The physiological functions of yeast cell walls and their applications

In livestock and poultry breeding, yeast has been used as protein feed for a long time. Adding yeast to the livestock and poultry feed can prevent gastrointestinal diseases in livestock and poultry, improve feed efficiency, and enhance production performance. In recent years, through extensive research, it has been found that the main components of yeast cell walls are glucans, mannan oligosaccharides, glycoproteins, and chitin. Among them, the active components glucans and mannan oligosaccharides have the functions of stimulating cellular immunity and humoral immunity, promoting the growth of beneficial bacteria, and inhibiting the growth of harmful bacteria. Practical production has proved that yeast cell extract is a very good immunostimulant and beneficial bacteria proliferator, and therefore can be used to produce green immune feed additives.

1.The main component of yeast cell wall

The yeast cell wall is a natural green additive obtained by collecting the yeast cells after their cultivation and proliferation, subjecting them to sound wave disruption, multiple cleaning and filtration, and then performing centrifugal separation on the soluble substances after undergoing high-temperature and acid-base treatment. The extracted cell wall is subjected to spray drying under specific temperature and pressure conditions. The product is a light yellow powder without any bitter taste. The main components of the yeast cell wall are glucans (57.0%), mannan oligosaccharides (6.6%), glycoproteins (22.0%), and chitin, accounting for approximately 85% of the dry weight of the cell wall. Other components such as proteins, nucleic acids, lipids, and ash account for about 20% of the dry weight.

The yeast cell wall is generally divided into three layers. The inner layer is a glucan layer, the middle layer contains abundant proteins, and the outer layer is a mannan oligosaccharide layer. The layers can partially interlock with each other.

1.1 Polysaccharide

The glucan layer is located close to the cell membrane on the inner layer of the cell wall and is the main component of the cell wall structure. There are two types of glucans in the yeast cell wall, both of which have branches. One type of glucan is composed of D-glucan molecules connected by β-(1-3) bonds, and has a small number of branches connected by β-(1-6) bonds. The other type of glucan has D-glucose molecules forming the main chain by β-(1-6) bonds, and has branches formed by β-(1-3) bonds. It is insoluble in the body, not absorbed, and does not produce viscosity.

1.2 Maltodextrin oligosaccharide

The mannose oligosaccharide binding part of the yeast protein forms a mannose-oligosaccharide-protein complex that covers the cell surface. The mannose oligosaccharide backbone is formed by α-(1-6) bonds, with a length of approximately 50 mannose units. The main chain is connected by 1 to 4 mannose units linked by α-(1-2) and α-(1-3) bonds. The mannose molecule at the core end is connected to N-acetyldiglycose through β-(1-4) bonds, which is attached to the asparagine in the protein polypeptide chain, forming a mannose oligosaccharide protein network structure. This network structure has antigen specificity. The various groups bound to the mannose branches often cause animals to produce specific antibodies due to different configurations. The specific side groups on the mannose oligosaccharide are the determinants of the yeast cell antigen.

1.3 Proteins, chitin, lipids and enzymes

Most of the proteins in the yeast cell wall combine with sugars to form mannan-protein complexes, which contain approximately 5% to 10% protein. The chitin in the yeast cell wall is a linear polymer of N-acetylglucosamine, and in the cell wall, N-acetylglucosamine is linked together by β-(1-4) bonds.

The lipids of yeast cell walls are mostly composed of neutral lipids such as acylated glycerol and sterol lipids. The content of lipids in the cell wall and the relationship between lipids and other components of the cell wall remain unclear.

The enzymes present within the cell wall include transglucosylase, α-galactosidase, acid phosphatase, L-asparaginase, amino acid peptidase II, propionyl-β-glucosidase, glucose amylase, etc., all of which are hydrolases that enable the cell to absorb nutrients within the cell. Additionally, the cell wall contains glucanase and protein disulfide reductase, which are enzymes related to cell wall proliferation.

2. The physiological functions of yeast cell walls

2.1 Stimulate and enhance immune function​

The yeast cell wall regulates the body's immune system by acting as an auxiliary factor for immune stimulation, thereby enhancing the immunity of animals. The yeast cell wall contains a large amount of β-3-glucan and mannan oligosaccharides, providing non-specific immunity against diseases caused by bacteria, fungi and viruses, as well as stress responses such as transportation, migration, vaccination, and climate change.

2.2 Maintain the balance of microorganisms within the body and control diseases

Dextran has a defensive function against microorganisms that invade the animal body. The mannans (MOS) in the yeast cell wall have the function of occupying attachment sites on the intestinal wall for harmful bacteria (such as Escherichia coli and Salmonella), thereby preventing pathogenic bacteria from settling and proliferating, maintaining the balance of the bacterial community in the animal's intestinal tract, and controlling the occurrence of intestinal diseases.

3. Applications of Yeast Cell Walls

The yeast cell wall can accelerate the development of the immune organs of fish and shrimp, increase the number of lymphocytes, and enhance both non-specific immunity and stimulate the production of humoral immunity in the body. The research conducted by Nureco Company has proved that the combination of β-(1-3) glucan and vitamin C in the yeast cell wall has a positive impact on the activity of macrophages. They conducted a two-week trial on two groups of fish. One group was the control group, which was given normal doses of vitamin C but no yeast cell wall, while the other group was given the yeast cell wall on the basis of the control group's feed. The results showed that the mortality rate (14.6%) of the fish fed with the feed containing yeast cell wall and vitamin C was significantly lower than that of the control group (35.5%).

Feeding yeast cell walls can increase the survival rate of Atlantic salmon infected with vibrio cold-water disease from 17% to around 70% (feeding yeast cell walls 3 weeks before infection). The combined use of yeast cell walls and vaccines has a stronger and more lasting immune protective effect, raising the survival rate of infected salmon to 90%. Adding a certain amount of yeast cell walls to the feed of cattle and sheep can enhance their disease resistance, prevent diarrhea in calves and lambs, and also increase daily weight gain and feed conversion rate. Adding yeast cell walls containing 2g of mannan oligosaccharides to the feed of calf and lamb daily, after 5 weeks of feeding, the number of Escherichia coli in the feces significantly decreases, and the respiratory disease rate drops from 9.9% to 3%.

The yeast cell wall mainly enhances the immunity of animals by stimulating the immune function and maintaining the micro-ecological balance, thereby improving the health condition of animals and increasing production performance, and fully exerting the growth potential of young animals. In the United States, Brazil and Taiwan of China, yeast cell wall has been widely used, and it has played a good role in preventing diseases, treating diseases and maintaining health in livestock and fish farming. Practical production has proved that yeast cell wall is a promising new type of additive, which is an immune promoter with dual functions of antibiotics and probiotics, and is increasingly valued by the academic community and the breeding industry.