Starch Is a Number of Materials

M. M. MacMasters.

Starch (in the form of tiny granules) is deposited in large quantities in the storage organs of various plants.

Many processing methods have been developed for separating the starch in pure form from the other plant materials, thus making starch available for industrial use. Cornstarch, for example, is obtained by a wet-milling process.

The application of this process to other cereal grains has been studied at the Northern Laboratory. With minor modifications, starch can be separated from grain sorghum and wheat by the method. Similar separation from barley, oats, and rye is possible although not economically practical at ordinary market prices of those grains.

Three methods are now in use in this country for the production of wheat starch from flour.

In one, the Martin process, which has been used for many years, the flour is mixed with water to make a dough about the stiffness of bread dough. It is allowed to stand for an hour or longer to permit the gluten to become cohesive. The dough is then kneaded under a water spray to wash out the starch, which is purified by sieving and gravity separation.

The second method is similar to the Martin process, but minor changes have been introduced to make it a continuous, rather than a batch, operation.

A third method, known as the batter process, was developed by C. E. Rist at the Northern Laboratory in 1943. It was adopted by industry during and after the Second World War, to produce wheat starch for conversion into glucose sirup, dextrose sugar, and industrial alcohol. Flour is mixed with water to form a uniform, elastic batter. When the batter is broken up mechanically in excess water, the starch and gluten separate quickly while the gluten forms small curds, or lumps. The starch is then removed on a mechanical screen and is further purified by gravity separation.

STARCH GRANULES vary in size and shape, depending on the kind of plant in which they have been formed. They are usually spherical or more or less egg-shaped, although often they appear flat in photographs.

In some plants, such as rice and wrinkled-seeded varieties of peas, compound granules are common. The groups of many simple granules are closely pressed together into globular masses, which only partly break up into the simple granules as the starch is removed from the plant.

The diameter of starch granules varies from 2 microns to more than 100 microns. (A micron equals 0.001 millimeter, or 39.37 millionths of an inch.) If 100 starch granules each 10 microns in diameter were laid in a row, the length of the row would about equal the thickness of a dime.

In bulk, starch is white, like new-fallen snow, but under the microscope the granules are transparent. Seen microscopically between crossed Nicol prisms, each granule is gleaming white with a dark cross dividing it into four segments. The cross may be regular or very eccentric, depending upon the shape of the granule. This appearance between crossed Nicol prisms, known as birefringence, indicates that there is an orderly arrangement of starch molecules within each granule.

STARCH GRANULES are heavier than water, a property used in purifying starch by gravity separation during processing of plant material for starch production. Dry starch seems light in weight, however, because a large amount of air surrounds the granules.

When they are heated in water, as in making paste, starch granules undergo peculiar changes. Birefringence is lost within a certain temperature range, which is characteristic for each kind of starch. The range usually is 9 to 18 F., and for most starches lies somewhere between the approximate limits 130 and 175 F. Birefringence disappears first around the intersection of the arms of the cross, then outward to the periphery of the granule. The temperature at which a granule has just lost all birefringence (and is therefore invisible when viewed between crossed Nicol prisms) is known as the gelatinization temperature. The individual granules of a starch sample have different gelatinization temperatures, but all lie within the gelatinization temperature range for that kind of starch.

As starch granules are heated in water above their' gelatinization temperature, they swell and usually lose some of their starch molecules into the surrounding water. The molecules seem to seep out, as the granules are not necessarily broken at this time. The greater the swelling, however, the more fragile the granules become. Sometimes the jostling of adjacent granules is enough to cause each of them to break into several large pieces. If the suspension of swollen granules is stirred hard enough, the granules break into fragments less than 1 micron in diameter. Such behavior partly explains the decrease in viscosity of starch paste during prolonged stirring a point of importance in industry.

Gelatinization and swelling of starch granules can be brought about at room temperature by treating the granules with solutions of certain chemicals, especially those that are alkaline.

Swollen starch granules form a viscous paste if the concentration of starch is high enough. The viscosity depends on the kind and concentration of starch and on how much the granules have swollen and the amount of breakage they have undergone. Under ordinary conditions, though, a paste containing 2 percent potato starch is viscous and long, while a 5 percent cornstarch paste is less viscous and short. A long paste forms "strings" if a rod is dipped into it and pulled out; a short paste breaks away as the rod leaves it.

Starch paste sets to a gel when it cools. The type of gel depends on the kind of starch used. Cornstarch, for example, forms a rigid, nearly opaque gel. Tapioca forms a clear gel, which does not hold its shape. Stiff gels, such as those of cornstarch, tend to lose water as they age.

WHEN WE SPEAK of starch, then, we speak of a number of materials, alike in a broad sense, but actually differing considerably from one another in both physical and chemical properties. We might compare the term "starch" with the term "dog" while all dogs are alike in a general way, a St. Bernard is quite different from a Pekingese.

All starches do have one thing in common : They can be broken down into glucose sirup containing dextrose sugar. If the breakdown is carried only a little way, dextrins are obtained. Their molecules are similar to those of starch but contain less glucose residues. Dextrins, glucose sirup, and dextrose sugar, both crude and purified, have important uses in industry.

Although starch can I be separated from literally hundreds of kinds of plants, only a few are economical sources of starch for industry. Corn, wheat, grain sorghum, potato, and sometimes the rice and sweetpotato starches are made in the United States. The tapioca and sago starches we use are imported.

About 98 percent of the starch produced in the United States is obtained from corn, because that grain is the cheapest and most plentiful source of starch that we have. If starch is to be used for the production of glucose sirup or dextrose sugar, any kind will serve, and about a third of the cornstarch produced in 1948, for example, was so used. In that year our corn wet-milling industry produced more than 3 1/2 billion pounds of starch, of which about 1,389 million pounds was sold as such, while the remainder was converted and sold as approximately 158 million pounds of dextrins, more than 1,333 million pounds of glucose sirup, and about 712 million pounds of dextrose sugar. A considerably smaller amount of glucose sirup was made from wheat starch. Relatively minor quantities of potato, wheat, and possibly rice starch were also produced, mainly for special uses.