LEAVENING AGENTS
Leavening agents cause baked products to increase their volume or 'rise'. They
do this mainly by introducing gas/vapour bubbles into a batter or dough - these
expand during cooking, giving the 'rise'. So, leavening is all about getting and
holding lots of air/vapour/gas in the product mixture and there are
essentially
4 mechanisms for doing this:
BAKING POWDER OR BAKING SODA
Both of these agents work in the same way to
produce the gas carbon dioxide. Acids in the cake mix react with the Baking Soda
(also a constituent of Baking Powder) and this results in tiny bubbles of gas
being released. The detailed difference between the two is covered in a separate
Baking Powder article
but essentially Baking Soda relies on acid being added to the mix (often in the
form of something like lemon juice) whereas Baking Powder contains its own acid
source in the form of Cream of Tartar - this gets activated as soon as it
comes into contact with moisture.
EGGS
In baking, eggs play a
number of roles and leavening is one of them. The leavening properties are
achieved in the main because of their ability to foam
and hold lots of air (whole eggs can be whisked to about 4 times their volume
and egg whites, to about 8 times their normal volume). When a cake mixture is beaten,
lots of air gets incorporated into the mix and this causes much of the rise
during baking.
Eggs exhibit another mechanism as well though and if you've
ever poured a lightly beaten egg into a hot pan, you will no doubt have noticed
that it expands very rapidly. Since there is very little air in a lightly beaten
egg the rise must be caused by something else and it is; firstly, the egg
whites contain about 90% water so steam gets produced (see below) but the heat
also cause proteins in the egg to unwind and solidify which also contributes to
the increased volume.
STEAM
Whenever there is water in a cake mixture then steam will get
produced (cooking temperature is always greater than 100 C). Steam takes up a
far greater volume than the liquid water it comes from and so the mixture rises.
A lot of the steam will escape but by the time it does, the cake will have
solidified its structure so its job will have been done.
YEAST
The final common leavening agent is yeast. Yeast is a living micro-organism which
consumes sugar and excretes alcohol and carbon dioxide as by-products'. Wine and
beer makers are interested in the alcohol, but bakers are interested in the carbon dioxide which gets release as tiny bubbles - perfect for our needs. The
reaction is
quite slow though which is why breads leavened with yeast, take a long time to prove
(rise).