cereal grain processing class lecture
TRANSCRIPT
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.