Cereal Grain Processing

Flour is a finely ground cereal grain or other starchy portions of

plants.

Flour, especially wheat flour, is a basic ingredient of baked goods.

In the production of refined flour, the starchy endosperm is separated

from the other parts of the kernel by milling through rollers.

In the production of whole-wheat flour, all parts of the kernel are

used.

Following milling, the particles of endosperm (called semolina) are

ground to flour and often bleached to imitate natural aging.

When flour is mixed with water to make dough, its protein content

is converted to gluten, an elastic substance that forms a continuous

network throughout the dough and is capable of retaining gas, thus

causing the baked product to expand, or rise.

Raw Materials

Although most flour is made from wheat, it can also be made from

other starchy plant foods.

These include barley, buckwheat, corn, lima beans, oats, peanuts,

potatoes, soybeans, rice, and rye.

Many varieties of wheat exist for use in making flour. In general,

wheat is either hard (containing 11-18% protein) or soft (containing

8-11% protein).

Flour intended to be used to bake bread is made from hard wheat.

The high percentage of protein in hard wheat means the dough will

have more gluten, allowing it to rise more than soft wheat flour.

Flour intended to be used to bake cakes and pastry is made from soft

wheat.

Lima beansWheat

Buckwheat Barley Corn

Rye

Oats

Sorghum

Millet

All-purpose flour is made from a blend of soft and hard wheat.

Flour usually contains a small amount of additives.

Bleaching agents such as benzoyl peroxide are added to make the

flour more white.

Oxidizing agents (also known as improvers) such as potassium

bromate and chlorine dioxide are added to enhance the baking quality

of the flour.

These agents are added in a few parts per million.

Self-rising flour contains salt and a leavening agent such as calcium

phosphate. It is used to make baked goods without the need to add

yeast or baking powder.

Most states require flour to contain added vitamins and minerals to

replace those lost during milling. The most important of these are

iron and the B vitamins, especially thiamin, riboflavin, and niacin.

Cereal Grain Processing

Cereals are seed grains grown principally as a food source. The

major cereal grains are wheat, corn, rice, oats, barley, rye, grain

sorghum, and millet.

Wheat, for example, is processed (milled) principally for the

production of purified flour used for the production of leavened

white bread.

Flour Milling

The basic principle of the present − day system of milling wheat into

flour was introduced less than 75 years ago.

The difference between the old and the new methods is great; a meal

is produced by pulverizing the grain, whereas the production of a flour

involves dissection of the kernels, whereby the inner starchy matter is

separated from the outer skins.

A wheat grain consists broadly of three main fractions,

the germ

the endosperm

and the outer skins

The germ is the vital part of the grain which gives rise to the new

plant when the grain is submitted to conditions favorable to growth.

The germ represents about 2.5 per cent of the grain.

The function of the endosperm is to serve as a source of food for

the young plant until such time as the root system is sufficiently

developed to withdraw the required nutrients from the soil.

The endosperm content of the grain is about 83 percent of grain.

The amount of endosperm which a miller removes as flour,

expressed as a percentage of the weight of the wheat used, is referred

to as the extraction.

A flour of 72 per cent extraction, for example, would be arrived at

by finishing the milling operation with 72 parts of flour and 28 parts

of by − products for every 100 parts of wheat sent to the mill.

It is not possible to remove from the grain a reasonable proportion

of the endosperm completely uncontaminated with ground − up skins,

that is free from “bran powder”.

Conversely, it is not possible to scrape the skins entirely free from

endosperm, and so the by − product, bran always contains adhering

endosperm.

Wheat Intake

According to the situation of the mill, the incoming wheat may

arrive by ship, by barge, by railway truck or by road transport.

The method of transferring the incoming grain into the large

storage bins, or silos will vary with the size of the mill and with

mode of transport.

In small mills where wheat is arriving by road and rail the grain

may be tipped into a pit or hopper which feeds a bucket elevator for

carrying upwards.

Wheat arriving at larger mills by water is conveyed from the ship

to the silos by a pneumatic system in which the grain is sucked up a

flexible pipe.

When wheat is discharged from the intake plant it passes through

automatic weighing machines and thence to aspirated sieving machines

which remove the coarser impurities and dust.

The grain is passed over sieving machines designed to remove

impurities which are smaller and others which are larger than the grain,

and during the sieving processes aspiration is applied whereby light

pieces of skin which may have become detached from the surfaces of

the grain, light seeds and dust are removed.

After undergoing this preliminary cleaning the grain is transported on

conveyors to the storage bins.

Wheat Cleaning: Dry cleaning

When wheat is required for milling it does not pass direct from the

storage bins to the mill but is circulated through the “screw − room”

where it is subjected to a vigorous cleaning and is then washed and

conditioned.

When wheat is required for milling it does not pass direct from the

storage bins to the mill but is circulated through the “screw − room”

where it is subjected to a vigorous cleaning and is then washed and

conditioned.

The first operation in screen − room is a sieving which is followed by

an aspiration. (This device works like a vacuum cleaner. The aspirator

sucks up foreign matter which is lighter than the wheat and removes

it)

The sieving machine is designed to remove miscellaneous impurities

which are either larger or smaller than wheat grains. The grain can then

with advantage be passed over a magnetic separator which will remove

any scraps of metal that may be in the wheat.

The next stage of the wheat cleaning process is performed by disk

separator.

A disk separator (a disc separator designed to catch individual grains

of wheat and reject larger or smaller material and then to a stoner for

removal of stones, sand, flints, and balls of caked earth or mud),

moves the wheat over a series of disks with indentations that collect

objects the size of a grain of wheat. Smaller or larger objects (grains

of other cereals, such as oats, barley and rye) pass over the disks and

are removed.

In addition, the wheat is passed through a machine known as a

scourer in which it is rubbed by beater against the inside of a metal

cylinder which often is lined with emery. The final cleaning operation is

brushing. The wheat is conveyed along a cylinder by means of a spiral

brush, the vigorous brushing which it receives loosening any remaining

dust and dirt (lightly adhering), thin skins (which are called beeswing)

and giving the wheat a polish.

Washing and Conditioning

Despite the intensive dry cleaning operations which are applied in

the screen − room, wheat may retain some types of impurities. These

may include dirt which is adhering firmly to the wheat, and small

stones and pellets (a small round ball) of soil which are sufficiently

similar to wheat in size to preclude their removal by sieving.

These impurities can be effectively dealt with washing.

The washing is effected by feeding the wheat into a worm

conveyor immersed in running water, the conveyor being so designed

that the upward motion of the water which it produces enables the

wheat to be carried the length of the worm without falling to the

bottom of the trough.

This treatment (washing) loosens and removes adhering dirt, while

stones which may be present sink and are removed by another worm

conveyor.

The washed wheat passes into a centrifugal machine known as

“whizzer” and the surplus water is removed and the damp wheat is

then subjected to heat in a “conditioner”.

The object of the conditioner is to bring the wheat to a physical

condition which enables the endosperm to be separated from the skin

more readily during the milling process.

The standard form of conditioner is a tall metal machine of

rectangular section which is fitted with a large number of air ducts,

hot water radiators or both.

Wheat is fed into the top of the machine and descends slowly by

gravity being heated in the earlier stages either by hot air or by the

radiators.

In the latter part of its travel the wheat is cooled by means of air

currents.

In a modern development of the conditioning process wheat is

wormed through a drum where it meets steam jets, whereby its

temperature is raised to 1200F to 1600F in less than one minute.

The wheat is then passed immediately into cold water. It is claimed

that the sudden rise and fall of temperature causes some loosening of

the outer skins and thus aids the clean removal of the endosperm

during the milling process.

If conditioning results in too much moisture, water can be removed

by vacuum dryers.

The Break System

Wheat of different grades and moistures is blended together to obtain a

batch of wheat with the characteristics necessary to make the kind of flour

being manufactured.

On entering the mill the conditioned wheat goes to a pair of large

corrugated metal rollers known as breaker rolls which rotate at different

speed of 2½ : 1; thereby exerting a shearing (to break under pressure) action

on the grain which passes between them. The effect of this action is to split open the grains [This category consists of pieces of the interior

which are still attached to the bran (hull orhusk)]

to scrape out some of the endosperm in the form of granules (semolina

and middlings or farina)

to form a small amount of endosperm in powder form that is as flour.

The stock leaving the rolls, i.e., the mixture of split open grains,

released granular endosperm and a very small amount of flour, is

conveyed to the first break scalper where it is sieved into its

component fractions.

The split open grains are then sent to the second break rolls, where

they are opened up more or less completely and more granular

endosperm and a little flour are released.

This mixed stock is sieved and the split open grains go to the third

break rolls where more endosperm is scraped from them.

After another sieving the residue of the grains, which is now little

more than flattened skins with some adhering endosperm, goes to the

fourth break rolls, which in many mills perform the final scraping.

The stock from the final break rolls goes to the corresponding

scalper and the residue remaining after the released endosperm has

been sieved away is bran.

Break Scalping, Purification and Reduction Systems

The removal of the granular endosperm from the broken open

grains is done by “scalping”, and is performed upon sieves or

“plansifters” that operate in a horizontal plane, or on rotating

cylindrical sieves, which are known as centrifugals.

The separated granular endosperm, which is termed semolina,

middlings or dunst according to its particle size, is graded and

purified on machines known as “purifiers”

A purifier by means of which this is accomplished consists of an

inclined sieve, the mesh size of which increases from the head to the

tail of the machine.

The sieve is enclosed in a casing and air is blown up through it.

Vibrating motors apply a reciprocating motion to the sieves that

hang in inclined position

Sieve hangers could be adjusted to vary the sieve inclinations and

strokes that move the material

Air currents drawn through the sieves fluidize and stratify the

material based on the particle size, specific weight and shape.

As the granular endosperm is conveyed down the sieve by the

reciprocating action, it is graded because of the different mesh sizes

over which it passes and, at the same time, small pieces of wheat skin,

which would otherwise pass through the sieve and thus contaminate

the sieved stock, are removed by the upward current of air.

Larger pieces of skin are held up by the air current and float along

to the end of the machine where they are removed.

The grinding of the purified semolina and middlings to flour is

performed upon smooth chilled − iron rolls, which constitute the

“reduction system”.

The number of reduction rolls varies with the size of the mill but

may reach sixteen. The powdered endosperm, i.e., the flour, is

removed by sieving, the sieving machines being known as “flour

dressers”.

When the streams of flour leaving the various reduction rolls are

all mixed together the resulting product is known as “straight − run”

flour.

The purer stocks from a few of the reduction rolls at the head of the

mill are separated from the remaining reduction flours and are mixed

together to form a “patent” flour.

Such flours represent a low extraction, e.g., 30 per cent to 50 per

cent, and sell at a higher price than “straight − run” flour, which is

of longer extraction.

Flour Treatment and Flour Bleaching

When freshly milled flour is stored it undergoes an aging effect

which whitens its color and at the same time causes it to produce a

stronger and more resilient dough when mixed with water, and hence

a dough that handles better in the bakery and gives bolder loaf.

Because of this it was at one time customary for the miller to give

his flour several weeks’ aging before he sent it to the baker.

Subsequently it was discovered that the improvement in dough

properties and the bleaching, which were obtained slowly by natural

aging, could be produced rapidly by adding to the flour certain

oxidizing substances.

Some of these of compounds, such as, ammonium persulphate and

potassium bromate, improve the baking quality but have no effect

upon the whiteness.

Others, such as nitrogen peroxide and benzoyl peroxide, bleach

the flour but have no effect upon dough properties.

Nitrogen trichloride and chlorine dioxide, both bleach and

“improver”.

It is now the standard practice for millers to use one or a

combination of these substances to produce the necessary ageing

effect and to bleach their flour at the time of manufacture and thus

avoid the disadvantages of protracted storage.

The proportions in which the improvers and bleachers are normally

used are:

Nitrogen peroxide: 5 parts per million

Benzoyl peroxide: 15 – 45 parts per million

Ammonium persulphate: 160 parts per million

Potassium bromate: 20 parts per million

Nitrogen trichloride: 60 parts per million

Chlorine dioxide: 30 parts per million

The chemical composition of wheat varies according to the variety

and to the conditions under which it is grown and harvested.

The chemical composition of a flour will depend upon the wheat

from which it was milled and also upon the extraction. The higher

the extraction of a flour produced from a given wheat the higher its

contents of protein, mineral matter and fiber.

When wheat flour is mixed with water about half its weight of

water a dough is formed which has elastic properties.

Wheat flour differs from all other cereal flours, because on

admixture with water, they yield plastic masses which are not elastic.

The reason for this difference is that in the presence of water the

Protein Quality

the two main proteins of wheat flour form a complex known as

gluten, which is both extensible and elastic.

During dough formation the proteins dispersed throughout the

flour are converted into a network of interwoven strands of gluten

and this serves as the skeleton girder work of the dough mass.

Obviously the physical properties of the dough will depend upon

the number of gluten girders which are present and upon the

properties possessed by these girders.

These two factors depend in turn upon the proportion of protein in

the flour and upon the nature of that protein.

As wheats may differ widely not only in protein content but also

in protein quality, the dough properties of a flour are determined by

the wheats from which it is milled.

Some wheats yield a gluten which is strong and tough, while others

give rise to a weak and flowy gluten.

The physical properties required in a dough depend upon the

purpose for which the dough is to be used and this, therefore,

determines the quantity and quality of protein called for in the flour.

If bread is to be made, the flour should contain a reasonably high

proportion of protein which furnishes a strong and elastic gluten.

Flour intended for confectionery work should be only moderately

proteinous and should yield a weaker and less elastic gluten, while

biscuit flours should be low in protein content and the glutens they

furnish should be weak and extensible.