Humans had used yeasts since ancient Egypt when people use them to leaven bread and ferment wine. Yeasts are valuable nutrients for animals due to their exceptional fermentative and nutritional properties. Various yeast species and their derivatives have been used in animal feed for more than 100 years.
Yeasts can stabilize the animal’s intestinal flora and reinforce immune modulation; it positively benefits animal health and well-being. Recently, their use has been increasingly sought on the market for the continuously growing demand for natural alternatives to treatments (e.g., antibiotics).
Consequently, the use of yeasts as feed ingredients has increased mainly in the last few decades.
Various types of yeast-derived feed additives in different forms have flooded the market. What differentiates them? What can they contribute to animal nutrition and health?
The main types of yeast derivatives products used in animal nutrition are:
Live yeasts as probiotics
Live yeasts are also known as active dry yeasts; they are generally added to animal feeds due to their probiotic effects. Probiotics are referred to live microorganisms, which confer a health benefit to the host when administered in adequate amounts. (World Health Organization).
Not all live yeasts are the same: specific yeast strains will have unique effects on targeted species.
One of the challenges of manufacturing probiotic yeast is to confirm it remains alive and active throughout the entire production process and through further feed processing and storage until it reaches the animal’s digestive tract, where the products are functional.
Whole-cell inactivated yeasts
The yeast biomass will be treated explicitly to no longer be alive after fermentation, thus avoiding further cell multiplication. In animal nutrition, most of these yeast products are typically from secondary fermentation, often obtained from the brewing industry and ethanol production. Inactivated yeasts from primary-grown fermentation are mainly applied for high-value food or biotech applications.
Inactivated yeasts are typically used as flavor enhancers in food manufacturing. At the same time, they have abundant proteins and B vitamins. There are lots of nutrients located in the cytoplasm of the yeast cell. The enzyme must break down yeast cells to release these nutrients so that the animal can absorb them.
Therefore, the bioavailability of whole-cell inactivated yeasts is less than more processed yeasts, like hydrolyzed yeasts and yeast extracts. The yeast strain and the production process (e.g., the drying conditions) can influence the end-product characteristics, structure, composition, and efficacy.
According to the inactivation process used (thermal, mechanical, chemical, etc.), there are different inactivated yeasts.
Autolysis is the process of self-digestion, in which the cell is lysis through the action of its own digestive enzymes, also called endogenous enzymes. The whole yeast is fragmented during the production process of autolyzed yeasts, and there is no separation process. The autolysis process is less oriented and controlled than a more advanced lysis process (hydrolysis), which is added exogenous enzymes. The proteins and nucleotides are only partially fragmented.
Applications of Autolyzed yeasts
Autolyzed yeasts are used in the food industry to enhance food flavors, such as can soups and savory snacks. They are also used in animal nutrition to improve feed palatability and help support gut health and digestion.
However, because the production process of autolyzed yeasts is less oriented and controlled (compared to further processed yeasts), this could cause variable product qualities and efficacies.
Hydrolyzed yeasts are made through the process of yeast cell digestion by both endogenous and exogenous enzymes. The selected exogenous enzymes are added during the production process to obtain the desired level of hydrolysis. Proteins and nucleic acids are fragmented into small peptides that provide highly digestible nutrients through an oriented and controlled process.
The strictly controlled process is crucial to confirm product consistency in terms of composition. Hydrolyzed yeasts are consist of the cell wall and the yeast extract as there is no separation step during the process.
Applications of Hydrolyzed yeasts
Hydrolyzed yeasts are essential protein sources for feed additives. Such alternatives are in great demand on the market for different reasons:
- To support the reduction of the environmental pollution of animal production
- To limit the usage of food-grade proteins
- To support the reduction of antibiotics use and improve animal health
Thanks to functional effects, hydrolyzed yeasts offer both nutritional and functional advantages to support the growth and health of young animals.
Due to the addition of specific enzymes during the production process, they deliver highly digestible proteins and are also a source of amino acids and nucleic acids. Indeed, in young animals, the immune system is not fully developed, and they require these highly digestible nutrients to enhance feed intake, gut integrity, and gut function.
The selection of the enzymes and the control of the process conditions will determine the product composition and functions.
These are why feeding hydrolyzed yeasts can ensure the supply of free amino acids and small peptides to maintain proper gut health during challenging situations:
- Providing necessary amino acids to support the gut mucosa mass and integrity.
- Providing the nutrients for gut microbiota development.
- Providing the defenses: the gut contains 70% of the immune cells of the body. Nutrients are crucial to maintaining this vital defense system.
Inactivated enriched yeasts
There is another specific type of inactivated yeasts，which is mineral- or vitamin-enriched yeasts. Yeasts can naturally produce Vitamin D when exposed to B-ultra-violet light like humans. In this process, natural sterols in yeast are converted into vitamin D. Yeasts can also incorporate trace minerals (selenium, chromium, iodine, etc.) within their cells, making them ideal for producing mineral-enriched yeast biomass.
Selenium-enriched yeasts are an essential source of organic selenium used in animal nutrition. S. cerevisiae is a selected strain of yeast. It can utilize inorganic selenium and incorporate it into the yeast proteins within organic seleno-amino acids (such as selenomethionine and selenocysteine). Selenium is added to the fermentation medium during the fermentation process. Proper yeast strain selection and specific process development lead to producing high-quality, concentrated, Se-enriched yeast.
Applications of Selenium Yeast
Selenium (Se) in its organic form has a much higher bioavailability (with benefits in the Se content of the resulting meat, milk, and eggs) and unique effects on metabolism. Organic selenium is a premium choice when it comes to feeding supplements. For example, in human nutrition, Vitamin D-enriched yeast products offer bakers the possibility of making bread a daily vegetarian source of Vitamin D as it can be added to baked products.
Yeast cell walls (YCW)
Yeast cell walls are the insoluble fraction of autolyzed or hydrolyzed yeasts obtained after remove the cytoplasmic content (yeast extract). The YCW composed 30-40% of the dry weight of the yeast cell. It is formed of two layers :
The external layer is rich in mannan oligosaccharides (MOS). MOS represents 20-30% of the YCW, depending on processing conditions. MOS is accountable for flagellated undesirable bacteria binding, parietal exchanges, porosity/permeability, and, more widely, protecting the yeast from the environment. The inner layer, rich in β-1,3-glucans and β-1,6-glucans (representing 20-35% of the YCW), confers rigidity and flexibility to the cell wall. It also contains about 2% chitin of the YCW, which encompasses a major role within the integrity of the cell membrane, affecting the “cohesion” of the wall. The structure and composition of the YCW have a direct influence on their biological properties and potential activity. Each YCW product has its specificities that lead to different functions and applications in animal nutrition. YCW is either issued from primary or secondary fermentation, affecting their properties and mode of action.
Applications of Yeast Cell Wall
YCW is particularly indicated in all animal species to:
- Help maintain digestive health
- Support feed utilization and growth performance
- Help reinforce the immune system and natural defenses
- Adsorb certain mycotoxins
- Support skin health/protection of fish
Yeast extracts (YE) are the water-soluble fraction of the yeast cell. They are mainly obtained from the centrifugation of hydrolyzed yeasts, depending on the post-fermentation process used. YE is used both as a source of nutrients and as a flavor enhancer. Rich in proteins (>60%) and peptides, they also contain a broad range of B-vitamins. About 50% of nitrogen of YE is in the form of free amino acids, notably available as a substrate for beneficial microflora in the gut of animals. YE contains nucleic acids (such as adenine) and nucleotides. They are also rich in minerals and can be used either in dry or liquid form. The type of YE obtained is strongly dependent on the nature of the enzyme(s) cocktail used and process conditions (hydrolysis duration, temperature, etc.).
According to the process, we will obtain a large variety of yeast extract for specific purposes, such as meat or cheese type notes in flavoring, or nucleotide rich fractions, etc. This more complex process makes YE more specific in terms of applications. They are widely used as food flavor enhancers, mainly for their glutamate effect and “umami” taste.
A Source Of Nucleosides And Nucleotides
Nucleotides are an integral part of DNA and RNA. Every organism must create new cells to stay alive through cellular division. Certain cells, like liver cells, can cover their nucleotide building blocks through the process.
However, intestinal mucosal cells, bone marrow cells, leukocytes, erythrocytes, or some brain cells need nucleotides from dietary origin. Moreover, additional demands are placed on new cell development under stressful conditions like health disorders, intensive growth, or breeding. This is also particularly important when the immune system is stimulated, and new cells and proteins(antibodies, etc.) must be produced to respond to a challenge (e.g., a bacterial infection). Nucleotide supplementation helps support these nutritional requirements.
Applications of Yeast Extract
Let’s take the instance of piglets, for which weaning is a significant source of stress. This stress can be expressed as a substantial drop in feed intake, intestinal villi atrophy, reduced capacity to digest/absorb nutrients, and impaired intestinal mucosa integrity. It is also a physiological stage where piglets have high body growth and intense digestive system development with a total cell renewal every five to six days. At the same time, as they are moving from milk to dry feed, this causes a substantial drop in nucleotide intake since milk is naturally rich in nucleotides. The supply in YE, rich in nucleotides, can play a role at various levels by Supporting feed intake due to the palatability effect of free glutamate Supporting the immune response.
Helping improve intestinal morphology by supporting cell multiplication Altogether, these benefits can reduce the risk of digestive disorders and support performance in the post-weaning phase.
Yeasts provide a versatile and valuable source of functional and nutritional ingredients for feed additives. Each yeast product has its specific benefits on animal growth performance and health, especially during challenging periods when animals are reared in suboptimal hygienic conditions or face stressful periods (i.e., weaning, disease challenges, etc.). There are vast amounts of pure yeast products, including live yeasts and yeast derivatives, are available in animal nutrition. Knowing the differences between products is crucial to choose the most appropriate ones regarding their potential roles and desired effects when added to animal feed.
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