Living, naturally occurring, microscopic, single–celled fungi, which feed on carbohydrates, breathe air and give off carbon dioxide and alcohol. Yeast is used for bread and beer–making. When yeast is used for bread–making, it converts the natural sugars in the flour to bubbles of carbon dioxide, these are trapped by and stretch, as they expand, the elastic mesh formed by the gluten in the flour, causing the bread to rise. Baking kills the yeast and sets the dough; as the bread cools, air replaces the carbon dioxide. Yeast can be chilled, dried and frozen and re–activates when conditions return to normal, but it needs warmth and moisture to thrive. It is retarded by the use of excess sugar or salt so follow recipes carefully. Wild yeasts are used to make sourdough breads, these are encouraged to grow on a piece of fermented dough or fruit and then used to leaven the bread dough they are incorporated into. Store fresh yeast in the fridge for up to 1 week or freeze for several months. Dried yeast will keep for about 1 year in a cool, dark place.