Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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Wheat Conditioning

0015 After cleaning, the wheat is conditioned (tempered)

by the addition of moisture (Figure 2). The precise

addition of the correct tempering moisture is of

fundamental importance to the milling process.

0016 The primary purpose of tempering is to optimize

the separation of bran from endosperm during

the milling process. Tempering also reduces bran

powdering, thereby reducing bran contamination in

flour, and imparting optimum texture to the endo-

sperm for efficient reduction into flour. Too much

tempering moisture reduces flour yield because com-

plete separation of bran from endosperm is difficult

to achieve, and sieving efficiency is reduced. Too little

tempering moisture results in bran powdering which

contaminates flour.

0017 Tempering water is added by spray nozzles to

wheat in an enclosed screw conveyor. Modern

tempering equipment features electronic control and

vigorous mixing action to insure that precise and

uniform addition of moisture is achieved. Following

tempering, wheat is stored in bins for a predetermined

time to allow uniform distribution of moisture within

the wheat kernel.

0018Optimum tempering moisture varies with wheat

type. Soft wheat is tempered to lower moisture than

hard wheat, because soft wheat stocks are inherently

more sticky and difficult to sift than hard wheat

stocks, and that tendency is exacerbated as moisture

content increases. Optimum tempering time may vary

from several hours to a day or more depending on

incoming wheat moisture content and hardness.

Water penetrates into the endosperm of soft wheat

more quickly than for hard wheat because soft

wheat endosperm is more porous. Therefore, tempered

soft wheat does not need to be rested as long before

milling as hard wheat. Wheat is often tempered in

two stages with a resting time between stages, par-

ticularly when a large amount of tempering moisture

is added.

0019After tempering, wheat is scoured. Scourers use

steel beaters to throw wheat against a wire screen.

Dirt and loosened bran, which would contaminate

the flour, pass through the screen. Commonly, as

shown in Figure 2, wheat is lightly sprayed just

prior to milling. This is to toughen the bran, and

reduce bran powdering. Following a brief period in

holding bins, the wheat is weighed, and passed

Preparation of wheat for milling

Wheat receival

Ship, barges

Rail

Truck

Storage

Magnet

Grain separator

Wheat bins

Conve

Conveyor

Light

Wheat

Large

Small

Light

Wheat

Broken, flax, cockle

Air

Disk separator

Oats and

barley

Stones

Large

Small

Cleaning

Magnet

Dampener

Scale

Scourer

Grain separator

Destoner

Conditioning

fig0002 Figure 2 A simplified wheat intake, cleaning, and conditioning diagram.

2538 FLOUR/Roller Milling Operations

through a magnet to remove ferrous metal fragments

prior to milling.

Mill Equipment

0020 Roller milling consists of a succession of grindings,

each followed by separation of the ground stock

according to its size and composition. The most im-

portant pieces of equipment common to all modern

flour mills are the roller mill, the plansifter, the puri-

fier, and the bran duster.

0021 The standard modern roller mill is a double-

mounted stand consisting of two pairs of steel rollers

mounted within a cast iron frame (Figure 3). The

roller pairs are mounted back to back. Each roller

pair is driven separately and has separate feeding

and adjustment mechanisms. Recently, roller mills

with four pairs of rollers, comprising two pairs

stacked vertically with the upper pairs feeding the

lower pairs directly, have become available. They

are gaining in popularity, particularly for use early

in the break system, and early in the reduction system,

which allows an economical way of increasing milling

capacity.

0022 The two rollers within each pair rotate in opposite

directions and at different speeds. The ratio of roller

speeds is known as the differential. Having one roller

rotate faster than the other imparts shear to the grind-

ing action. The slower roll holds the stock, while the

faster roll compresses and shears it.

0023 The surface of the rollers may be smooth or corru-

gated, with flutes cut in a spiral pattern along the

length of the roller. The spiral prevents the rollers

from locking, and imparts a cutting and shearing

effect. The number of flutes per centimeter and the

shape of the flutes vary widely, depending on the stage

of the milling process, the type of wheat being milled,

and the desired flour extraction rate. Reduction rollers

are usually smooth, although the surface is slightly

rough (frosted) to impart some shear to the grinding

action. For the most effective performance, break

rollers are run at a differential of about 2.5:1, whereas

reduction rollers are run at a differential of about

1.25:1.

0024A modern plansifter consists of as many as 30

layers of sieves, rotating in a horizontal plane. The

sieves are arranged in sections, with some of the sifter

units comprising up to eight sections. The aperture of

the sifter clothing ranges from about 1600 mm down

to less than 100 mm. Coarse clothing is composed of

wire mesh, while finer clothing is composed of nylon

or silk.

0025Stock enters the sifter at the top. The rotating

motion of the sifter works the finer material through

the top sieve. The coarse material is drawn away and

the finer material proceeds to a series of progressively

finer sieves where further size separations are made.

0026Bran dusters use impact to remove endosperm that

remains attached to broad bran at the end of the

break system. The same machine, when used to

remove endosperm from shorts (fine bran from the

end of the break system), is commonly referred to as a

shorts duster. Bran or shorts is fed into a horizontal

Feed

Air

Feed

rolls

Grinding

rolls

Ground material

Pure

semolina

Composite

particles

Overs of

sieves

fig0003 Figure 3 Schematic views of a modern roller mill (left) and a modern three-deck purifier (right).

FLOUR/Roller Milling Operations 2539

cylinder, the lower part of which is perforated. Within

the cylinder, finger beaters attached to a central shaft

rotate, and scrape endosperm from bran or shorts.

Endosperm is pushed through the apertures, and bran

or shorts passes though the cylinder and is collected

separately.

0027 Purifiers separate pure endosperm particles from

bran-rich particles of comparable size (Figure 3).

This is done by taking advantage of the greater

density of pure endosperm particles.

0028 Purifiers comprise a sieve frame mounted with a

slight downward slope. The frame is clothed with

progressively coarser material from head to tail, and

an air current passes upwards through the sieves. The

frame oscillates longitudinally. The frame oscillation

and the air currents cause the stock to stratify, with

the lighter bran-rich particles rising to the surface.

The heavier endosperm particles fall though the

sieve if the aperture is large enough, whereas the

bran-rich material floats over the end of the sieve

frame.

0029 The combination of the air flotation and sieving

action allows the purifier to separate the incoming

mixture into material that is progressively coarser

and more contaminated by bran from head to tail.

Modern purifiers have two or three decks of sieves to

enhance efficiency of separation.

Flour Milling

0030Figure 4 gives a simplified version of a typical hard

common wheat flour mill flow, designed to give a

flour extraction rate of about 75%. The milling

process comprises the break system, intermediate

processing, which includes purification and sizing,

and the reduction system.

0031The wheat kernel is opened on the first break

rollers. The ground material passes to the first

break sifter, where particles are separated according

to size. The largest particles, which consist of wheat

bran and adhering endosperm, proceed to the second

break rollers. Each successive break and sifting pas-

sage separates more bran from endosperm. Immedi-

ately following the last break sifter, the bran proceeds

to a shorts duster to remove some of the adhering

endosperm.

0032The primary purpose of the break system is to

separate endosperm from bran as efficiently as

possible, although a small amount of break flour is

Break

Roll stand

Sifter

Purifier

M1 SIZ

M2 M4

Sifter

Germ

Shorts

Sifter

Sizing

M4 Sifter

Bran

M4 Sifter

Shorts

Grader Purification

Milling process

Reduction Product outloading

fig0004 Figure 4 A simplified hard common wheat mill diagram. B, break; BD, bran duster; GR, grader; P, purifier; SD, shorts duster;

SIZ, sizing; M, middling. Corrugated rollers are cross-hatched and smooth rollers are open.

2540 FLOUR/Roller Milling Operations

produced. Most of the finer stock produced by the

break system consists of particles larger than flour

known as semolina (coarser particles) and middlings

(finer particles). Semolina and middlings are separ-

ated by a sifter, referred to as a grader, according to

particle size, before proceeding to purifiers.

0033 Purifiers separate pure endosperm particles from

brain-rich material. As a result, essentially pure endo-

sperm is sent to the beginning of the reduction system.

This allows the production of a large proportion of

highly valued, highly refined white flour.

0034 The coarse bran-rich material from purifiers pro-

ceeds to the sizing (scratch) system. The objective of

the sizing system is not to make flour, but to remove

bran adhering to middlings. This is accomplished

by light grinding on finely corrugated rollers. The

ground stock is then sifted and purified before con-

tinuing on to the reduction system.

0035 The heart of the flour milling process is the reduc-

tion system, where most of the flour is produced. The

reduction system reduces the carefully sized and puri-

fied middlings to flour particle size. This is done

gradually by a series of successive grinding and sift-

ings. Germ, which is flattened rather than reduced by

smooth reduction rolls, can be recovered by sifting.

0036 Grinding conditions must be carefully controlled

to produce the maximum amount of highly valued

prime-quality flour. If grinding is too severe, bran

contamination is increased, reducing flour brightness.

Severe grinding can also result in excessive starch

damage which adversely affects flour functionality.

Flour Dividing

0037 A flour mill may produce a single or multiple grades

of finished flour from the milling of a given wheat or

wheat mix. These finished flours are prepared by

combining various flour streams that are produced

at different stages of milling (Figure 5).

0038Individual flour streams, when compared with

each other, exhibit variable processing quality. Flour

millers may combine all flour streams to produce a

single flour known as a ‘straight-run’ or ‘straight-

grade’ flour. A more complex alternative is to

produce several flours with different properties by

judicious blending of streams. Figure 5 shows that

each flour stream leaving a sifter can be directed to

any one of three flour conveyors. Each continuous

line represents a given flour stream flow to a desig-

nated flour conveyor. In this example, three flour

streams each go to conveyors for flour #1 and flour

#2, respectively, while two streams are directed to the

conveyor for flour #3. This is known as ‘divide’ or

‘split-run’ milling and allows millers with demanding

clients to increase return by closely targeting specific

processing requirements.

Flour Additives

0039Mills commonly add additives to flour to improve

nutritional value or to assist in achieving specific

processing quality. As mentioned earlier, endosperm,

the main component of flour, is poor in vitamins

compared to the bran and germ. Many countries

have legislation requiring the addition of vitamins

and minerals to improve the nutritional properties

of flours.

0040Bleaching agents like benzoyl peroxide may be

added to give the flour a whiter appearance. Other

additives impart better functionality. These include

various enzymes (alpha-amylase of either fungal or

grain origin) and oxidizing agents. Additives are

available in powder form, and are dispensed directly

into flour conveyors using powder feeders.

Divide milling

Sifters

Diverter

valves

Flour

conveyors

Flour #1

Flour #2

Flour #3

fig0005 Figure 5 An example of divide milling, where three custom flours are being produced from a single wheat mix.

FLOUR/Roller Milling Operations 2541

Soft Wheat Milling

0041 Soft wheat milling is similar to hard wheat milling,

but intrinsic differences between soft wheat and hard

wheat require some modifications to the milling pro-

cess. Soft wheat breaks down more quickly than hard

wheat, and soft wheat stock is stickier and fluffier,

which makes it more difficult to sift than hard wheat

stock. In addition, the endosperm of soft wheat ad-

heres more strongly to bran, reducing flour extraction

rate expectations by up to 2% unless the break system

is extended.

0042 Soft wheat must be fed more slowly to the mill to

facilitate sifting and to insure that stock flows freely

through the mill. The rapid breakdown of soft wheat

increases the yield of flour from the break system and

reduces the yield of semolina and middlings. As a

result, purifiers are of less importance and are often

absent from the flow.

Durum Wheat Milling

0043 The same milling equipment is used for durum wheat

milling as for common wheat milling, but the mill

flow is very different. The product preferred for pre-

mium pasta and couscous is uniformly sized semo-

lina, free of bran. Durum wheat flour produced from

semolina mills is a lower-valued byproduct.

0044 Durum wheat is very hard, which facilitates a high

yield of semolina with minimal production of flour.

The break system for durum wheat is extended to

allow gradual breakdown of the wheat kernel to

achieve a maximum production of semolina and a

minimum production of flour. Purified semolina

from the break system is uniformly sized and made

free from adhering bran by a succession of sizing

passages, gradings, and purifications. Most of the

semolina is from the sizing purifiers, making durum

mills readily recognized by the large number of puri-

fiers.

Rye Milling

0045 Rye grain is milled into various forms of meal and

flour for application in the production of a variety of

baked products, particularly in northern and eastern

Europe. Dark rye bread, pumpernickel, and a host of

other bread products containing rye offer a unique

combination of texture and flavor that is truly

characteristic of rye.

0046 A rye mill also uses grinding and sifting equipment

similar to that used in wheat flour milling. Rye milling

is similar to the milling of soft wheat. The endosperm

is very soft and sticky, making it very difficult to grind

and sift efficiently.

0047Because rye is so soft, a significant proportion of

the total flour is produced at the beginning of the flow

following each break passage. Rye endosperm is so

soft and plastic that smooth rolls tend to flatten mid-

dlings rather than reduce them. As a result, all the

grinding passages in a rye mill use corrugated rolls,

including the reduction rolls.

0048Rye endosperm breaks down quickly into fine par-

ticles, and these particles lump together. Therefore,

purification passages are of no value and purifiers are

absent. Rye has gluten, but it imparts much less elas-

ticity to dough than wheat gluten. Rye flour color is

inherently dark. As a result, rye flour produces bread

loaves that are denser, darker, and heavier in texture

than wheat flour bread.

Milling Byproducts

0049The byproducts of milling receive little attention, but

they are an important economical consideration in

flour milling. These include impurities from the

cleaning house (screenings), mill feed (bran and

shorts), and germ.

0050Screenings, with the exception of metal, stones,

and mud balls, are usually ground and sold as animal

feed. The mill feed, which comprises large bran flakes

produced from the break system and finer bran par-

ticles, or shorts, are also primarily used as animal

feed. Depending on market conditions, mill feeds

also find use for foods such as breakfast cereals and

high-fiber specialty wheat products.

0051Wheat germ is a highly valued byproduct because

of its excellent nutritional composition. As a result,

many modern mills have sophisticated germ recovery

systems.

Debranning (Preprocessing)

0052Wheat debranning, or preprocessing, a process in

which the bran layers are removed sequentially

by friction and abrasion operations in modified rice

polishers prior to roller milling, has enormous

potential in wheat milling. Debranning has gained

considerable acceptance in durum wheat semolina

milling in particular. The yield and refinement of

semolina are significantly improved, allowing

more efficient milling of top-quality durum wheat,

or, alternatively, the use of lower-quality durum

wheat to produce semolina within customer specifi-

cations. There is less conclusive evidence that de-

branning improves the milling performance of

common wheat.

0053Regardless of wheat class being milled, an advan-

tage of using debranning is lower capital investment

because simplifications of the mill flow (the break

2542 FLOUR/Roller Milling Operations

system is almost eliminated) allows more compact

plants for a given capacity. In addition, the bypro-

ducts of debranning have the potential to be more

valuable than byproducts from conventional roller

milling. Rather than removing all bran layers

together, as in conventional roller milling, during

debranning the individual bran layers are stripped

off in sequence. Each bran layer has distinct physico-

chemical and nutritional properties, giving debran-

ning byproducts great promise as novel food

ingredients.

Milling Process Monitoring and Control

0054 All electrical motors driving various pieces of equip-

ment are sequence-controlled and interlocked to pro-

vide ease of operation, control, and protection. This

allows sequential starting and stopping of the motors

and stopping of the process in a failsafe manner in

case of interruptions due to a malfunction or for any

other reason. Additionally, a number of process con-

trol functions are incorporated that may, depending

on the extent of use of various types of sensors,

switches, high-level and low-level indicators, allow

the entire process to be automated. A conventional

control system involves a mimic process diagram

for visual display of the process, and a hard-wired

electromechanical relay control system for control

functions.

0055 Since the early 1980s there has been a gradual

transition to control flour mill automation with com-

puterization. Mimic panels with small pilot lights are

now being replaced by computer graphics for visual

display of the process status. Hard-wired control

relays are being replaced by programmable logic

controllers (PLC). PLC control with PC operator

interface enables performance of all the functions

described above. Additionally, it offers the flexibility

of changing control functions, if required, through

simply making programming changes rather than

changing the hard wiring. It also allows full integra-

tion of the remaining critical functions relating to

process control and performance evaluation, includ-

ing product yield calculations and automated roll gap

adjustment.

0056 Another important advance is online quality moni-

toring. Factors such as moisture content, protein con-

tent, and flour color can be monitored continuously

using automated online samplers allowing flour

millers to meet customer specification efficiently.

0057 Computerized process control has greatly reduced

labor requirements in flour mills. Many mills have

gone to so-called lights-out operation, where the mill

runs for extended periods without staff present in the

mill building.

See also: Biscuits, Cookies, and Crackers: Nature of the

Products; Bread: Breadmaking Processes; Cereals:

Breakfast Cereals; Dietary Fiber: Bran; Food

Fortification; Milling: Principles of Milling; Pasta and

Macaroni: Methods of Manufacture; Rye; Wheat: Grain

Structure of Wheat and Wheat-based Products

Further Reading

Canadian International Grains Institute (1993) Grains and

Oilseeds: Handling, Marketing, Processing, 4th edn,

vols 1 and 2. Winnipeg, Manitoba: Canadian Inter-

national Grains Institute.

Dexter JE and Wood PJ (1996) Recent applications of

debranning of wheat before milling. Trends in Food

Science Technology 7: 35–41.

Fabriani G and Lintas C (eds) (1988) Durum Wheat: Chem-

istry and Technology. St Paul, Minnesota: American

Association of Cereal Chemists.

Hoseney RC (1994) Principles of Cereal Science and Tech-

nology, 2nd edn. St Paul, Minnesota: American Associ-

ation of Cereal Chemists.

Matz SA (ed.) (1991) The Chemistry and Technology of

Cereals as Food and Feed, 2nd edn. McAllen, Texas:

Pan-Tech International.

National Association of British and Irish Millers (1976) The

Practice of Flour Milling, vol. 1. Richmond: Dimbleby

Printers.

Palmer GH (ed.) (1989) Cereal Science and Technology.

Aberdeen: Aberdeen University Press.

Pomeranz Y (ed.) (1988) Wheat: Chemistry and Technol-

ogy, 3rd edn, vols 1 and 2. St Paul, Minnesota: American

Association of Cereal Chemists.

Posner ES and Hibbs AN (1997) Wheat Flour Milling.

St Paul, Minnesota: American Association of Cereal

Chemists.

Yamazaki WT and Greenwood CT (eds) (1981) Soft Wheat:

Production, Breeding, Milling and Uses. St Paul, Minne-

sota: American Association of Cereal Chemists.

Analysis of Wheat Flours

K R Preston and P C Williams, (formerly of) Grain

Research Laboratory, Winnipeg, Manitoba, Canada

Introduction

0001Flour, produced by the selective removal of the germ

and the outer layers (bran) from the endosperm of

wheat during the milling process, is used in the pro-

duction of a wide variety of baked, extruded, and

sheeted products throughout the world. Each of

these products may require different types of flour

FLOUR/Analysis of Wheat Flours 2543

to produce an acceptable product, as shown in

Table 1. The ability of a flour to produce a specific

product is determined by the inherent characteristics

of the wheat, the effects of environment during

growth, harvesting, and storage on wheat properties,

and the milling techniques used to produce the flour.

Flour (and wheat)-testing procedures have been

developed to predict or measure the effects of these

factors on flour-processing quality and to insure

health standards have been met.

0002 The most common applications of flour analysis

include:

0003 In wheat-breeding programs where new cultivars

are evaluated for selection.

0004 In government and private laboratories respon-

sible for monitoring quality and health standards,

providing information for segregation of wheats

and flours and providing information to buyers.

0005 In flour mills for selection of wheat lots and the

blending thereof and to insure that flours pro-

duced meet customer specifications and health

standards.

0006 In end-user facilities to verify that flour specifica-

tions have been met and that health-related

standards are maintained.

0007 This article provides an overview of quality and

health-related testing procedures commonly used in

flour analysis with emphasis on their use and inter-

pretation. This review is restricted to common

(hexaploid) wheat flour testing and does not include

durum wheat which is normally milled to semolina

for the production of pasta and couscous. Detailed

information on wheat and flour test procedures are

available from approved methods manuals published

by the two largest associations involved in cereal

chemistry – the American Association of Cereal

Chemists (3340 Pilot Knob Road, St Paul, MN

55121, USA http://www.aaccnet.org) and the Inter-

national Association for Cereal Science and Technol-

ogy (PO Box 77, Wiener Str. 22a, A-2320 Schwechat,

Austria, http://www.icc.or.at).

Flour Quality Tests

0008Flour quality tests are designed to predict or deter-

mine milling efficiency and end-product suitability.

These tests can be subdivided into three categories

including analysis of basic constituents (proximate

analysis), processing-related parameters, and analysis

for specific additives. The most common of these tests

are summarized in Table 2.

0009Flours are also commonly subjected to physical

dough tests which provide information involving the

viscoelastic properties of flour dough and to small

laboratory-scale end product processing tests which

provide the most accurate estimate of product quality.

Proximate Analysis

0010Proximate analysis of flour normally involves the

determination of moisture, protein, and ash content.

Measurement of fiber components (endogenous and

that added from other sources) is also becoming

increasingly important in view of their nutritional

importance. Determination of oil and carbohydrate

composition, the latter of which is determined by

difference (i.e., 100  sum of the five constituents

listed above) is not normally carried out and will

not be discussed further.

0011Flour moisture content is fundamental informa-

tion, since variance in moisture content affects the

proportion of all other constituents. Analytical infor-

mation for flour is normally reported on a specified

moisture basis, while the amount of flour required for

physical dough or for end-product testing is normally

weighed to a specified moisture basis. Moisture con-

tent is also important for both economic reasons and

flour storage. Selling higher-moisture flour is nor-

mally more profitable to mills. Flour will deteriorate

rapidly if moisture exceeds 15% in moderate climates

and 14% in tropical climates. Flour moisture can be

determined by air-oven methods. For more rapid

analysis, near-infrared reflectance (NIR) can be used.

0012The protein content of flour is an essential piece

of information concerning the end-use potential of a

tbl0001 Table 1 Major uses of common wheat flour classified by

protein content and physical dough strength

Flour type

Uses

Strong, high-protein Pan breads, blending

Medium strong, medium-protein Hearth breads, flat breads,

steamed breads, noodles

Weak, low-protein Cakes, cookies, dumplings

Soft wheat noodles (udon)

Approximate protein ranges for high-, medium-, and low-protein (14%

moisture basis) flours are >12%, 10–12%, and <10% respectively.

Products listed with each flour type are intended as a general guideline.

Many of the products listed are often produced from the other flour types.

tbl0002Table 2 Common quality flour tests

Constituents Parameters Additives

Moisture Flour color Potassium bromate

Protein Wet gluten Ascorbic acid

Ash Gluten index Riboflavin

Fiber Starch damage Thiamin

Particle size Niacin

Amylograph peak viscosity Iron

Falling number Folic acid

Maltose value

Gassing power

Fat acidity

2544 FLOUR/Analysis of Wheat Flours

sample within a flour type or class. For example, with

bread-wheat flours, protein content is generally posi-

tively associated with baking quality parameters such

as loaf volume and bread score, as shown in Figure 1.

In contrast, with cake and cookie flours the relation-

ship between quality and protein content is negative.

However, protein quality, as well as quantity, exerts a

major impact upon end-product quality and must

also be considered. Protein quality can be assessed

by physical dough and end-product tests, described

later in this article.

0013 For many years the Kjeldahl test and modifications

thereof, involving the measurement of ammonia

nitrogen released by acid hydrolysis, has been univer-

sally accepted as a method for the determination of

the protein content (N  5.7) of wheat and flour.

Since the last edition of this encyclopedia, improve-

ments in the instrumentation of the Dumas test have

resulted in the introduction of a number of high-

temperature combustion nitrogen analyzers (CNA).

The increased accuracy and precision of these instru-

ments (relative to the Kjeldahl method), combined

with the elimination of the requirements for fume

exhaust, drainage systems, and the use of highly cor-

rosive chemicals, have led to increasing adoption of

the Dumas test as the accepted reference method for

protein determination in many commodities, includ-

ing flour. In addition, the Dumas method offers im-

provements in the time per test (3 min, as compared

to 2 h for the Kjeldahl test), while the method uses no

offensive chemicals. The instruments are semiauto-

mated, which enables virtually continuous analysis

throughout the working day. For more rapid analysis,

NIR can be used to predict protein content from

whole and ground grain and flour.

0014The ash test is used in the milling industry as a

measure of milling efficiency and in classification of

mill streams for the production of various types of

flours. Ash content is primarily a measure of the

efficiency of separation of the low-ash endosperm

from the high-ash bran. Flours with low ash content

(less than about 0.40%, 14.0% moisture basis) are

termed patent flours and are derived from the inner

endosperm of the wheat kernel. This flour is used

where a bright white color is needed in the end

product.

0015Straight-run, or straight-grade flours, which are

most commonly produced worldwide, can have ash

contents varying from approximately 0.48 to over

0.60%. They are derived from the entire endosperm

with varying degrees of bran inclusion and normally

represent wheat extraction rates of 74–80%. High-

ash flours (> 0.60%), involving high wheat extraction

rates (80–95%), are produced where a darker-colored

product is preferred by the consumer (e.g., brown and

wholewheat breads), for traditional high-extraction

flour products such as flat breads in the Middle East

and chapattis in the Indian subcontinent or where

government policy requires high extraction rates for

economic reasons. As with protein, ash content is a

common flour measurement used by mills to meet

customer specifications. In some countries, flour

color is used in place of ash content as a measure

of milling efficiency. Flour ash content is normally

determined by weight difference after incineration

of a sample for several hours in a high-temperature

(550–590



C) muffle oven.

0016Crude fiber and dietary fiber have not been com-

monly measured in flour as a routine test. However,

measurement of dietary fiber is becoming increasingly

important in flours to which high-fiber ingredients

have been added for nutritional considerations (e.g.,

high-fiber breads and breakfast cereals). At present,

a number of methods are used to measure dietary

fiber but no single procedure has gained widespread

acceptance.

0017NIR analysis is now in commercial use as an inline

instrument in modern flour mills. The NIR instru-

ments are calibrated to provide continual data for

moisture, protein, and ash contents of flour during

1000

950

900

850

800

750

700

650

600

9 101112131415

Loaf volume (cc)

Flour protein content (%)

fig0001 Figure 1 Relationship between loaf volume of remix-

processed, straight-dough bread, and flour protein content

(14% moisture basis) for Canadian red spring wheat straight-

grade flours. Reproduced from Flour, Encyclopaedia of Food Sci-

ence, Food Technology and Nutrition, Macrae R, Robinson RK and

Sadler MJ (eds), 1993, Academic Press.

FLOUR/Analysis of Wheat Flours 2545

the milling process. The inline instruments are used in

conjunction with benchtop computers (PCs) to moni-

tor and control flour composition. The information is

used by millers to control mill-flow. As well, NIR

instruments have been calibrated for the prediction

of flour starch damage and wet gluten content.

Table 3 summarizes typical data for proximate analy-

sis of flour, using a scanning spectrophotometer.

0018 Flour is an ideal medium for NIR analysis. Due to

the sensitivity of NIR analysis to particle size, the

technology also affords an excellent means of

monitoring the consistency of the production of all

products of a flour mill.

Processing Related Parameters

0019 Several tests provide useful information concerning

the functionality or end-use potential of flour. The

most widely used of these processing-related param-

eters are listed in Table 2. They can be grouped into

parameters that are influenced mainly by milling

extraction (flour color), protein content (wet gluten),

wheat texture (starch damage and particle size), a-

amylase activity (amylograph peak viscosity and fall-

ing number), wheat texture and a-amylase activity

combined (gassing power and maltose), and storage

conditions (fat acidity).

0020 In some countries (e.g., UK) flour color is regarded

as being a more reliable indication of the purity of

flour than the ash test and is used in place of the latter

in flour specifications. Flour color is also used in

conjunction with the ash test to give a better indica-

tion of potential product quality, in particular flour

whiteness and brightness. Measurement of flour color

is considered particularly important in many Asian

mills producing flour for various types of noodles

where consumer preference is strong for a bright

white product (e.g., dried noodles) or a bright, light

yellow product (e.g., alkali ‘Chinese’ noodles).

0021 Flour color can be determined with the Satake/

Kent-Jones color grader which involves measurement

of the reflectance of a flour-water slurry. Other

common methods include the Agtron test, and the

Minolta, or tri-stimulus test. The determination of

tri-stimulus color values of flour or products thereof,

using reflectance spectrometers, is now being widely

used. Brightness (L*), red/green chromaticity (a*),

and yellow/blue chromaticity (b*) can be rapidly

and simultaneously measured.

0022Wet gluten is formed by hand or machine washing

of flour in a dilute salt solution. The resulting visco-

elastic product consists mainly of insoluble proteins

which are of primary importance in determining

dough rheological properties. Wet gluten content is

closely related to flour protein content (in the ratio

of approximately 3:1). The hand-washing method is

used widely in many countries in place of protein con-

tent for flour specifications due to its simplicity and

low cost. The yield or the ‘feel’ of the resulting gluten is

also useful as an overall indication of inherent quality

and as an indication of damage caused by proteolytic

enzymes injected into the wheat kernel by the wheat

bug (Cinch bug, Suni bug, etc., Eurygaster sp.).

0023About 30 years ago Perten Instruments introduced

the Glutomatic instrument, as a mechanical washing

method for determination of wet gluten content. The

test method has been modified to provide a gluten

index value. The gluten index value is becoming

widely used as an indication of gluten (flour) strength.

In this procedure, wet gluten prepared by machine

washing of flour or ground grain is centrifuged

through a sieve. The percentage of gluten remaining

on the sieve after centrifugation, relative to the total

gluten content, is defined as the gluten index value.

High values (90–100) indicate strong gluten charac-

teristics while low values (up to 30) indicate weak

gluten characteristics.

0024Damaged starch is caused by the rupture and abra-

sion of starch granules during the milling process. The

amount of damaged starch is closely related to the

texture of the wheat. Hard-textured wheat gives flour

with much higher starch damage than soft-textured

wheat. The degree of flour starch damage, in addition

to protein content, is a primary determinant of the

ability of flour dough to absorb water (damaged

starch absorbs approximately three times the amount

of water compared to intact starch granules). This is

tbl0003 Table 3 Accuracy and precision of near-infrared analysis of flour

Parameter r

SEP b a Bias RPD SET CV %

Moisture 0.990 0.119 1.017 0.232 0.006 9.71 0.025 0.25

Protein 0.992 0.132 0.990 0.102 0.009 11.43 0.044 0.38

Ash 0.880 0.0189 1.020 0.0091 0.0005 2.86 0.012 2.40

Wet gluten 0.908 1.33 0.956 1.17 0.322 3.51 0.79 2.50

Starch damage 0.947 0.318 0.997 0.019 0.003 4.64 0.057 0.64

SEP, standard error of prediction; b, slope; a, intercept; RPD, standard deviation of reference data/SEP; SET, standard error per test; CV%, coefficient of

variability.

2546 FLOUR/Analysis of Wheat Flours

particularly important in bread production where

high water absorption is desirable and in the produc-

tion of ‘soft’ wheat products such as cakes and

cookies where low water absorption is desirable.

0025 Damaged starch is also important in the baking of

leavened products since, due to its susceptibility to a-

amylase attack, it provides a source of sugar to the

yeast. Damaged starch is measured by digestion with

an excess of amylases (a and b) followed by measure-

ment of the resulting released sugar. The Tripette &

Renaud (Chopin) model RT instrument is also used

for the determination of starch damage. The Model

RT employs an iodine reaction for the prediction of

starch damage.

0026 Flour particle size (granulation) is also strongly in-

fluenced by wheat texture. Harder wheat gives larger

average particle size than softer wheat. Milling condi-

tions may also influence particle size (in particular, the

choice of sieves). Finer flours generally absorb water

faster than coarse flours. Flour particle size is measured

by a variety of techniques including sedimentation,

sieving, light refraction, and light scattering.

0027 Enzymes present in flour can have a major impact

upon processing quality. The most important enzyme

in flour is a-amylase. High levels of a-amylase are

usually detrimental to processing quality, particularly

in the case of bread products where dough becomes

difficult to handle (sticky) and shows a tendency to

collapse during baking. This is mainly attributable to

the formation of dextrans, which are products of

starch hydrolysis. High levels of a-amylase are most

commonly associated with the presence of sprouted

wheat in the milling grist. Very low levels of this

enzyme may also be undesirable in bread flours

since insufficient sugar will be released for gas

production by yeast. a-Amylase activity in flour is

normally estimated by measuring the starch-pasting

characteristics of flours. The most common measure-

ments include amylograph peak viscosity Hagberg

(now Perten) falling number and the Rapid Visco-

analyzer. In all three methods, lower values indicate

higher enzyme activity. Where activity is low, suffi-

cient a-amylase (from fungus or malted wheat or

flour) may be added to give a predetermined amylo-

graph peak viscosity equivalent to a falling number

value of about 250 Brabender units.

0028 The ability of a-amylase to produce sugars during

fermentation is also dependent on the degree of starch

damage, since intact starch granules are less suscep-

tible to attack. Gassing power and maltose or dia-

static value are measurements of the effects of these

two factors. These measurements are commonly used

to assess the ability of flour to maintain gas produc-

tion by yeast and to make adjustments in a-amylase

or sugar levels if necessary.

0029Other enzymes also have an impact on flour qual-

ity. However, in most cases differences in activity

between different flours do not have a major impact

on processing quality. Exceptions include proteases

and certain oxidases. Endogenous flour proteolytic

enzymes do not appear to have a major impact upon

flour quality but, as noted earlier, injection of these

enzymes by insects into the wheat kernel can cause

extensive damage (weakening) of the gluten proteins.

Polyphenoloxidases can cause browning reactions

while lipoxygenases can bleach flour dough due to

oxidation of pigments. High levels of the former

enzymes, due to sprout damage or inherent variety

characteristics, can cause product discoloration, par-

ticularly in the case of wet noodles. Lipoxygenases

are often added to bread flours (as bean or pea flour)

as a whitening agent.

0030During long-term storage of wheat and flour, a

gradual deterioration in quality may occur, depending

upon conditions. This is indicated by increased levels

of free fatty acids, acid phosphates, and amino acids.

Fat acidity shows the largest changes during the early

stages of deterioration and is used most commonly to

diagnose this process. The most common method of

measuring fat acidity in flour involves extraction of

free fatty acids with toluene followed by titration

with dilute alcoholic alkali.

Additives

0031Ingredients may be added to flour by mills as im-

provers and to enhance the nutritional value of the

flour. The most common improvers include bleaching

agents to whiten the flour and oxidants which are

added to improve the baking quality of bread flours.

Soft wheat flours are sometimes chlorinated (to a

specified pH value) for specific products.

0032Bleaching agents such as benzoyl peroxide are not

measured since they react rapidly on contact with flour

pigments. Oxidative improvers (added in the p.p.m.

range) are measured by millers to insure that correct

amounts have been added. The most commonly used

oxidants include ascorbic acid, potassium bromate,

and azodicabonamide (ADA). The use of the latter

two oxidants has been banned in many countries due

to health concerns, leaving ascorbic acid as the only

widely accepted chemical additive for oxidation. This

has resulted in the widespread use by the baking indus-

try of oxidative enzymes obtained primarily from

fungal sources to enhance or supplement the action of

ascorbic acid. Ascorbic acid is quantified by measuring

the decolorization of 2,6-dichlorophenolindophenol

while bromate and ADA are measured by reaction

with potassium iodide at low pH.

0033Vitamins and minerals may be added to flours to

replace those removed with the bran and germ layers

FLOUR/Analysis of Wheat Flours 2547