Both starch and sucrose contain glucose, but in very different forms. Sucrose contains only two sugar subunits linked together: glucose and fructose. The chemistry of their linkage prevents the addition of further sugar subunits. The carbohydrates that make up starch consist of repeating glucose sugar units linked together in such a way as to form long polymers. These seemingly small chemical differences result in large differences both in baking and in the variety of biochemical reactions that occur in your body.
Sucrose
Sucrose is a disaccharide sugar molecule, "di" meaning "two" and "saccharide" meaning "sugar." Disaccharides contain a bond between the first carbon or the C1 atom of one sugar, and a hydroxyl or OH group at any position on the other sugar. Sucrose has a unique linkage in that the bond between the C1 carbon of glucose and the OH on the C2 carbon of fructose results in a molecule with no other reaction sites. It is a non-reducing sugar. Unlike other disaccharides, sucrose cannot make long chains of itself; there are no sucrose polymers.
Starch
Starch consists of long chains of single sugar subunits linked together. Starch is a polysaccharide. Just like "disaccharide" refers to "two" sugar molecules linked together, "polysaccharide" refers to "many" sugar molecules linked together. In starch, these glucose molecules link together either in a straight chain or a branched chain fashion.
Flour
Flour contains starch. Different types of flour contain different amounts of starch. The starch content of the flour depends upon what type of wheat made that flour. Hard wheat contains high levels of protein, making its flour excellent for breads, while soft wheat contains high levels of starch, making better flour for cakes. Examples of hard wheat include durum, hard red spring and hard red winter, while examples of soft wheat include soft red winter wheat and soft white wheat. Soft wheat makes cake flour, having a very high starch to gluten ratio, which bakes into a fine, crumbly cake texture. The interplay between protein, starch, sugars and other added ingredients helps determine the final texture and taste of the baked product.
Cellulose
Starch and cellulose contain the same glucose-based repeat units. The only difference between the two molecules is the spatial orientation of these glucose links. Starch's glucose units all orient in the same direction, while cellulose's glucose units rotate 180 degrees relative to the last repeat unit. This stereochemical difference has major biological significance. While you can eat and digest starch and the glucose molecules it contains, you cannot digest a single molecule of cellulose. You body's enzymes recognize and break down the bonds linking each of the glucose molecules in a starch molecule, but they don't recognize and cannot break the 180-degree rotated bonds in cellulose.
Flour and Sucrose
In general, most wheat and grain flours contain very little total sugar, none of which is sucrose. The starch in flour consists of glucose polymers. Some of the starch polymers may break down forming glucose monomers, accounting for some sugar in flour. However, because sucrose requires both glucose and fructose to form, the breakdown of starch does not create sucrose. Some flours, such as carob flour, contain high levels of sugar, over 10 times the amount of sugar in wheat flour. However, in general, the carbohydrates in flour consist mostly of glucose sugar molecules in the form of starch.
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