FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 7 : Specific Cereals

Milling of Wheat

The milling of Wheat consists in the separation of bran and germ from the endosperm and reduction of endosperm to fine flour.

Various steps are involved in making the wheat flour. These steps are shown in Figure.

The traditional procedure for milling wheat in India has been stone grinding to obtain whole wheat flour.

In modern milling, the wheat is subjected to cleaning to remove various types of impurities together with damaged kernels.

• Vibrating Screen: This removes bits of straw and other coarse materials and second screen removes foreign materials like seeds.
  
  
  


• Aspirator: It lifts off lighter impurities in the wheat. The stream of grain is directed across screens while air sucks off the dust and lighter particles.
• Disc separator: After the aspirator it moves into a disc separator consisting of discs revolving on a horizontal axis. The surface of the discs indented to catch individual grains of wheat but reject larger of smaller material.


• Scourer: The wheat then moves into the scourer, a machine in which beaters attached to a central shaft throw the wheat violently against the surrounding drum, buffing each kernel and breaking off the kernel hairs.


• Magnetic Separator: The stream of wheat next passes over a magnetic separator that pulls out iron and steel particles contaminated during harvesting.


• Washer stoner: high-speed rotators spin the wheat in the water bath. Excess water is thrown out by centrifugal force. Stones drop to the bottom and are removed. Lighter material float off leaving only the clean wheat.


• Tempering: Wheat is tempered, before the start of grinding, the process in which moisture is added. Tempering aids in separation of the bran from the endosperm and helps to provide constant controlled amount of moisture and temperature throughout milling. The percentage of moisture, length of soaking, time and temperature are three important factors in tempering with different requirement in soft, medium and hard wheat.


• Entoleter: Discs revolving at high speed in the scourer aspirator hurl the wheat against finger like pins. The impact cracks down any unsound kernel which is rejected.


• Grinding bin: The “first break” rolls of a mill and are corrugated rather than smooth, break into coarse particles.


• Sifter: The broken particles of wheat and bran go into a box like sifter where they are shaken through a series of cloth or screens to separate larger from the smaller particles. Larger particles are shaken off from the top by leaving the final flour to shift towards the bottom.


• Purifier: The top fractions and particles of endosperm graded by size are carried to separate purifiers. In a purifier a controlled flow of air lifts off bran particles while cloth screen separate and grade coarse fractions by size and qualities.


• The down purifier: Four or five additional break rolls with successively final corrugations and each followed by a sifter are usually used to rework the coarse stock from the sifter and reduce the wheat particles granular middlings as free from bran as possible.

Sifters, purifiers and rollers reduces wheat until the maximum amount of flour (72.0%) is separatedSifters, purifiers and rollers reduces wheat until the maximum amount of flour (72.0%) is separated.

Figure : Milling of wheat

Source: From wheat to flour, 1965, Wheat Institute, Chicago, Illinois.