A baker's yeast preparation, through the electron scanning microscope, reveals a multitude of distinctive cells (Fig. 1), ovoid in shape. Some of them have buds or the scars that they have left. They vary in size from 6 to 8 Am. One cubic centimeter of compressed yeast with a 30% dry matter content contains about 10 billion cells. Transmission electron microscopy reveals the following ultrastructure (Fig. 2), from the outside towards the inside:
The cell wall. Composed of glucans and mannans bound to proteins, it provides physical external protection. It is completely permeable to water, minerals, and small organic molecules. The cytoplasmic membrane is made up of glycolipids and glycoproteins. It controls the exchanges between the intracellular and extracellular media. It is characterized by a selective permeability: it allows water and some solutions to circulate while retaining the large molecules. The cytoplasm is a colloidal substance in which a multitude of biochemical reactions take place and which contains organelles in suspension:
The nucleus contains the chromosomes (carriers of genetic information), which transmit hereditary characters and control the synthesis of proteins. The ribosomes, sites of protein synthesis
The endoplasmic reticulum and Golgi bodies, a network of membranes involved in the secretion of proteins.
The mitochondria, energy-producing bodies in the cell in the presence of oxygen
The vacuoles, places where different storage substances are found.
The composition of yeast depends on its type and the conditions in which it is stored. Table 2 gives average indicative values for samples of fresh European yeast.
The proteins contribute to a considerable degree of potential metabolic activity because they are made up of a high proportion of enzymes. The protein content is therefore directly related to fermenting power and the ability to produce biomass (see Sec. IV. C and Sec. V.B.).
The carbohydrates are mainly:
Glucans and mannans, wall constituents;
Glycogen, a storage macromolecule usually found in animal cells, which is used when there is a long-term deficiency of nutrients; Trehalose, a disaccharide which is called up preferentially when there is a short-term deficiency. The storage of this sugar is very important whenever the yeast cell has to undergo stress, such as drying, high osmotic pressure, or freezing.
The lipids, particularly lipoproteins and phospholipids, are involved in the makeup of the cytoplasmic membrane and in maintaining its properties in the various processes used for drying active yeast.
Percent 30.0 to 33.0
Proteins/DM (nitrogen x 6.25) Carbohydrates/DM
6.5 to 9.30 40.6 to 58.0 of which glutathione 35.0 to 45.0 of which glycogen trehalose
4.0 to 6.0 of which phospholipids 5.0 to 7.5 of which potassium
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