This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Adsorption and impurities in the solution. Freundlich states that "in the vast majority of cases the foreign substance lowers the rate of crystallization." The rate of crystal growth is retarded because of adsorption of the foreign substance by the crystals. If the substance is strongly adsorbed, crystallization may be retarded to such a rate that it is practically abolished, even though the solution is very supersaturated with the crystallizing substance. Since the adsorption prevents crystal growth, if a precipitate is formed in the presence of a substance strongly adsorbed by the precipitate, it will crystallize in a much finer state of subdivision. Sometimes the substance is more strongly adsorbed at one face than at another, so that this face grows more slowly than is customary and thus becomes comparatively larger in relation to other faces. This explains why crystals from pure solutions may have different shapes from those obtained from impure solutions. Speaking of sugar manufacture and crystallization of sucrose, Zitkowski says, "If the content of non-sugars in the solution is high enough crystallization may be prevented even though evaporation is carried to the point of dryness." The addition of other carbohydrates, such as dextrose, levulose, and starch, to sucrose solutions retards the crystallization of the sucrose. The extent to which they delay crystallization depends upon how strongly they are adsorbed and the proportion added.
Crystallization of sucrose in sugar manufacture. In sugar manufacture, when it is time to start crystallization in the mass of boiling sirup, the sugar boiler may lower the pressure, which causes violent boiling or agitation, or he may add a sirup or water to change the concentration or temperature. He may also seed the sirup by adding a sufficient amount of small sugar crystals. The crystals when first formed are too small to be visible in the sirup but grow to visible size. This may be in a few moments. The sugar boiler controls the temperature, pressure, and density to bring the crystals to the desired size. If they are too small when finished, too much sugar is lost in washing the liquid from the crystals. If they are too large, the public does not care for them.
Grades of sugar. Rice states that there is a surprising lack of information about the various grades of sucrose sugar produced and their specific uses, probably because the information has not been given general circulation. He lists 36 grades of sucrose, to which may be added many more grades made for a specific use with a particular equipment. The following classifications are those given by Rice.
The refined sugar distributed for household use is somewhat smaller in grain size than that found some years ago, because the general demand has been for a more rapidly dissolving sugar. As the finer grain gives a whiter appearance, the tendency has been to produce a finer sugar. Fine Granulated is produced in the largest quantity of any grade. In the same class, each with increasingly finer grain or crystal size, are Extra-fine Granulated, Berry or Fruit or Fruit Powdered (it is designated by all these names), and Coating. Rice states the name Fruit Powdered has nothing to do with fruit nor is the sugar powdered. The "Coating" is used where unusually fine crystals are desired.
Four sugars, produced especially for use when high-temperature cooking is necessary in making clear hard candies and very white fondants or pan-coated goods, or when an extremely hard grain must be used, are Coarse, Standard, Medium, and Manufacturers' Grade. Manufacturers are tending to use Manufacturers' Granulated, which varies from the others only in that it is the finest of the four and will dissolve much faster. In this same class are the Sanding sugars. Sanding sugars are used to sprinkle upon the surfaces of many soft candies and fruit products to prevent their sticking together. They are also used to sand gum drops and fruits which are to have further quantities of sugar crystallized upon their surfaces. The crystals of Sanding sugar are generally nearly perfect and exact in size.
There are generally produced three grades of machine powdered sugar differing only in the degree of fineness: Coarse Powdered, having about the fineness of Coating sugar; Standard Powdered, being somewhat finer; and XXXX, commonly known as Confectioners', being as fine as practical to produce with the ordinary type of sugar mill. A still finer powdered sugar, made with an improved powdering mill, is sometimes designated as 6X or Special XXXX. This last sugar is particularly valuable in preparing cold icings, because they remain softer longer than when prepared with coarser powdered sugars. The smaller the particle size the greater the amount of moisture held, because of the proportionally greater surface area. Rice states the number of X's applied to different sugars means nothing in actual fineness. Powdered sugars may be treated to prevent caking. Usually 3 per cent of cornstarch is added. A tri-calcium phosphate has been introduced for this purpose and 1 per cent of it is more effective than 3 per cent of cornstarch. Its use is not as yet permitted in some states.
Rice describes a new and different sugar called transformed sugar. It is produced in the same way as refined sugar up to the point of crystallization. It is then treated to give a very small grain with exceedingly irregular surface, the surface being penetrated by cracks or recesses which cause the grain to crumble easily, and, because of the relatively large surface, to dissolve almost instantly when dropped into water. The particles may be crushed easily and are used in chocolate coatings. "The crevices in these grains are very small and naturally are full of air. When this sugar is used for creaming with shortening in the production of cake or in very dry dough for biscuits, this finely divided air cannot collect and escape in large bubbles but acts as a leavening agent and reduces the necessity for most, if not all, other leavening agents." Because of its fine and fluffy condition it can replace powdered sugars in various prepared drinks and food mixtures. At present there are seven grades of transformed sugar, varying in fluffiness and color.
Soft brown sugars are graded according to color and range from a nearly white No. 1 to No. 15, which is as dark as roasted coffee. The grain of these sugars is softer than that of refined sugar and the percentage of invert sugar increases with increasing darkness in color.
Amorphous sugar. When sucrose is melted, or when solutions of sucrose are heated to high temperatures and then allowed to cool, crystallization does not occur immediately. A very brittle, very hard, solid, transparent mass is formed. This form of sucrose is known as amorphous sugar. It crystallizes very slowly, sometimes taking several months or years to crystallize. Candies of this type are extremely hard. Some candies are cooked to stages between those of the soft crystalline ones and the hard amorphous forms. Caramels are examples of this type of candy -they are fairly soft, yet are not crystallized.
Crystallization in candy making. Large crystals are the result of growth, for which several days or weeks may be required. The growth of the crystals is favored if the solution is not stirred, if the supersatura-tion is not great, and if no crystals are added to the solution. Since small crystals are desired in candy making, the procedure followed is the opposite of that for obtaining large crystals. The period of crystallization is short, usually not over 30 minutes is required, the supersaturation is great, and the solution is stirred after supersaturation is attained.