In spite of its inert aspect, a block of yeast is, in reality, formed from a gigantic number of unicellular organisms visible only under a microscope. A small 1cm cube weighs about 1g and contains 10 billion living cells of yeast!
Each cell, which is a living being, of a spherical or ovoid form,
is nothing but a tiny and simplified fungus the size of which does not exceed 6 to 8 thousandth of millimetre.
When yeast is examined under an electronic microscope, one can see from the outside to the inside, like most vegetal cells,
a cellular wall, a cytoplasmic membrane, a cytoplasm, a nucleus, vacuoles, ribosomes and mitochondria.
The cytoplasmic membrane protected by the cellular wall, permits exchanges with exterior. The cytoplasm, a sort of jelly, forms
the very substrate of the cell life. The nucleus, which contains chromosomes (elements which bear genetic specifications), regulates the transmission of hereditary characteristics and
the greatest part of the reactions produced inside
the cell. Vacuoles store reserve substances of various types.
Mitochondria are the real power stations of the cell when the latter operates in presence of oxygen. Their role consists in using the sugars available in yeast to produce energy and thus ensure the growth of the cell.
Yeast composition depends on its type and its shell-life conditions. The table below presents average indicative values for fresh yeast picked up on the European market.
Yeast, like any living organism, lives thanks to the presence of oxygen (aerobiosis); but it also has the remarkable ability of being adaptable to an environment deprived of air (anaerobiosis).
To cope with its expenditure of energy, it can use different carbon substrates, mainly sugars: glucose is the best favoured food of Saccharomyces cerevisiae; saccharose is immediately transformed into glucose and fructose by an enzyme which yeast has released; maltose is the main endogenous substrate of French bread fermentation; it gets into the yeast cell thanks to a specific permease to be split afterwards into two molecules of glucose by maltase.
The conditions of oxygenation of the environment generate two types of metabolism:
Then yeast is in presence of air, it produces, from sugar and oxygen, carbon dioxide, water and a great amount of
It is the metabolic process
In these conditions the oxidation
of glucose is complete:
All the biochemical energy potentially contained in glucose is freed. Thanks to this energy, yeast ensures its life. But it can also use it to synthesize organically, that is to say start its growth and multiply. It will then have to find other nutritive elements in its environment, mainly nitrogen.
When there is no oxygen available, yeast can nevertheless use sugars to produce the
energy it needs to be maintained in life. This metabolic process was defined by Pasteur as being the fermentation process. Sugars are transformed into carbon dioxide and
The glucose oxidation is incomplete:
The alcohol which has been formed still contains a great amount of energy. Only a part of the biochemical energy potentially present in glucose which was freed (about 20 times less than for respiration). It ensures a minimum level but does not enable yeast to multiply rapidly. The oxygen which is incorporated into the dough during the kneading stage is consumed in a few minutes by yeast. It is therefore the fermentation metabolic process which is implied in the bread making process. It is accompanied by the production of secondary metabolites, some of which react on the physical properties of dough, while some others give bread its characteristic flavour.