|MadSci Network: Biochemistry|
Wheat flour used in making bread is high in protein types called gluten and glutenin. Other grains do not have these breadmaking proteins, except for rye flour which has a much lesser amount than wheat. Gluten and glutenin proteins bind poorly to water, unlike many other food proteins. The viscosity of the oil primarily determines the speed and ease with which you can knead the oil into the dough. The mechanical energy of kneading breaks the proteins loose from their native state and allows the salt and water to hold the configuration open so that strong cross-linking can occur on heating. The fat (oil) has somewhat the opposite effect in creating a water-impassable layer. When the dough is cooked, certain sulfur-containing amino acids can bond across protein molecules (cross-linking) and form a permanent 3-D structure. Compressibility of the "crumb" is primarily related to the "stretchiness" of the protein structure and to the number of cross-links formed. Too many cross-links and the crumb is stiff and unyielding (brittle). The fat can separate the proteins so that cross-linking is not likely, while water allows them. So the amount of fat and amount of water are related to compressibility. Compressibility is also related to the porosity of the structure, which is determined by the amount of rising occurring. Typically "toughness" means "strong" and "stretchy" ("ductile"). An easily deformed structure can be "weak" and "stretchy" (e.g., anglefood cake) or "weak" and "brittle" (breaks without stretching). The viscosity of the oil is determined by the degree of saturation more than the molecular weight of its fatty acids. Canola oil is highly unsaturated, while cottonseed oil is very saturated. The less viscous oil also presents less steric (geometric) hindrance to the flour proteins, so is easier to work in. The function of fats in bread is "shortening" or reduction in the toughness of the dough. So long as the fats are liquid when used and don't contain emulsifiers, the properties of the end product (other than taste) should not be affected too much by the type of oil used. Much more important factors in determining compressibility are: 1) the amount of salt; 2) the amount of water; 3) the amount of kneading; 4) the amount of rising. Finally, wheat bread contains whole wheat, which contains more bran fiber. This coarsens the crumb of the final bread and makes the 3-D structure less homogeneous. This would lessen compressibility and make a denser crumb.
Try the links in the MadSci Library for more information on Biochemistry.