The Chemist Looks at Starch

R. J. Dimler.

When a chemist looks at starch, he tries to find out what it really is, why it behaves the way it does, why some starches are different from others. He draws a mental picture of the chemical structure of starch to see how it is put together. With this picture and his studies of the behavior of starches, he can show the way to new products, new uses, and more satisfactory adaptations of the present uses of starch.

Starch is a type of compound the chemist calls a polymer; that is, one in which the molecules are built up by the chemical union of hundreds or thousands of molecules of a simpler substance. In starch, this simpler substance, or building block, is D-glucose, or dextrose, which combines with itself chemically through a dehydration reaction to make the starch molecule.

Starch can be broken down again to dextrose by acid hydrolysis or by enzyme action. The first is a step in the commercial production of dextrose. The latter is involved in the digestion of starch in the body. All starches are alike in that they are built up from dextrose and can be broken down, therefore, into dextrose.

The chemist has a diagrammatic picture of the dextrose molecule ( shown in the first figure) , which he uses to indicate how starch molecules are built up. The carbon atoms (C) are numbered for ease in indicating the position of linkages. The relative positions of the hydroxyl (OH) groups and the number of carbon atoms distinguish one simple sugar from another, such as dextrose from galactose, each of which is a 6-carbon sugar (or hexose) , or from xylose, which is a 5-carbon sugar (or pentose). When two molecules of a simple sugar, or monosaccharide, are linked chemically, the result is a disaccharide (such as maltose) or, with two different monosaccharides, sucrose (cane or beet sugar) or lactose. Maltose is obtained from starch by enzyme action; its structure typifies the kind of chemical linkages between the dextrose units in starch.

A fragment of a starch molecule as the chemist diagrams it is charted also on page 130. Here the dextrose units have been combined by a type of linkage known as an a-1,4-glucosidic linkage. It links carbon atom No. 1 of one dextrose unit to carbon atom No. 4 of the next. For many years, the chemist thought starch consisted of only one type of molecule. Now he knows that in starch there are at least two different types, amylose and amylopectin. Since the discovery of methods by which the two types can be separated from the starch granule, the chemist can look at the amylose and amylopectin individually.

The most satisfactory processes for separating, or fractionating, starch depend on the ability of certain organic chemicals to combine physically with the amylose fraction, giving an insoluble complex that can be removed mechanically from the mixture. The amylopectin fraction remains in the solution, from which it can be removed if desired. Among the chemicals that have proved particularly useful for separating amylose from amylopectin are butanol, Pentasol, 1-nitropropane, 2- nitropropane, nitrobenzene, and stearic, oleic, and other fatty acids.

With butanol, for example, the fractionation procedure consists of first saturating a 3-percent solution of starch in hot water with the butanol (about 10 percent). The mixture is allowed to cool slowly for about 24 hours, when the amylose-butanol complex forms, usually as microscopic needles or rosettes, which are removed by centrifuging. The crude amylose fraction thus obtained can be purified by redissolving it in hot water saturated with butanol. On slow cooling, the insoluble complex again forms and is removed as before. The wet cake, or thick paste, of either crude or purified amylosebutanol complex is carefully dried, frequently by dehydration with ethyl alcohol. Special methods have been developed for this step to keep the product from being horny or gritty and therefore hard to dissolve or react with chemicals, as for the production of acetates or other derivatives.

After the amylose-butanol complex has been removed, the amylopectin fraction remains in solution in the water saturated with butanol. To recover the amylopectin fraction in dry form, the solution is added to a large excess (5 to 10 parts) of ethyl alcohol with vigorous stirring. The amylopectin separates, or precipitates, in finely divided form. It is removed and dried, again using special methods to insure a light, fluffy powder.

NOW THAT the chemist has the amylose and amylopectin fractions before him, he can look at their molecular structures. At this point he reminds us that these are fractions of starch, not definite compounds. Each fraction probably contains a mixture of rather similar molecules, which vary in size and other fine points of structure. The chemist's molecular pictures therefore represent composites, which show the distinctive characteristics of the two fractions. The most important difference he sees between amylose and amylopectin molecules is in the way they are put together. Amylose has linear or straight-chain molecules, while amylopectin molecules are branched, perhaps bushlike in shape. As we shall see, this difference has a marked effect on the behavior of the two fractions.

All the dextrose units in amylose are linked one to the next by the a-1,4-glucosidic linkages that are predominant in starch. Roughly, 500 or more dextrose units are thus combined like the links of a long chain.

Amylopectin molecules seem to be combinations of short lengths of the same type of chain, containing 20 to 25 dextrose units per length. The short lengths have been linked together, the No. 1 carbon atom of the end dextrose unit on one chain being attached by an a-glucosidic linkage to the No. 6 carbon atom of a dextrose unit somewhere along the next length of chain, as shown on the next page. That gives a branched, more or less spherical structure, in which the exact arrangement of the branches is still unknown. The entire amylopectin molecule apparently contains more than 50 of these short lengths of chain linked together, giving a total of more than 1,000 dextrose units.

These are large molecules macromolecules from the chemist's viewpoint. Yet they are still very small in terms of actual measurements.

If starch molecules are so exceedingly small, how can we picture the details of their structures? The chemist has drawn his picture from the way the starch fractions behave. Much of the behavior of the amylose and amylopectin fractions of starch is what would be expected of linear and branched molecules, respectively. In addition, some properties point to the type of linkage involved in the branching.