The use of yeast and its products as feed additives began in the mid-1970s and was first used as protein supplementary for ruminants. Yeast cells are rich in protein, B vitamins, fats, sugars, enzymes and other nutrients. At present, more than 500 species of yeast have been discovered, belonging to 41 genera. However, not all yeasts can be used as feed additives, when applying in production, attention should be paid to the selection of strain types, commonly used ones include Candida and Debarymyces, Torulopsis sphaeroides and Rhodotorula. The use of yeast to produce single-cell protein (SCP) has the advantages of fast growth and short farming cycle. Multiple tests have shown that yeast, as a feed additive, can promote the growth of microorganisms in the stomach, prevent and treat gastrointestinal diseases in poultry and livestock, improve health, and improve production performance. However, due to the bitter taste and poor palatability of feed yeast, its application in production has been limited.

Yeast cell wall is a special by-product extracted from soluble substances during the production of beer yeast, it accounts for 20% to 30% of the dry weight of the entire cell. It plays an important role in maintaining cell morphology and cell-cell recognition. Gertien et al. (1999) studied the structure of yeast cell wall at the molecular level and believed that it is a dynamic and controllable structure. Its structure and composition can be strictly regulated and can respond extensively to environmental changes. Now yeast cell wall feed additives have attracted widespread attention. Yeast cell wall is a light-yellow powder, compared with yeast feed, it has no bitter taste and good palatability. It can be used as an immune promoter to improve animal health by stimulating and enhancing the body's immunity to improve production performance, especially to enable young animals to exert their growth potential. Yeast cell wall is increasingly used in feed.

1.Chemical composition

In the production process, the method of breaking the walls of yeast cells has also been receiving attention, and the most commonly used method at present is the sonic shock method. Under the premise of keeping the yeast cell wall intact as much as possible, the cells are shattered using sonic shock. The shattered yeast solution is washed and filtered multiple times to remove bitterness and impurities, and then undergoes high temperature, acid, alkali treatment, centrifugation and other operations. After obtaining the cell wall, the finished product can be obtained by spray drying.

The main active components of the yeast cell wall are glucan accounting for approximately 57.0% of its dry weight, mannan oligosaccharide accounting for approximately 6.6% of its dry weight, glycoproteins accounting for approximately 22.0% of its dry weight, chitin and other components proteins, nucleic acids, lipids and ash account for less than 20.0% of its dry weight.

2.Mechanism of physiological functions

The main physiological functions of the yeast cell wall are performed by mannan oligosaccharide and β-glucans. They perform their respective functions in the animal body and can respond to diseases caused by bacteria, fungi and viruses, as well as stress caused by changes in transportation, vaccination, climate, etc. and generate non-specific immunity.

2.1 Mannan oligosaccharide

Mannan oligosaccharide is oligosaccharides composed of mannose and glucose. They generally contain 2 to 10 monosaccharides, and the monosaccharides are connected by α-1,2, α-1,3 and α-1,6 bonds. Mannan oligosaccharide is easily soluble in water but not easily soluble in organic solvents. Its viscosity gradually decreases as the temperature rises. It has unique gelling properties. It is generally stable under physiological pH and normal feed processing conditions. Its physical and chemical properties vary slightly depending on the source.

Mannan oligosaccharide contains a large number of chemical bonds that cannot be cut by digestive enzymes and can hardly be digested and utilized in the small intestine. After entering the later part of the digestive tract and being concentrated, they can be selectively fermented and utilized by beneficial bacteria in the animal's digestive tract flora, participating in metabolism in the form of releasing organic acids and CH₄ ,CO₂ and H₂ to provide energy. At the same time, the acidic substances produced by fermentation will reduce the pH value of the entire intestinal tract and inhibit the growth of harmful bacteria. Mannan oligosaccharides can also greatly affect the immune system of animals, can interfere with the colonization of intestinal pathogens, and are powerful improvers of intestinal microflora. The main component of the non-immune defense system of the gastrointestinal tract is the endogenous microbial flora. Beneficial microbial flora covers the gastrointestinal mucosal epithelium to block the attachment of pathogenic microorganisms, while mannan oligosaccharide contribute to the proliferation of beneficial intestinal bacteria. proliferation. Therefore, mannan oligosaccharide is called "pathogen adsorbents" or "pathogen scavengers", and these functions of mannan oligosaccharides have also been confirmed in a series of experiments.

Mannan oligosaccharide can also increase the release of cytokines, which coordinate the activities of different immune cells. Mannan oligosaccharide can increase the concentration of interleukin (IL-2), which causes T cells to proliferate and differentiate. In addition, mannan oligosaccharide can enhance the activity of IFN (interferon). IFN is a cytokine released by activated T cells. The appearance of IFN promotes the migration of white blood cells, body fluids and proteins to the site of infection, and activates macrophages, promoting them to kill surrounding bacteria.

Cooney's research found that mannan oligosaccharide can chelate aflatoxin released from the gastrointestinal tract and can also bind to zearalenone, and the binding force has a non-linear relationship. A large number of in vitro experiments have shown that the degree of adsorption of toxins by yeast cell walls is related to the type of mold and the preparation of the cell wall. The most effective one is lipated mannan extracted from the cell wall of brewer's yeast.

2.2 β-Glucan

β-glucan accounts for about 12% to 14% of the total weight of cells. It is a polysaccharide with a special structure. It is usually composed of 10 to 20 monosaccharides and has a relative molecular mass of 6,500 to 7,500. Most of them are insoluble colloidal particles. The monosaccharides in β-glucan are connected by β-1,3 bonds and β-1,6 bonds. Due to the special bonding method and the existence of molecular hydrogen bonds, β-glucan has a spiral molecular structure, its special configuration has a strong stimulating effect on the animal immune system.

There are a large number of macrophages in the reticuloendothelial system of most animals. Under normal circumstances, macrophages are inactive. When β-glucan binds to macrophages through cell surface glycoproteins, macrophages are activated to destroy, absorb and remove damaged, aging and dead own cells and pathogenic microorganisms that invade the body through phagocytosis, and induces the body to produce a series of cellular immunity and humoral immune responses, so β-glucan is also called Immunopolysaccharides. In addition to activating natural killer cells and improving the phagocytic capacity of monocytes, neutrophils and macrophages in the blood, β-glucan can also affect the content of cytokines in circulating blood and can also affect the secretion of lymphocyte cytokines in vitro culture. β-glucan can promote the secretion of interleukin and interferon by spleen lymphocytes and enhance the effect of cytokines. At the same time, β-glucan can enhance T lymphocyte function and promote lymphocyte transformation and proliferation.

2.3 Proteins, chitin, lipids and enzymes

The protein content in the yeast cell wall is about 5% to 10%, most of the proteins and sugars are combined to form a mannan oligosaccharide-protein complex. The chitin in the yeast cell wall is a linear polymer of N-acetylglucosamine, N-acetylglucosamine is linked with β-1,4 bonds in the cell wall. The lipids of yeast cell walls are mostly composed of neutral lipids such as acylated glycerol and sterol lipids, the factors that determine the lipid content in the cell wall are not yet clear. Enzymes present in the yeast cell wall include invertase, α-galactosidase, L-asparaginase, amino acid peptidase II, propionyl-β-glucosidase, glucoamylase, etc., which are all hydrolases, allowing cells to take up nutrients within the cells. In addition, they also include glucanase, protein disulfide reductase, etc., these enzymes are related to cell wall proliferation.

Application of yeast cell wall in aquaculture

3.1 Application in fish

Yeast cell wall can accelerate the development of immune organs in fish and shrimp and increase the number of lymphocytes, which can not only enhance non-specific immunity, but also stimulate the production of humoral immunity in the body. A large number of experiments have confirmed that yeast cell walls and their extracts play a very good role in disease prevention, treatment and health care in livestock, poultry and aquaculture. Yeast cell walls have been widely used in the United States, Brazil and China.

Experimental research by Nureco shows that the combination of β-glucan and vitamin C in yeast cell walls has a positive impact on macrophage activity. The mortality rate of fish fed with yeast cell wall diet (14.6%) was much lower than that of the control group (35.5%). Experiments by the Brazilian ICC Company have confirmed that feeding yeast cell walls can increase the survival rate of Atlantic salmon infected with cold-water vibriosis from 17% to about 70% (feeding yeast cell walls about 3 weeks before infection), and yeast cell wall is used in combination with vaccines, it has a stronger and more lasting immune protection effect, increasing the survival rate of infected salmon to 90%.

Foreign scholars have conducted a large number of experiments on salmon and trout, confirming that yeast cell walls can enhance the immunity of fish. The test results of Robertsen et al. showed that β-glucan extracted from yeast can enhance the resistance of Atlantic salmon to infection by Aeromonas salmonicida, Vibrio anguillarum and Yersinia ruckeri. Engstad et al.'s study proved the presence of β-glucan receptors on Atlantic salmon macrophages. After injecting yeast glucan into Atlantic salmon, Brattgjerd et al. found that the phagocytic activity of macrophages in the test fish increased significantly under in vitro conditions. Anderson et al. used injection and immersion methods to inoculate brook trout with immune polysaccharides, the resistance of the test fish to artificially infected live bacteria was significantly enhanced. Jeney et al. used dextran as an adjuvant in combination with inactivated vaccines, and the results proved that it could significantly improve the non-specific immunity of rainbow trout. Kawakami et al. used inactivated bacterium, fungal glucan, chitin and Freund's complete adjuvant to inoculate crucian carp respectively. The test found that the immune fish in various treatments were resistant to fish-killing Pasteurella infection. Significantly enhanced. The test results of Samuel et al. showed that β-glucan can significantly enhance the resistance of Trichogaster trichopterus.

Chinese scholars have also conducted many controlled experiments on the impact of yeast cell walls on fish immunity. Chen et al. reported that injection of β-glucan can improve the ability of channel catfish to resist Edwardsiella infection. Chen Changfu et al. (2004) explored the regulatory effect of oral immune polysaccharide (yeast cell wall) on the immune response of carassius auratus gibelio. The results showed that adding 150.0 mg of immune polysaccharide (yeast cell wall) per kilogram of body weight, It has a strong regulatory effect on the immune response. Compared with the control fish, not only the agglutination antibody titer in the serum has increased, but also the phagocytosis percentage (PP) and phagocytosis index (PI) of white blood cells in the blood increased significantly.

As feed additives, yeast cell wall and its extract can promote fish growth, reduce mortality, reduce nitrogen emissions in feces, and prevent pollution. Yeast cell wall enhances animal immunity and improves animal health by stimulating immune function and maintaining microecological balance, thereby improving production performance and allowing the growth potential of young animals to be fully utilized. The effects of yeast cell walls and their extracts on fish production performance have also received much attention. Yoshida et al. (1995) reported that mannan oligosaccharide can significantly improve catfish cell activity and feed utilization efficiency. Zhang Qixin et al. (1997) conducted a trial of feeding oral immune polysaccharides in the factory farming of flounder and stone flounder. They added immune polysaccharides to the flounder feed. After 100 days of feeding, the survival rate, the total length and weight in the test group were increased by 0.9%, 1.28% and 2.82% respectively. Zhang Qixin (2003) conducted a comparative farming experiment in which immune polysaccharides were added to turbot feed. After 210 days of feeding, the average body length, weight, and survival rate in the test group were increased by 14 mm, 32.6 g and 4.8% respectively. Other studies have shown that the mortality rate of trout fry after being attacked by cold water pathogens is as high as 25% when the body weight is 1 to 7 g, but adding 0.7% MOS to the diet can reduce the mortality rate of trout fry at this stage to 1%. Zhang Hongmei (2004) studied the effects of adding different levels of yeast mannan oligosaccharides to the healthy carp diet on its production performance. The results showed that adding MOS to the diet can improve the production performance of common carp. With the increase in the amount of MOS added, survival rate increased first and then dropped. When the addition amount was increased to 0.2%, the survival rate reached a maximum of 99.40%, and then the survival rate decreased.

3.2 Application in shrimp

Chen Changfu has done a lot of work on the impact of yeast cell wall on aquatic animals. Not only did he study its impact on some fish, he also conducted a test on the effect of oral immune-polysaccharide (yeast cell wall) on Penaeus vannamei against Vibrio harveyi infection. The results show that adding immune polysaccharide (yeast cell wall) to the shrimp feed can significantly improve the activities of acid phosphatase (ACP) and alkaline phosphatase (ALP) in the serum and muscle of the white-leg shrimp, and extremely significantly improve the activities of ACP, ALP and peroxidase (POD) in the hepatopancreas, which enhance the ability to resist Vibrio harveyi infection. Xu Dixin et al. (2004) injected immune polysaccharide (yeast cell wall) as an immune stimulant into Procambarus clarkia (cray fish) and found that the ACP and ALP activities in the hepatopancreas increased significantly. Liu Heng et al.(1998) used immune polysaccharides extracted from seaweed as a feed additive to immunize Penaeus vannamei in an oral form. The results showed that, except for a few cases, the phenoloxidase, bacteriolytic, superoxide dismutase, and antibacterial activity of the test group were higher than those in the control group. Although this immune polysaccharide is extracted from plants, it has similar structure and properties to microbial polysaccharides. Liu Donghui (2002) discussed the effect of the combination of brewer's yeast β-glucan preparation and vitamin C on the growth and disease resistance of Penaeus monodon. The results found that the combination of brewer's yeast β-glucan preparation and vitamin C can significantly improve the growth rate, survival rate and serum protein level of Penaeus monodon and significantly reduce the FCR, and its effect was equivalent to that of the high-dose VC group. In the challenge test, the combination of brewer's yeast β-glucan preparation and vitamin C greatly improved Survival rates of shrimp injected with white spot virus.

3.3 application in other aquatic animals

Except for fish and shrimp, there are few studies on the application of yeast cell wall and its extracts in aquatic animals. Zhang Qixin (2002) added 5% oral immune polysaccharide to the diet of stripped hybrid abalone seedlings, and the average shell height and survival rate increased by 1.55mm and 3.2% respectively compared with the control group. Sun Hushan (2001) injected zymosan (yeast cell wall) into Chlamydomonas scallops and measured the activities of phenoloxidase (PO) and myeloperoxidase (MPO) involved in immune defense in the serum and blood cells of Chlamydomonas scallops. The results showed that only 1 hour after the injection of zymosan, the PO activity in the serum of the test group was significantly higher than that of the control group, while there was no PO activity in the blood cells, indicating that zymosan has an enhancement effect on the activity of PO and MPO in the hemolymph of Chlamydomonas scallops. Beijing Jiawei Biotechnology Co., Ltd. conducted a feeding test on soft-shell turtles with feed added with immune polysaccharides. As a result, the number of soft-shell turtle deaths in the test group was significantly less than that in the control group. The analysis concluded that the addition of 0.2% of the immune polysaccharide had better effects on disease prevention and It is best to consider saving farming costs.

Prospect

Adding a small amount of yeast cell wall or its extract to feed can effectively improve the microbial flora in the digestive tract of aquatic animals, reduce the amount of pathogenic bacteria such as Escherichia coli and salmonella in the digestive tract and aquatic products, and significantly improve the disease resistance of aquatic animals. It can also improve production performance such as weight gain and feed conversion rate. Moreover, the yeast cell wall is a natural component and does not cause pollution, it has good application prospects in animal nutrition. However, it should be noted that the optimal addition amount of yeast cell wall will be different for different species, ages and physiological conditions. The optimal addition amount should be determined after sufficient testing. Other ingredients or additives in the diet may have synergistic or antagonistic effects on the function of the yeast cell wall, which requires in-depth research.