General characteristics of yeast
Sugar present in the wort undergoes fermentation with the participation of yeast, which are single-celled microorganisms belonging to the class Ascomycetes. Yeast is a non-taxonomic group of single-celled fungi that have lost their mycelial structure due to their transition to a lifestyle in liquid and semi-liquid substrates rich in organic matter. Yeast cells can take on various shapes, such as oval, elliptical, or elongated, and their shape and length (ranging from 6 to 11 micrometers) depend on the specific yeast species and the conditions of their development. The ratio of cell mass to cell volume influences the rate of metabolic exchange between the cell and the surrounding environment, which in turn affects yeast’s metabolic activity.
The appearance of yeast cells can serve as an indicator of their physiological state. In industrial conditions, young, mature, budding, old, and dead cells can coexist simultaneously. Mature cells typically possess the highest fermentative energy.
A yeast cell consists of a membrane, cytoplasm, and nucleus. The outer part of the membrane is composed of polysaccharides such as mannans and a small amount of chitin, while the inner part consists of proteins, phospholipids, and lipoids. The membrane regulates the cell’s content and has selective permeability, distinguishing it significantly from ordinary semi-permeable membranes. The thickness of yeast cell walls can be up to 400 nm.
Forms of yeast cells:
The cytoplasmic membrane has a thickness of 7-8 nm and is located beneath the cell wall, separating it from the cytoplasm. The plasma membrane is the primary barrier that regulates the osmotic pressure within the cell, enabling the selective movement of nutrients from the environment into the cell and the expulsion of metabolites from the cell. The plasma membrane consists of a bilayer of lipids that includes protein molecules. The lipids are oriented with non-polar ends facing inward toward each other and polar ends facing outward.The movement of substances across the cytoplasmic membrane occurs through molecular diffusion (along a concentration gradient) and as a result of active transport involving specific enzymes. In the case of active transport, substances can enter the cell against a concentration gradient. For example, amino acids can easily penetrate the cell from the environment, even when their concentration in the cytoplasm is 100-200 times higher than in the nutrient medium.
Appearance of yeast cells under a microscope
The cytoplasm has a heterogeneous structure and a viscous consistency. Its colloidal nature is due to protein substances. In addition to proteins, the cytoplasm contains ribonucleoproteins, lipoids, carbohydrates, and a significant amount of water. The cytoplasm of young cells appears homogeneous externally. As cells age, vacuoles, uniform granularity, fat, and lipoid granules appear. Important enzymatic processes take place within the cytoplasm and its organelles (chondriosomes, microsomes, vacuoles) and inclusions.
Mitochondria (chondriosomes) can have a granular, rod-like, or filamentous shape. Mitochondrial membranes consist of proteins (80%) and lipids (20%).
Polyphosphates, ribonucleic acid (RNA), and deoxyribonucleic acid (DNA) are also present in mitochondria. Mitochondria replicate independently, replicating their mitochondrial DNA and producing their own proteins. Nutrient cells that enter the cell are adsorbed and accumulate in chondriosomes, undergoing rapid transformation due to the concentration of the corresponding enzymes in these areas. Mitochondria are responsible for the complete tricarboxylic acid cycle and the most crucial energy reaction, oxidative phosphorylation. Therefore, they are considered the cell’s primary “powerhouse.” Reactions for activating amino acids in the synthesis of proteins, lipids, and other compounds also occur here.
Microsomes (ribosomes) are inclusions in the form of submicroscopic granules composed of lipids, proteins, and ribonucleic acids (RNA), which facilitate the synthesis of proteins from activated amino acids received from the mitochondrial system.
The nucleus is a small spherical or oval body surrounded by cytoplasm and insoluble in it. Nuclear structures are separated in the form of inclusions of deoxyribonucleic acid (DNA) and its protein (histone) in large amounts of RNA. DNA contributes to the transmission of genetic information and the preservation of the microorganisms’ properties. Transcription (the synthesis of informational RNA molecules by reading information from DNA using RNA polymerase) occurs in the nucleus, as well as DNA replication during cell division.
An essential organelle of the cell is the vacuole – cavities filled with cell sap and separated from the cytoplasm by a vacuolar membrane. The shape of vacuoles changes due to the movement and concentration of cytoplasm. In young cells, vacuoles consist of numerous small cavities, while in old cells, they become one very large vacuole. Cell sap is a watery solution of various salts, carbohydrates, proteins, fats, and enzymes. Various compounds that undergo enzymatic transformations, as well as waste products of life, are concentrated in vacuoles.
In young yeast cells, there is usually no fat, while in mature cells, it is present only in a few cells in the form of small droplets, and in old cells, in the form of large droplets.
Glycogen is a reserve nutrient of yeast, accumulated when yeast is cultured in sugar-rich environments and quickly consumed when sugar is scarce. In young cells, there is little glycogen, while in mature cells, there is a significant amount (up to 40%).
Requirements for yeast used in alcohol production include: high fermentative energy (yeast must rapidly and completely ferment sugars), anaerobic respiration, resistance to the by-products of their metabolism and the by-products of other microorganisms, as well as changes in the composition of the environment. They should also be capable of tolerating high concentrations of salts and dry matter in the wort, and the ability to fully ferment raffinose when processing molasses. If yeast is harvested from mature mash and used for baking, they must possess high regenerative capacity and meet the requirements for baking yeast in terms of storage stability, leavening power, freeze-thaw stability, and maltase activity.
Increasing the fermentative activity of yeast can be achieved through various methods, including mutagenesis, hybridization, and more. Hybridization is the most promising method for obtaining yeast with desired properties because when two parental yeast species are crossed, hybrid yeast strains with pre-desired properties can be selected.
