Yeast is capable of living with or without oxygen!
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Without air
Under conditions of anaerobiosis (in the absence of air) the sugar is chiefly converted into alcohol to the detriment of the level of energy released. This is the case during bread-making. The yeast is unable to find any additional oxygen and the sugar supplied by the flour is converted into alcohol (which evaporates during baking) and into carbon dioxide, all by-products of the metabolic process known as fermentation.
During the baking process, the bread rises as a result of the production of carbon dioxide. Here too, energy is released but in lower quantities, sufficient to live but insufficient to multiply.
SUGAR --> CO2 + ALCOHOL + LOW ENERGY
With air
Under conditions of aerobiosis (in the presence of air), the yeast breathes and multiplies abundantly with no alcohol formation. The sugar upon which it feeds is converted into carbon dioxide and water. This phenomenon is accompanied by a major release of energy enabling it to grow and multiply through “budding”.
When the two cells are the same size, they separate and the budding process continues.
This metabolic process is referred to as respiration. Yeast producers make use of this process in order to encourage cell multiplication.
SUGAR + OXYGEN --> CO2 + WATER + HIGH ENERGY
