Leaveners produce a desirable texture by
introducing a gas which is carbon dioxide, into the batter or dough by one
of the three means - chemical, organic or physical.
Baking soda and baking powder are the
primary chemical leaveners. In these leaveners, an alkaline ingredient
(baking soda or baking powder) interacts with an acid (already present in
baking powder, or an ingredient such as buttermilk, sour cream, yogurt, or
chocolate). The alkalis and acids produce a gas, which is carbon dioxide,
when combined in the batter. As the item is baked, this gas expands,
giving the baked goods their characteristic texture, known as "crumb".
This process of expansion happens rapidly; hence, many items prepared with
chemical leaveners are called "quick breads".
Double-acting baking powder got its name
because a first action occurs in the presence of moisture in the batter
and a second action is initiated by the presence of heat. That is, it
reacts once when it is mixed with the batter's liquids and again when the
batter is placed in a hot oven.
Organic leaveners, are living organisms that
feed on sugars present in flours or as added sweeteners, producing alcohol
and carbon dioxide. Unlike chemical leaveners, organic leaveners take a
substantial amount of time to do their job. They have to grow and
reproduce sufficiently to fill the dough with air pockets.
Yeast and sourdoughs are organic leaveners,
which means that they must be "alive" in order to be effective. Organic
leaveners can be killed by overly high temperatures and, conversely, cold
temperatures can inhibit their action. The temperature of the dough and
its environment must be controlled carefully. Yeast will not function well
below approximately 65 to 70oF (18 to 21oC) , and
above 110oF (43oC) yeast is destroyed.
Two types of yeast are used in the
professional bakery. The dry (or granulated) yeast and fresh (compressed)
yeast. Dry yeast, in bulk or packets, should be refrigerated. It will keep
for several months, which makes it suitable for kitchens that only
occasionally make their own bread. Fresh yeast, on the other hand, is
quite perishable and can be held under refrigeration for only 7 to 10
days. It may be frozen for longer storage but should be allowed to return
to room temperature before it is used.
The basic physical leavener is steam, which
is produced when liquids in a batter or dough are heated. This causes the
air pockets to expand. Steam leavening is critical in sponge cakes and
souffles. It also plays a vital role in the production of puff pastry,
croissant and Danish. In these products, the steam is trapped, causing the
layers to separate and rise.