Wednesday, April 22, 2009

LEAVENING AGENTS

LEAVENING AGENTS

Leavening is the production or incorporation of gases in a baked product to
increase volume and to produce shape and texture. These gases must be
retained in the product until the structure is set enough (by the coagulation of
gluten and egg proteins and the gelatinization of starches) to hold its shape.
Exact measurement of leavening agents is important because small
changes can produce major defects in baked products.

YEAST
Yeast is the leavening agent in breads, dinner rolls, Danish pastries, and similar
products. This section discusses the characteristics of yeast.
Fermentation is the process by which yeast acts on sugars and changes
them into carbon dioxide gas and alcohol. This release of gas produces the
leavening action in yeast products.The alcohol evaporates completely during
and immediately after baking.

Fermentable sugar in bread dough comes from two sources:
1. It is added to the dough by the baker.
2. It is produced from flour by enzymes that break down the wheat starch
into sugar.These enzymes are present in the flour and/or are added by the
baker in the form of diastatic malt.
Yeast is a microscopic plant that accomplishes this fermentation process
by producing enzymes. Some of these enzymes change complex sugars
(sucrose and maltose) into simple sugars. Others change the simple sugars into
carbon dioxide gas and alcohol.The following formula describes this reaction
in chemical terms:
C6H12O6 → 2CO2 + 2C2H5OH
simple sugar carbon dioxide alcohol
Because yeast is a living organism, it is sensitive to temperatures.
34°F (1°C) Inactive (storage temperature)
60° to 70°F (15° to 20°C) Slow action
70° to 90°F (20° to 32°C) Best growth (fermentation and proofing
temperatures for bread doughs)
Above 100°F (38°C) Reaction slows
140°F (60°C) Yeast is killed

Yeast is available in three forms:
1. Fresh yeast, also called compressed yeast, is moist and perishable and
is preferred by professional bakers. It is usually purchased in 1-lb (450 g)
cakes.
2. Active dry yeast is a dry, granular form of yeast. Active dry yeast must be
rehydrated in 4 times its weight of warm water [about 110°F (43°C)]
before use.When using active dry yeast in a bread formula, use part of the
water in the formula to dissolve the yeast. Do not add additional water.
3. Instant dry yeast is also a dry granular form of yeast, but it does not have
to be dissolved in water before use. It can be added in its dry form because
it absorbs water much more quickly than regular dry yeast. It also
produces more gas than regular dry yeast, so less of it is needed. Instant
dry yeast is sometimes called rapid-rise or quick-rise yeast.
In this book,when yeast is required in a formula, fresh yeast is specified.To
substitute dry yeast, use the following guidelines.
• To convert fresh yeast to regular active dry yeast, multiply the quantity by
0.5. For example, if the formula calls for 1.5 oz fresh yeast, multiply by 0.5
to get 0.75 oz active dry yeast.
• To convert fresh yeast to instant dry yeast, multiply the quantity by 0.35.
For example,if the formula calls for 40 g fresh yeast,multiply by 0.35 to get
14 g instant yeast.
Yeast contributes flavor in addition to leavening action.

CHEMICAL LEAVENERS
Chemical leaveners are those that release gases produced by chemical
reactions.

Baking Soda
Baking soda is the chemical sodium bicarbonate. If moisture and an acid are
present, soda releases carbon dioxide gas, which leavens the product.
Heat is not necessary for the reaction (though the gas is released faster at
high temperatures). For this reason, products leavened with soda must be
baked at once or gases will escape and leavening power will be lost.
Acids that react with soda in a batter include honey, molasses, buttermilk,
fruit juices and purées, and chocolate. Sometimes cream of tartar is used for
the acid.The amount of soda used in a formula is generally the amount needed
to balance the acid. If more leavening power is needed, baking powder, not
more soda, is used.

Baking Powder
Baking powders are mixtures of baking soda plus an acid to react with it.They
also contain starch, which prevents lumping and brings the leavening power
down to a standard level. Because baking powders do not depend for their
leavening power on acid ingredients in a formula, they are more versatile.
Single-acting baking powders require only moisture to be able to release
gas. Like baking soda, they can be used only if the product is to be baked
immediately after mixing.
Double-acting baking powders release some gas when cold, but they
require heat for complete reaction. Thus, cake batters made with these can
incorporate the leavening agent early in the mixing period and then stand for
some time before being baked.
Do not include more baking powder than necessary in a formula because
undesirable flavors may be created. Also, excess leavening may create an
undesirably light, crumbly texture. Cakes may rise too much and then fall
before they become set.

Baking Ammonia
Baking ammonia is a mixture of ammonium carbonate, ammonium bicarbonate,
and ammonium carbamate. It decomposes rapidly during baking to
form carbon dioxide gas, ammonia gas, and water. Only heat and moisture are
necessary for it to work. No acids are needed.
Because it decomposes completely, it leaves no residue that could affect
flavor when it is properly used.However, it can be used only in small products
that are baked until dry, such as cookies. Only in such products can the
ammonia gas be completely driven off.
Because ammonia releases gases very quickly, it is sometimes used in
products in which rapid leavening is desired, such as cream puffs. Use of
ammonia enables the baker to lower the cost of such products by reducing the
quantity of eggs.However, the quality of the resulting goods is lowered.
Storage of Chemical Leaveners
Baking soda, powder, and ammonia must always be kept tightly closed when
not in use. If left open, they can absorb moisture from the air and lose part of
their leavening power.They must be stored in a cool place, because heat also
causes them to deteriorate.

AIR
Air is incorporated into a batter primarily by two methods: creaming and
foaming.This air expands during baking and leavens the products.
1. Creaming is the process of beating fat and sugar together to incorporate
air. It is an important technique in cake and cookie making. Some pound
cakes and cookies are leavened almost entirely by this method.
2. Foaming is the process of beating eggs, with or without sugar, to
incorporate air. Foams made with whole eggs are used to leaven sponge
cakes, while angel food cakes, meringues, and soufflés are leavened with
egg white foams.

STEAM
When water turns to steam, it expands to 1,100 times its original volume.
Because all baked products contain some moisture, steam is an important
leavening agent.
Puff pastry, cream puffs, popovers, and pie crusts use steam as their
primary or only leavening agent. If the starting baking temperature for these
products is high, steam is produced rapidly and leavening is greatest.

1 comment:

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