Milling of Flour by gyvwpsjkko


									                                       The Milling of Flour

Historical Development
The milling of flour from grain has its origins in the early history of all major civilisations. It is
generally regarded as the oldest known industry. The basic milling technique was to grind the
grain between two stone surfaces. This enabled the tough fibrous bran skin to be separated
from the endosperm, which then was ground into a fine powder. This method, utilising saddle
stones or pestles and mortars, appears to have developed independently in a number of
areas around the world.

The progression from these simple implements to rotary stone mills occurred in Europe, the
Middle East, Central Asia and the Far East. The size of the mill stones increased in the Middle
Ages as animal, water and wind power was harnessed.

White flour as we know it today was first produced in Hungary and Germany in the 18th
Century. It became so popular that the exported product sold in Europe for up to ten times
the value of unrefined wholemeal. White flour was manufactured by arranging a number of
stone mills to form a series of grinding passages. The grinding severity of each passage was
adjusted so that the grain was broken down progressively, and in this way the separation of
bran skin from endosperm was enhanced. Purifiers were introduced to remove bran from
intermediate stocks, and the remaining material, which had a high endosperm content, was
ground into white flour on additional stone mills. The process was referred to as the 'Gradual
Reduction System of Milling'.

In modern flour mills, millstones have been replaced by steel rollers. Old style reel sifters
have been replaced by modern plansifters. The design of purifiers has been radically
improved, and many new ancillary machines have been incorporated into the milling system.
The same basic principles of the Gradual Reduction System of Milling, however, are still
universally employed. Even the world's most advanced flour mills are still utilising processing
technology that was fully developed as long ago as 1930. The flow diagrams of flour mills in
all countries have been made progressively less complex over the last fifty years, with the
exception of the Japanese flour milling industry, which produces exceptionally white flour of
very low ash content. The development of the flour milling industry has been associated with
advances of equipment design, which has improved operating capacity and efficiency, but no
major technological breakthrough has occurred.

Flour Milling in Australia
The Australian flour milling industry has changed considerably since the 1870s, when more
than 500 flour mills were operating, powered by steam, water or wind.

Mill ownership has evolved from small family concerns to large diversified companies. Those
remaining in the industry have extended their operations into fields such as stockfeed milling,
bakeries, household flour, packaged cake mixes, pasta and biscuit production, starch, gluten
and glucose manufacture, and livestock production. Australian owned companies produce the
majority of the flour milled in Australia.

The decline in the number of flour mills operating in Australia has been largely due to the
application of new technology and the search for economies of scale. It is no longer feasible
for each town to have its own mill to supply the local population. Another important factor
was the steady loss of export flour markets in the Asia/Pacific region when countries in these
areas set up their own mills following World War II.

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The total domestic market for flour products has experienced steady growth under the
influence of rising population.

Currently, around 1.25 million tonnes of flour is milled for domestic human consumption each
year. Use of flour for industrial purposes, such as starch, gluten and their derivatives, has
increased significantly in recent years and now amounts to about 460,000 tonnes a year.

Flour Milling Processes

Wheat Cleaning
Before milling starts, the wheat is tested for protein content and other characteristics and
graded into various categories. It is then cleaned to remove foreign material such as stones,
dust and weed seeds. A number of grades are usually then blended together to produce the
desired type of flour. Soft, low protein wheats are most suitable for biscuit flours, while flours
for bread making are usually made from harder, higher protein wheats.

Wheat Conditioning
Water is added to the wheat blend in a process called conditioning or tempering. The amount
added is usually between 2–4%, depending on the initial moisture content of the grain and
the type of wheat being milled. The wheat is then allowed to lie in the conditioning bins for
between 10 and 20 hours. The bran layers are toughened by the addition of water,
minimising shattering during milling and the endosperm is made more friable (crumbly),
requiring less power to be ground into flour.

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Flour Milling
The separation of the endosperm material from the bran layers of the kernel and the gradual
reduction in size of the endosperm particles is achieved by a complex combination of grinding
rolls (break and reduction rolls), sifters, purifiers, and other equipment.

The tempered wheat is fed into roller mills which house two steel cylindrical rolls with
longitudinal grooves (flutes) cut into their surface. As wheat passes between the rolls which
rotate towards each other at different speeds, the bran is gripped by the flutes on the slower
rolls. The friable endosperm is torn away into granular chunks (semolina) or fine powder
(flour) by the overtaking flutes on the faster roll.

Rotating plan sifters, which incorporate a number of superimposed sieves of varying sizes,
grade the ground stock into bran fragments, coarse semolina, fine semolina and flour
according to particle size. The larger bran fragments are processed by a further three or four
break roll-sifter passages, until all of the endosperm is removed.

The purpose of the 'break' system is to break open the wheat grains to release semolina
particles and leave bran in the largest particle size possible, thus minimising bran
contamination in the semolina. The granular stocks produced are predominantly semolina
interspersed with bran particles. Although undesirable, some flour is produced at this time.
The main production of fine flour comes from the later reduction stage.

To purify the mixed granular stocks of semolina and bran coming from the break system,
they are passed through the purification system. A purifier is essentially a long rectangular
machine with oscillating sieves through which a current of air is drawn. Semolina stock is
conveyed along the length of sieves and the shaking motion, together with the upward
current of air, causes the stock to stratify. Bran fragments are floated above a layer of pure
semolina which gradually passes through the sieves. The bran fragments pass over to the
end of the sieves.

The particles of purified semolina are then crushed to fine flour by the 'reduction' system.
This is comprised of roller mills fitted with smooth rolls operating at slightly different speeds.
Flour is removed by sifting the ground stock over fine mesh nylon screens. Course mesh
sieves separate the bran flakes. The size of any intermediate product is gradually reduced by
subjecting it to repeated roll and sifting treatments. Up to eight to twelve passages may be
employed, depending on the design of the plant.

Stock failing to pass through the flour sieves at the completion of the reduction system
becomes offal (pollard), as does the course material (bran) rejected at the termination of the
break system.

All the mill products, including flour, wheat-germ and bran are either bagged or stored in bulk
bins. Flour is normally stored for at least three days before being delivered to bakeries and
food processing factories. The ageing process has a beneficial effect on the flour for most

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