It’s a basic ingredient of bread. Its function is  to produce the fermentation of the dough by introducing microscopic cells called Yeast. They take the sugars contained in the dough and produce carbon dioxide and alcohol out of them. Alcohol is not important in breadmaking, but carbon dioxide is, to cause the expansion of dough.

Of the many yeasts that exist in nature, there are two which are important in breadmaking:

Candida Milleri: We usually call it “natural yeast”, although all yeasts are natural. It is in the grains of wheat and other grains in the fields. In the dough (flour and water) this yeast wakes up and starts to multiply and produce gas. From earliest times, it’s been used to make bread and fermented beverages. It is not easy to produce a bread without acidity using this yeast. I explain this in detail in the chapter devoted to natural yeast on this site.

This yeast is used to make “Sourdough” bread, very famous in San Francisco.

Saccharomyces cerevisiae: also called “beer yeast” and “commercial yeast”. It has been used since ancient times in the preparation of the said alcoholic beverage, as well as others. This yeast has been used for centuries in Europe and America to leaven bread as well. Initially, the foam and deposits of the fermentation tanks were collected once the process was complete, and so it was used in the nearby bakeries. Centrifugation and packaging methods were developed in modern times which allow yeast to last much longer and be carried to bakeries everywhere.

This yeast is similar to the ones occurring naturally on fruit skins, and also causes spontaneous fermentation. Sometimes winemakers prefer to use bread yeast to ferment their wine instead of the original yeast coming from the fruits in order to ensure the results as bread yeast behaviour is more predictable than in wild yeasts.

Bread yeast known today no longer comes from breweries, but is produced in large tanks in a controlled solution of molasses.

Fresh, moist and compressed yeast, presented in packs of half a kilogram or a pound and, commonly used in bakeries around the world, is also well known in homes and used for making bread for the family. It must be kept in the refrigerator. It has the disadvantage of being short lived.

Later, dry yeast in the form of small granules arrived. It has a much longer shelf life than the fresh yeast, and does not need refrigeration. It can be stored in a tightly closed glass jar. This yeast is very dry to touch, but it is not completely dry, otherwise the cells would die. It contains a low moisture amount (7%) that keeps it alive. Therefore, it is vital to keep it in a sealed container to prevent the yeast from total dehydration.

To use, you must first dissolve the yeast with some warm water before adding it to the dough.

Note: The granules of traditional dry yeast can be grinded into powder using a blender, and then added to the dough without previous dissolution.

Recently, the use of instant dry yeast was popularized, which does not require prior dilution and wakes up immediately upon receiving water. It is used in home bread machines. It comes in very small granules and in a light brown colour. Instant dry yeast can be added, undissolved, to the other ingredients before kneading.

For reasons of practicality, this is the yeast that I always use. It comes in airtight vacuum sealed packages of one to two pounds. I always have some empty and clean small glass jars with screw cap to put all the yeast in them. I shut them tight and keep them in a cool dark place. I only put one in the refrigerator for everyday use. I always take care not to leave the bottle open unnecessarily. I close it tightly as soon as I took the yeast I need and return it to the refrigerator.

Indeed, but since the bottle is open frequently,the yeast often runs the risk of becoming dehydrated. The cold prevents the yeast from dehydrating. When it is exposed to air  instead of losing moisture condenses humidity of the air.

Except for long process breads, the amount usually recommended is one tablespoon per kg flour, approximately 2% of the total flour. This amount varies considerably with the room’s temperature. If it is hot, this amount can be halved.