The daily output of the Minneapolis mills is so enormous that every effort is made to maintain uniformity of character and quality in the flour. A special expert with several assistants is employed for this purpose. He grinds the samples of wheat by little mills designed for the work. In the selection of wheat, the quality of flour desired must be borne in mind.

The mixing of the wheat is another preliminary process. This is the most recent and scientific method of keeping the different grades of flour uniform from year to year. The practice seems to have been adopted in the eighties, and it became well established in the United States and Europe during the

early nineties. The mixing is done by means of the elevator machinery. Each mill generally has its own elevators for storing the grain. Different bins receive the different grades of wheat from the cars. The mixing and other manipulation of grain may take place in the elevators, or by a separate system of machinery in the mill proper. The quality of wheat varies so much with climate and season that it is practically impossible to prevent corresponding variations in flour without mixing. In our country much winter wheat is grown, and the best grade of flour can be made from it only by mixing it with the hard spring wheat. The spring wheats bear a higher and more uniform percentage of gluten, and herein lies their great value for mixing purposes. It is also claimed that spring wheat flour is more regular in the time required to mature in the bread dough. These are the reasons why spring wheat brings a higher price in the markets.

Three Fundamental Processes are passed through by the grain in the milling: (1) Cleaning; (2) tempering; and (3) grinding or milling proper.

CLEANING. In this three objects are held in view: The removal of foreign seeds from the grain; the securing of clean wheat berries that are free from dust and other adherent foreign matter; and the removal of small particles of bran which would drop off afterwards and find their way into the flour. A special machine has been designed for the removal of each kind of foreign seed, such as that of other grain and of weeds. In the main, two different methods are used in the removing from the wheat berry all undesirable matter adhering or attached to it. In each method, machines adapted to the purpose are utilized. One method is known as dry cleaning, in which the wheat is passed through scourers. In the other method, the wheat is washed with water and subsequently dried. Each method has its advantages for different conditions of the grain, but some millers wash all wheat.

TEMPERING consists of putting wheat in the best of condition for milling. The coats of the berry must be so tough that the bran flakes out in one large piece in the grinding, and the interior of the grain must be in such condition as to give the largest yield of flour. There are nearly as many methods of tempering as there are mills, for each miller uses a process that

will yield the results which he desires in the final milling. Heating the wheat to a certain temperature is a part of the tempering process, and moisture in some form is always applied. This may be accomplished by one or more applications of water, of steam, or of both water and steam.

MILLING PROPER.-The wheat is passed between corrugated steel rolls, each of which moves at a different speed from its mate. The berry is not crushed, but ruptured and flattened out, so that its interior can be separated from the bran coats in the largest pieces possible. As much of the interior as is thus separated from the bran coats is sifted out, and the residue is again passed through steel rolls so that more of the interior may be separated. This is what is meant by gradual reduction.

The interior of the berry which has been separated from the branny portion is known as "middlings." This material is now passed through the middlings purifier, which removes any particles of bran that may be present, the cellulose structural material of the interior of the berry, and the germ of the grain. The latter would give the flour a yellow appearance, and impair its keeping qualities. After the middlings have been purified, they are reground, and again purified. These processes are repeated until the material is of such fineness that it will pass through the finest silk bolting cloth. The material is tested at every stage of the process, and finally the finished flour is again tested before it is shipped to the trade. An expert can determine its quality largely by feel and color. Only the largest mills have facilities for making chemical tests. The smaller millers frequently have their products tested at chemical laboratories.

The Bleaching of Flour is a process of recent origin, and there has been considerable controversy as to its merits. The most common method employed is to pass air through an electric discharge of high voltage and low amperage. This results in the formation of oxide of nitrogen. The treated air is piped to an agitator or spraying machine, through which the flour is passing in a thin running stream. The latter operation requires from 7 to 10 seconds, and during this time the flour is aged and whitened. It is claimed that the only effect upon the flour is the decolorization of its oil. Bleaching gives whiteness only, and it does not enable the miller to increase the amount of flour,

nor to change lower grades into higher ones.' Of the many processes for bleaching, the only ones having any industrial value seem to be "based on the use of peroxide of nitrogen, prepared either by chemical action or by the action of a flaming are upon atmospheric air." Flour naturally gro vs whiter as it grows older.

2

The Dust Collector.-The milling of wheat always produces flour dust. The ignition of these particles suspended in the air caused disastrous explosions in the Minneapolis mills during 1877-78. This led to the development of the dust collector, the first form of which was a filtering diaphragm. Its essential principle is now the vortical or rotary air current, which masses and precipitates smaller particles than the finest filter could arrest.

The Grades of Flour most usually made are four in number: (1) Patent; (2) first class; (3) second class; and (4) red dog. The feeds comprise the remainder of the milled product. The basis of flour grading is mainly its purity, that is, its freedom from the bran and germ portions of the wheat kernel. The best flour comes from the center of the grain. The strongest gluten is nearest to the outside of the kernel, but the outside can never be perfectly separated from the bran. The degree of purity varies with the different processes of milling. The equipment of the miller, his special process of milling, and the market to which his products go determine the number of grades of flour that he makes. He may omit some of these four grades, or he may further separate them and make a larger number of grades. Patent flour, for example, may be separated into first and second patent. The requirements of some buyers are for a flour that is sharper or more granular, while those of other buyers are for a flour with fine soft granulations and very white color. After all of the flours have been collected from the various machines into their respective grades, they are conveyed to the packing bins, from the bottom of which they are drawn by automatic power packers into packages varying in weight from 2 to 280 pounds.

1 Letters. Frank W. Emmons, Washburn-Crosby Co., and John E Mitchell. Alsop Process Co.

2 Sci. Am., S. 61:25263 (1906).

The Large Typical Mill of the Winter Wheat Belt.'-A milling plant of five buildings and six grain storage tanks or silos, having a storage capacity of 300,000 bushels and a daily milling capacity of 1,500 barrels, involves a capital investment of about $200,000. The buildings include: The mill, 88 by 42 feet, and five stories high; the warehouse, 98 by 42 feet, and three stories high; the power house, 68 by 72 feet, and 25 feet high, with a tile smoke stack 125 feet high; and the grain elevator, 48 by 42 feet, and 118 feet high. The steel tanks or silos for storing the grain are each 30 by 60 feet. All of these tanks are connected at the top with a gallery for delivering the grain, and at the bottom with a conveyor belt for discharging the grain. Four thousand bushels of grain can be received and discharged per hour. It is moved from car to elevator and conveyors by means of steam shovels. The number of men required when the mill is running day and night consists of 3 officials, 5 office employees, and 35 other employees.

When the grain is received in the mill, it is given one cleaning over ordinary separators, and then stored. Before grinding, it is given such additional separations as may be required. It takes about one hour to complete the milling. The finished product is either loaded into cars or stored in the warehouse, and it is disposed of to the local and foreign trade. The average amount of wheat carried is about 200,000 bushels, and the probable average amount of flour on hand is about 10,000 barrels. Mills located at interior country points depend largely for their supply of wheat upon the deliveries of farmers and of country grain merchants in contiguous territory. About 5 per cent of the capital invested is devoted to products other than those of wheat. About $25,000 of cash must be ready for immediate requirements. Five to 10 per cent of the business is done on credit. The average expenditure for salaries and wages amounts to about $45,000 per year. Approximately 5 per cent of the original cost is annually charged to the depreciation of plant and equipment.

One of the largest mills in the world is the "A" mill of the Pillsbury-Washburn company of Minneapolis. The ordinary capacity of this mill is 15,000 barrels of flour per day, but

1 These data are furnished by the Kansas Milling Company, Wichita, Kansas.