Yeast

Yeast is a single-cell fungus that feeds on simple sugars to produce carbon dioxide and alcohol. It is used to ferment grains and fruits to make wine, beer and spirits, and as a leavening agent to make a range of breads and baked products.

There are two major types of food yeasts commonly available: the first is a non-leavening yeast known as brewer's yeast, whilst the other is a leavening yeast known as baker's yeast. Leavening yeast may be sold as a fresh block, which must be used within a few weeks of purchase, or as dried granules, which may be stored for up to a year. Fresh leavening yeast may also be sold as a 'starter yeast', which are traditionally used to make sourdough or sweet breads known as starter breads.

Baker's yeast may be used to make a variety of breads, rolls, cakes and sweet dough mixtures. When the yeast is added to a mixture of dough, a fermentation process begins as the yeast feeds on the sugars and produces carbon dioxide gas, which causes the dough to rise. However, before adding yeast to any recipe, it is always best to proof it to make sure that it is active. This may be done by adding a small amount of yeast to some water warmed to 40-45°C (hot water will kill the yeast) along with a dash of honey or sugar. This mixture should be allowed to sit for approximately 10 minutes; if it begins to foam, bubble up or increase in size, then the yeast is active and may be used for leavening dough. However, if no reaction occurs, then the yeast is probably dead and should be thrown away.

It is also worth noting that the altitude plays a large part in preparing leavened dough; the higher the altitude, the less air pressure there is pressing on the dough, and the faster and higher the dough will rise. In areas of high altitude, the amount of yeast may need to be decreased by up to a half in order to allow the dough to rise more slowly prior and during baking.