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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) .
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 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 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 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.
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 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 fermentations. Extracts are rich in amino acids, vitamins and trace minerals and function as growth stimulants for microorganisms.
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.
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