wheatstruc, nv, c and m

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.

1Prepared by Hab2 S.

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.


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


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.


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.


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.


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.


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.


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.


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 .


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 .


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.


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.


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.


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.


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.


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


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.


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.


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.


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.


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.


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.

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.


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."


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.


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.


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.


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 .


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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