile are why different fermented products (e.g., breads, beers, wines, whiskies) all taste and smell differently from each other. There are many different metabolites secreted, depending on the composition of the media fermented and the conditions of the fermentation process, including peptides, alcohols, esters and organic acids. In the case of beer fermentation, these extracellular metabolites are washed away from the yeast cells and are contained in the beer and are not part of the brewers yeast. Just remember that Yeast Culture is unique among all other yeast products and must be evaluated as a fermented product containing "undefined nutrients" and not based on its "known nutrient" profile like protein, vitamin, and amino acid profile.Nutritional Yeast
The nutritional yeast products consist of yeast biomass or pure, dead yeast cells which are fed for their nutrient value. They include primary dried yeast, brewers dried yeast, torula dried yeast and whey yeast.
Yeast cells have long contributed to the nutritional value of fermented foods, like breads and beers. In some societies, "cloudy" beers make a major contribution to daily nutritional needs. The cloudy sediment of yeast cells provides essential B-vitamins, minerals and amino acids. And during the middle ages, infants were often fed the sediment from cloudy beer to keep them healthy and avoid nutritional deficiencies.
Yeasts are a good source of protein or amino acids. Approximately 40% of the weight of dried yeast consists of protein. The quality of yeast protein is excellent for a vegetable protein and it is about equivalent in quality to soybean protein. Both are rich in lysine, and are excellent supplements to cereals, whose proteins are generally low in lysine. As with other plant proteins, yeast protein is low in the sulfur amino acids, but supplementing dried yeast with 0.5% methionine can raise its protein quality up to that of casein. However, there is a limit to how much yeast can be fed, because about 20% of the crude protein nitrogen in yeast is in the form of nucleic acids. Nucleic acids can cause problems if over fed, because excessive nucleic acid intake results in elevated uric acid levels in the blood. High levels of uric acid tend to crystallize in the joints and in man and this can cause gout and arthritis or even renal stones.
While the nutritional value of yeast was recognized early, the identification of the nutritional factors which cured certain nutritional diseases did not take place until the early 20th century. That was when the B-vitamins were discovered. Several of these vitamins were first extracted and characterized from yeast, including biotin, niacin, pantothenic acid, and thiamin. Yeast has long been recognized as a rich source of natural B-vitamins.Primary Dried Yeast
Primary dried yeast refers to yeast (usually Saccharomyces cerevisiae or Candida utilis) which is intentionally grown and harvested as a nutritional yeast source and is not a by-product of another industry. It is generally propagated on sugar substrates, like molasses, in aerobic bioreactors much like active bakers yeast, but dried at high temperature to kill the yeast cells. It is predominantly used in the food industry for food enrichment and is generally too expensive for use in the feed tradeBrewers Dried Yeast
Brewers yeast is a by-product of the beer and ale brewing industry. After the beer is fermented, the yeast (Saccharomyces cerevisiae) is recovered from the fermentation vats, dried at high temperature to kill the yeast, and sold to both the food, health food and animal feed trades as a specialty protein, vitamin and mineral supplement.Torula Dried Yeast
Torula yeast refers to a special yeast species (Candida utilis) which is often grown on the waste-water from the paper industry, called sulfite liquor. The waste liquor is high in pentose or five-carbon sugars and growing the yeast in the liquor reduces the biological oxygen demand on the waste, making it easier to dispose of – it also provides a marketable by-product. At one time, the feed industry was the primary market for Torula yeast, but now, most of it is used by food manufacturers. It got its name from Torulopsis utilis, which used to be the yeast’s name before it was reclassified by the taxonomists.Whey Yeast
Whey yeast is yeast grown on whey lactose and consists of yeast from the species Kluyveromyces marxianus (formerly classified as Kluyveromyces fragilis). At one time there was considerable whey yeast available to the feed trade, but its availability in recent years has almost disappeared. However, as whey waste-streams become a bigger problem in the future, we may see an reemergence of whey yeast as a feed ingredient.Yeast Cell Nutrients
When whole yeast cells are fed, like brewers yeast or active dry yeast, their primary nutritional contribution comes from the proteins, peptides, vitamins and minerals contained within the cell ... the intracellular biochemicals found in the yeast cell. Thus, for these nutrients to become available the yeast cell must be lysed or broken open so that the contents within the cell become available for digestion and absorption.
This can happen in two ways: 1) protease and glucanase enzymes from microorganisms in the digestive tract can break open the cell via "hydrolysis" from the outside-in, and 2) the enzymes within the live yeast cell can cause "autolysis" of the yeast cell by digesting the cell-wall from the inside-out.
When dead yeast cells, like brewers dried yeast, are fed, hydrolysis is the only way the intracellular yeast nutrients are made available. However, when live yeast cells are fed, both hydrolysis and autolysis may play a role in rupturing the cell for digestion.
As for digestibility or availability of the intracellular yeast nutrients, the nutrients from live cells are probably more available … assuming the cell is lysed. The reason for this is heat. The higher the temperature used to dry the yeast, the more "denaturization" or destruction of the nutrients. It is not that live yeast cells, per se, are more nutritious than dead yeast cells, but that dead yeasts are generally dried at much higher temperatures, which is why they are dead, and nutrient availability is generally lower.Special Purpose Yeast Products
Although yeast is not a source of vitamin D activity, it does contain a sterol, ergosterol, which is converted to form vitamin D2 (ergocalciferol) when irradiated with ultra violet light. In the past, Irradiated Dry Yeast served as an important source of vitamin D activity, until it was driven out of the market place by cheaper synthetic vitamin D3 (cholcalciferol) .Selenium Yeast
Yeast is also a good source of dietary selenium. The selenium in yeast is generally in the form of selenomethionine, which is an organic form of selenium with selenium replacing the sulfur in the methionine molecule. Selenium is required for the activation of an enzyme system that has protective effects on the liver and other tissues. It appears that the selenium activated enzyme, glutathione peroxidase, prevents oxidative damage of the cell membrane and subsequent premature aging of the cell.
Brewers yeast selenium played an early role in animal nutrition, especially in pet food manufacturing. The fact that brewers yeast contains appreciable amounts of B-vitamins and selenium often accounted for its inclusion in many animal feed formulations. It was frequently used in early pet food and specialty products as a natural selenium source before sodium selenite became widely used.
Commercial "high selenium" yeasts are manufactured and sold through health food stores and sometimes added to vitamin/mineral supplement tablets. While bakers yeast may contain one or two parts per million (ppm) selenium, commercial "high selenium" yeasts are available containing as much as 2,000 ppm selenium, 75% of which is organically bound.Chromium Yeast
Chromium in yeast is present in the organic form called the "glucose tolerance factor" and is important in the regulation of sugar metabolism. It consists of trivalent chromium complexed with biologically active peptides, amino acids and niacin, and appears to act in conjunction with insulin to facilitate efficient metabolism of carbohydrates. It appears important for older people, diabetics, and children of diabetics since they either have a lower tissue chromium content, a lowered ability to absorb chromium, or a higher incidence of impaired glucose tolerance. Although it is not known exactly how it works, studies indicate that individuals who consume chromium in the organic form have a reduction in blood sugar and insulin dependency, and a reduction in serum cholesterol and triglycerides. Recent research trials indicate that organic chromium, either as high chromium yeast or chromium picolinate, may reduce stress in cattle and reduce fat deposition in swine.Phaffia Yeast
Phaffia rhodozyma, known as Phaffia Yeast, is the latest yeast product to enter the feed industry. This yeast produces a red pigment used in trout and salmon feeds for its red pigmentation of the meat. This red pigment is a carotenoid called "astaxanthin". Phaffia yeast is more expensive than the synthetic form of the carotenoid, but limited data suggests that astaxanthin from ruptured yeast cells may be a more effective pigmentor since it is in an organic matrix.Yeast Fractions
Yeast extracts and autolysates are produced from whole yeast cells, either debittered brewers yeast or primary grown bakers yeast, and are used extensively in the food industry for flavor enhancement. Yeast extracts consist of the intracellular components of the yeast cell, with the yeast cell-wall removed. Yeast autolysates consist of ruptured or lysed cells and contain both the intracellular and cell-wall fractions. Both contain 5'-nucleotides and glutamate which enhance flavor recognition. Yeast extracts are also used as microbial stimulants in the fermentation industries and microbiologists use them in their laboratory growth media to optimize bacterial growth.
Yeast cell-walls remaining as a by-product in the manufacture of yeast extracts are often called yeast hulls or yeast ghosts. They consist predominantly of beta-glucans and mannans, with some chitin and protein. Yeast ghosts are often used in wine making to avoid "stuck" fermentations due to accumulating octanoic and decanoic acids. These acids are adsorbed onto the ghosts which prevents their inhibitory effect on fermentation.Autolysates:
These yeast products consist of whole yeast cells which have been broken open (lysed) by means of letting the yeast cell destroy itself using its own autolytic enzymes (autolysates), by using acids or enzymes to hydrolyze the cell (hydrolysates), or by rupturing the cell using osmotic pressure due to suspending the yeast cells in a high salt solution, called plasmolysis (plasmolysates).
They contain both the cell contents and the cell-wall of the cell. Autolysates are the cleanest of the three types of products, because hydrolysates and plasmolysates are generally high in salt or sodium content due to the salt gradients used in plasmolysis or the use of bases to neutralize the acids used to make hydrolysates.
Autolysates are used extensively by the food industry for their ability to enhance food flavors, especially in soups and snack products. This enhancement is due to the yeast’s nucleic acid content – the 5’nucleotides. The nucleotides add "savoriness" to food by accentuating the effects of glutamic acid or monosodium glutamate to enhance flavors.Yeast Extracts:
Yeast extracts consist solely of the intracellular contents of the yeast cell, following lysis of the cell, with the cell-wall removed. They are used by microbiologists in the preparation of microbial growth media and by some industrial and pharmaceutical fermentations. Extracts are rich in amino acids, vitamins and trace minerals and function as growth stimulants for microorganisms.Yeast Cell-Walls:
The yeast cell has a carbohydrate shell around it called the yeast cell-wall. It is often called a "glucan shield" and consists mostly of beta-glucans and mannans. These are structural polysaccharides (chains of sugar molecules) similar to starch or cellulose. Beta-glucans are chains of glucose sugar molecules, just like starch, but the sugars are joined together with different linkages (beta-1,3 and 1,6 linkages instead of alpha-1,4 and 1,6 linkages). Therefore, different enzymes are needed to break them down into absorbable sugar molecules. Mannans are chains of a different sugar molecule called mannose.
The yeast cell wall is thought to have a unique ability to adsorb or bind with certain things in the digestive tract, especially bad things like toxins, anti-vitamins, viruses and pathogenic bacteria, and is presumed to have a protective or flushing effect in the gut. The mannan fraction of the yeast cell wall is also thought to be a special polysaccharide which is selectively consumed by good bacteria in the gut. These good bugs, in turn, suppress or kill the bad bacteria like salmonella. This is the same concept behind a product called fructooligosaccharide or FOS. The yeast product is similarly called "MOS", the acronym MOS standing for mannanoligosaccharide (an oligosaccharide is a polysaccharide which is only 3-10 sugar molecules long instead of hundreds of molecules, suggesting that it is a short chain mannan). When these short-chained mannans are fed, they are not digested by the animal, but are consumed by select bacteria in the gut which grow rapidly and have a probiotic effect against bad bugs.References and Suggested Reading
Kreger van Rij, N. J. W. (ed.). 1984. The Yeasts: A Taxonomic Study. Elsevier Biomedical Press, Amsterdam, Holland.
Reed, G. and T. W. Nagodawithana. 1991. Yeast Technology (2nd ed.), Van Nostrand Reinhold, New York.
Peppler, H. J. 1983. Fermented feeds and feed supplements. In Biotechnology vol 5, G. Reed (ed.), VCH Publishing Co., Weinheim, West Germany.
Dearstyne, R. S. and C. O. Bollinger. 1938. Some effects of feeding yeast fermented mash to laying pullets. North Carolina Agri. Experiment Station, Technical Bulletin No. 55.
Johnson, E. A., D. E. Conklin and M. J. Lewis. 1977. The yeast Phaffia rhodozyma as a dietary pigment source for salmonids and crustaceans. J. Fish Res. Board of Canada 34:2417-2421.
Johnson, E. A., T. G. Villa, M. J. Lewis and H. J. Phaff. 1978. Simple method for the isolation of astaxanthin from the basidiomycetous yeast, Phaffia rhodozyma. Appl. Environ. Microbiol. 35:1155-1159.
Peppler, H. J. and C. W. Stone. 1976. Feed yeast products. Feed Management 27(8):17-18.
Mertz, W. and K. Schwartz. 1955. Impaired glucose tolerance as an early sign of dietary necrotic overdegradation. Arch. Biochem. Biophys. 58:504-506.
Mertz, W., W. Woepfer, E. E. Roginski and M. M. Polansky. 1974. Present knowledge of the role of chromium. Federation Proc. 33:2275-2280.
Doisy, R. J., D. H. P. Streeter, J. M. Freibes and A. J. Schneider. 1976. Chromium metabolism in man and biochemical effects. In Trace Elements in Human Health and Disease, vol. 2, A. S. Prasad (ed.), Academic Press, New York.
Chang, X. and D. N. Mowat. 1992. Supplemental chromium for stressed and growing feeder calves. J. Anim. Sci. 70:559-565.
Page, T. G., L. L Southern, T. L. Ward and D. L. Thompson, Jr. 1993. Effect of chromium picolinate on growth and serum and carcass traits in growing-finishing pigs. J. Anim. Sci. 71:656-662.
Nikl, L., L. J. Albright and T. P. L. Evelyn. 1991. Influence of seven immunostimulants on the response of coho salmon to Aeromonas salmonicida. Dis. Aquat. Org. 12:7-12.