Yeast is the baking most important ingredient because it is responsible of bread volume, structure, aroma and taste
It produces carbon dioxide
metabolizing the sugar of the dough. The gas evolution causes the dough expansion because it remains trapped in its protein matrix
It causes the dough maturation
producing that light structure associated with leavened products
It gives the bread his characteristic aroma
producing secondary metabolites during the leavening.
The yeast quality is expressed in these terms:
Gassing Power: The good yeast must produce the adequate amount of carbon dioxide for each kind of leavened dough
Preservation: The good yeast must keep its properties during all the shelf life, at a temperature of 0 +10° degrees
Reliability: The good yeast must preserve his characteristics unchanged during time
Physical Features: The good yeast must be white and crumbly
Microbiological purity: The good yeast must be produced by pure strains in controlled hygiene and safety conditions to avoid other microorganisms contamination
In Egypt, during the time of the first Pharaohs, it was discovered by chance that a flour dough left standing for some time, expanded his volume becoming softer and more airy.
After cooking the bread taste and aroma were much better. This phenomenon, mysterious at this time, the leavening, was used in the bread production: for centuries men have used a small portion of the dough to start each new bread dough.
It is only in the XIX century that the microscope discovery gave to Pasteur the chance to demonstrate that this strange phenomenon was caused by yeast, a living organism able to ferment sugar into carbon dioxide and alcohol.
Pasteur discovered that yeast was responsible also for wine and beer fermentation, explaining why bakers used the by-products of brewing and wine making to leaven their bread .
At the beginning of the XX century the biologists discovered that yeast reproduce rapidly in the presence of air, without producing alcohol.
By this knowledge scientists and yeast manufacturers have created the modern yeast industry, selecting pure strains for the specialized needs of the baking industry.