to within ten years the system of making flour was exclusively by what is now known as " low grinding" or " low mill ing," which is the ordinary system of making flour by one grinding and of separating the meal by a single bolting. In this operation, however, the wheat-germs and much of the bran are reduced too fine for complete separation, resulting in an impure flour which, though it will not keep, is preferred by many for domestic use.
111kh more modern system of high grinding avoids as far as possible the production of flour during the first passage of the grain through the millstones. The principle involved in this process is that of a system of successively graduated crackings of the wheat, with alternate separations of the bran and flour as rapidly as they are pro duced.
ilfilling is the system in which the grain receives more crack ing than in low milling and more pressure than in high milling. Each of the above methods is best suited to certain kinds of grain—low milling for soft wheats, and high milling for hard wheats.
or is the improved modern method of making flour. This is the system, of shearing the wheat-berries by grooved roils and of mashing and grinding the fragments by smooth rolls, all of which have differential face velocities.
Disintegration is a system in which neither stones nor rolls are used, the wheat-grains being violently thrown against one another and struck by a hard body moving with a high velocity (p. 45).
The Decortication of Grain as a preliminary to grinding has been prac tised from the earliest times. The Romans pounded and rasped the grain with pieces of brick, and also with sand, in mortars. In England pearl barley was prepared by submitting the grains to a grating action, but without crushing them, in a mill having a stone like a grindstone, roughened on its circumference, revolving in a grater-like metallic cas ing in which the serrated edges of the metal pointed inward and upward. In Germany decortication was done between millstones set close enough to rasp of the bran, but not to mash the kernel. In Mexico the practice is first to soften by immersion in lye or lime-water, and then to remove the hulls of the shelled maize by hand-roller mashing. The hulls are also removed by a thrashing and grating action in the hominy-mill. A Prus sian practice is to decorticate by centrifugal action, whirling the grain at a high velocity against a perforated grater, through whose meshes the dust and bran are driven.
Structure of the to understand the action of the decor ticating-mill, a brief description of the nature and structure of the grain will be necessary. A grain of wheat is composed of the following membranes or coats and enclosed parts, which are clearly shown in the magnified sections on Plate 4 (figs. 1, 2). Naming them in order, beginning with the external skin (fig. 2), we have (t) the epidermis, (2) the efiicarfius, and (3) the endo carpus, these three together forming the outer skin of the grain; (4) the testa membrane; (5) the embryo membrane, containing in its cells the sub stance to which the name "cerealine" has been given by its discoverer, the French chemist Mege Mouries; and (6) the embryo or germ. These six distinct parts constitute what is commonly called the "bran." Within these, and making up the entire central body of the grain, are (7) the flour cells, to which the name of per/SPEW/1MM has been given. The three outer coats and the outer of the two membranes are composed principally of ligneous tissue, and constitute from three to five per cent. of the entire volume of the grain. The presence of cerealine in flour impairs the qual ity of the bread; the importance, therefore, of eliminating it in the process is apparent. Practically, flour which contains no cerealine is white, of fine texture and of palatable flavor, and will retain these qualities unim paired for any length of time. The difficulty of entirely removing these coats from an unbroken berry of wheat by any machine acting upon its exterior may be clearly seen by reference to Figure I, which represents the average normal size of the berry, also a section and cross-section of the same, magnified eighteen diameters, exhibiting the relative thicknesses of the outer coats, the flour-cells massed within, and particularly the pecu liarly looped, infolded outline of the longitudinal groove running the full length of one side of the berry. It will be seen that a mechanical scraper may readily remove the outer coats from the rounded parts of the berry, but could not be so contrived as to penetrate the interior of the fold for removing every part of these coatings.