Introduction

In recent years, the interest in bio-additives has been increased by optimizing the use of organic additives or new ingredients arising from biotechnological processes, which leads to the application of living organisms and their metabolites in diverse industries. Notably, poultry production faces significant challenges to meet the global demand for meat and eggs due to the steady increase in the global population. Many countries are gradually phasing out prophylactic antibiotics in animal feed to overcome antibiotic resistance in livestock (1). Baker’s yeast from Saccharomyces cerevisiae (S. cerevisiae) contains essential nutrients that could be used as a source of amino acids and vitamins in the diet, as well as immune enhancers. Baker’s yeast was investigated as a potential feed additive for poultry production (2-5). First, baker’s yeast or S. cerevisiaeincreased poultry performance by increasing feed intake and nutrient absorption in the digestive system by increasing the morphological features of the intestines (4-6). Second, S. cerevisiae could be an alternative to antibiotics by reducing risks from various diseases (7). Last, the use of baker’s yeast for feed fermentation was also discovered (8,9). The application of baker’s yeast was shown in studies of fermented cottonseed meal (10), fermented white rice (11), fermented brown rice (8,12), fermented rice bran (13), and deoil rice bran (9). Moreover, the positive effects of baker’s yeast on laying hens were documented. The triglyceride and cholesterol in the blood of laying hens decreased, egg quality increased, and egg production was recorded in the studies of Mirza et al. (4) and Yalcin et al. (14). As mentioned above, the application of baker’s yeast previously proves its functions in health and growth performance. In addition, baker’s yeast is considered one of the technological solutions to develop organic, clean, and sustainable agricultural production methods while also improving the quality of poultry products.

Therefore, research on baker’s yeast has become popular. This review summarizes all aspects of baker’s yeast S. cerevisiae and its roles in poultry performance and provides transparent, applicable information for broilers and laying poultry.

Nutrition compositions

According to Walker et al. (15), yeasts are single-celled microorganisms and are eukaryotes with diameters ranging from 3 to 40 microns. As imparted by Klis et al. (16), yeast contains a high level of digestible protein and an abundance of vitamins and essential elements. Remarkably, yeast is a crucial protein and amino acids source for poultry growth. Baker’s yeast, in particular, is high in thiamin and riboflavin vitamins, as well as nicotinic acid, pantothenic acid, biotin, Mg, and Zn. The dry matter content of dried baker's yeast was 91%; crude protein was 44.4%; ether extract was 0.52%; crude fiber content was 0.33%; crude ash content was 5.01%; calcium content was 0.28%; total phosphorus content was 1.21%; and metabolizable energy was 10.79 MJ/kg, which were analyzed in the study of Yalcin et al. (17). The sources of β-glucans, metabolites, and several industrial enzymes are also widespread in yeast (18). Chitin, β-glucans, and α-mannan are the main polysaccharides in yeast cell walls, accounting for 90% of their weight (dry matter). Yeast cells can detect and interact with each other by the carbohydrate component β-mannan of mannoprotein, which also allows us to determine the yeast's immunological specificity (19).S. cerevisiae is currently widely used as a protein source in animal diets to improve growth performance and is formed from inactivated cells. Although S. cerevisiae contains high amounts of protein and other nutrients, it is not used for human nutrition directly (20). The use of living yeast cells detoxifies mycotoxins, other bacterial toxins, and their receptors in the mucous membrane and Vibrio cholera toxin. Diets containing these toxins cause damage in several organs; however, the presence of S. cerevisiae has been shown to reduce this damage and minimize animal stress while also providing enzymes, vitamins, and protein (21). The detailed composition and amino acid contents of yeast S. cerevisiaeare shown in table 1 and table 2 (22).

Table 1: Chemical composition of baker’s yeast, active dry

Table 2. Essential and nonessential amino acids in baker’s yeast, active dry

Mode of action of baker’s yeast

The mechanism of yeast can be classified into two parts: pharmacokinetics and pharmacodynamics (23). Suarez and Guevara (23) showed that the use of yeast as a probiotic and prebiotic is determined by its biological regulatory effect through its various mechanisms: the elimination of pathogens from microbial antagonism, the stimulation of the animal's immune system, the attachment and removal of pathogens and an increase in the activity of enzyme-specific bacteria. It was discovered that yeast, S. cerevisiae, yeast cell wall, and yeast derivatives had a positive effect on broiler growth in trials, confirming that yeast can modulate the growth of gut microbes and assist the process by which volatile fatty acids are produced to improve nutrient absorption (24). In figure 1, biotherapeutic mechanisms of action are used by yeast in pathogen control.

Figure 1: Biotherapeutic mechanisms of action used by yeast in pathogen control (23).

The mode of action for poultry production is divided into two primary principles. This is illustrated in figure 2, and the first action is to improve poultry performance. The second action is to improve poultry health. Thus, it has a function to improve poultry production. Saccharomyces cerevisiae promotes the immune system in animals by modulating and altering cytokines. This recent advancement for livestock immunity partly helps control animal diseases. The pH level in the gut is also decreased by using Saccharomyces cerevisiae (25). Enzymes from yeast and their metabolites, including vitamins and amino acids, could affect poultry performance (23).

Figure 2: The mode of action of Saccharomyces cerevisiae for poultry production (25).

The applications of baker’s yeast for broiler poultry

In particular, yeast or baker's yeast is a potential feed additive for poultry diets that has been explored by various scientific studies worldwide. To improve poultry performance, baker's yeast could be supplemented directly in the diets. The use of baker's yeast as a probiotic in the poultry diet showed promising results in poultry performance. First, baker's yeast adjusts the intestinal microflora of poultry and controls bacterial pathogen colonization in the gastrointestinal tract of poultry. Thus, it improves meat quality and sensory characteristics.

On the other hand, the results have been inconsistent when probiotic or prebiotic yeasts have been used as an alternative to antibiotics in feed for healthy, stressed, or diseased broilers. This was due to various factors, including the yeast (S. cerevisiae) composition, the method of yeast processing, the dosage or level of administration, feed ingredients, kind of breed and age of the birds, and variations in the poultry rearing environment (26). Table 3 shows the details of baker’s yeast supplementation in the poultry diet (27-34).

Table 3: The recommended amount of Saccharomyces cerevisiae supplemented in the poultry diet

Table 3 shows the basic information from previous research on S. cerevisiae. There have been few recent studies on waterfowl for meat purposes, such as geese and ducks. Most research has shown a positive effect of Saccharomyces cerevisiae on broiler poultry growth performance, particularly in chickens, turkeys, and Japanese quails. However, research on meat characteristics and meat quality is limited. The improvement in broiler growth performance was evident through feed intake, daily weight gain, and feed conversion ratio. Specifically, Saccharomyces cerevisiae fed directly into broiler diets might be utilized in place of antibiotics to improve growth performance and nutritional digestibility (6). The increase in average weight gain in live yeast-supplemented groups with a reduced feed conversion ratio throughout the finisher phase might be associated with increased nutrient retention in broilers. Yeast is an excellent source of small peptides containing free amino acids. This ensures a rapid rate of digestion and absorption, which could significantly enhance feed utilization. According to a recent study by Sousa et al. (35), feeding broilers yeast resulted in increased body weight gain and a decreased feed conversion ratio during the finisher phase, but no alterations in feed consumption were identified in broilers. In the same study, yeast improved protein digestibility in broilers.

Similarly, supplementing broiler diets with yeast-derived products may boost body weight gain and feed conversion ratio (36). Moreover, Wang et al. (37) found an increase in nutritional digestibility, histomorphology, nutrient absorption, and physiological responses of broiler chicks given yeast at various graded supplements, which led to the improved growth performance of broilers. He et al. (6) showed that improvements in intestinal morphology, including increased villus height, were beneficial for nutrient utilization, stress resistance, and gut barrier function in livestock animals. Thus, it improves the growth performance.

Applications of baker’s yeast for laying poultry

The benefits of using Saccharomyces cerevisiae were observed in broiler poultry and laying poultry. Some previous studies showed that adding yeast cell walls to the diet of laying poultry increased the layer's productivity while also improving egg quality both internally and externally, thereby contributing to increased profitability. According to Koiyama et al.(38), supplementation with 553 ppm yeast in a layer diet could result in maximum egg production. Furthermore, a maximum albumen height of 8.06 mm and a maximum Haugh unit was determined at the 547-ppm supplementation and 535-ppm supplementation levels. The study also showed that the advantageous findings could be explained by the effect of mannan oligosaccharides in the gastrointestinal tract. The mannan-oligosaccharides played a role in preventing pathogenic bacteria from attaching to the intestinal wall. It also combines 1,3- and 1,6-D-glucans to stimulate the immune system and increase nutrient digestibility. Eventually, this improves the hen’s productive performance.

Additionally, Bidura et al. (39) found that supplementation with yeast or Saccharomyces strain increased egg production (hen-day production) and the total egg weight in layers. According to Bidura et al. (40), supplementing laying duck diets with 0.20-0.30% Saccharomyces strain increased the egg mass, yolk color, yolk, and eggshell weight and interestingly decreased the cholesterol in egg yolk. In the study of Ezema and Eze (41), supplementation of 1.0 g/kg yeast for layers showed the optimum egg production results. Furthermore, according to Özsoy et al. (42), groups were given 0.1, and 0.2% yeast obtained the most significant egg weights compared to the control group (Table 4). In the same study, fatty acids, including myristoleic (C14:1) and hexadecenoic acid (C16:0) concentrations, were higher in the 0.1 and 0.2% yeast-supplemented groups than in the control group (43,44).

Table 4: The effect of Saccharomyces cerevisiae on the Haugh unit

According to Liu et al. (45), the improvement of egg quality, hatching rate, and increased health of chicks might be related to enhanced egg quality and eggshell, which prevented the harmful bacteria from accessing the eggs. An increase in yolk pigment was also recorded when Saccharomyces cerevisiae was added to the diet. This was due to the presence of carotenoids in yeast that enhanced the color of the egg yolk (46). Lu et al. (47) also recorded that supplementation with yeast improved immunoglobulin A (IgA) in egg yolks. Thus, improving the chick’s immune system and improving the performance of old layers. The older the chicken is, the fewer eggs it can lay. Duan et al. (48) also confirmed that older layers reduce egg production because absorption and digestibility decreased as their age increased.

Therefore, supplementation with yeast was used for this purpose because yeast contains enzymes and organic acids (45). Moreover, the use of Saccharomyces cerevisiae also affected the amount of cholesterol in eggs. The effect of yeast on the egg yolk cholesterol level was shown in the study of Yalcin et al. (14). There was a significant reduction in egg yolk cholesterol in the groups fed 1 and 2 g/kg Saccharomyces cerevisiae: yolk egg cholesterol was 14.58 mg/g yolk and 14.54 mg/g yolk, respectively. These values were lower and better than those without yeast 16.78 mg/g. Supplementation with Saccharomyces cerevisiae yeast or its products reduced the cholesterol levels in egg yolks. This can be explained by how yolk cholesterol was reduced by reducing the absorption and synthesis of cholesterol in the gastrointestinal tract. Ezema and Eza (41) showed that Saccharomyces cerevisiaein diets reduced the egg’s cholesterol content. Supplementation with 0.6 g, 0.8 g, and 1.0 g of S. cerevisiae resulted in the following cholesterol contents: 510.60 mg/dl, 595.70 mg/dl, and 476.60 mg/dl, respectively. Compared to the non-yeast treatment, which resulted in a cholesterol level of 824.50 mg/dl. Moreover, the addition of yeast autolysate at a concentration of 4 mg/kg^-1 resulted in the reduction of 14.2 mg g^-1 yolk cholesterol compared to the control treatment without yeast autolysate 16.4 mg g^-1 yolk. Yalcin et al. (49) also suggested that Saccharomyces strains remove cholesterol.

The application of baker’s yeast in fermented feed

Maicas (50) demonstrated that traditional fermentation techniques for wines, beers, and fermented food are facilitated mainly by Saccharomyces cerevisiae strains, the most prevalent and commercially available yeast. Yeasts are also critical in treating wastewater and the creation of biofuels. From a biological standpoint, fermentation occurs when the pyruvate created during glucose metabolism is broken down into ethanol and carbon dioxide by yeasts (and some bacteria).

Notably, yeast or baker’s yeast is used for diet directly and fermenting agricultural products useful for animal nutrition. Baker's yeast itself contains a high nutrient value for animal diets. Furthermore, baker's yeast plays a vital role in the fermentation process of agricultural products or byproducts to improve the nutrient values of those products through the abovementioned process. The application of baker’s yeast (S. cerevisiae) on carcass traits was also recorded and had numerous positive influences on carcass weight, carcass percentage, and carcass composition. According to Linh et al. (11), after fermentation, filtrate from yeast-fermented rice contains a highly nutritious byproduct that helps local chickens improve their growth performance and carcass characteristics. It notably improved 20% over the diet without aqueous yeast-fermented rice. This was due to the fermentation process improving nutrients from aqueous and solid fermented products. In the same study, the carcass weights of crossbred chickens increased with increasing levels of yeast-fermented rice in the diets. In particular, the treatment with 4% aqueous yeast-fermented rice increased the chickens' carcass, thigh, and breast weights. When using S. cerevisiae to ferment brown rice, Christ-Ribeiro et al. (8) recorded an increase in protein, ash, and fiber content and decreased carbohydrate and lipid content. Azrinnahar et al. (9) also recorded the improvement of feed conversion ratio and body weight gain of broilers fed with fermented feed, with a maximum 10.18% increase in supplemented groups compared to the control group. Yeast secretes enzymes during the fermentation process, creating nutrients available for bird absorption.

Additionally, some secretions from the yeast cell wall, such as mannan-oligosaccharides and fructooligosaccharides, can prevent the action of intestinal pathogens. The experiment from Sun et al. (10) with different yeasts yielded the same results while using baker’s yeast to ferment the cottonseed meal. The feed mixture increased the number of beneficial bacteria, which improved the broiler's gut health and eventually improved the growth performance. According to Kang et al. (13), fermented rice bran has a positive effect on the growth performance of broilers. A significant effect on white blood cells and lymphocytes was also recorded. Based on the results, Kang et al. (13) suggested that the effect might be due to the composition of fermented rice bran, which included more than 120 antioxidants, vitamins, proteins, minerals, carbohydrates, and essential fatty acids.

Bloodparameters

According to Kang et al. (13), hematological parameters are significant markers of an animal's pathological, physiological, and nutritional response. The changes in these parameters can interpret the impact on the animal performance of nutritional components and additives delivered in the diet. Yeast, which is utilized as a probiotic and prebiotic in poultry diets, acts as a stimulator of bile acid secretion. Yeast is used to recovering bile acid, resulting in more cholesterol as the precursor of the bile acid. This aids in the reduction of blood serum cholesterol levels (9). In some research, it is the notable reason for reducing cholesterol (29). The effect of baker’s yeast (S. cerevisiae) on blood parameters has been shown in previous studies through cholesterol, albumin, and hemoglobin concentrations. The most significant effect of baker’s yeast (S. cerevisiae) is reducing cholesterol in plasma and increasing albumin. However, the reduction effects are not similar. This is due to many internal and external factors, such as poultry breed, baker's yeast, and environment. It is possible that giving live yeast to broilers will help improve their lipid metabolism. Ahmed et al. (30) also recorded that the variable results could be attributed to various factors, such as the dose or kind of yeast used, the experimental settings, the primary food, or the breed of poultry utilized. According to the findings of He et al. (6), Live yeast prevented cholesterol oxidation, resulting in decreased lipid deposition in blood vessels. Thus, his study proposed that live yeast might have anti-cholesteric influences on chickens. According to Bolacali and Irak (2), feeding baker's yeast increased the amount of albumin in quail blood. The presence of high albumin levels in male quail may be correlated with its beneficial effects on protein metabolism, and the presence of a high protein nutrient in the autolysate may indicate his finding.

Mulatu et al. (29) showed that normal hematological parameters could be used to determine an animal's nutritional status. Furthermore, Al-Nasrawi et al. (5) reported that various biochemical features of experimental birds suggested a considerable reduction in cholesterol levels in the blood compared to the control without yeast. The presence of glucan rolls in the diet, and more specifically in the cell walls of yeasts, might be responsible for lowering cholesterol levels. As a result of interaction with bile salt acids in the intestinal tract, cholesterol is produced. This stimulates the production of cholesterol (5). The fermentation process for glucan fiber occurs within the intestinal tract, allowing for the production of short-chain fatty acids such as propionate, acetate, and butyrate, which are then taken into the liver via the portal vein.

Furthermore, this fiber remains in the gut, resulting in reduced sugar absorption, followed by a decrease in insulin levels in the blood. This will further result in a decrease in the production of cholesterol. In addition, undissolved fibers result in the decreased absorption of fats, including cholesterol, through the viscous intestines, further lowering cholesterol levels (5).

Use of baker’s yeast against disease and promote immunity

Various scientific studies on yeast-based diets have consistently shown that the immune system is greatly improved. Baker's yeast improves poultry health by improving the immune system's response and preventing the entry of poultry diseases. Several previous studies have studied the effect of baker's yeast on the immune response. The microbial population that resides within the poultry digestive system is essential for gut homeostasis and various physiological activities within the bird and the bird's overall health and well-being. They are crucial in the digestion of nutrients, the inhibition of pathogens, and the interaction of the gut-associated immune system (51). As concluded by Wang et al. (37), the addition of 0.5 g/kg yeast (S. cerevisiae, 1.0 £1010 CFU/g feed) could be used to improve gut morphology. Thus, it is possible to improve poultry health. It is widely known that mannan-oligosaccharides produced by yeast, S. cerevisiae, effectively prevent pathogenic bacteria from adhering to the intestinal wall and stimulate the immune system when combined with 1,3 and 1,6-D-glucans in yeast (38).

According to Suarez and Guevara (23), the addition of S. cerevisiae in meals, which may contain some toxins, could help prevent severe organ damage caused by these toxins. The adsorption to S. cerevisiae cell walls and the changes in the composition of the blood membrane after patulin exposure all contributed to the mycotoxin-removing effect (21,52,53). S. cerevisiae was chosen for its potential to alleviate animal stress while also delivering vitamins, enzymes, and proteins. In addition, Delgado et al. (54) have shown that the use of S. cerevisiae as a nutritional additive also has a prebiotic effect that helps to minimize the occurrence of diarrhea. When diarrhea occurs, the duration of the diarrhea is reduced. The inflammation caused by Salmonella lipopolysaccharide was alleviated (55) after the addition of 0.5 g/kg diet yeast S. cerevisiae (1.0 £ 1010 CFU/g feed), and the performance of E. coli lipopolysaccharide was improved (37) by using 0.5 g/kg yeast (S. cerevisiae, 1.0 £1010 CFU/g feed). Additionally, it has been observed that yeast can help control the spread of undesirable intestinal infections in broiler chickens by providing competitive binding sites to pathogenic bacteria (26). Moreover, the reduction of NF-κB expression and ileal toll-like receptor four was recorded in the study of Wang et al. (56) when using 0.5 g/kg live yeast (S. cerevisiae) in the diet. Kiarie et al. (7) showed that in broilers subjected to coccidia vaccination, supplementation with 5 g/kg yeast S. cerevisiae autolysate in the diet increased the secretion of IgA from the intestinal mucosa, as well as the cell-mediated and humoral immune responses. Therefore, oocyst shedding was decreased. Using S. cerevisiae in broiler diets at concentrations of 0.5 and 1.0 g/kg, He et al. (6) reported that yeast (S. cerevisiae) could be used as an alternative to antibiotics in broiler chickens to improve serum antioxidant capacity, intestinal morphology, and immune function. Yeast contains mannan-oligosaccharides and β−1,3/1,6-D-glucan, which are considered the molecules responsible for animal immune stimulation (6). Its function in improving immunoglobulin production in poultry was also investigated (57). Thus, yeast has the function of improving the immune system and its response. He et al. (6) concluded that adding 1000 mg/kg yeast to the diet improved the IgM concentration for Arbor Acres male broilers at 21 days of age and improved the IgG concentration at 42 days of age. In the research of Sun et al. (10), serum IgG of Arbor Acres broilers for both the ages of 21 days and 42 days was increased when supplemented with baker’s yeast.

Conclusion

As a potential feed additive, baker's yeast will play a crucial role in poultry production by enhancing poultry performance through increased nutrient absorption and digestibility. Poultry health will also be elevated through the improved immune response and normal blood parameters. Applying baker's yeast in a diet could improve both the production and reproduction aspects. Baker yeast could be supplemented in the diet directly or by using it to ferment agricultural products that will further increase the nutrient composition of those ingredients.

Acknowledgment

We appreciate Tra Vinh University for supporting us in this review. We also thank Mr. Mark Anthony Mercado from the Philippines for his valuable help checking grammar for this manuscript.