The yeast Saccharomyces cerevisiae, often recognized as baker’s or brewer’s yeast, functions as a microscopic factory, converting simple sugars into a vast array of molecules. These molecules serve purposes deeply intertwined with human industry, health, and nutrition. For millennia, humans have leveraged the unique biochemical pathways of yeast to produce food and beverages. The molecules produced by this single-celled fungus range from small organic compounds that define flavor to large structural polysaccharides that modulate the human immune system.
Molecules of Fermentation
The most widely known molecules produced by yeast result from anaerobic metabolism, a process called alcoholic fermentation. When yeast consumes sugars in an oxygen-deprived environment, it executes a two-step biochemical conversion to generate cellular energy. The final products are ethanol and carbon dioxide (CO2), which have formed the foundation of the brewing, winemaking, and baking industries for centuries.
The process begins with the breakdown of glucose into pyruvate. Pyruvate is then converted into acetaldehyde, releasing carbon dioxide. Finally, acetaldehyde is reduced to ethanol, regenerating the coenzyme needed to sustain the initial breakdown of glucose.
In the beer and wine industries, ethanol is the desired product, while carbon dioxide is responsible for the characteristic carbonation. In bread making, the gaseous carbon dioxide causes the dough to rise and gives baked goods their light, airy texture. The ethanol produced largely evaporates during baking. Fermentation also yields secondary metabolites, such as higher alcohols and esters, which contribute significantly to the unique aroma and flavor profiles of fermented foods and beverages.
Essential Nutritional Compounds
When consumed directly as nutritional or brewer’s yeast, the cell is a concentration of beneficial compounds. Yeast biomass is rich in high-quality protein, constituting 35% to 60% of its dry mass. This protein is considered complete because it contains all nine essential amino acids required by the human diet, making it a valuable non-animal protein source.
Yeast is also a source of B-complex vitamins, which are water-soluble compounds acting as cofactors in numerous metabolic reactions. These B vitamins include thiamine (B1), riboflavin (B2), niacin (B3), and pyridoxine (B6), all important for energy metabolism and nervous system function. While yeast synthesizes most B vitamins, certain strains are often fortified with cobalamin (B12) to meet the needs of diets where B12 is deficient.
Ergosterol, a sterol found in the cell membranes of fungi, is the biological precursor to Vitamin D2 (ergocalciferol). When yeast biomass is exposed to ultraviolet (UV) light, ergosterol converts into Vitamin D2, which is preserved in the inactive yeast product. This process makes UV-irradiated yeast a unique, non-animal source of this fat-soluble vitamin, necessary for calcium absorption and bone health.
Immune Signaling Molecules
The yeast cell wall yields large, non-digestible molecules that interact directly with the human immune system. The most significant of these are the Beta-glucans, specifically the \(beta\)-1,3/1,6-glucans, which form the structural backbone of the yeast cell wall. These polysaccharides are recognized by the innate immune system as microbe-associated molecular patterns (MAMPs) because they are not produced by the human body.
Beta-glucans interact with Pattern Recognition Receptors (PRRs) on the surface of immune cells, such as macrophages and dendritic cells. This binding initiates a complex intracellular signaling cascade, which can lead to the production of pro-inflammatory cytokines and the modulation of immune responses. The effect is one of signaling, where the molecule primes or “trains” the immune cells for a more effective response to subsequent challenges.
These large molecules are not broken down for energy but are taken up by specialized immune cells in the gut, which then transport them to various immune organs like the spleen. The \(beta\)-glucans are then fragmented into smaller, soluble particles that continue to interact with and modulate the function of other immune cells. The yeast cell wall also contains mannoproteins, or mannans, which engage with different PRRs, further contributing to the immune-modulating properties of whole yeast cell products.