Flour is obtained from wheat kernel by the process of milling. Flour milling has two objectives. First, it is a process that separates the endosperm from bran and germ. Second, it involves grinding the grain to fine flour. Ideally, milling separates out as much endosperm as possible without damaging starch granules, but this is difficult to do. In fact, commercial milling operations are able to extract only an average of 72 pounds of flour for every 100 pounds of wheat, for a so-called extraction rate of 72 percent, even though the endosperm makes up 85 percent of the wheat kernel.
Wheat consignment contains impurities like stones, iron particles, small seeds like argimone, rat droppings etc to about 2-4%. These impurities should be removed before milling process starts as once grinded it’s impossible to remove them & they can cause wear & tear to machines.
Different types of wheat are blended in order to achieve required quality of flour as per the consumer requirement. The blend of wheat the miller selects before milling is called "GRIST". Milling process starts from:
1. First cleaning
3. Second cleaning
a. Break rolls
b. Scratch system
c. Reduction rolls
f. Bran finisher
1) Cleaning the wheat:
It is being done to remove unwanted materials from wheat like stones, metallic impurities, other seeds etc. This is done through a series of steps.
a) Magnetic separator: The main function of Magnetic separator is to remove all the Ferrous materials.
b) Separator: Separator constitutes of vibrating screens which are employed to remove bits of wood, straw etc.
c) Aspirator: Air currents act as a kind of vacuum to remove dust & lighter impurities.
d) Destoners / Gravity separators: It helps to separate heavy stones & lighter particles that may be of the same size of wheat. It uses the principle of gravity.
e) Disc separator: Disc separator uses the principle of difference of shape /length of impurities i.e it removes impurities that are of the same size of wheat but with different shapes (like Oats & Sorghum).
f) Scourer: The scourer before the dampner helps to removes outer husks, crease, dirt and any smaller impurity which are attached to bran with an intense scouring action. Currents of air pull all the loosened materials away.
g) Entoleter: Nowadays most of the mills are equipped with equipment called entoleter. It destroys insects/eggs in the wheat by impact.
2) Tempering / Conditioning of the cleaned wheat:
After the cleaning where all the admixtures & adhering dirt has been removed, wheat is tempered or conditioned. It is done to achieve following easy & clean separation of bran & germ from endosperm. The reason for adding water to the wheat is to take the advantage of the physical differences between the germ, bran and endosperm. Tempering makes the bran tough and the endosperm mellowed, thus allowing the endosperm to separate easily. It is being done by adding water in precise amounts to the wheat and allowing it to rest for a period of 8 - 24 hours depending on the type of wheat. The conditioning time depends on (a) structure of the kernel- harder the kernel more would be the tempering time (b) natural moisture content- drier the kernel more is the tempering time (c) intensity of machine. The conditioning of wheat must be closely monitored as variations of more than 0.2 % above or below the desired moisture can cause problems in the milling process. If moisture is too high, break releases, mill capacity, protein recovery and the flour yield may decrease and if the moisture is low, ash goes up with the resultant poor colored flour.
3) Second cleaning:
Second cleaning consists of flow of wheat kernels from tempering bins to break rolls with the objective of polishing off all the dirt & loose bran particles followed by passing the grains to break roll action.
a) Break rolls The first rolls in milling are break rolls. The object of the break system is to open the grain & scrap out the endosperm carefully from the bran pieces, without fragmenting them. Most of the mills have 4-5 break rolls. The pressure on break rolls gradually increase from first to last break in order to achieve gradual & almost complete removal of endosperm form bran. In order to avoid unnecessary cutting of bran roll gaps is gradually reduced from first to last break. After each grinding operations on break rolls, the ground stock is graded in a plansifter & graded products go to different destinations. The coarse break stock goes to next break passage, flour is separated out as finished product semolina & middlings are purified & dust is ground on reduction rolls.
b) Scratch system: The composite semolina from purifiers is treated on scratch rolls. The scratch rolls are can be either fluted or smooth, depending on the hardness of wheat. The function of scratch passage is to produce maximum amount of purifiable semolina and least amount of flour.
c) Reduction system: The reduction rolls are smooth rolls. Purified stocks as well as the dust from break system are brought onto reduction passages. The reduction rolls have a roll differential of 1:1.25. The ground stock is graded in a plansifter. The branny stock is sent to the collecting passage, the over tails of flour sieve sent to next middling passage, while flour is separated out as finished product.
d) Purification: It is essential when flour of low ash content is required to be produced. The stocks to purifiers come from first three break passages and comprise of pure endosperm, particles, endosperm particles to which a piece of bran is adhering (composite semolina) and pure bran pieces. The differences in their densities & air resistance among these components are made use of to separate pure endosperm particles which are subsequently ground into flour on head reduction rolls, whereas the composite semolina is sent to scratch rolls & the branny stock to break rolls.
e) Sieving: It is being done on plansifter. Plansifter is a stock of sieves on which ground stocks are graded according to size. The stocks thus graded are directed to different machines such as purifiers, reduction rolls, bran finisher as well as packing as finished products depending on the characteristics of the product.
f) Bran finisher: It helps the last break and reduction rolls in recovering the endosperm from the coarse and fine bran.
Picture source: http://www.wheatflourbook.org/DesktopModules/IM.aspx?I=462&M=0