Yeasts are saprophytic unicellular fungal micro-oganisms. The species most commonly used for baking is Saccharomyces cerevisiae, an ascomycete (spores develop within asci (from Gk. askos bag)) member of the phylum Thallophytes. Baker's yeast is able to grow with or without air (oxidative or fermentative metabolism respectively), a wider range of carbon sources being metabolised aerobically.
Yeast was derived historically from the top yeasts used in brewing, and would be obtained from the local brewery. The term 'top yeasts' refers to the vigorously fermenting yeast cells that are taken to the top of the vat by the carbon dioxide gas. Nowadays industrial production of yeasts ensures better consistency and greater quantities of the products. Manufacture begins with a culture in liquid suspension. This is centrifuged to increase the concentration of the solids and produce cream yeast, which is used in industrial scale food production. The cream yeast can then be dehydrated to varying degrees depending on the qualities required in particular yeast end products, whilst taking care to maintain the viability of the living organisms. These further products take three different forms, each with characteristics to suit particular food manufacturing requirements, but they give similar results in home baking. Yeast is vital in causing expansion of the dough, but it also affects the taste and shelf life of the loaf. All categories of yeast are referred to by a variety of names:
Compressed, cake, wet, fresh, or live yeast
Comes as a beige coloured, soft, putty-like substance. May be used directly with other ingredients or dispersed in tepid water first. Typical moisture content 70%. Shelf life: several weeks stored at 5°C.
Dried active, or active dried yeast
Manufactured from 'press cake' (compressed yeast). Comes as small, light beige coloured, smooth spheroidal granules of up to 2 mm diameter. Typically moisture content 8%. Rehydrate in water at about 40°C. Shelf life: two years stored at room temperature.
Fast action dried, or instant dried yeast
Manufactured from 'press cake' (compressed yeast). Comes as smaller, light beige coloured strands up to 0.4 mm diameter and 1.5 mm long. Typical moisture content 5%. Use directly or blend in 5 parts of water at about 40°C. Shelf life: two years stored at room temperature. Typical proprietary brands of fast action dried yeast contain additives such as:
Sensitivity to temperature
Yeast can survive freezing temperatures in the range -18 to - 2°C. Activity increases through the range 20 to 40°C. At higher temperatures the death rate increases and it is killed rapidly at 55°C or above (some time will elapse before the heat penetrates into dough placed in a hot oven, of course).
Choosing which type to use
All types can be relied on provided that they are used before their expiry dates. The dried types are of more consistent quality, so it is best to become practised at baking before you try to use live yeast. I prefer to buy Doves Farm Quick Yeast because its 'bulk' packaging gives better economy than brands which come in 7 gm sachets.