The use of this organic binder is common to counteract the effects of mycotoxins and improve health and welfare in animal production
In recent years, a wide variety of feed additives for animal nutrition have been developed with different properties and mechanisms of action, in order to reduce mycotoxins exposure from post-harvest contaminated food or feed. Among these additives, mycotoxins binders are included. Mycotoxins binders are used to adsorb toxins, forming complexes that are subsequently excreted through the feces; thereby preventing and reducing the absorption of mycotoxins through the gastrointestinal tract (Kolawole et.al, 2022). A mycotoxin binder possesses a variety of characteristics that, when combined, interfere with the effects of mycotoxins without causing nutritional imbalances or toxicity in animals. With this, the mycotoxins binder is capable of: neutralizing the action of toxins and mutagens, filtering toxins while preserving useful elements (such as sugars and amino acids), and also having a positive effect on the intestinal microbiota (Solovyov et.al, 2020). Clay minerals, polymers, yeast cell walls, and agricultural waste products are commonly used as mycotoxin binders. Yeast cell walls are one of the main organic binders utilized to mitigate the toxic effects of mycotoxins (Kolawole et.al, 2022).
Yeast and its derivate products posses a variety of actions, including antioxidant, immunological, antitumor and antimicrobial properties. Additionally, the components of yeast contribute to improve productive performance and nutrients digestibility in the animals. Saccharomyces cerevisiae is the most prominent yeast species which has been utilized for centuries in various food industry processes, from beer to bread manufacturing. Recently, yeast and its derivatives have been used as a complementary feed additive in animal production, providing benefits for health and welfare (Broadway, Carroll, & Burdick, 2015) .
The yeast cell wall (Saccharomyces cerevisiae) is composed of lipids, proteins, chitins and polysaccharides such as glucan and mannan (Kolawole, Siri, Petchkongkaw, Meneely, & Elliott, 2022) . This wall is made up of layers: The outer layer consists of a smooth membrane and below it lies the mannan-protein complex. The inner layer is comprised of a complex of amorphous mannans β-glucans (β(1,3) and β(1,6)-glucans) and proteins that are interconnected through a network of glucan-integrated microfibrils. Polysaccharides are the main component of the yeast wall, constituting approximately 90% of the cell wall of Saccharomyces cerevisiae (Solovyov et.al, 2020).
Within the polysaccharides, there is glucan, which is a branched glucose polymer located adjacent to the plasma membrane. This polymer is the most important structural component as its removal leads to the destruction of the yeast. On the other hand, another common polysaccharide is mannan, which is not essential for the integrity of the cell wall and is located on the exterior of the yeast. The final component is chitin, which is a polymer of N- acetylglucosamine found in areas associated with budding scars (the process by which yeast forms daughter cells through cloning). Only 10% of the cell wall mass consists of proteins, and several of them are enzymes (mostly mannanoproteins, with mannan being their constituent molecule) (Solovyov et al., 2020).
The polysaccharide coating of yeast has a high adsorption capacity for high molecular weight mycotoxins (such as T-2 toxin, Ochratoxin A, Zearalenone, and Deoxynivalenol), while it has limited adsorption of low molecular weight mycotoxins (such as Fumonisins and Aflatoxin B1). Yeast cell walls are widely used as enterosorbents in animal feed, and due to their selectivity compared to inorganic enterosorbents, they do not cause long-term complications and do not result in the loss of essential substances from the chyme (Solovyov et al., 2020).
It is suggested that the cellular components of the yeast cell wall use non-covalent hydrogen bonds and hydrophobic or ionic interactions to adsorb mycotoxins. It should be noted that the adsorption capacity of yeast or its derivative products depends on their origin, strain, pH, surface area, binding sites, growth conditions, and the concentration of cell wall components (such as mannanoproteins, lipids, chitin, and β-glucan) (Kolawole et al., 2022).
Different yeast species possess varying adsorption capacities, which are not solely determined by their intrinsic characteristics but also depend on the specific type of mycotoxin present. Saccharomyces cerevisiae is a yeast that has demonstrated effectiveness against ochratoxin A, zearalenone, and deoxynivalenol under laboratory conditions. Other yeasts have shown different affinities for various mycotoxins. Lachancea thermotolerans significantly reduces OTA in vitro, as do yeasts such as Hanseniaspora uvarum, Pichia anomala, and P. Kluyveri. Among the mentioned yeasts, P. anomala decreases the biosynthesis of aflatoxin B1 produced by Penicillium flavus. Based on the aforementioned information, Saccharomyces cerevisiae is undoubtedly the most commonly selected option for mycotoxins control (Pfliegler, Pusztahelyi, & Pócsi, 2015).
Through the use of different yeast species, the industry selects genetically suitable strains to fulfill the adsorption function. Therefore, there is a meticulous process of genetic design and cultivation to ensure that the yeast has a cell wall with a wide range of affinity for certain groups of mycotoxins (Martin, Lagorce, & François, 2020). In addition to this genetic factor, environmental conditions, particularly pH, play a role. The functionality of the yeast cell wall is effective at a pH close to neutral, unlike in an alkaline environment. Acidic conditions (found in the digestive tract) are conducive to the function of glucans, which is one of the reasons why yeast or its derivatives are considered for addition to feed additives (Piotrowska, 2021). Several authors state that the amount of cell wall is positively related to wall thickness and cell diameter, so as the amount of cell wall increases, the adsorption of mycotoxins improves (Pfliegler, Pusztahelyi, & Pócsi, 2015). Along with the cell wall structure, the organization of glucan (α-d-glucan and β-d-glucan) plays an essential role in adsorption and modulates the binding strength of the cell wall-mycotoxin complex (Devreese, De Backer, & Croubels, 2013). Additionally, these polysaccharides are directly related to immune cells and bind to pathogens to prevent their adhesion to the gastrointestinal tract (Broadway, Carroll, & Burdick, 2015). Recent studies assert that the successful action of the adsorbent with the mycotoxin does not depend on the use of whole yeast but rather on the components included within the cell wall. It is suggested that by preserving the intact yeast cell wall or the non-viable cell, the adsorption capacity is more effective (Pfliegler, Pusztahelyi, & Pócsi, 2015). Finally, it has been demonstrated that β-d- glucan is the fraction directly involved in the sequestration of zearalenone, and based on other in vitro assays, glucomannans effectively bind to mycotoxins such as DON, T-2 toxins, zearalenone, and ochratoxin (Devreese, De Backer, & Croubels, 2013).
Selectivity is one of the several enriching properties possessed by yeast cell wall. Many mineral-based mycotoxin binders have low selectivity for mycotoxins and adsorb macro and micronutrients, complicating animal nutrition and health. In contrast, yeast and its derived products not only provide protective support against mycotoxins but also contribute a high nutritional content to the diet. As an ecological advantage, yeast cell wall is biodegradable, so mycotoxin-binder complexes do not accumulate in the environment through feces, as is the case with inorganic binders. Additionally, yeast and its derived products have a broad spectrum of action against mycotoxins (DON, OTA, ZEN, and AFB1), unlike inorganic binders (Xu et al., 2022).
In conclusion, yeast and its derived products prove to be a feed additive for animal nutrition with a highly selective mechanism of action against mycotoxins. The effect generated by this type of binder not only stands out for its broad spectrum of action but also for the vast nutritional content that promotes intestinal health and, consequently, animal performance. There is no doubt that yeast cell wall is an excellent choice with a detoxifying effect once it enters the animal and when it is excreted (biodegradable).
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