SHARON CROXFORD
Chemistry from the Kitchen: Yeast and Bread
August 2nd 2008 13:00
Yeast and bread
Saccharomyces cerevisiae is the single-celled fungi used in brewing and baking. Yeast is the raising agent used in most bread making. It works by metabolising simple carbohydrates (sugar) for energy and producing alcohol and carbon dioxide (CO2). The heat during cooking or baking bread forces both the CO2 and alcohol out of the final product. Sounds simple, but yeast is a very sensitive microscopic soul. Whilst a little sugar can stimulate its activity, too much will slow it down. Salt and fat also retard its action, but heat is the thing that kills it.
The different yeasts available for baking are fresh, or compressed yeast, active dried and instant dried yeast. Fresh yeast has a relatively short shelf life, 1-2 weeks, and needs to be kept in the refrigerator. It is best warmed and brought to activity in warm water (35oC/95oF), with a little sugar or flour. Active dried yeast needs a bit of a kick start and water at about 40-42oC/105-110oF will bring it to life. Instant dried yeast can be added directly to the flour mixture. Most yeast will come with manufacturers’ instructions.
Saccharomyces cerevisiae is the single-celled fungi used in brewing and baking. Yeast is the raising agent used in most bread making. It works by metabolising simple carbohydrates (sugar) for energy and producing alcohol and carbon dioxide (CO2). The heat during cooking or baking bread forces both the CO2 and alcohol out of the final product. Sounds simple, but yeast is a very sensitive microscopic soul. Whilst a little sugar can stimulate its activity, too much will slow it down. Salt and fat also retard its action, but heat is the thing that kills it.
The different yeasts available for baking are fresh, or compressed yeast, active dried and instant dried yeast. Fresh yeast has a relatively short shelf life, 1-2 weeks, and needs to be kept in the refrigerator. It is best warmed and brought to activity in warm water (35oC/95oF), with a little sugar or flour. Active dried yeast needs a bit of a kick start and water at about 40-42oC/105-110oF will bring it to life. Instant dried yeast can be added directly to the flour mixture. Most yeast will come with manufacturers’ instructions.
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