In summary for yeast to survive it needs:
• Moisture
• Food
• Warmth
• Time
The salt in the dough controls the yeast activity.
Yeast is a microscopic, single cell organism classified with fungi. The variety of yeast used in Bread making is Saccharomyces cerevisiae. The yeast cell is roughly oval in shape measuring 7-14 microns in length; one gram of yeast contains over 15,000,000,000 individual cells!
The yeast cell wall is double layered and is permeable to certain water soluble substances, in this way food materials (as discussed in starch & enzymic action) may be absorbed by the cell. Once yeast has the presence of warmth combined with a soluble food source, the cells will grow and multiply, the by product of this growth is – CO² (Carbon Dioxide). This process of growth is known as osmosis.
(Carbon dioxide given off provides structure to the dough by expanding it, giving it a spongy like texture.)
Traditionally yeast has three principle functions in a dough:
• It produces carbon dioxide gas which expands the dough to the required volume and gives it the light sponge-like texture necessary to produce bread with good eating qualities
• It matures or develops the dough through the action of fermentation on the gluten structure
• It provides flavour through the production of complex chemical compounds as by-products of the fermentation process
Effects on yeast
It would be worth noting the various influences on yeast in the fermentation process. We have already talked about food being produced for the yeast through the addition of water – to take the possible food source into solution, and the enzymatic action that breaks down the damaged starch cells to provide the main source of food.
Factors:
• The amount of moisture in the dough, if there is insufficient then the amount of available food may be restricted
• The temperature of the dough is vital, too cold and the yeast is retarded, too hot and the yeast is killed (Gas production is possible up to 40ºC, at 60ºC the yeast is destroyed)
• The amount of available food. If there is little food present, the yeast will simply ‘burnout’
• pH. Yeast fermentation rate is stable over a PH range of 4.4 to 6. acidic Outside this range the gassing rate decreases (ingredients such as Bunspice may play a role here)