Yeast is a tiny plant. A single cell fungus which is so small that there are about 1,510 cells per gram. There are very many different types of yeast but the one used to ferment dough is called Saccharomyces cerevisiae. Under anaerobic conditions, that is, with the exclusion of oxygen, this organism is capable of the production of carbon dioxide gas and alcohol from lower sugars (monosaccharides). It is the gas production facility that is of most importance in the fermentation of dough.

Yeast can be purchased either as Fresh Yeast in a compressed block form, with moisture content of about 70%, Active Dried Yeast (in granular form) or as Instant Active Dried Yeast. The blocks of fresh yeast are usually 1 kg each and they should be stored at either about 4°C, when the yeast will remain in good condition for some 3-4 weeks, or deep frozen, when it will be useful for at least 3 months. Storage in a normal refrigerator at 4°C has the problem that there is considerable drying from the product, causing build up of ice on the colder parts of the refrigerator and desiccation of the yeast blocks. Active Dried Yeast has a moisture content of about 8% and, in a sealed bag, will remain viable for 3 or more years losing its activity at about 1% per month. Instant Active Dried Yeast has a moisture content of about 5% and in a sealed pack will last a long time losing its activity at about the same rate as Active Dried Yeast.

Fresh yeast, in the compressed block form, if it is in good condition should be in one piece, not crumbly. It should be an even creamy beige colour, smooth and fresh smelling. (In an advanced state of decay it becomes a dark exceedingly evil smelling liquid!). If it has crumbled and looks grey and dry, it is stale. It may still work but will not ferment well, i.e. its gassing power will be reduced.

When using fresh yeast it is best to disperse it in water, in the ratio of about 1:5 of water. To revitalise frozen yeast, use tepid water and leave it to stand for about 15 minutes.

Active Dried Yeast is at least twice as potent, on a weight for weight basis as fresh yeast but when dried it looses some of its activity. It should be reactivated by dispersing in water at around 40°C with a little sugar for 10-15 minutes. Active Dried Yeast should never be rehydrated with hot water.

Instant Active Dried Yeast, by its mode of manufacture and the coating with an emulsifier, allows it to be rehydrated very easily. It may be added directly to the dough but like Active Dried Yeast there is a 5 to 15 minutes delay before it is as active as the fresh yeast. On a dry weight basis it is almost as active in gassing power as fresh yeast.

Although convenient to store, the dried yeasts are more expensive than fresh yeast.

Dispersions of yeast should never be made in salt water. Even a 2% salt solution can kill the cells or cause significant retardation of the gassing power. Sugar solutions also inhibit the yeast. The sugar solution concentration should not exceed 5%.