March 8, 2009
Yeast we can
By Chris Tan
Q Why can't you use yeast for making cakes or cookies, rather than baking powder and baking soda?
Janet Lee
A Yeast, baking soda and baking powder are all leavening agents. They act to produce gas that makes batters and doughs bubbly and light, but they are not interchangeable.
Baking powder blends a dry acid salt, a dry alkali and a filler, usually cornstarch. When it dissolves in liquid, the acid and alkali react to form water and carbon dioxide.
Bicarbonate of soda, or baking soda, is an alkali that breaks down when it meets acid ingredients, such as vinegar, fruit juice, brown sugar and sour cream, to form water and carbon dioxide. As long as there is enough acid to react with, baking soda is four times as powerful as baking powder, weight for weight.
Ammonium bicarbonate (chow fun) works much like baking soda, but produces ammonia and carbon dioxide, and no water, resulting in a drier texture. It is used mostly for small items such as cookies - the ammonia may not have completely diffused and evaporated from large items by the end of baking.
Yeast is a living organism that feeds on carbohydrates in a batter or dough. It produces carbon dioxide and ethanol as respiratory by-products. In bread recipes, the fermentation (rising) periods nurture the yeast and allow many different and delicate biochemical reactions to take place that affect the flavour and texture of the bread.
Each kind of leavening works best in a particular environment, so you cannot substitute them for one another at will. Chemical leaveners produce quick rises but are somewhat sensitive to a batter's pH (level of acidity). They must be measured carefully as too much can make a cake over-rise and collapse, or result in off-flavours. Yeast is slower and needs more careful cosseting, but it is more versatile. It also assists in flavour development during fermentation.
Many cakes from around the world use yeast as a leavener, such as Austrian kugelhupf, Eurasian breudher and Polish babka. However, these arguably straddle the boundary between cake and bread, as do baking powder-risen quickbreads such as muffins, and soda breads leavened with baking soda, such as Irish soda bread and Aussie damper.
A very few items use all three leaveners: recipes for char siew pau dough often call for yeast, baking powder and ammonium bicarbonate, while recipes for huat kueh sometimes use yeast, baking powder and baking soda.
Plain to rise
Q How can I convert plain flour to self-raising flour?
Jenny Tan
A You can approximate commercial self-raising flour by mixing all-purpose flour with baking powder, in the ratio of 110g to one level teaspoon.
I prefer to mix flour and baking powder myself instead of buying self-raising flour. Baking powder loses potency over time, and it is difficult to gauge how long ago the self-raising flour was blended and how long it has been on the shelf. Different kinds of baked goods need different amounts of leavening to reach the right texture, so it is always better to control this yourself.
Mix it right
Q Some recipes require yeast to be mixed with warm water and sugar before being poured into the dough. In others, yeast is just mixed in. What is the difference?
Deborah Koh
A The relatively large granules of active- dried yeast need an initial proofing in warm water to hydrate and liven up before being mixed with the dough. Also, because of how they are dried, the granules contain a certain fraction of dead cells that cannot be activated. The proofing gives their remaining healthy brethren a head start to make up for the dead weight.
Instant yeast, pictured below, is much finer-grained and hydrates much faster, so it can be mixed straight into the dry flour before the other ingredients are added. This also protects it from coming into direct contact with sugar, salt or acid ingredients as high concentrations
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