wheatstruc, nv, c and m
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Historic documents confirm that wheat is the earliest field crop used for human food processing .
It also became the leading grain used for human consumption due to its nutritive profile and relatively easy harvesting, storing, transportation, and processing, as compared to other grains.
The earliest varieties, grown 12,000-17,000 years ego in the Near East, were Triticum monococcum (einkorn) and Triticum dicoccum (emmer).
Continued breeding resulted in the development of new varieties around the world that often became adapted to areas previously unsuited for the cultivation of wheat.
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The main wheat varieties grown today are Triticum aestivum, subspecies vulgare, which is a hexaploid with six groups of seven chromosomes in each group.
This species includes hard red winter, hard red spring, soft red winter, and white wheats.
Another wheat durum is a tetraploid, containing four groups of seven chromosomes totaling 28 chromosomes.
The botanical name of durum wheat is Triticum durum.
A limited area is planted with the soft white wheat variety of Triticum aestivum, subspecies compactum, commonly known as club wheat.
Currently about 4000 different wheat varieties are grown around the world. 2Prepared by Hab2 S.
Data related to the morphology of the wheat kernel
and proximate analyses vary in different research
reports.
This variability is likely due to the different types and
growing conditions of wheats analyzed.
In general, there are about 30,000 cells in a wheat
kernel, and their content varies significantly
depending on their location in the kernel .
The morphology of the wheat kernel is unique and as
such creates technical (milling) challenges in
separating the endosperm and the germ from the
outer fibrous layers, commonly named the ''bran." 3Prepared by Hab2 S.
The presence of the crease (about 25% of the
kernel surface), which extends almost to the
center of the wheat kernel, requires special
consideration in grinding.
The wheat germ (about 24% of the kernel weight)
is located on the dorsal side.
The wheat germ parts are the embryo, with
rudimentary roots and shoots, and the scutulum,
which is a transport organ of nutrition to the
embryo during sprouting.
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The wheat kernel outer botanical coats (about
78% of the kernel weight) consist of several
distinct cellulose-rich layers.
The outermost layer, the pericarp (fruit coat), is
made up of the outer pericarp, which includes the
outer epidermis, hypodermis, thin-walled cells,
and the inner pericarp, which includes
intermediate-size cells, cross layers, and tube
cells (inner epidermis
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The inner layers are the seed coat (testa) and
nucellar epidermis (hyaline layer) .
Between the nucellar epidermis and the starchy
endosperm we find the aleurone layer, having high
soluble protein and mineral contents.
The aleurone layer constitutes about 58% of the wheat
kernel.
This layer is botanically similar to the endosperm, but
it is difficult to separate from the bran by conventional
milling techniques.
Depending on the kind of wheat, the thickness of the
aleurone layer varies. 7Prepared by Hab2 S.
Mechanical damage or hydrolysis with cellulase
of the aleurone thick cell wall allows access to
proteins within the aleurone layer .
Although nutritious, incorporation of a fraction
with a large percentage of aleurone layer
adversely affects the baking quality of flour .
The endosperm of the kernel was also shown to
follow a gradient in ash, protein content, gluten
characteristics, and baking quality.
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Many wheat kinds and classes, available around the world, vary in quality as a result of climate, irrigation, specific variety characteristics, growing conditions, harvesting, and handling.
Presently, wheats are graded differently in exporting and importing countries .
In some countries the government is involved in setting limits for contaminants in imported wheats.
In others, mainly exporting countries like United States, government officers inspect, according to official standards, all exported wheat; domestically traded wheat is inspected upon request only.
The current grading system covers eight classes of wheat:
durum, hard red spring, hard red winter, soft red winter, hard white, soft white, unclassed, and mixed wheat.
Durum, hard red spring, and white wheat are further divided into subclasses.
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According to the U.S. standards for wheat, the definitions for the classes and subclasses are as follows:
1. Durum wheat: all varieties of white (amber) durum wheat.
This class is divided into three subclasses: (1) hard amber durum wheat this subclass
designates durum wheat with 75% or more of hard and vitreous kernels of amber color;
(2) amber durum wheat this subclass is durum
wheat with 60% or more but less than 75% hard and vitreous kernels of amber color;
(3) durum wheat durum wheat with less than 60% hard vitreous kernels with amber color. 10Prepared by Hab2 S.
2. Hard red spring wheat: all varieties of hard red
spring wheat.
This class is divided into the following three subclasses:
1 dark northern spring wheathard red spring wheat with
75% or more dark, hard, and vitreous kernels;
2 northern spring wheathard red spring wheat with 25%
or more but less than 75% dark, hard, and vitreous
kernels;
3)red spring wheat hard red spring wheat with less than
25% dark, hard, and vitreous kernels.
3. Hard red winter wheat: all varieties of hard red
winter wheat. There are no subclasses in this wheat class.
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4. Soft red winter wheat: all varieties of soft red winter wheat. There are no subclasses in this wheat class.
5. Hard white wheat: all hard endosperm white wheat varieties. There are no subclasses in this class.
6. Soft white wheat: all soft endosperm white wheat
varieties. This class is divided into the following three subclasses: 1 soft white wheat soft endosperm white wheat varieties
that contain not more than 10% of white club wheat 2white club wheat soft endosperm white club wheat
containing not more than 10% of other soft white wheats
3 western white wheatsoft white wheat containing more than 10% white club wheat and more than 10% other soft white wheats.
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7. Unclassed wheat: any variety of wheat that
is not classified under other criteria provided in
the wheat standards
There are no subclasses in this class.
This class includes any wheat that is other than
red or white in color.
8. Mixed wheat: any mixture of wheat that
consists of less than 90% of one class and more
than 10% of one other class or a combination of
classes that meet the definition of wheat.
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The value of wheat depends upon its milling and
flour end use quality.
This can be accurately determined through actual
milling and baking tests.
The miller has to assess wheat quality and
evaluate its suitability to produce, individually or in
a blend, final flour specifications.
In addition, the miller has to determine the
expected wheat-processing performance in the mill,
the resulting flour extraction, and other qualities
such as color, particle size, and starch damage. 15Prepared by Hab2 S.
Flour extraction is the proportion of the wheat
recovered as flour during milling.
The following are tests of importance to the miller
for evaluating wheats and flours:
experimental milling, physical, chemical, physical-
chemical, dough rheology, and the baking test.
Wheat and flour testing can follow different official
methods such as those of the American Association
of Cereal Chemists (AACC), the International
Association of Cereal Chemists (ICC), or the
Association of Official Analytical Chemists (AOAC). 16Prepared by Hab2 S.
1. Test weight: quality test which is basically a rough measure of density of grain in terms of weight per volume, i.e., the weight (lb.) per volume bushel (Winchester bushel in U.S.; Imperial in Canada).
The hectoliter weight (hL), indicating the weight in kg/hL (100 L), is used in the metric system countries.
No uniform conversion factors between test weight and hL weight values are possible due to differences in kernel shape, size, and procedures for determination of these values.
2. Thousand kernel weight (TKW): a quality test to determine the potential milling value of wheat.
Weight of 1000 kernels gives an indication of kernel density and its consequent flour yield.
The advantage of TKW is that the weight can be expressed on a desired-moisture basis.
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3. Kernel size distribution: the size distribution
of kernels in a wheat sample can be determined
using a stack of sieves. The ''theoretical flour yield"
can be determined by the total value of multiplying
the percentage above each sieve by a factor .
The factors can be calculated using multiple
regression analysis for a mill, based on a database
in which percentages of wheat sizes are the
independent variables and the actual flour yields
are the dependent variables .
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4. Kernel hardness: a relative term, which is related to the disintegration of the endosperm during its separation from bran and germ.
Currently, hardness values are determined by near-infrared refraction (NIR) or mechanical crushing instruments such as the single kernel characterization system (SKCS).
They are used to identify variation of wheat characteristics in the trading system as well as indicate processing characteristics
5. Assessment of the milling quality of wheat is performed using an experimental unit using a sample of about 1000-1500 g.
Experimental milling can give a preliminary indication whether a wheat alone or in a mix of wheats complies with a required quality.
An experimental mill should be differentiated from a laboratory mill that is a milling unit with a fixed setting, where all wheat samples are treated in the same manner during milling. 19Prepared by Hab2 S.
5. Assessment of the milling quality of wheat is performed using an experimental unit using a sample of about 1000-1500 g.
Experimental milling can give a preliminary indication whether a wheat alone or in a mix of wheats complies with a required quality.
An experimental mill should be differentiated from a laboratory mill that is a milling unit with a fixed setting, where all wheat samples are treated in the same manner during milling.
Flour samples produced with laboratory mills in a relatively short time can be used for further testing but do not provide information on the wheat-milling properties.
Official methods explain the procedures for using experimental mills and should be followed rigidly, preferably by the same operator .
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Improved experimental mills are fitted with technical
parameters of the commercial mill where the wheat
is expected to be processed.
Accurate sampling, tempering, and controlled
environment in the facility and uniform practices
ensure reproducibility and confidence in the results.
Flours from experimental milling procedures could
be used for further rheological and baking tests.
6. Other physical and chemical evaluation tests
performed in the mill laboratory include those for
moisture, protein, ash, fatty acids, amylase
activity, Falling Number, and gluten quantity
and quality.21Prepared by Hab2 S.
It is important to preserve the quality and economic
value of wheat as it moves from the field into storage at
the processing mill.
If not properly stored, insects, moisture damage, or
other conditions may cause losses. Moisture and
temperature are two main factors that influence the
development of grain molds and insects in stored
wheat.
In some areas of the world, where wheat is harvested at
a high moisture content, wheat should be carefully
dried to a moisture below 12.5%, a level regarded as
safe for storage.
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B. Blending
Usually a mill is designed for milling wheat of a
certain class and physical characteristics.
However, a mill designed for one class of wheat
(e.g., hard or soft) does not ensure uniformity of end-
product quality.
Wheat arriving at the mill usually varies in quality
and requires blending to deliver a "wheat mix" of
uniform qualities.
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Wheat blending is the initial step in providing
bakers with a uniform flour.
Accordingly, mills prepare "wheat mixes" of certain
protein levels or other quality characteristics.
There are different methods of blending.
Some millers blend wheats directly in storage bins,
others before grinding.
Wheat blending just before the milling process is
mainly applied when the components of the "wheat
mix" differ in endosperm hardness and require
adjustments of moisture levels and tempering times
prior to milling. 24Prepared by Hab2 S.
C. Cleaning Intensive cleaning of the wheat before milling ensures
that bacteria, mold, undesired seeds, infested kernels, shrunken and broken kernels, and other foreign materials do not contaminate the mill products or damage the equipment.
Separation in the mill cleaning house is based on the following differences between whole sound wheat kernels.
D. Conditioning Conditioning, a process that adjusts the moisture level of
wheat before milling, achieves a mellow endosperm and tough bran.
Bran that absorbs proper amounts of moisture becomes elastic and will not splinter during grinding to contaminate the flour with fine particles.
Mellow endosperm breaks off the bran during grinding, and less power is required to reduce large pure particles to flour.
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On the other hand, an excessive moisture level softens the wheat endosperm to a degree where it does not have the resistance to break down to sharp particles that is important for efficient sieving and separation from the bran.
Another objective of wheat conditioning is to equalize the hardness of the different kernels in the wheat mix before processing.
If the moisture content and hardness of wheat lots in a mix are significantly different, they might be treated separately during the conditioning process.
Different methods could be used to condition the wheat before milling.
Heating the wheat, application of warm water, application of live steam, or just intensive mixing of wheat and water are some of the methods used to increase the amount and rate of water penetration into the kernel.
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Moisture pick-up by wheat capillary action increases
slightly and linearly with increasing water temperature .
The increase from the initial temperature of 26.7°C is
approximately 2% at 30°C and 4% at 90°C for each
variety of wheat.
Excessive heat (above 65°C) results in gelatinization of
starch and protein denaturation.
The current method most frequently used is termed
''tempering."
According to this procedure, a calculated amount of
water is added to the wheat, which is then intensively
mixed in a continuous mixer in order to maximize a
uniform dispersion of the water on all wheat kernels.27Prepared by Hab2 S.
Wheat flour milling is a process that consists of controlled breaking, reduction, and separation.
The objective during milling is to separate the branny cover and germ of the wheat kernel from the endosperm.
Breaking of the wheat kernel is affected by corrugated cast steel rolls that gradually separate the endosperm, bran, and germ.
Reduction of relatively pure endosperm to particles smaller than 180 mm is achieved by using smooth rolls.
Segregation between the kernel parts occurs in sifters and purifiers.
In sifters, sieves separate particles of different size. In purifiers with sieves and air, differences in size,
specific gravity, and shape of particles are used to separate particles of pure endosperm and those which include different ratios of bran and endosperm. 28Prepared by Hab2 S.
None of the kernel fractions coming out of the mill are completely pure, and each contains some parts of the others.
The level of purity of each product at the end of the mill is one of the measures of mill efficiency.
Flour extraction in the mill is measured as percentage
of flour produced based on a quantity of wheat that is
either dirty, dry, clean, or cleaned and tempered.
The basis used for calculation of the extraction rate
should be stated with the results.
Another measure is the gain/loss or the difference
between the wheat arriving in the mill and the total
weight of products shipped out. 29Prepared by Hab2 S.
There should be a gain of total product weight after the milling process as a result of the difference between the moisture content of the wheat arriving at the mill and the cumulative moisture content of all final products.
The flour-milling process consists of numerous stages
that can be divided into the following sub-processes: breaking, grading, purification, sizings, reduction, mill feed handling, germ recovery, and flour dressing.
Materials at different stages of the milling process differ in quality or in the ratio of bran to endosperm and particle size.
The efficiency of gradual separation between the endosperm, bran, and germ is directly related to the length and the number of stages in the process.
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Segregation of the intermediate materials to
different grinding stages is based on their size
and the amount of undesirable bran and germ
particles.
In an optimal system each of the materials would
be treated individually.
However, grinding rolls, sifters, and purifiers are
manufactured to standard sizes, and this causes
mill designers to compromise on the number of
separations in respect to quality and quantity of
the intermediate materials. 31Prepared by Hab2 S.
Accordingly, the extent to which intermediate
materials are subdivided in the mill is a function of the
mill capacity.
If the mill capacity is too small, different stages would
be underloaded with standard size equipment, and in
this case products that are only slightly different
should be combined.
The initial grinding stages in the milling process are
named "breaks."
The breaks are used in the grinding steps of the milling
process to separate the bran, germ, and endosperm
from each other. 32Prepared by Hab2 S.
The success or failure is measured in the level of
achieving, as efficiently as possible, complete
separation between the kernel parts.
In the conventional milling of hard and durum
wheats, the objective is to produce minimal
amounts of flour in the breaks but a maximum of
clean endosperm chunks.
However, with soft wheat, because of the softer,
less dense endosperm, the percentage of flour
extracted from the breaks in conventional milling
is higher than that from hard and durum wheats. 33Prepared by Hab2 S.
One study reported that hard, soft, and durum wheats produced on the first three breaks are 49.8, 44.7, and 77.4 and 5.7, 10.5, and 2.0% of sizings and flour, respectively.
Starting with the first break, the objective is to open
the kernel.
The shape and depth of the first break roll corrugations
should be selected to fit the size of the kernels.
Optimum results in the first break are achieved if the
kernels are fed to the gap between the rolls
horizontally, held by the corrugation of the slow-
moving roll, and opened exactly at the crease by the
fast-moving roll. 34Prepared by Hab2 S.
Optimum for the second break rolls and the
subsequent breaks is feeding the material
(endosperm attached to a flake of bran) directly to a
precisely adjusted gap where with the right pressure
the fast-moving roll scrapes the endosperm from the
bran.
As the bran flakes get smaller toward the final
breaking stages and the endosperm layer attached
to it becomes thinner, gradually smaller
corrugations are used (or a larger number of
corrugations per inch of roll surface). 35Prepared by Hab2 S.
Optimally conditioned wheat and the right
corrugations, pressure, and differential minimize
splitting of the bran to particles of a size that can be
sieved through with the flour.
Good results in conventional milling are obtained when
most of the endosperm free bran consists of large
flakes.
Conventionally with a longer break system, up to six
stages in hard wheat and seven in durum wheat mills,
it is possible to grind the material fed to the rolls in a
less severe manner.
Roll surfaces should be maintained in good condition to
ensure good flour extraction and quality. 36Prepared by Hab2 S.
Depending on the quality of the steel and the type of milling technology used, corrugated rolls should be refurbished every 36 months of milling.
Other factors that influence the need for refurbishing are roll surface allocation, feed rate per unit, severity of grinding, wheat hardness, and presence of stones or other impurities in wheat.
Recent advances in metallurgy that allow casting of harder outer surfaces for corrugated rolls extend the time between refurbishing up to 8 months.
Even when the mix in the mill is changed drastically in wheat size and kernels are smaller or larger than normal, usually mills will continue using the existing corrugations, keeping many exiting variables unaltered. 37Prepared by Hab2 S.
Generally, the gap between the rolls will be adjusted
intuitively by the miller based on his or her experience.
A few studies were conducted to evaluate the first roll
action and the different parameters that could effect
conventional milling of different kinds of wheat.
Grinding of soft and hard wheats on a set of rolls at
different rotating speeds indicated that better
separation between bran and endosperm occurred on
the first break with a lower speed and smaller diameter.
Wheat moisture is another important factor that affects
the grinding process for common and durum wheat
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1. Sieving In the sifter, particles of the grounded material are
separated according to size.
Sifters are available in two, four, six, and eight sections.
Modern sifters are more sanitary than those used in the past, which often were a source of infestation.
The sieves in a sifter section are divided into groups. At the top of the section, there are usually coarser
sieves separating the larger material that flows out of the sifter through a side channel.
The material passing through the sieve is either transferred out of the machine or directed down to finer-aperture sieves for a further separation. 39Prepared by Hab2 S.
Below each sieve, a backwire is attached to the frame on which hard rubber balls, plastic elements, or cotton pads bounce to keep the sieve clean.
''Throughs," a stream passing through the upper sieves in a break stage sifter, is a mixture of flour and chunks of endosperm to which often some bran is also attached.
While the "overs" of the top sieves are transferred to the next break for additional scraping of endosperm, the mixture of the throughs is segregated, based on particle size differences on lower sieve groups in the section.
This is evident from a schematic view of a first break sifter section where six materials that differ in quality and size flow out. 40Prepared by Hab2 S.
Graders are sifter sections used to handle mainly materials
directed from the breaks.
A blend of medium-sized and fine sizings as well as
middlings is directed to the graders.
Materials from primary breaks are directed to the first
grader.
Materials from secondary breaks (e.g., the third or fourth)
are directed to second or third graders.
The main objective of the grader is to remove the remaining
flour from the middlings and to separate the granular
material to narrow particle size ranges for better efficiency
in the purifiers.41Prepared by Hab2 S.
At the head end of the milling system granular intermediate materials of the same size range are directed to machines called purifiers.
The different size groups differ also in the amount of pure endosperm, bran, and such particles of endosperm to which bran is still attached.
The more similar the particles are in size, the more effective is the purifier performance.
The purifier's main purpose is to separate particles into fractions of pure endosperm, a mixture of particles to which bran is attached, and bran particles.
This is achieved by using sieves and air currents. The purifiers classify the material into several
fractions according to size, shape, and specific gravity. The endosperm particles, essentially free from bran
and germ, are spouted to smooth rolls, where they are ground into flour. 42Prepared by Hab2 S.
Other particles to which bran and other outer layers
of kernel adhere are delivered to different pairs of
rolls ("sizings") for careful reduction and separation
of the bran.
4.Sizings
The material at each of the sizing stages is a mixture
of particles close in size range, some pure
endosperm, and others still with attached bran.
The objective of the sizing stages is to reduce the
particle size and, during reduction, to separate the
still attached bran from the endosperm. 43Prepared by Hab2 S.
Material from the sizing stages can be diverted to
purifiers, to middlings for final reduction, or to flour
as a final product.
However, the miller tries to refrain from severe
grinding in the sizing stage to avoid production of
flour that may be contaminated by the presence of
bran.
Some millers use corrugated rolls on sizing stages,
while others use smooth rolls.
Smooth rolls will have a more delicate effect and
produce lower-ash flour than corrugated ones. 44Prepared by Hab2 S.
When corrugated rolls are used in sizings stages, the
corrugation features are adjusted to the particle size
and the bran adhering to them.
5 Middlings or Reductions
Coarse and fine pure endosperm particles from
breaks, purifiers, sizings, and reductions in the mill
are reduced to flour on smooth rolls.
The outer layer of smooth rolls is of ''softer" steel
than that of corrugated rolls.
The "softer" steel, which includes more carbon
molecules in the cast, "loses" them with time, thus
keeping a rough surface. 45Prepared by Hab2 S.
In general, the reduction system substantially affects the quality of the end product through the compression and shear applied on the endosperm matrix of protein in which starch granules are embedded.
In hard wheat the adhesion between the starch granules and the protein matrix of the endosperm cells is stronger than in soft wheat.
Therefore, flours from soft wheat disintegrate easier in milling and produce finer flours than those of hard wheats.
Millers adjust the flowsheet and mill equipment to produce flours of coarser granulation from weaker wheats and finer granulation from stronger wheats to achieve optimum results in baking. 46Prepared by Hab2 S.
Starch damaged by milling absorbs five times more water
during the dough process and is susceptible to diastatic activity
by enzymes that decompose starch to dextrin, oligosaccharides,
and simple sugars during the dough preparation.
When present at an excessive level, damaged starch has an
adverse effect on dough and bread quality.
Because of its harder cell structure, hard wheat endosperm
generates flour with more damaged starch by the action of high
roll pressure or high impact forces during the reduction stages
of the mill.
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6. Air As a Means of Processing Machine location and product transfer in the mill are
optimized by maximizing the use of gravity flow for intermediate materials.
For vertical transfer of materials positive or negative pneumatic systems are used.
Negative pneumatic systems are usually used for the transfer of all intermediate materials in the grinding unit.
Properly designed and efficient air-handling systems for pneumatic conveying or suction in various locations in the mill reduce significantly the energy consumption of the operation.
In a modern mill about 10 times more air weight than wheat weight is moved through the system.
Accordingly, it is essential to maintain the relative humidity at about 65% and temperature at about 25°C (77°F) in the mill to control moisture evaporation in intermediate and final products.
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In locations where extreme humidity levels or temperatures exist, air control units should be installed in the mill.
If intermediate stocks are too dry or too wet this affects the sieving efficiency, the breaking up of the bran, and accordingly the final quality of the flours.
Mill Control Control of mill performance is a continuous chore of
the miller who sets methods and procedures to achieve optimal performance.
As an example, when changing wheat mixes in the mill, the flours are directed to a set-off bin until the mill is adjusted for the new wheat mix.
The mill flours are directed to the set-off bins also upon starting and shutting down the mill.
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The reason for such measures is to prevent
production of off-grade flours while the mill is
underloaded.
The flour in the set-off bins is reblended to the main
stream at a very low rate.
Scales to weigh wheat at receiving point, before and
after cleaning, tempered wheat, and final products
could indicate changes in loads, extraction levels,
and any other problems in each section of the mill.
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On-line instrumentation to determine moisture,
protein, ash, and color ensures uniformity of raw
materials and final products.
Evaluation of the mill technological performance is
measured by using the ash content of wheat,
intermediate materials, individual flour streams, and
final products.
The significant difference in ash content among the
three main parts of the wheat kernel endosperm,
bran, and germ is used as a measure to determine
the level of the separation efficiency from each other. 51Prepared by Hab2 S.
However, in the past, because no other accurate tools were available, ash was used as a criterion of flour quality.
Flour ash was an inconclusive parameter and in the past created significant economic losses to millers and bakers.
The reason is that ash values of flours are not directly related to the flour end user's specifications.
Millers compromised on flour extraction to supply flour within specifications from good baking quality wheats that inheritably had higher endosperm ash.
Today, fast and accurate instrumentation to determine flour qualities such as color, starch damage, rheological characteristics, and baking qualities is widening the parameters for flour specifications. 52Prepared by Hab2 S.
The objective in milling is to achieve as high as possible flour extraction with the lowest contamination of bran and germ that increase ash content.
The ash curve is a mean to express cumulative ash of the flour streams in the mill.
To construct the ash curve the streams are arranged in increasing order of ash content, and they are weighted based on the extraction of each into a function that is a relationship between the cumulative ash content of a number of streams and the related total flour extraction fig .
The miller's objective is to reach an ash curve that is flat and start to turn upward at the highest possible flour extraction.
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While the ash values and curve are an indication of the mill separation efficiency between the endosperm and bran, the granulation curve is a function of mill adjustment and screen selection.
The granulation curve (Fig. 4) expresses the disintegration of the wheat kernel at different stages of the milling process.
The curve is drawn as a graph where the horizontal axis shows the various sieve apertures in micrometers, and the vertical axis shows the cumulative percentage tailovers of the respective sieves.
The granulation curve shows the particle size distribution of the ground material.
By drawing granulation curves for each of the grinding stages, the miller can monitor variability in kernel disintegration and make the necessary adjustments in the system.
The data to construct the granulation curve can be generated with an experimental sifter. 55Prepared by Hab2 S.
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The miller sieves the stock from under the rollstand
on a stack of sieves and then calculates the
percentages of all the quantities remaining on the
sieves and the material in the bottom pan from the
total weight.
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If a different set of sieves is used for the separation of
a grounded stock, different points will be allocated on
the same graph to determine a change in the amount
overtailing from each sieve.
The shape of the curve does not depend on the sieve
aperture, but on the sample granulation distribution.
The miller draws the granulation curves of the mill for
each wheat mix at the time when mill performance is
optimum.
Granulation curve analysis can generate the following
information: (1) corrugation condition, (2) mill balance,
(3) roll adjustment, and (4) sieve area, aperture,
division, and efficiency of the sieving stages. 58Prepared by Hab2 S.
A. Flours Flour quality is a subjective concept that relates to
final product usage. For different types of bread around the world specific
wheat characteristics and flour qualities are required. Quality parameters such as color, protein,
granulation distribution, gluten quantity and quality, and starch damage play a role in the suitability of flour for the baker.
Another important factor besides the determination of quality is the concept of flour uniformity.
For the commercial baker uniformity of flour supplied is more important than variations in characteristics such as premium protein or reduced starch damage. 59Prepared by Hab2 S.
Flours from the different stages in the mill are not identical in physical appearance, chemical analysis, or baking properties.
These flour streams are composed of varying amounts of different parts of the wheat kernel.
In the case that all the flour streams are blended to one composite, the result is a ''straight-grade flour."
The quality of the straight-grade flour is directly related to the quality of the processed wheat.
It is possible to combine these flour streams in different ratios to produce simultaneously two or more final flours that differ in color, ash content, protein content, dough-handling properties, and bread baking characteristics.
This method of producing more than one final flour from one wheat mix is called "split milling" or "divide milling."
60Prepared by Hab2 S.
In wheat-importing countries the method of split milling is used to accommodate the requirements for flour qualities of different end uses.
In wheat-growing countries such as the United States split milling is not frequently used since the wide variety of wheat types accommodate different end uses.
In the United States the common types of flours produced in a mill are patent, first clear, and second clear.
Amounts and types of final products vary among mills are a result of differences in flow-sheet, adjustments, and kinds of wheat milled.
Flour streams from the head end middlings, primary sizings, and in some cases that of second and third breaks originate from the center of the wheat kernel.
61Prepared by Hab2 S.
The blend of these flour streams is called ''patent flour."
Patent flour is about 77% of the total flour, is the whitest, and contains the lowest relative amount of ash (0.38-0.42%, corrected to 14% moisture basis( m.b.).
Other flour streams of the process that contain a higher percentage of the endosperm parts adjacent to the bran and germ are distinguished from the former by higher ash and protein contents, darker color, and inferior baking qualities.
These flour streams can be combined to make up "first-clear flour."
First-clear flour is about 20% of the total flour and contains about 0.75% ash.
"Second-clear flour," made up of the rest of the streams, is 3% of total flour and contains up to 1.2% ash (14% m.b.). 62Prepared by Hab2 S.
The ratio between patent, first clear, and second
clear could vary substantially in percentages in other
instances and, accordingly, in ash and quality.
Blending part or all of the first clear into the patent
comprises the "baker's patent.“
Control of flour particle size distribution is a
parameter the miller controls by wheat selection,
tempering, mill flow, and mill adjustment.
63Prepared by Hab2 S.
The miller subjectively blends the flour streams
from different stages in the mill to make up the final
products.
Optimum flour granulation distribution is an
important parameter for the baking process.
Drastic change in granulation effects water
absorption, water retention during fermentation,
proofing, and quality of finished breads.
The mill adjusts product granulation to the kind of
additives added during dough preparation and to
the types of breads baked. 64Prepared by Hab2 S.
The ash content does not affect the baking quality of
the flour; it relates basically to the level of bran in
the flour.
Ash content of flour is a very valuable test for mill
control.
However, in many cases flour ash is used in flour
quality specifications disproportionately to its value
and significance in baking.
This creates a situation where millers are constrained
to lower flour extraction when using good baking
quality wheat of inherently high endosperm ash. 65Prepared by Hab2 S.
Flour color depends on wheat cleanliness,
tempering level, finesse of flour, and the amount of
bran particles it contains.
Too much fine bran effects flour shade, producing
a darker shade.
Frequently during the mill operation the miller
slicks a flour sample and wets it. This method,
called the Pekar test, is used by the miller to
evaluate the color and amount of bran particles in
the flour.
Change in mill ambient conditions could also affect
flour color. 66Prepared by Hab2 S.
In addition, flour carries a yellow cast due to the
presence of carotene.
Natural aging during storage of the flour for up to 2
weeks or usage of different bleaching agents, where
permitted, could overcome this problem.
In mills where microingredients are added to flour
according to customers' specifications, they are
introduced into large-capacity, high-speed batch
mixers during final blending and before load-out.
67Prepared by Hab2 S.
In some countries improvers and enrichments are
fed into the flour in the mill or in the blending
facilities before load-out.
The powders are added to the flour with great
accuracy and uniformity by special feeders.
Modern systems use programmable logic controller
(PLC)-controlled feeding systems.
At the end of the milling process the
microingredients are conveyed by air and
introduced and mixed into the flour by special
agitators. 68Prepared by Hab2 S.
B Bran Commercial bran differs from the botanical outer
layers of the wheat kernel.
The bran that is removed during the various stages of
the milling process is made up of fractions that differ
in size and endosperm content.
Bran is described using factors such as minimum
protein, minimum fat, maximum fiber, and maximum
moisture.
In the United States "wheat mill bran" would be a
product that includes all offal fractions from a typical
mill. 69Prepared by Hab2 S.
According to the American Feed Control Officials [66], wheat mill run consists of the following: minimum protein, 13.0%; minimum fat, 4.0%; maximum fiber, 9.5%; and maximum moisture, 14.0%.
The loosely held embryo part of the germ can be extracted relatively easily, but the soft scutellum, high in fat and protein, is difficult to separate from the endosperm and the bran .
70Prepared by Hab2 S.
The American Feed Control Officials define proximate
analysis for all other by-products from the milling process.
Specifications will vary from country to country based on
milling technology, feed regulations, kind of wheat used,
and climatic conditions.
C . Wheat Germ
The germ constitutes about 2.53% by weight of the wheat
kernel depending on the size of the whole kernel.
The two main parts of the wheat germ are the embryo and
the scutellum.
71Prepared by Hab2 S.
The embryo and the whole germ differ in size, shape,
and the level at which they are embedded into the
kernel among the different kinds of wheat.
The mill flow is designed to separate whole embryos
during the breaking stages.
The moist, soft, and easily flattened embryos are
directed in the mill flow, usually from a purifier, to a
pair of smooth rolls with low differential, where they
are flaked.
The small flakes are extracted in the sifters over a 14
US mesh sieve (1410 mm). 72Prepared by Hab2 S.
According to definitions of the Association American Feed
Control Officials, pure wheat germ that is used primarily
for human food should contain a minimum of 30%
protein.
In some mills the germ is separated with an impact
machine ahead of the first break roll.
After impaction the material is sifted on a sifter, where it
is separated into different fractions.
The coarse material is diverted to the first-break coarse,
the intermediate material to first-break fine, and the fines
containing the embryos to a smooth pair of rolls, where it
is flaked for separation.73Prepared by Hab2 S.
A .Protein Various classes of wheat are intentionally bred and
selected for a specific composition, usually to meet end-use requirements for a product.
For example, commercial soft wheats are maintained at low protein levels, although certain soft wheats are associated with genes for high protein and are used as germplasm in breeding programs to develop high-protein hard wheats.
Protein content in a single variety of wheat can vary from 7 to 20% depending upon growing environment and fertilizer use.
The high-protein hard wheat is higher in protein in all constituents except the germ.
74Prepared by Hab2 S.
Constituents of wheat grains are not distributed
uniformly.
The pericarp (bran) is high in pentosans, cellulose, and
ash.
The aleurone is a botanical part of the endosperm, but
during milling it is removed with the bran.
With an increase in extraction rates, protein, fat, and
fiber increase, whereas carbohydrates decrease.
It is commonly accepted that the protein content of
straight-grade flour is about 1% less than that of the
wheat used by the mill. 75Prepared by Hab2 S.
It is high in protein, lipids, pentosans, and ash, thus contributing significantly to the nutritional quality of bran as a feedstuff. Starch is found in the endosperm.
The outer endosperm (subaleurone) is higher in
protein than the inner portion.
The embryo and scutellum, which make up the germ,
are high in protein, lipids, reducing sugars, and ash.
Because of the structure of various parts, milling
extraction rates affect flour composition.
76Prepared by Hab2 S.
The miller controls variation in flour protein by adjusting
wheat protein, wheat size, and wheat-blending methods.
The protein ''difference" between the whole kernels and
flour is larger for smaller size kernels .
In cereals only wheatand to some extent ryehave storage
proteins that form the gluten network in flour and water
doughs, which has the unique properties of elasticity and
strength to produce yeast-leavened bread.
Storage proteins comprise 85% of wheat endosperm
proteins and consist of gliadin (alcohol-soluble) and
glutenin (alkali- or acid-soluble) fractions.
77Prepared by Hab2 S.
The amino acid composition, Glutamic acid and proline are highest in the endosperm.
Lysine, argenine, aspartic acid, and alanine are lowest in the wheat and flour.
Lysine is the limiting essential amino acid in wheat and most cereals.
B .Lipid Lipid contents of wheat grains typically range from 2 to
4%.
Lipid material is not dispersed evenly throughout the grain.
The embryo (germ) contains 30% of its weight as oil. Commercial germ is in the 10-11% range.
The endosperm is lowest in oil, and the outer layers have an intermediate lipid level between the germ and the endosperm.
78Prepared by Hab2 S.
Wheat germ oil includes a high proportion of unsaturated fatty
acids.
C .Vitamins and Minerals
Vitamins are found in high concentrations in wheat germ and
bran, and minerals are especially concentrated in the bran.
Whole kernel data for each are influenced by kernel size and
the ratio of bran to endosperm, which may be higher in small
kernels.
Kernel size can be influenced by environmental stress or
genetic factors.
Milling and the degree of flour extraction will also affect
vitamin and mineral analysis on flour and other milled
products. 79Prepared by Hab2 S.
A. Durum Wheat Milling
Usually drum wheat is milled into a granular product
called semolina for pasta production.
Depending on the pasta manufacturing system, ranges
of semolina granulation and particle distribution will
vary.
Regulations by the U.S. Federal Drug Administration
define semolina as a product made only from durum
wheat that passes through a No. 20 sieve, not more
than 3% passing through a No. 100 sieve.
Its moisture content is not more than 15% and
maximum dry ash content is 0.92%. 80Prepared by Hab2 S.
Durum wheat is also milled to flour of a granulation
finer than 200 mm in some parts of the world for local
bread baking.
The extraction of final products based on wheat
entering the durum semolina mill ranges from about
65.70, 10, and 25.20% of semolina, flour, and bran,
respectively.
Couscous is made from very coarse durum semolina
with a particle size range between 550 and 1100 mm.
Couscous is not extruded, but is coagulated and
steamed in granular form. 81Prepared by Hab2 S.
The granulation distribution of the semolina affects
water absorption of the particles during hydration in
a pasta-production process.
Subsequently, it also affects the drying of the pasta
and its quality.
Optimum semolina granulation for each pasta
product is a major concern of the miller and pasta
manufacturer.
Common semolina particle size for long pasta is finer
than 630 mm and for short goods finer than 350 mm.
82Prepared by Hab2 S.
Durum wheat semolina is evaluated based on speck
count, protein level, and ash.
The origin of specks in the semolina could arise from
different sources.
Generally about 45% originate from discolored
germs, 25% discolored endosperm, 15% bran
particles, 10% grit, and 5% other sources.
Ergot, when present in wheat, could show up as
specks in the semolina.
83Prepared by Hab2 S.
Durum and spring wheat, like other cereals that
might go through the flowering period during
cold and wet weather, could be infected by the
fungus Claviceps purpurea or ergot.
Ergot is a fungus that produces alkaloids toxic to
humans and animals when it invades spring
wheat, durum wheat, and rye.
84Prepared by Hab2 S.
The word ''ergot" is applied to both the fungus and the
disease that the fungus causes.
Hard wheats are more vulnerable to ergot attack than soft
wheats.
Hybrid varieties are more susceptible presumably because
they have smaller anthers with less than sufficient pollen for
quick fertilization, resulting in sensitivity to ergot attack.
Millers use different methods such as gravity tables and
color sorters to separate ergot from the wheat.
According to U.S. Department of Agriculture Standards for
Grain, ergoty wheat is wheat that contains more than 0.05%
percent ergot.85Prepared by Hab2 S.
The specks have an adverse effect on the aesthetic
appearance of pasta and, to some extent, the
resistance to breakage of long varieties.
Grit content in the granular semolina is also a
quality measure.
Grit originates from ground stones not separated
from the wheat during cleaning.
Grit in semolina could damage the pasta extruder's
surface.
86Prepared by Hab2 S.
Durum milling is substantially different from flour
milling.
To achieve maximum extraction of granular
endosperm, more break and corrugated sizing
stages are used.
The tail-end materials in the mill that could not
be extracted as semolina are usually ground on
smooth rolls to flour.
87Prepared by Hab2 S.
Although the total cumulative break release would be the
same, the release on the individual breaks is lower than in
flour milling.
The number of purifiers used in semolina milling is
significantly higher than in conventional flour milling.
The purifier is the machine from which the final semolina is
extracted.
In durum milling the miller sends material to purifiers with
much narrower particle size ranges than in flour milling to
differentiate more sharply between the different
characteristics of materials based on size, shape, and specific
gravity. 88Prepared by Hab2 S.
B .Soft Wheat Milling
The soft wheat milling process differs from that for
hard wheat because of the softer kernel endosperm.
Soft wheat is milled to flour that is used mainly for
the manufacture of baked goods not requiring a
developed structure during fermentation.
Protein contents of flours produced in the soft wheat
mill ranged from 4.7 to 9.1% and patent ash
contents from 0.23 to 0.42% (14% m.b.).
Soft wheat kernels are wider and have a lower
specific weight than hard wheat kernels. 89Prepared by Hab2 S.
Accordingly, cleaning machinery must be adjusted to the
physical characteristics for efficient separation of unmillable
materials.
The endosperm structure of soft wheat is not vitreous and
dense, allowing water to penetrate at a faster rate than in
hard wheats through the capillary spaces in the endosperm.
Therefore, tempering time to reach a milling moisture is
very short for soft wheat, usually about one half of the time
required by hard wheat.
In cases when the natural moisture of the wheat is high, only
a limited amount of water is sprayed on the wheat about 30
minutes before milling to toughen the bran.90Prepared by Hab2 S.
Endosperm of soft and hard wheats fracture
differently during the milling process.
Hard wheats are more crystalline and break into
large chunks of endosperm while soft wheat
endosperm is amorphous and crumbles into smaller
particles.
The soft endosperm disintegrates during the milling
process with less pressure.
As a result, soft wheat produces finer flour particles
with lower levels of starch damage compared to
hard wheat. 91Prepared by Hab2 S.
In countries where soft wheat flours are used for
bread baking, the miller is aware that he or she has to
control the starch damage of the flour.
This is done by applying heavy roll pressures in the
reduction system.
Also, the starch protein bond in soft wheat is weaker
than that in hard wheat.
With proper impact force, it is possible to separate the
granules from the protein matrix in which they are
embedded.
During milling more flour from breaks and less sizing
production are the main characteristics of soft wheats
compared to hard wheats. 92Prepared by Hab2 S.
The sifter effective area in a soft wheat mill is relatively
larger than in the hard wheat mill.
This should overcome difficulties in sieving of fine flours.
Some millers overcome the difficulties of sifting soft
wheat materials by using centrifugal sifters.
The centrifugal sifters might have advantages over
regular gyrating sifter boxes.
The action of a centrifugal machine, in which a counter
rotating rotor throws the stock against a cylindrical
sieve, allows efficient separation, especially in the poorly
flowing stocks of the soft milling flow.
93Prepared by Hab2 S.
In general, purifiers are not used in soft wheat mills.
In cases where they are incorporated in the flow they
treat only the small amount of sizings from the primary
breaks.
The less rigid endosperm attached to the bran in the
tail end breaks is difficult to separate with conventional
grinding rolls that might splinter the bran.
Impact dusters are used before the third, fourth, and
fifth break rolls to achieve more flour extraction.
In general, more impactors are used in a soft wheat
mill between the rolls and sifters to increase flour
extraction compared to hard wheat milling.94Prepared by Hab2 S.
There is a new technical approach to the separation of
the three main parts of the wheat kernel: endosperm,
bran, and germ.
The new technology applies intensive and accurate
abrasion of the wheat kernel bran. The miller can
selectively remove wheat pericarp layers from the outside
in.
The objective of the new technology is to break up the
structure of the kernel in such a way that the crease
''structure" will stay intact.
This technology reduces to a large extent the number of
machines in the mill. 95Prepared by Hab2 S.
The benefits of such a technology are reduced
capital investment, shorter milling process,
reduction in energy, reduction of a-amylase content
of flour when partially sprouted wheats are used,
and reduction of fragments and bacteria count in
flours.
The rapid developments in electronics and
instrumentation are implemented in the mill for
rapidly sensing online the quantitative and
qualitative characteristics of mill products.
96Prepared by Hab2 S.
Evaluation of intermediate and final mill products
allows the development of mill automation and
control.
Near-infrared reflectance, fluorescence imaging,
microwave, and electronic weighing are some of the
current and future areas of development.
97Prepared by Hab2 S.
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