References:

On Food and Cooking, pages 284-300; 308-310; 313.

Hardness

What is hardness?
What are flours of different hardness used for?
Which wheats produce "hard" flours?
What is semolina?

Gluten

What is gluten made of?
How do molecules in gluten attach to each other?
What do oils do with respect to gluten?
How does bread go from dough to solid bread during baking?

Flour varies for a property called HARDNESS, which is essentially a measure of protein content. Hard flours have high protein content. Soft flours have low protein content. The higher the protein content of the wheat, the lower the starch, and thus there are fewer starch granules. When the wheat with high protein content is milled (ground up), it produces large chunks of protein that a free of starch grains. A strong gluten forms when flours with high protein content are mixed with water, and are thus preferred for bread making. Hard wheats, like bread wheat (Triticum aestivum), makes up the great majority of the wheat crop in the US. The hardest kind of wheat is Durum wheat (Triticum durum). Flour of durum wheat is too hard to make bread with, and thus durum wheat is usually milled into a substance called SEMOLINA, which is used to make pasta.

Soft wheats are usually used for cake flours and used to make things like cakes and biscuits; things meant to be tender and crumbly. All purpose flour is a blend of soft and hard flours. These days hard and soft components of flours can be separated, and thus hard and soft flours can be made from almost any flour by separating out hard and soft components and then mixing the back into specific combinations.

Flours traditionally have been "aged". It is known that if the flour is left to age for a couple of months, it will have better baking properties. In this age of cost control, flour is not left ot age naturally, but the effects of the aging process are created using chemicals like iodate and potassium bromate. This treatment affects the chemical bonding properties of gluten in the same manner as aging, allowing the gluten to form stronger more elastic doughs.

Flours (other than pasta flours) are bleached white, usually with chlorine dioxide gas.

Gluten

Gluten is composed of two proteins found in wheat endosperm, GLIADIN and GLUTENIN. Gliadin proteins consist of a few thousand atoms while glutenin proteins have up to a million atoms. They are both, by any measure, very large molecules. Proteins are chains of amino acids. There are 20 different amino acids, each with slightly to greatly different chemical properties. One amino acid, cysteine, has a SULFHYDRYL group (-SH) attached to one of it's carbons. Proteins crosslink to each other by the sulfur on the sulfhydryl group of a cysteine in one protein attaching to that of another, forming a DISULFIDE BOND.

                R--SH    HS--R   (--R is the rest of the cysteine molecule)

The hydrogens come off when dissolved in water

                R--S       S--R

and the sulfurs bond to each other

                R--S-S--R

joining the 2 cysteines and thus joining the 2 proteins containing the cysteines.

Different glutens are cross linked to each other and linked to gliadins by disulfide bonds to form long chains.

Parts of proteins form coils. Parts of proteins fold over on itself, with coils and folds maintained by weak molecular interactions like hydrogen bonds.

Thus in gluten, we have springy sorts of molecules linked to each other into long chains. They are spongy because they interact with each other to form coils and folds that are weakly held together. If we apply a force, we can stretch the gluten, which can spring back to its original shape.

So when we bake bread, we add the yeast, which grow, giving off carbon dioxide gas as a product of metabolism. If this were all we had, we wouldn't get what we call bread, as there is no place for the carbon dioxide gas to accumulate. We need to add small pockets of air to the dough, pockets that can grow when carbon dioxide is produced. We add these pockets by kneading the bread. Kneading also seems to cause the gluten molecules to line up in a more regular fashion, forming structures that are highly elastic and not easily deformed. This stiff dough is ideal for bread. Note that if the dough is kneaded too much, the gluten can break down. This happens because too much water gets into the gluten and degrades disulfide bonds.

When dough is baked, lipids (oils) are needed to help lubricate the gluten so that layers of gluten that form will slide past each other. Temperature is important during baking. If temperature is too low, the dough will expand to its maximum before the gluten and the starch in the dough solidify. At the correct temperature, the yeast will undergo one last burst of activity as they heat up, causing "oven rise". As the yeast die at 140 degrees F, the starch starts to solidify, forming a solid structure around the bubbles of carbon dioxide in the dough. The gluten is still doughy, but it too solidifies at 160 degrees F. If the gas production ceases before the bread starts to harden, the bread will collapse. The timing has to be just right. The relationship between the solidification of starch and gluten have to be just right, and the yeast just happen to die at 140 degrees F!

Sour Dough

The sourness of dough has been an issue from the onset of bread production. Through most of history, sour breads were not held in high regard. However, the the San Francisco area during the gold rush, supplies of yeast were low, and old dough had to be used as a source of leavening. As it happens, such dough cultures get infected with microbes from the local environment, causing the sourness. This happened in San Francisco. This particular sourdough became quite popular, however, and is still produced from old dough in the region as it was during the gold rush.

Since there is a market for the San Francisco type of sourdough, it has been analyzed extensively. It turns out that the predominant yeast in San Francisco sourdough is not regular baker's yeast, Saccharomyces cerevisiae, but another yeast called Saccharomyces exiguus. S. exiguus does not metabolize maltose (maltose is a disaccharide consisting of 2 glucose molecules). S. exiguus is acid tolerant. Since starch is a long chain of glucose molecules, maltose is produced when starch in wheat is degraded). As a consequence, the dough has come to be populated by a group of acid producing bacteria (acetic acid (vinegar) and lactic acid being the two types of acid produced). The acid producing bacteria require maltose as an energy source.

So in sourdough bread we have a little bread ecology (ecology looks at interactions among organisms). If regular baker's yeast dominates the dough, we do not get sourdough, as baker's yeast metabolizes ("eats") all the maltose. If S. exiguus dominates, it leaves the maltose, which allow the acid producing bacteria to invade the dough. The acid producing bacteria make the dough acid, which favors S. exiguus because it is acid tolerant. A nice symbiotic relationship! The acid is what makes the dough "sour". The sour dough has been quite ecologically stable over the years, so for some reason it must be difficult for other organisms to invade the dough and upset the "sourdough ecosystem".