Chemicals From Starch and Sugar

E. A. Talley, I. A. Wolff.

Starches and sugars belong to the important group of substances known as carbohydrates. Because they are an essential part of all plants, carbohydrates occur everywhere and in a supply that is replaced each year.

The name of the group comes from the simplest formula, CH₂O, for the central compound, D-glucose or dextrose, as it is known commercially. D-Glucose contains only carbon, hydrogen, and oxygen. In most of the principal members of the group, hydrogen and oxygen occur in the same ratio as in water. In fact, the French, who did the early research in this field, named the group hydrate de carbone (hydrate of carbon). The Germans translated it as Kohlenhydrate, and in English it became carbohydrate.

Dextrose or D-glucose, is outstanding among the carbohydrates, both chemically and biologically. It is the product of photosynthesis in the green leaves of plants in the presence of sunlight. In the process, the raw materials are carbon dioxide and water; oxygen is formed as a byproduct. The energy of the sun is stored in green leaves in this way.

Sucrose, the sugar that is obtained from sugarcane and sugar beets, is made up of one unit of glucose and one unit of fructose. These two simpler sugars differ only in the structural arrangement of their atoms. Sucrose is found in the sap of nearly all plants and is stored in fairly high concentration in the sugar beet.

Glucose is the fundamental unit of two other important plant materials, cellulose and starch, which differ only in the structural arrangement of the glucose units. Cellulose is a structural building material of plants. Starch is the form in which carbohydrates are stored in many plants. It occurs as small granules, which are rather easily separated in pure form from the rest of the plant.

MANY OF THE INDUSTRIAL USES of starch as such are based on its physical characteristics: It can form a thick paste in water, and it is a white, finely divided, and free-flowing powder that can be incorporated in foodstuffs. Its most important chemical characteristics are that it may be broken down into simpler units and that it is a polyhydroxy alcohol. Starch may be broken down into its simplest unit, D-glucose, by acid or enzyme hydrolysis. By reaction with suitable chemical reagents, such as acid anhydrides or alkyl halides, is possible to prepare, from starch, ester and ether derivatives, the properties of which differ greatly from those of the raw material itself although the size of the starch molecule is essentially retained. If the substitution is limited, the product, like starch itself, is insoluble in organic solvents but dispersible in water. More completely substituted starch derivatives, on the other hand, are usually more soluble in organic solvents and less soluble in water.

In 1948 more than 8 million pounds of cornstarch was used by the explosives industry, part as a nonexplosive ingredient in fireworks and dynamite, but much of it, after nitration, as an explosive agent. These starch nitrates, or nitrostarches as they are sometimes less correctly called, are esters formed by the reaction of nitric acid with starch in the presence of either sulfuric acid or phosphoric acid as a catalyst. The use of phosphoric acid leads to starch nitrates, which are more stable than those obtained with sulfuric acid.

Starch phosphate, another ester of starch with an inorganic acid, also has interesting and valuable properties. When phosphorus oxychloride is allowed to react with starch granules in the presence of pyridine, which is an organic base, approximately one hydroxyl group in each of the repeating units of the starch molecule can be converted to a phosphate ester. This starch mono-phosphate, insoluble in hot or cold water and in organic solvents, and containing strongly acidic groups, has valuable ion-exchange properties. That is, the compounds can be used to soften water or to remove the metallic ion of many inorganic salts from solutions containing them a desirable procedure in such industrial processes as the demineralization of sugar sirups. The nature of the starch phosphate obtained under similar conditions of reaction is greatly influenced by the variety of starch used. The starches with smaller granules show the greatest reactivity with the phosphorylating reagent.

Starch normally swells in hot water to form a paste. Water-resistant starches that are completely unaffected by boiling water, or even by heating above the boiling point in an autoclave can be prepared by treating starch with appropriate chemical reagents. Organic diisocyanates, such as hexamethylene diisocyanate, react with starch in the presence of pyridine to form cross links, or bridges, between the carbohydrate chains. These cross-linked products are notably inert, and may find uses as dusting powders, insecticide carriers, or fillers for plastics. Treatment of starch under suitable conditions with such other reagents as formaldehyde, glyoxal, dibasic acid chlorides, or phosphorus oxychloride can also convert it to products that swell little if at all under conditions that completely gelatinize untreated starch.

Acetates of whole starch or of each of its fractions may be prepared by treatment with acetic acid-acetic anhydride mixtures in the presence of catalysts. The acetates of whole starch or of amylopectin are too brittle and weak to form fibers or plastics. But the amylose triacetate ( and other saturated aliphatic triesters) can be formed into cellophane-like films, which are lustrous and pliable. Amylose triacetate can also be converted into fibers resembling acetate rayon by a process known as dry spinning, in which a viscous chloroform solution of the amylose triacetate is forced through tiny holes of a spinneret into a hot-air chamber for evaporation of the solvent. The resulting threads are wound on a reel. Mixed acetyl with higher acyl radicals ( such as the mixed acetate-propionate of amylose) are thermoplastic and can be molded into plastic articles. More extensive use of the valuable properties of the amylose derivatives depends on economic factors.

Other esters of starch ( such as the propionate, benzoate, xanthate, palpitate, thiocyanate, sulfate, and chloride esters) and ethers (such as the methyl, ethyl, and benzyl derivatives ) have been prepared. Many of these are being investigated with a view to possible industrial use.

Of particular importance are esters of the starch-containing unsaturated groups. Starch methacrylate can readily be polymerized to form insoluble materials, and may thus be of value as a plastic or as a coating compound.

An interesting compound is obtained by the reaction of chloroacetic acid with starch in the presence of sodium hydroxide. The product, called carboxymethyl starch, is said to be a potential replacement material for various natural gums in foods, cosmetic preparations, pharmaceuticals, and some other products where a stable thickening agent, constant in properties from batch to batch, is required.

The allyl ether of starch, another important unsaturated derivative, is prepared by heating starch, a strong alkali solution, and allyl chloride in a closed vessel. The ether formed is a doughy material, which dissolves readily in organic solvents. Sprayed or painted on surfaces, the solutions form a film that dries tack-free in a short time. On exposure to air, the material behaves like a drying oil in that it takes up oxygen to form an insoluble hard resin. It is useful as a coating material, an adhesive, and an ingredient in printing inks. Sulfur may be substituted for oxygen in the polymerization process. Mixtures with sulfur may be used to form rigid plastics and laminates.

Allyl starch has been successfully tested for the following applications:

As surface coatings for furniture finishes, interior wood finishes, and metal finishes.

As bronzing liquid, thermosetting adhesive, overprint and finishing varnish, and printing-ink vehicle.

Other miscellaneous uses such as solvent-proofing paper and grease-proofing paper, fast-drying undercoats, plastic for wood and other laminates, leather dressing, synthetic enamels, and dope for aircraft finishes.

For each of those uses, proper formulation ( addition of plasticizers, resins, driers) must be used.

Samples and limited commercial quantities of allyl starch are available.

Allyl sucrose (made from ordinary cane or beet sugar) has properties similar to those of allyl starch. Unlike allyl starch, allyl sucrose is compatible with drying oils.

Alkoxides of starch have been prepared in which a metallic atom or ion takes the place of a hydrogen atom on a hydroxyl group of the starch. Sodium starchate has been prepared by heating the starch in an organic solvent containing dissolved sodium hydroxide. Other metal derivatives have been prepared from the sodium derivative. The evidence indicates that only the hydroxyl on the second carbon atom of the anyhydroglucose unit is attacked. The sodium atom may be replaced by treatment with metal salts, alkyl halides, and other reagents. The copper derivative is a mildew-proofing agent. Other derivatives are being studied, and new uses are being sought for compounds prepared by use of the starch alkoxides as intermediates.

THUS FAR WE HAVE DISCUSSED the chemical products from starch in which little or no breakdown of the molecule has occurred. One possible product from the more drastic degradation of starch is 1-glucosan, an anhydride of the beta form of glucose. When starch is heated to 350 C. in an evacuated system, 60 percent of the original weight distills over as a brown viscous sirup. Treating this distillate with boiling acetone gives crystalline 1-glucosan, which, after recrystallization, is a white, alkali-stable, anhydro sugar that contains three hydroxyl groups. The crystalline material is neutral in reaction, free from unstable reducing substances, and stable to light and air. On acid hydrolysis, it is converted quantitatively to dextrose. The process for the preparation of 1-glucosan from starch has received considerable study in the research laboratory. Because no expensive chemicals need be used in the degradation, but only heat and simultaneous distillation under reduced pressure, economical production will probably be possible if suitable uses are found for it or its derivatives.

The cleavage of starch, dextrose, or sorbitol by hydrogen at high temperatures and pressures in the presence of a suitable catalyst, such as finely divided nickel, results in their conversion in good yield to a mixture of propylene glycol, ethylene glycol, and glycerol. In times of national emergency when our supply of glycerol from natural fats is inadequate, the process may have particular significance. In Germany during the Second World War, a mixture of glycols and glycerol, obtained by the hydrogenolysis of carbohydrates and called "glycerogen," was marketed as an antifreeze and for other purposes.

Levulinic acid, a 5-carbon y-ketoacid, has been prepared industrially by heating dextrose-containing materials or starch in aqueous solutions of mineral acids. It is used pharmaceutically in the form of its calcium salt and has promise as a chemical intermediate for use in plastics and in the synthesis of other organic chemicals.