Scott wrote:
picked up a copy of Baking 9-1-1 by Sarah Phillips,
<http://www.amazon.com/exec/obidos/tg/detail/-/0743246829/qid=11268086...>

>and came across a statement that piqued my interest. In her recipe for
>sugar cookies, she writes "most cookies use baking powder for leavening;
>I chose to use baking soda. It causes the cookies to spread rather than
>puff, perfect for a crispy cookie, and have a wonderful buttery flavor
>you can't get with baking powder."

Baking soda in the absence of any acid salt will make the cookie dough
slightly alkaline. Now alkaline conditions will alsoto promote
Maillard's reaction promoting the formation of flavors.
>The ingredients a unbleached AP flour, baking soda (1/2 tsp), baking
>powder (1/4 tsp), salt, unsalted butter, sugar, an egg, vanilla extract,
>and an optional 1/2 tsp lemon extract.
>First, what could the acid be? What would interact with the baking soda?

If baking soda is heated:
2NaHCO3 ( heat) = Na2CO3 + H2O + CO2, it does not need any acid to
release the gas which can contribute some leavening action.

>If nothing, why add baking soda?

It is added primarily to generate CO2 but it has other effect on the
cookie dough also.
Baking soda is used alone in some cookie formulation but in my
experience is not the best leavening salt; I prefer ammonium
bicarbonate instead as it can produce more gases per molecule of the
salt
NH4HCO3 ( heat) = NH3 +CO2 + H2O
Therefore for every molecule of ammonium bicarbonate it releases three
molecule of leavening gases; ammonia, carbon dioxide, and steam .
Where as two molecules of baking soda as shown above will only release
a molecule each of carbon dioxide and steam.. Therefore in per molecule
basis ( NaHCO3)its not efficient. Besides the sodium ion has a
tightening effect on the protein due ionic bonding with the amino acids
in the protein molecule which can confer some stabilizing effect .
Now if the thesis is cookie spread, the reason for that is the cookie
dough alkalinity which will tend to loosen the gluten structure but not
much exhibited by baking soda actually.
The pH of the ammonium bicarbonate leavened cookie is slightly higher
and so promote more dough relaxation allowing it to spread better
resulting in flatter cookie with wider diameter than the ones leavened
with baking soda.
As the ammonia does not form a tigher ionic bond than the sodium oon
the cookie texture is more loose and so tends to spreads more.
The only advantage of baking soda against bicarbonate of ammonia (
baking ammonia) is that it does not leave any ammoniacal smell to the
cookie.
But I don't see that the spread factor( the ratio of the cookie
diameter to its height is significant with baking soda and that can be
influenced also by the amount of fat present in the recipe as well as
the nature of the flour used.

>Second, it DOES have baking powder,

That is self explanatory, baking soda alone does not leaven the cookie
dough efficiently but needs a boost from the baking powder. Besides it
needs a higher temperature for the baking soda to evolve the needed
carbon dioxide so if used alone and baked at moderate baking
temperature you will not get what you expect in terms of leavening
performance.. In addition the reaction of baking powder once its
hydrated allow for the formation of some gas cells which are expanded
by the released steam and CO2 from the oven heat release of baking soda
and from residual amount of baking soda in the baking powder

>too, so what's she talking about about choosing to use baking soda?

I would say that her selection is opinionated based upon her limited
experience in chemical leavening application .. If she happened to be
well experienced and more knowledgeable with the chemistry of
leavening agents she should not say that! I

>know that some recipes use both, but (a), baking soda is often used to
>also neutralize an acid,

You are right, baking soda is used in baking with acidulant for the
efficient release of the carbon dioxide

>and (b) she seems to imply she was substituting
>one for the other. OK, maybe that's a nitpick

Her employment of two CO2 generating agents is due to the fact that
with a slightly alkaline dough due to the formation of sodium carbonate
will promote the cookie to brown faster resulting in a nice cookie
color if baked at just moderate temperatures. If she is only employing
baking powder it will appear to be pale colored as the slightly acidic
dough does not promote the cookie crust to brown .
Besides the crumb color will be not pale colored as well for the same
reason.

> Third, by saying that
>baking soda makes the cookies spread rather than puff, doesn't that mean
>it ISN'T a leavening agent here?

I have already stated above the reasons for spreading . but its
presence will add a bit more of CO2 to added baking powder.

> And finally, how on earth would baking
>soda add "a wonderful buttery flavor"???

It is self explanatory,s the uses butter here which when it heated
volatilizes the aroma components , and is the main source of butter
flavor.
But it does not hold if you use a bland tasting fat in the dough and I
had confirmed thate extensively in the past.
Butter flavor can arise if you use substances that contains it such as
added butter flavor, the use of some margarines and surely of pure
butter and anhydrous milk fat.
Even the use of full cream milk and buttermilk seems to generate the
buttery aroma also in cookies.
I have used baking soda /baking powder tandem in the past to leaven
some cookies containing vegetable shortening and I get a different
smell , artificial and unappetizing more soapy in character. These is
due to minimal fat breakdown resulting in the release of fatty acids
which will combine with the sodium ion to forms soapy tasting cookie