What is yeast?

What is yeast?Yeast is an ovoid or spherical unicellular microscopic fungus. The particularity of yeast is that it is a living organism!

Like those found in humans, yeast cells are living and natural. They need air to multiply, but the absence of air also has consequences on their development.

A living organism
A living organism

Although barely larger than a pinhead, brewer's yeast, or Saccharomyces Cerevisiae, is a major player. Yeast is everywhere: food, healthcare, cosmetics. Zoom onto the infinitely small...

Although it looks inert, this block of yeast consists of a multitude of living organisms, scientifically called "micro-organisms". The yeast cell is egg-shaped and is not visible to the naked eye. Its size in fact does not exceed 6 to 8 thousandths of a millimetre, barely bigger than a pinhead! A 1 cm cube weighs around 1g and it alone contains more than 10 billion living yeast cells!


There are several yeast species. The most well-known is Saccharomyces cerevisiae. However, there are many other types of yeast. Etymologically, "saccharo" comes from sugar, "Myces" from fungus and "cerevisiae" means "brewery" in latin. More commonly saccharomyces cerevisiae are called "brewer's yeasts" and "baker's yeasts" but they may also be called "budding yeast" after their means of reproduction. Yeast often conjures up the world of baking in people's minds. However, by selecting strains and developing multiplication techniques, yeast finds many other applications in such varied activities as food processing, flavouring, pharmaceuticals, animal health, etc.

The composition of a yeast cell
The composition of a yeast cell

Yeast consists of a cell wall surrounding its membrane which protects yeast from external factors.
 Take a close look at this micro-organism!

This wall consists of:

  • an outer layer of mannoprotein, associated with glucanes
  • an inner layer of glucanes associated with chitin
  • a cytoplasmic membrane with high protein complex content.

yeast cell scheme

Yeasts are eukaryotic organisms. The cell nucleus contains 16 linear chromosomes.

The development of yeast
The development of yeast

Yeast is capable of living in the presence or absence of oxygen! Explanations.

Without air

When there is no air, sugar is mainly transformed into alcohol, to the detriment of the energy released. This happens in the case of bread-making. The yeast cannot find any more oxygen. The sugar provided by the flour is transformed into alcohol (this evaporates during baking) and carbon dioxide, proving the metabolic process of fermentation. In baking, this production of carbon dioxide causes the dough to rise. Here again, energy is released, but in low quantity enough to live but not to multiply.


With air

In the presence of air, yeasts breathe and multiply abundantly, without forming alcohol. The sugar they feed off is transformed into carbon dioxide and water. This phenomenon is accompanied by tremendous release of energy to allow them to grow and multiply by budding. When the two cells have reached the same size, they separate and cell budding continues. This metabolic process is called respiration. It is used by yeast manufacturers to multiply the cells.