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Yeasts have been fed to animals for more
than 100 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.
The purpose of this article is not to argue efficacy for yeast products, but to provide a
better understanding of what the products on the market are and describe their different
uses.
Yeasts are microscopic fungi -- single-cell organisms of the plant kingdom 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 (Kreger van Rij, 1984).
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 live yeast cells
per gram to over a million per gram. Several species have proven very beneficial to man,
while a few "imperfect" yeasts are 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. It is also the
yeast used by breweries to make beer, 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, previously classified as Kluyveromyces
fragilis. 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 and bread leavening are aerobic processes where
the yeast converts oxygen and sugar, through oxidative metabolism, into carbon dioxide and
usable energy for efficient yeast cell growth. 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 sugar into ethanol and
carbon dioxide and the yeast grow very slowly. To optimize ethanol production, the
fermentation or growth media is maintained without oxygen; but to maximize carbon dioxide
or 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.
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.
Nutritional Yeast Products:
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, therefore, 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, 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 thiamine. Yeast has long been recognized as a rich source of natural
B-vitamins.
Irradiated Yeast:
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 a methionine amino acid molecule containing selenium. 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 (Mertz and Schwartz, 1955). It consists of
trivalent chromium complexed with biologically active peptides and niacin, and appears to
act in conjunction with insulin to facilitate efficient metabolism of carbohydrates (Mertz
et al., 1974). 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 (Doisey, et al., 1976) . 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 (Chang and Mowat, 1992; Page et
al., 1993).
Active Dry Yeast:
Active dry yeast (95% dry
matter) is the predominant viable yeast available to the feed industry. Although wet yeast
cake (30% dry matter), and to a lesser extent yeast cream (20% dry matter), are used
extensively by the bakery trade, active dry yeast is the form of live yeast used in most
animal feeds. All three forms of live yeast can be used, however, to manufacture Yeast
Culture, which will be discussed later.
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 and fluid-bed dried yeasts 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 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 active dry yeast.
Yeast Culture:
Yeast culture is a unique
yeast product because it is a yeast-fermented product rather than yeast biomass … a
fermented product similar to beer, wine or bread. It is the fermentation metabolites
produced by the bakers yeast (Saccharomyces cerevisiae) which gives it value, not the
yeast itself. When fed to animals, it functions as a digestive stimulant by enhancing the
growth of digestive bacteria in the rumen and lower digestive tract. This improvement in
microbial activity improves the digestive efficiency and productivity of the animal due to
improved nutritional status.
Flavor Enhancers:
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.
Pigmentation:
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 (Johnson et al., 1977, Johnson et al., 1978).
Yeast Ghosts:
Yeast cell-walls
remaining as a by-product in the manufacture of the manufacture of yeast extracts are
often called yeast hulls or yeast ghosts. They consist predominantly of 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.
Yeast beta-glucans, from the yeast cell-wall, have recently gained notoriety in the
aquaculture industry. Recent studies in Canada with yeast beta-glucans injected into trout
suggest that they might have a significant effect as an immunostimulator, potentially
reducing mortality in commercial fish farms (Nikl et al., 1991).
References Cited:
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.
Copyright 1997 - Charlie W. Stone
Related Links:
Diamond V Mills : Yeast Culture
Manufacturer
ASAS : Journal of Animal Science
ADSA : Journal of Dairy Science
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