Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and 1,500 species are currently identified. They are estimated to constitute 1% of all described fungal species. Yeasts are unicellular organisms which evolved from multicellular ancestors, with some species having the ability to develop multicellular characteristics by forming strings of connected budding cells known as pseudohyphae or false hyphae. Yeast sizes vary greatly, depending on species and environment, typically measuring 3–4 µm in diameter, although some yeasts can grow to 40 µm in size. Most yeasts reproduce asexually by mitosis, and many do so by the asymmetric division process known as budding.wiki >>
HOW TO SELECTFresh, dried granular or easy-blend yeast (which has finer grains than dried granular yeast), according to the recipe. Fresh yeast is reckoned to give the best flavour – it should be firm and moist, with a cream colour. Avoid any that is dark or dry and crumbly. Granular yeast is more convenient than fresh yeast, as it keeps for longer. Easy-blend yeast doesn’t need proofing (see below) – it can be added directly to the dough mix. Fresh yeast and dried granular yeast are not suitable for bread-making machines.
HOW TO STOREKeep dried yeast in a cool, dark, dry place, and use within its best before date – beyond this, it may fail to make the bread rise. Fresh yeast is highly perishable and should be kept in the fridge. Use within a maximum of 2 weeks, or within the use by date.