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The Story of Flour MillingClick on the image
below to view "The Story of
Flour Milling" which is a six page booklet (reproduced courtesy of the Bread Research Institute of Australia).
The Story of Flour Milling
The term flour is
generally used to describe any edible substance in powder form. Of all
the cereals milled for flour, wheat is by far the most versatile as hydrated
wheat protein is capable of producing a dough that retains gas under pressure.
The other cereals are milled, and used at various times in cooking, but
they do not give elastic doughs with the same properties as wheat. They
are therefore not suitable for baking risen products such as bread or
cake unless combined with wheat flour. To most Australians "flour" means
wheat flour.
Milling grain dates
back to prehistoric times. Originally the grain was pounded between two
stones to form a meal, then later the Romans invented the quern, an arrangement
of two millstones the upper one of which was turned by slave or beast.
Once the grain was crushed it was sifted through horsehair sieves to produce
different grades of flour from the preferred finest white to wholemeal.
The invention of the
water-powered mill goes back to about the birth of Christ. Windmills first
appeared in Europe around 1300 and were widely used until the invention
of the steam-engine in 1751 provided a more efficient power source.
Stone Grinding has
been retained by some mills due to the popularity of stone ground products
within a sector of the market.
STONE GRINDING
Stone grinding consists of feeding grain into the recessed centres of
two grinding stones placed one on top of the other. The top flat stone
rotates slowly gradually pulverising the grain while the bottom stone
is furrowed with a series of grooves which distribute the grains outwards
over the whole face of the stone. The furrows also helps to cool the grain
during grinding.
ROLLER MILLING
The development of the roller mill in the mid 19th century revolutionised
the milling industry. The system of roller milling is still in use today.
Before milling commences, samples of various wheats are analysed and blended
so as to produce flours best suited for the intended purpose. The wheat
is thoroughly cleaned then tempered by moistening with water to toughen
the outer protective "bran" layers. Extra water is added to soften the
"endosperm" so as to assist control the degree of starch damage during
milling.
This endosperm is
a crystalline starch and protein matrix that is food for the young wheat
plant, and also for yeast during fermentation, and for humans. In small
particles, it is called flour. The grains are broken open by passing them
between fluted steel "break" rollers. After each of the "breaks", a series
of sieves are employed to grade the stock, with several separations employed.
Endosperm released is sent to "purifiers" and then to "reduction" rollers,
and particles with bran attached sent to another "break" roller. The residue
after the last of the "break" system is termed "bran". "Reduction" rollers
gradually reduces the particle size of the endosperm and separate flour,
small bran and germ. The residue of the reduction system is often termed
"pollard".
The flour, sieved
through fine silk or nylon, is collected from various mill-streams and
blended to give the type of flour desired. In Australia it is customary
to carefully select the wheats as required, and then combine all the flour
streams to produce a straight run flour. To improve colour and baking
quality, flour may be bleached by the addition of small quantities of
benzoyl peroxide as permitted by the Pure Food Regulations.
The aim of the miller
is to separate as much good quality white flour as possible from the grain
without excessively damaging the starch.
Milling processes
remove most of the bran layers and germ which together amount to about
12-15% of the wheat grain. The shape of the wheat grain, which has a crease
down its length, prevents a perfect separation of bran and germ from the
endosperm so a clean white flour of 85% extractor is not possible. Depending
on the milling technique, up to 50% of the germ may be retained in white
flour.
EXTRACTION RATE
The number of parts by weight of flour that is produced from 100 parts
of wheat is termed the flour yield or extraction rate. In Australia the
average extraction rate is about 76% with a range of 70-80%. Other countries
maintain different extraction rates. In New Zealand it is 78%, producing
flour with more branny flecks and poorer colour, and in Japan the rate
may fall as low as 60%, producing a very soft, fine, white flour of lower
nutrient content.
Wheat varieties are
developed for both agronomic and end use quality. These varieties are
segregated into several main classes of Australian milling grade wheat:
- Australian Prime
Hard: must be of specified hard-grained varieties and have more than
13% protein (mainly segregated in Qld., Northern N.S.W.).
- Australian Hard:
must be of specified hard-grained varieties and have more than 11.5%
protein (grown in all states).
- Australian Premium
White (A.P.W.): a multipurpose class of intermediate grain hardness.
Selected varieties must have a protein level above 10% (grown in all
states).
- Australian Standard
White (A.S.W.): is a multipurpose class wheat of intermediate grain
hardness (grown in all states).
- Australian Soft:
must be of the specified soft-grained varieties and have less than 10%
protein. (Irrigated and high-rainfall areas in all states except Qld.). W.A. used solely for export market.
Other special segregations
of Varieties are made for particular end uses.
Durum, used mainly
for Pasta, is a separate botanical species noted for its extreme hardness
and the yellow pigment of its endosperm. It must contain 13% protein (grown
in Northern N.S.W.).
The following chart
emphasises the importance of two separate wheat characteristics - hardness
and protein content - for different end uses.
Thus, a wheat of hard
grain and high and good quality protein is required for bread manufacture.
Hardness is important in the milling process, high protein and good quality
protein in the baking process. Bread made from high protein flour usually
has better volume, softer crumb and better keeping qualities.
MOISTURE:
The wheat grain contains natural moisture, the amount depending on the
weather conditions during growth and at harvesting. Australian wheats
have a lower moisture content (9.5-12%) than wheats grown in many other
parts of the world. Wheat is conditioned and the moisture content adjusted
prior to milling to prevent the bran from crumbling, thus affecting its
end use quality and darkening the flour.
PROTEINS:
Normally breadmaking flours contain from 10-15% protein. White soft flours
may extend the range down to 7.5%. The proteins of wheaten flour are unique
for their ability to form the elastic material gluten, structurally important
in flour-based products. Gluten consists of protein with almost equal
part of glutenin (a glutelin protein) and gliadin (a prolamin).
STARCH (COMPLEX
CARBOHYDRATES):
The endosperm of the wheat grain has a high complex carbohydrate content.
When heated with water to about 55°C starch gelatinises. During the baking
of bread and other flour products, the degree of gelatinisation is dependent
on the internal temperature attained by the dough and the amount of water
available. Cooked starch is more readily digested.
DIETARY FIBRE:
The bran layers of the wheat grain contain large amounts of dietary fibre
consisting of cellulose, hemicellulose and lignin so wholemeal flour contains
up to three times as much fibre as white flour. The endosperm contains
cell wall material which in addition to some fine bran provides the dietary
fibre content of white flour. VFAT OR OIL (LIPIDS):
Wheats contains only 2-4% fat or oil most of which is found in the germ.
White flour contains approximately 0.8-2%. The higher levels in meals
and germ can cause deterioration through rancidity unless stored correctly.
VITAMINS:
Wheat is an important source of vitamins of the B group and of the vitamin
E. In Australia a high proportion of these vitamins is retained in white
flour. Thiamine is the most important and modern milling techniques retain
50% or more. All types of Australia flour and bread can be classed as
"Thiamine Donors". All flour used for breadmaking is now required, by law,
to be fortified with thiamine to a level of 6.4mg/100g. Because most of
the niacin occurs in the aleurone layer in a biologically unavailable
form, wholemeal flour cannot be considered a better source of this nutrient.
MINERALS:
The mineral content of flour increases with the extraction rate and dietary
fibre content. The aleurone layer contains approximately 60% of the grain's
minerals and the endosperm 20%. In wholemeal flour the phytic acid content
of the bran layers affect the availability of the extra minerals particularly
iron, zinc and calcium.
ENZYMES:
Like other living matter, wheat and, therefore, flour contains enzymes
that affect protein, starch and fats. In baking yeast fermentation (a
process brought about by the action of living cells) is completely dependent
on enzymes. The most important enzymes in wheat are the diastatic enzymes
(the amylases). Their function is to break down some of the sugars, principally
maltose, to sustain fermentation with adequate gas production (inadequate
gassing would result in poor loaf volume).
 Bread
is made from flour or meal, usually with the addition of yeast or leaven.
It is the oldest, commonest, most
convenient and cheapest form of food, with a history dating back more
than 10,000 years. Through the ages cereal grains, various grasses, roots
and seeds have been ground into coarse flours for bread.
Today the commercially
baked leavened loaf is the common form of bread in most countries. In
India, the middle East and parts of Asia the traditional unleavened flat
breads are being replaced to some extent by commercial breads.
INGREDIENTS
The basic
ingredients for bread are flour, salt, yeast and water. Optional ingredients
include sugar, milk powder, fats, dry gluten, bread improvers, egg, dried
fruits, cheese, spices and flavourings.
FLOUR
Flour may
be made by grinding or milling wheat, corn, barley, rye, millet, beans,
nuts, or tubers - starchy roots of plants (sweet potatoes) but the term
"flour" usually denotes wheaten flour unless the name of another food
is mentioned. Wheat flour contains a unique complex of proteins which,
on hydration, produces gluten, and elastic substance which may be stretched
to provide structure for a leavened loaf. Wheat, is the only flour which
have the capacity to form this type of structure.
Almost all bread
baked in Australia is made from wheaten flour or meal. By law flour
for breadmaking must be fortified with Thiamine.
YEAST
Saccharomyces
cerevisiae, also called Bakers yeast, is added to ferment dough producing
carbon dioxide thus enabling the dough to rise.
Sourdough bread
is becoming popular again for its slightly sour flavour and denser texture.
A spontaneous leaven is made by combining flour and liquid to form a
host for wild yeasts floating in the air. These yeasts produce carbon
dioxide which, in turn, leavens the dough. Sometimes a piece of dough
is retained for addition to subsequent doughs as the leaven. (For more
detailed information, see "Understanding Yeast, another title in this BRI series.)
SALT
Salt is added
to bread doughs for flavour, to stabilise fermentation and to strengthen
gluten. Additions normally range from 1½ - 2½ of flour weight. In mechanised
production salt is necessary to control fermentation and assist in dough
handling. Salt-free bread is available in some areas for special diets
but its bland flavour makes it unacceptable to most consumers. Reduced
salt and low salt breads usually present a more attractive alternative.
MILK
Specially
heat-treated milk powders are used in commercial manufacture of bread.
This adds some extra protein and calcium and contributes to crust colour.
In Australia to conform with Pure Food Regulations bread labelled "Milk
Bread" must contain 4% of the flour weight as non-fat milk solids. In
some states Vienna bread must contain specified amounts.
GLUTEN
Gluten is
added to bread dough to increase the protein content when necessary.
FATS
The addition
of fat improves bread quality and is important in modern, rapid dough
development processes. Two per cent is normally added but this may be
increased to 12% of the flour weight in some buns and rolls. Fat increases
loaf volume and improves keeping qualities and texture by softening
the crumb and crust.
SUGAR
Small amounts
(1-2%) of can sugar (sucrose) are sometimes added to help yeast action.
During fermentation the sugar is converted to glucose and fructose.
Larger amounts of sugar are added to sweet buns and fruit loves.
ADDITIVES
On the basis
of safety and technological need the National Health and Medical Research
Council has approved a small number of chemicals for use in breadmaking
and specifies the quantity of each permitted. These include enzyme supplements
(malt derivatives or fungal enzymes), gluten-modifying agents (eg ascorbic
acid, sodium metabisulphite and L-cysteine), yeast foods (eg ammonium
chloride) and crumb softeners (eg lecithin and glycerol monostearate).
Rope and mould inhibitors
(propionates) may be used in hot weather to inhibit the growth of micro-organisms
which cause bread spoilage.
PROCESSING
MIXING
For thousands
of years bread was made by hand in the home or by the local baker. After
mixing and kneading the dough was bulk fermented for several hours to
allow it to mature. In large commercial bakeries the dough is mixed
at high speed to develop its structure mechanically. This intensive
mixing eliminates the need bulk fermentation.
During kneading
and fermentation gluten is stretched and made elastic to allow more
gas to be held in the dough.
PROOFING
After mixing
the dough is divided into pieces of predetermined weight, rounded and
allowed to rest for a few minutes. The dough pieces are then moulded
into loaves and placed in the pans ready for proofing. This process
takes place ideally in a warm, moist atmosphere. Enzymes present in
the flour and the yeast convert carbohydrates from flour to simple sugars.
The yeast, acting on the sugars, causes carbon dioxide to be formed
which is trapped in the dough to give it a light, spongy texture.
BAKING
When the dough
enters the oven marked physical changes take place. As the temperature
of the dough rises, the yeast works faster and produces large quantities
of gas. This is a temporary action as very soon the temperature in the
loaf rises sufficiently to prevent further fermentation and finally
kills the yeast before baking is complete. Meanwhile the gases produced
expand increasing the size of the loaf. During baking starch gelatinizes
and gluten coagulates forming a semi-rigid, three-dimensional structure.
As the exterior of the loaf reaches a higher temperature it dries out
and a crust is formed. Dextriniation of the starch and Maillard reaction
between the amino acid and reducing sugars contribute both flavour and
colour to the crust.
PACKAGING
After baking,
bread must be cooled before it is sliced and bagged. Sliced bread has
gained popularity for its convenience and as most of it is sold in supermarkets,
wrapping ensures hygienic handling. Unsliced and crusty breads are available
from specialty shops and in-store bakeries.
TYPES OF BREAD
Bread is sold in standard
loaf weights. Pure Food Regulations and Bread Acts define certain bread
types. The following are the most popular: white, mixed grain wholemeal,
protein-increased (may be white, mixed grain or wholemeal), rye, fancy
bread and rolls.
Many wholemeal breads
are made from 100% wholemeal flour however, 90% wholemeal and 10% white
is permitted.
Brown breads must
contain at least 50% wholemeal flour.
Mixed grain and kibbled
varieties are usually classified as white breads unless labelled wholemeal.
High-fibre breads
are made according to the Standards with the addition of bran or other
approved seed coat material.
Rye breads must contain
at least 30% rye flour or meal. Some types may contain up to 100%, but
these are usually labelled as such.
Breads with reduced
salt (sodium) are also available in some areas.
Yeast comes from
the Sanskrit word 'yas' which means 'to seethe'. It is neither plant nor
animal but a single cell, microscopic fungus. Yeast is a leaven because
it creates gas bubbles in dough which causes the dough to rise.
Since before civilisation,
trace amounts of wild yeasts have blown freely in the air. Yeast comes
from fermenting matter in nature. When it lands on a suitable host and
is provided with sugar, oxygen and warmth it begins to grow and reproduce.
As it grows, it produces carbon dioxide, water and alcohol- a process
called fermentation from the Latin 'fevere' meaning 'to boil or 'seethe'.
The fermenting action is destroyed by temperatures above 43C and the alcohol
and water are evaporated during the baking process.
Archaeologists have
found that Triticum boeoticum, the wild wheat ancestor of all other wheat,
first grew in the fertile valley above the Persian Gulf (Ancient Mesopotamia,
now the areas of Kurdistan and Iran) prior to man. Because wild yeast
is so easy to obtain, one can readily picture Mesopotamians 9000 years
ago leaving mounded dough lying in the sunshine while heating an oven
and attending to other chores... only to find it had become when seeing
it again! They had put nothing living in it but had grown so it must be
alive. The esteem they received for producing the 'staff of life' must
have increased with their ability to achieve a 'miracle'.
As time progressed,
ancient bakers found that adding a little honey to dough would increase
the activity of the wild yeast. More rising was produced, making an even
lighter bread.
This custom spread
from Ancient Mesopotamia, west of Danube river valley, then to the lakeside
dwellings in Switzerland and south into Egypt where Archaeologists have
found, in Egyptian tombs, honey bread in the shapes of men and animals
such as gingerbread men are made today. From Egypt, honey bread was carried
to Greece where it was improved by use of finer flour due to better milling.
Greek sailors and merchants carried it to ancient Rome and from there
legions took it to the boundaries of their empire.
Sourdough bread originated
with the discovery that a piece of dough kept from previous day's baking
would become sour, and could be added to fresh dough as a leaven. This
was called a starter.
Use of a sourdough
starter to make a wheat or barley dough was common in Ancient Egypt and
the resulting bread, called ta, constituted the main diet of the Egyptian
labourer.
Australian pioneers
used sourdough leaven on ships and in the gold rush camps where it could
be produced easily and kept active by small additions of fresh dough.
1849 gold miners in
the U.S., the 'Forty-niners' were also called "sourdoughs".
After centuries of
using sourdough starters, Egyptians found that adding froth from beer
to dough created a product even lighter than sourdough.
It was this bread
along with sourdough bread plus beer and onions which 'built the pyramids'.
Ancient historians such as Pliny reported that barbarian countries like
Spain and France drank beer had bread 'lighter that that made elsewhere'.
There are many strains
of yeast. Brewer's yeast is used for beer making. Torula yeast, favoured
usually as a health drink, is made from wood cellulose or from sugar.
It is high in protein plus almost all of the B complex vitamins. Yeast
used for baking, bakers yeast, is saccharomyces cerevisiae.
Modern Yeast Production
In 1859, the
famous, French scientist, Louis Pasteur, discovered that yeast was the
organism which caused fermentation. In Denmark in the late 1800's methods
were developed for isolating and culturing pure strains of yeast for use
in the brewing industry. These techniques were soon applied to the selection
and propagation of baker's yeast. In 1921, s Danish scientist, Soren Sak,
developed a new method of yeast production called Differential Fermentation.
This technique provided the basis of the process by which much of today's
yeast is produced.
Today's baker's yeast
is made in four main steps:
1. Molasses is sterilised
with steam before being passed through clarifiers for removal of sludge.
The resulting fluid, called wort, is stormed in tanks under sterile conditions.
2. Inoculum, the pure
culture used to start the production of commercial yeast, is inoculated
into a seed fermenting tank which contains the sterilised wort plus other
nutrients. When the desired number of yeast cells have been produced,
the contents of the seed fermenting tank's contents are transferred to
the main fermenting tank.
3. The seed yeast
in the main fermenter continues to be fed wort and other nutrients and
at the same time, high volumes of sterilised air are pumped into the fermenter
to provide the necessary oxygen to assist in the rapid growth of the cell
population. The yeast cells duplicate themselves every 90 minutes. Two
hundred gram seed yeast can grow in 150 tonnes in five days. This is enough
yeast to make 10 million loaves of bread.
4. Separation occurs
at the end of the fermentation process when the many tonnes of yeast in
the main fermenter need to be harvested from the brew. This is accomplished
with large, centrifugal separators. The yeast cells are then washed with
water several times to remove all waste products and produce light, creamy
coloured suspension of yeast called cream yeast.
Cream yeast is stored
in refrigerated tanks to ensure the high activity is maintained. It is
then transported in insulated, stainless steel tankers to large, commercial
bakeries and kept in refrigerated holding tanks at 3°C while being constantly
agitated.
Forms of Yeast
One of the first recorded
instances of dried yeast was the Romans' preparation of it during grape
harvests. Millet or wheat bran was mixed with grape juice, allowed to
become contaminated by the air and then dried in the sun. The resulting
cakes were soaked in water when needed. In a similar process, early New
Zealand settlers in the U.S.A. made a wild yeast preparation of hops,
eye, Indian corn and water into dough which was allowed to ferment then
sun-dried and cut into cakes for future use.
The type of dried
yeast used today is credited to the Hungarian, Max Fleischmann. Unlike
the unstable and uncertain wild yeast, Fleischmann's yeast was a reliable
product made of brewer's yeast.
Today, dried yeast
is made from cream yeast. Cream yeast is 'dewatered' over a rotary vacuum
filter drum and then fed into a drying process which result in the production
of granules or High Activity Dry Yeast. This is then packed into 5000g
and 10kg foil vacuum packs. It is ideal for bakers who do not have refrigerated
storage space readily available or are located in more remote areas and
require a highly active, reliable product which is convenient to use.
Two metric teaspoons of dried yeast equal 30g of fresh compressed yeast.
Like compressed yeast, it is also packaged in small quantities (7g sachets)
for baking bread at home. It should be stored in a dry area preferably
below 20°C. Most dried yeast available in Australia had been packed under
vacuum after being flushed with nitrogen and should maintain its quality
for at least a year provided the seal is not broken or punctured.
In recent years, two
types of dried yeast have been developed specifically for the baking industry.
Active dry yeast (A.D.Y.) works more quickly than traditional dried yeast.
It requires no refrigeration, has a long life and must have water added
to it before use. It comes in 500g vacuum packs which once opened, should
be refrigerated and used within a few days.
Instant active dry
yeast (I.A.D.Y.) Works more quickly than active dry yeast, does not require
refrigeration, has a long shelf life and should be added directly to the
dough without adding water first. It is packed in the same manner as (A.D.Y.)
and, after opening, should be refrigerated and used within a few days.
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