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Salt in Bread Dough

"Maggie Glezer" <glezer@mindspring.com>
Fri, 14 Jan 2000 09:49:37 -0500
v100.n003.2
Salt in Bread Dough
By Maggie Glezer
(Originally published in the Bread Bakers Guild of America Newsletter)

Salt is such a minor ingredient in bread that few bakers stop to think 
about its exact role in bread-making.  However, sometimes focusing a light 
on a seemingly unimportant aspect of bread-making can illuminate the whole 
bread-making process.  Such is the case with salt.  Salt exerts an 
influence on almost every stage of bread-making, and every aspect of 
bread.  Why is salt one of bread's cornerstones?  What importance does it 
have beyond being a flavoring element?  This article will attempt to 
explain and clarify some of the chemistry governing salt's interactions 
with bread dough.

Salt's primary purpose in bread is to evoke and enhance the bread's flavor. 
To most Americans, saltless bread is insipid and virtually inedible, but 
adding only approximately 2% of the flour weight in salt to the average 
bread formula manifestly changes the perception of bread's flavor, 
eliciting the full spectrum of complex flavor notes, including a sweetness 
that would be otherwise absent.  It is interesting that the addition of 
salt to bread is a relatively new preference.  Medieval bread was almost 
never salted because salt was very expensive and difficult to procure; 
thus, salt-less bread was preferred.  According to Professor Raymond 
Calvel, professor emeritus of l'Ecole Francaise de Meunerie, French bread 
formulas started to include salt only at the end of the eighteenth century.

Besides flavoring the bread, bakers have long noted salt's alteration of 
certain dough characteristics.  Unsalted dough mixes faster, has little 
resistance to extension and feels sticky.  Bakers who delay the salt 
addition during mixing find that once salt is added, the dough tightens, 
becoming more difficult to stretch, but also becomes stronger, and is thus 
capable of stretching farther without ripping.  (Testing by cereal 
scientists confirms this seemingly contradictory observation: salted doughs 
are both more resistant to extension and more extensible once 
deformed.)  During fermentation, salted doughs rise more slowly, an 
occurrence usually solely attributed to salt's dehydrating effect on 
yeast.  To understand how salt affects these changes, and to see if our 
assumptions hold true, we will need to take a look at the interactions 
within the dough on a molecular level.

Table salt is a type of crystal made up of chlorine and sodium ions, or 
charged atoms.  In its crystalline state, salt's ions are positioned in a 
stable, geometric lattice.  However, when mixed with an appropriate solvent 
such as water, salt dissolves, meaning that the ion lattice is forced apart 
by the solvent and the individual ions become enveloped by the 
solvent.  This is exactly what occurs in a dough: crystalline salt is 
quickly dissolved by the dough's liquid into sodium and chloride ions.

The presence of any type of dissolved material, including ions, in the 
dough's liquid phase affects the function of the yeast and lactobacilli 
living in the dough (all doughs, not just sourdoughs, contain acidifying 
bacteria which contribute to the bread¹s flavor).  In an unsalted dough, 
water will move freely into the yeast or bacteria cell.  However, if salt 
is added to the dough, osmotic pressure, determined by the amount of 
material dissolved in the dough's liquid, will increase, drawing out some 
of the cell's water and thus partially dehydrating it.  Higher osmotic 
pressure also limits the amount of fermentable sugars able to pass into the 
cell.  These two effects--a loss of cell pressure and a decrease in 
sugars--combine to slow the overall rate of fermentation of both 
organisms.  If the percentage of salt added to a dough becomes too high, 
excessive dehydration will eventually kill the yeast and bacteria.

Most scientists believe that at 2% of the flour weight or less, salt alone 
does not significantly alter either the yeast's gassing power or the 
bacteria's acid production. A study measuring the gas production in a 
fermenting dough has shown that gas production is retarded by only about 9% 
in a dough containing 1.5% salt (based on the flour weight).

Although salt's osmotic effect on fermentation reduction may be minor, it 
must be taken into consideration when attempting to maximize the build up 
of fermentation byproducts in pre-ferments.  Thus, salt is always omitted 
in sponges, poolish, biga, and most other pre-ferments to ensure the 
greatest possible production of byproducts.

If the osmotic pressure exerted by the salt does not significantly change 
the fermentation rate of the dough, why does the dough rise so much more 
slowly when salt is added?  This phenomenon can be attributed to salt's 
direct effect on the gluten protein network.  Salt strengthens, tightens 
and compacts the gluten protein network, making it more resistant to 
pressure exerted by the build up of gaseous carbon dioxide.  In salted 
doughs, gas production may be approximately equal to unsalted doughs, but, 
since the gluten protein network itself is less extensible, the dough is 
more resistant to the stress created by the internal gas buildup.

How does salt strengthen the gluten protein network?  This is where the 
cereal science becomes murky.  Although cereal chemists have been studying 
doughs for many years, there is still no real understanding of bread dough 
on a molecular level.  Dr. O.K. Chung of the U.S.D.A. in Manhattan, Kansas, 
one of the leading experts on cereal lipids, has called the biochemistry of 
doughs "a huge puzzle," where every possible biochemical reaction is 
occurring at once, each one influencing the whole.  In addition, every 
chemist has his or her own pet theories, none of which are strictly 
proven.  So the layman must tread only where the path is well trampled.

The gluten in wheat is unique among the cereal proteins, because, when 
hydrated, it is capable of bonding with itself to form a viscoelastic 
web-like structure.  "Viscoelastic" means that the web is both viscous and 
elastic: When a wheat dough is stretched out and released, it will either 
flow into a new configuration or retract back into its original shape.  The 
gluten web can also trap and secure air bubbles, preventing them from 
migrating to the surface of the dough and releasing their gas.  It is this 
last characteristic that allows a dough to be leavened by the carbon 
dioxide produced by the yeast.

The most widely-accepted current theory holds that the lower the dough's 
pH, the more positively-charged the gluten structure will be.  A typical 
dough has a pH low enough (approximately 5) for the gluten protein to carry 
some positive charge.  Because like charges repulse, the protein filaments 
in a typical dough repulse one another, resulting in a more loosely 
organized and less interconnected web.  When salt is added to a dough, some 
of the negatively-charged chlorine ions will bond with the 
positively-charged sites on the gluten protein, neutralizing the overall 
charge.  With the repulsive forces eliminated, the web will tighten, 
compact, and bond with itself more strongly.  A more bonded, compact gluten 
web can better withstand the force exerted by the swelling air bubbles in 
an actively fermenting dough, and thus will expand more slowly.

Thus, while salt does slow the dough's expansion during fermentation, the 
long held belief that salt retards the yeast's gas production has been 
shown to be of only minor consequence to the fermentation rate.  Instead, 
the primarily cause of the slow down has been shown to be a tightening of 
the dough's gluten structure, induced by salt's neutralization of the 
structure's charge.  It still remains to be seen how this alteration of the 
dough's backbone affects its formation during mixing, or how a modification 
of the gluten impacts other dough constituents, especially the dough's 
lipids (fats) and enzymes.

Although the explanations in this article have been greatly simplified, 
hopefully the reader has come away with some insight into the chemistry of 
bread dough, and now has a heightened appreciation of the complexity 
surrounding even the most prosaic ingredients.

[Editor's Note: Maggie Glezer has a new book coming out in the fall.  It is
currently titled Artisan Baking Across America published by Artisan 
Publishing ... Reggie]