Yeast biomass and yeast-derived products in broiler nutrition: nutritional value, functional effects, sustainability, and practical limitations

Yeast biomass and yeast-derived products are increasingly considered in broiler nutrition as nutrient ingredients and functional feed components. However, they should not be treated as a single category because whole biomass, hydrolysates, extracts, cell-wall fractions, and yeast culture differ substantially in composition, digestibility, and biological effects. This review evaluates these materials in terms of classification, nutritional value, functional effects, and sustainability. Yeast biomass can provide substantial crude protein and relevant amino acids, but its practical value depends on species, substrate, processing route, and digestible amino acid supply rather than crude protein alone. Processed yeast products may enhance nutrient accessibility or deliver bioactive compounds such as β-glucans, mannans, and nucleotides that can influence gut health, immunity, oxidative status, and, in some cases, growth performance. Nevertheless, responses are inconsistent and strongly product- and context-dependent, with benefits often more evident under challenge or stress conditions than in low-challenge environments. From a sustainability perspective, yeast biomass has potential for side-stream valorization, reduced land-use dependence, and circular bioeconomy integration, but environmental and economic advantages remain conditional on substrate choice, energy input, and process efficiency. Overall, yeast biomass is best regarded as a complementary protein source, whereas many yeast-derived products are more appropriately viewed as targeted functional ingredients.