National National's Exploration of Starch As a dry granule or powder, as a swollen granule, as a molecule in solution, as a film or binder. Starch is unique among basic raw ma terials in its many faceted nature. To one man it may be an inert powder, to another a thickening agent, and to an other a binder. Each man looks upon starch in a different way because each of its facets carries a distinct set of char acteristics which govern its value in various applications. In some instances these properties are inherent in native starch but in many cases they must be acquired or magnified through phys ical or chemical modification. If we examine the various uses for starch we find that they depend upon the characteristics associated with one or more of four basic facets. These include: • the dry granule or powder • the swollen starch granule • the starch molecule in solution • the dried starch film AS A DRY GRANULE OR POWDER Starch is found in tubers, roots, or seeds of plants in the form of minute granules or cells. The size and shape vary depending upon the particular plant source. Some idea of their size can be realized when you remember that there
are 25,400 microns in one inch. Since the average corn starch granule is Ι5μ in diameter, you should be able to line up about 1700 granules end to end in 1 inch. One pound of corn starch con tains about 700 billion granules. This corresponds to a surface area in the neighborhood of 4000-5000 sq. ft. These granules are also edible. Corn starch granules are digestible in the human body giving a caloric value comparable to that of sugar. This large surface area and digest ibility—coupled with low cost—are ma jor factors in the use of starch in the food and pharmaceutical industries as a diluent for active ingredients such as flavors, as a fluidifying agent, dusting powder, and as a means of keeping two or more active materials physically separated as in baking powder. The surface area and low cost also lead to applications in cosmetics, dyestuffs, printing, etc. Low Moisture Starch. Besides pos sessing a large surface area, starch granules have a strong affinity for mois ture. This is attributable to the abun dance of hydroxyls in the starch mole cule. The amount of water which can be adsorbed by starch is a function of the type of starch and the relative hu midity, as shown in Table I.
Photomicrographs of starch granules
Table 1
Moisture adsorbed by various starches at different relative humidities—72T—expressed as grams water adsorbed on 100 grams anhydrous starch R.H. (%) STARCH Potato Corn Tapioca
90 28 23 24
70 19 17 18
40 13 12 13
20 8 8 8
Powdered corn starch is normally marketed at about 1 1 % moisture. As a result, at high humidities it has an abil ity to adsorb moisture. To increase its capacity for moisture vapor, powdered corn starch is frequently dried down to 7V2%, 5%, or even as low as Và% and sold in moisture-proof bags for use where it is desired to extract moisture from some other material, or protect active powders from the deleterious effects of moisture. Starch is also dried to these levels to permit its use in systems where the moisture found in normal starch might have an urîdesired effect such as in the preparation of microporous plastics. Another factor in the use of starch powder with other dry powders is its "mobility" or ability to flow. This mobility is increased by drying and decreased by moisture adsorption. At 12% moisture corn starch still retains some flowability, an important factor in its use in many areas as a powder. Hydrophobic Starch. By introducing hydrophobic groupings on the granule surface, as in National's DRYFLO®, it is possible to obtain a powder which flows like a liquid and is highly hydrophobic. Such a product greatly extends the usefulness of starch in the granule form. Although not wetted by water, it still retains the capacity of the parent starch to adsorb moisture. DRYFLO extends the usefulness of starch as a diluent, fluidifying, or processing aid by being able to adsorb much larger amounts of moisture than ordinary
© National Starch and Chemical Corporation
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AN EXPLORATION OF STARCH starch without losing its free flowing nature and by being water repellent. Molding Starch. The granules may also be modified so they will not be mobile. By spraying them with a small amount of food grade oil they will pack very easily and will accept an impres sion to form a mold. This permits the use of starch as a molding powder in the manufacture of gum candies. The starch serves not only to form the mold but to remove moisture from the candy, hastening the setting process.
AS A SWOLLEN GRANULE To properly understand the be havior of starch as a swollen or dis persed granule a knowledge of its chemistry is essential. Starch is polymeric in nature. It is a polysaccharide whose basic monomeric unit is glucose. These units are combined through glucosidic linkages with the. elimination of water. There are two types of polymers found in starches. One is amylose in which the anhydroglucose units are linked end to end to form a linear or straight chain polymer, which contains in the neigh borhood of 200-2000 anhydroglucose units. The other type is amylopectin. It is a branched polymer in which the usual end to end linkages are supple mented by branches about every 20-30 anhydroglucose units. While there is considerable uncertainty regarding its exact size, it is undoubtedly many times larger than amylose. Both polymers contain an abundance of hydroxyl groups which are capable of associat ing with each other by means of hydro gen bonding. This is normally not an important factor with amylopectin, since the branched structure prevents the molecules from getting together close enough for a significant number of the hydroxyls to associate. However, the size and linearity of amylose are conducive to close alignments which permit many of the hydroxyls of one molecule to hydrogen bond with those of another. Both amylose and amylopectin in their dissociated form are able to dis perse in hot water forming sols. How ever, on cooling, the amylose molecules tend to associate with each other via inter-molecular hydrogen bonds. In di lute solution this results in the forma 54
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tion of large complexes composed of two or more molecules which lose their solubility characteristics and precipi tate from solution. At high concentra tions they form rigid networks in which water is entrapped, giving a gel-like structure. See Figure 1. Amylopectin, however, has very little tendency to associate, and as a result, forms stable sols. See Figure 2. The granules are insoluble in cold water. When a suspension of starch in water is heated, the forces holding the granule together are weakened, per mitting the granules to absorb water and swell to many times their original size. During this process, the smaller polymers leach out of the granule and eventually the granules rupture or col lapse losing part of their hydrating ca pacity. During the process' of granule swelling, the aqueous suspension of starch becomes heavy or thickens and its clarity increases. This process con tinues until the granules begin to col lapse. The body or viscosity decreases and finally levels at some equilibrium value on continued cooking. As this loss in viscosity occurs, the dispersion changes from a pasty salve-like con sistency to a more fluid, rubbery sol. The properties associated with the highly swollen granules are utilized extensively in many applications where starch is used as a thickening agent or stabilizer. The salve-like consistency and relatively high viscosity associated with the swollen starch granules gov ern their use in the food field in salad dressings, pie fillings, canned foods, etc., and in products such as chemical specialties, cosmetics, pharmaceuticals, textile printing pastes.
Amylopectin molecules (figure 2)
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