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

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roasted ground flaxseed, salt, water, and honey. In

China, oil from cold-pressed flaxseed has long been

used for stir-frying. In North America, flaxseed con-

tinues to be an ingredient of Red River Cereal

breakfast staple introduced in 1924.

0020 Acceptability studies show that baked products

such as yeast breads, muffins, and cookies containing

flaxseed are increasingly well liked by North Ameri-

can consumers. This is in keeping with the growing

predominance of whole wheat and multigrain breads

in preference to white bread in several regions. Ex-

perienced sensory panelists give flaxseed and whole-

wheat breads comparable ratings in crust color,

texture, tenderness, porosity, and loaf shape.

0021 During the 1990’s, a ‘functional’ bread, Burgen

Soy-Lin

, was developed and commercialized in Aus-

tralia as one outcome from a study of the effects of

phytoestrogens on the symptoms of menopause. Four

slices of this 10% fiber bread daily contributed 2.5 g

of ALA and contained enough soy flour and flaxseed

to supply a significant amount of phytoestrogens

(220 mg) as isoflavones from soy together with the

lignan precursors from flaxseed. By April 1997, Bur-

gen

Soy-Lin

bread had captured 5.3% of the total

Australian bread market and was proving successful

in the UK.

Recipe Modifications

0022 Most wheat-flour recipes including yeast breads and

quick breads such as muffins, tea biscuits and cookies

can comfortably accommodate a 10% replacement of

wheat flour by flaxseed. However, when using flax-

seed in food products, it is important to recognize two

factors. First, flaxseed is an oilseed containing a

generous amount of fat, and second, flaxseed is

hydrophilic or water-loving.

0023 When substituting flaxseed for wheat flour in

baked goods, it is prudent to reduce the amount of

fat called for in the recipe by the amount contained in

the flaxseed. A useful ‘rule of thumb’ is the fact that

3 tbsp (1 tbsp ¼15 ml) of whole flaxseed, which is

41% oil, replaces 1 tbsp of salad oil. However,

3 tbsp of ground flaxseed will replace only 2/3 tbsp

(2 tsp) of salad oil. This is because flaxseed is aerated

during grinding so that ground flaxseed weighs less

per unit of volume, as shown in Table 1. Accordingly,

it replaces a little less fat than an equal amount of

whole flaxseed.

0024 The nature of the fat being substituted by the oil in

flaxseed may also influence the amount replaced.

Hydrogenated shortenings, like salad oils, are 100%

fat. However, because they are aerated to make them

easier to cream in cake and cookie production, they

contain about 10% less fat per unit volume than salad

oils. The standard tablespreads butter and margarine

are 80% fat with an allowable 16% water, so their fat

content per unit volume is less than either hydrogen-

ated shortening or salad oil. Thus, added flaxseed

with its 74 g of oil per cup (1 cup ¼240 ml) (Table 1)

theoretically replaces the volume of a fat type as

specified in a standard recipe such that flaxseed

would replace a greater volume of butter or margar-

ine than it would of hydrogenated shortening or salad

oil. Nevertheless, the balance of ingredients in a

sweetened, shortened baked product can tolerate as

much as a 25% increase or decrease in an ingredient

without making the product fail.

0025Considering the affinity of flaxseed for water,

bakers recommend that whole flaxseed for yeast

breads be conditioned with a water presoak for at

least 30 min at 18



C using a 2:1 ratio of water to

flaxseed. The soaking water, which contains some of

the soluble gums, then serves as the liquid in the

recipe. When ground flaxseed is used, water should

be added to approximate 75% of the flaxseed weight.

Some authors suggest that the yeast used be increased

by 25% if 10–15% of whole flaxseed is replaced by

ground flaxseed.

Storage Stability

0026The usefulness of flaxseed as a staple food ingredient

is reinforced by the fact that it appears remarkably

stable to storage. In one study, flaxseed from a mix-

ture of several varieties grown in Canada in 1996 was

ground with a pin-mill and packed in 60-lb (27.3-kg)

triple-wrapped plastic-lined paper bags. Samples

were stored for 128 days at ambient temperatures

under conditions similar to those used by commercial

bakeries. The mixed flaxseed contained 43% oil

consisting of 60% 18:3n-3 (ALA), 16% 18:2n-6,

and 15% 18:1n-9. Table 2 shows that, despite its

substantial content of unsaturated fatty acids,

ground flaxseed showed little or no evidence of

oxidative deterioration during 4 months of storage

at room temperature. Peroxide values, free fatty

acids, and conjugated double bonds did not change

appreciably. These chemical results were verified

by the lack of difference in odor intensity measured

by a trained sensory panel that examined water/

ground flaxseed slurries from all storage intervals

concurrently. Furthermore, a consumer panel of 36

volunteers, using a duo–trio test, could not detect any

difference between 10% flaxseed yeast breads pre-

pared from ground seed that had been stored at either

0 or 128 days.

0027While volatile compounds increased during the

storage of ground flaxseed, the level of total volatiles

after 4 months’ storage was 10–25 times less than

values reported for stored vegetable oils that con-

tained appreciably less oxidation-susceptible ALA.

2528 FLAXSEED

Increases over time were least in the dienals, which is

usually one of the major subclasses resulting from

polyunsaturated fatty acid oxidation. Among individ-

ual compounds, hexanal, which is often a quality

indicator, was found only at the p.p.b. level, margin-

ally close to reported sensory thresholds.

0028 What accounts for the oxidative stability of flax-

seed remains a question. In a sequel to the above

study, the fatty acid composition and the tocopherols,

known antioxidants, in ground mixed flaxseed were

examined after 11 and 20 months’ storage at ambient

temperatures. ALA levels were within the normal

variability range after 20 months’ storage, and the

tocopherols remained essentially unchanged. The

authors suggested that they were either not used or

in some way regenerated.

Baking Stability

0029 The ALA in flaxseed, whole or ground, appears to be

reasonably stable to baking temperatures. For

example, the proportion of ALA in a muffin mix

where flaxseed made up 28.5% of the formula

remained unchanged after baking for 2 h at 178



despite the fact that more oxygen was consumed by

flaxseed muffins during baking than those which

were flaxseed-free. Biological evidence also supports

the baking stability of ALA. No difference was found

between the plasma fatty acid profiles of college

women eating 50 g of raw, ground flaxseed added to

cereal, soup, juice or yogurt daily for 4 weeks and

those eating the same amount baked in bread. Both

groups showed a lowering of serum total cholesterol

and LDL cholesterol.

0030 Several studies suggest that the lignan precursor

SDG is stable to baking. For example, SDG levels in

the center of flaxseed breads proved similar to those

in the crust, even though the latter had a greater heat

exposure. Lignan production, measured by in vitro

fermentation, reflected the amounts of raw flaxseed

added in both commercial and home-baked products

(baked at 190



C) and pancakes (205



C), supporting

stability to baking. Furthermore, bioavailability was

confirmed in this series of studies when nine women,

for periods of 1–8 days, ate 5, 15, or 25 g of flaxseed

either added raw to applesauce or baked in muffins or

bread. While urinary lignan excretion increased as the

amounts of flaxseed eaten increased and with

repeated flaxseed consumption, there was no signifi-

cant difference in urinary lignan excretion due to

baking conditions.

Safety

0031Flaxseed contains two substances that may interfere

with the absorption of certain nutrients, although

both substances are present in flaxseed at levels that

appear to be of little consequence in the human diet.

Linatine inhibits the utilization of pyridoxine (vita-

min B

). However, when 30 adults consumed 45 g of

flaxseed daily in muffins for 5 weeks, serum pyridox-

ine levels did not change significantly from baseline.

The other substance, phytic acid, is known to form

insoluble complexes with calcium, copper, iron, mag-

nesium, and zinc in the intestine, thus reducing their

absorption. There is no evidence that this occurs to

any appreciable extent in humans.

0032Flaxseed also contains cyanogenic glycosides, which

are a group of natural substances found in more than

2500 plant species that can release cyanide, a poison-

ous compound, under certain conditions. Several

agronomically important crops such as cassava, sor-

ghum, flaxseed, and stone fruits like peaches, plums,

cherries, and apricots contain the glycosides. There is

no evidence that the cyanogenic glycosides pose a

health problem for healthy, nourished populations

in developed countries. Baking and other processing

methods apparently destroy or volatize the cyano-

genic glycosides. Indeed, muffins containing 25 g of

ground flaxseed had no detectable levels of the glyco-

sides after baking for 15–18 min at 230



Recommended Intakes

0033There is growing concern that Western populations

have an unbalanced intake of o-3 fatty acids relative

to the o-6 fatty acids. The current dietary o-6/o-3

tbl0002 Table 2 Chemical and sensory indices of the storage stability of ground mixed flaxseed

Parameter Storage day at 23 þ2



0 33 66 96 128

Peroxide value (meq kg

1

) 0.20 0.21 0.12 0.16 0.12

Free fatty acids (%) 0.34 0.48 0.31 0.35 0.31

Conjugated double bonds (absorbance at 268 nm) 0.20 0.19 0.19 0.19 0.19

Odor intensity

3.1 3.6 3.2 3.2 3.5

Adapted from Malcolmson LJ, Przybylski R and Daun JK (2000) Storage stability of milled flaxseed. Journal of the American Oil Chemists’ Society 77:

235–238.

1, none; 5, intense; nine-member trained panel; two replications.

FLAXSEED 2529

ratio ranges from about 10:1 to 25:1 in typical North

American diets. Health Canada recommends a ratio

of 4:1 to 10:1, whereas a joint Food and Agriculture

Organization/World Health Organization committee

recommends a ratio of between 5:1 and 10:1. Flax-

seed consumption can increase total o-3 fatty acid

intakes and reduce the o-6/ o-3 ratio, as shown in

Table 3.

0034 The US Institute of Medicine set a daily Adequate

Intake of 1.1 g of ALA for women and 1.6 g of ALA

for men. These recommended intakes can be achieved

by consuming as little as 8 g (1 tbsp) of ground

flaxseed or 2.5 g (1/2 tsp) of flaxseed oil per day. Up

to 10% of the Adequate Intake can be derived from

foods containing EPA and DHA.

0035 In most clinical trials of the health effects of

flaxseed, volunteers have consumed 5–50g of ground

flaxseed or 60 ml or flaxseed oil daily. Based on

the findings of these published reports, an intake of

about 8–16 g (1–2tbsp) of ground flaxseed daily is

recommended. Ground flaxseed is the preferred

form because it is easily added to foods and

supplies the full complement of nutrients associated

with flaxseed’s positive health effects. Flaxseed oil is

rich in ALA but does not contain dietary fiber or

lignans.

Summary

003 6 Flaxseed is a functional food valued for its pleasant,

nutty taste, unique nutrient profile, and health bene-

fits. Flaxseed is a rich source of ALA, the essential o-3

fatty acid, and the lignan, SDG. It also provides pro-

tein and dietary fiber. Newer research suggests that

flaxseed may help in the prevention and management

of chronic diseases such as coronary heart disease,

diabetes mellitus, certain types of cancer, and

autoimmune diseases. Flaxseed’s high ALA content

can make a significant contribution to total o-3

fatty acid intake. The lignan precursors and ALA in

flaxseed appear to be stable to common storage and

baking conditions. Consumers can purchase flaxseed

as whole seeds, ground flaxseed, flaxseed oil, in

capsule form and in baked goods, cereal mixes, and

pasta products.

Seealso: Analysis of Food; Antioxidants:Natural

Antioxidants;RoleofAntioxidantNutrientsinDefense

Systems; Cholesterol:FactorsDeterminingBlood

CholesterolLevels; Coronary Heart Disease:Etiology

andRiskFactor;Prevention; Diabetes Mellitus:Etiology;

TreatmentandManagement; Essential Fatty Acids;

Fatty Acids:DietaryImportance; Functional Foods;

Prostaglandins and Leukotrienes; Storage Stability:

MechanismsofDegradation;ParametersAffecting

StorageStability

Further Reading

Clark WF, Parbtani A, Huff MW et al. (1995) Flaxseed:

A potential treatment for lupus nephritis. Kidney

International 48: 475–480.

Cunnane SC, Ganguli S, Menard C et al. (1993) High

a-linolenic acid flaxseed (Linum usitatissimum). Some

nutritional properties in humans. British Journal of

Nutrition 69: 443–453.

Cunnane SC, Hamadeh MJ, Liede AC et al. (1995) Nutri-

tional attributes of traditional flaxseed in healthy young

adults.AmericanJournalofClinicalNutrition61:62–68.

deLorgerilM,RenaudS,MamelleNetal.(1994)Mediterra-

nean alpha-linolenic acid-rich diet in secondary prevent-

ion of coronary heart disease. Lancet 343: 1454–1459.

Jones J (2000) Update on defining dietary fiber. Cereal

Foods World 45: 219–220.

Malcolmson LJ, Przybylski R and Daun JK (2000) Storage

stability of milled flaxseed. Journal of the American Oil

Chemists Society 77: 235–238.

Payne TJ (2000) Promoting better health with flaxseed in

bread. Cereal Foods World 45: 102–104.

tbl0003 Table 3 Contributionofflaxseedto o-3and o-6fattyacidintakesandthe o-6/o-3fattyacidratiooftheNorthAmericandiet

FoodItem o-3 contentoffood

(g) o-6contentoffood(g) o-6/o-3ratioofthediet

Commonlyeatenfoods

Breakfast(cereal,milk,orangejuice)0.1060.333

Lunch(roastbeefsandwichwithpickleand

tomatoslicesplusonelargeapple)

0.285 2.187

Snack (4 oz (112-g) of potato chips) 0.216 13.584

Dinner (pizza, salad) 0.983 8.831

Total 1.59 24.935 16:1

Adding flaxseed changes the o-6/o-3 ratio of the diet

Flaxseed, whole, 1 tbsp 2.5 0.72 6:1

Flaxseed, ground, 1 tbsp 1.8 0.52 8:1

Flaxseed oil, 1 tbsp 8.0 2.24 3:1

Used with permission of the Flax Council of Canada.

The o-3 and o-6 fatty acid contents of foods were calculated using the US Department of Agriculture’s Nutrient Database for Standard Reference,

Release13,availableontheInternetatwww.nal.usda.gov/fnic/foodcomp.

2530 FLAXSEED

Przybylski R and Daun JK (2001) Additional data on the

storage stability of milled flaxseed. Journal of the

American Oil Chemists Society 78: 105–106.

Rickard SE, Jenah M, Tou JCL, Nesbitt PD and Thompson

LU (1998) Anticancer effects and availability of flaxseed

lignans. Proceedings of the Flax Institute 57: 8–14.

Stewart S and Mazza G (2000) Effect of flaxseed gum on

quality and stability of a model salad dressing. Journal of

Food Quality 23: 373–390.

Thompson LU, Seidl MM, Richard SE et al. (1996)

Antitumorigenic effect of a mammalian lignan

precursor from flaxseed. Nutrition and Cancer 26:

159–165.

Vaisey-Genser M and Morris DH (1997) Flaxseed – Health,

Nutrition and Functionality. Winnipeg, MB: Flax Coun-

cil of Canada, also available on the Council’s web site at

www.flaxcouncil.ca.

FLOCCULATION

B M Baraniak, Agriculture Academy, Lublin, Poland

E Walerian

czyk, Instytut Przemyslu Cukrowniczego,

Warszawaul, Poland

This article is reproduced from Encyclopaedia of Food Science,

Background

0001 Flocculation is a preliminary treatment of colloidal

systems, employed to destabilize the sol structure by

adding reagents that encourage particle growth,

before separation of the solid phase is attempted by

filtration or sedimentation. Particles of colloidal

dimension (0.5 10

5

m) frequently form stable dis-

persions which do not settle and are not amenable to

conventional filtration processes. (See Filtration of

Liquids.)

Mechanism of Flocculation

Colloid Destabilization by Ions

0002 The stability of a colloid is related to its zeta poten-

tial, which involves the charged surface of the par-

ticles and the oppositely charged layer of counter ions

adjacent to it, constituting an electrical double layer.

A reduction of zeta potential results in destabilization

of the stable colloid structure, and may be caused

either by the specific adsorption of counter ions – a

nonspecific effect of neutral electrolytes – or by en-

meshment of particles within a sweep floc. Since the

charge of counter ions is important, it is apparent that

polyvalent ions will be more effective than singly

charged ions: Na(I) < Ca(II) < Al(III).

Colloid Destabilization by Polymers

0003 A general mechanism for the destabilization of

colloidal sols by high-molecular-weight synthetic

polymers, termed polyelectrolytes, is described by the

theory of bridging. The polyelectrolyte is adsorbed at

several sites, not covering the whole area of the col-

loidal particles. As the polyelectrolyte molecules are

relatively long, loops or chains protrude beyond the

double layer. Thus, flocculation takes place through

hydrophobic or hydrogen bonds, and any compression

of the double layer that can be induced by increases in

ionic strength should enhance bridging by allowing

closer particle approach (Figure 1).

0004Polyelectrolyte adsorption may, in some cases, in-

crease the stability of the colloid. A strongly adsorbed

polymer may increase the diameter of the diffuse layer,

and the larger double layer will generate repulsive

(a) (b)

fig0001Figure 1 Bridging efficiency of polyelectrolytes: (a) low ionic

strength; (b) high ionic strength. Reproduced from Flocculation,

Encyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic

Press.

FLOCCULATION 2531

interreactions, leading to increased stability. The

amount of polyelectrolyte applied in relation to the

colloidal concentration is critical for destabilization

or restabilization of colloid structure. Overdosing is a

real danger, and the optimum concentration of poly-

electrolyte required for flocculation should be very

small (not in excess of 1 mg l

1

Aggregation Mechanism

0005 The aggregation of the destabilized colloid structure

is the second step in the final process of separation

of the solid phase, and it is ultimately dependent

on two factors: (1) frequency of collision between

particles; (2) effectiveness of the interactions causing

particle–particle adhesion. In general, there are

two mechanisms by which adhesion contacts occur:

(1) aggregation by thermally induced motions,

termed ‘perikinetic flocculation’; (2) contacts be-

tween particles in heterodisperse systems by differen-

tial settling under gravity, termed ‘orthokinetic

flocculation.’

0006 In real systems, the initial stage of flocculation

would be perikinetic, but with increasing floc growth

the orthokinetic model becomes dominant.

Flocculants and Coagulants

Polyelectrolytes

0007 Polyelectrolytes that are applied practically as floc-

culants are mainly water-soluble polyacrylamides,

polyphosphates and modified natural polymers –

gelatines, chitosan, and carrageenan, as well as

starch and cellulose derivatives. Polyelectrolytes are

classified as cationic, anionic, and nonionic according

to the nature of the functional groups along the

polymer chain. Predominant among these are

polyacrylamide copolymers of acrylamide and acry-

late, or monomers containing ammonium groups

(Figure 2).

0008 A polyelectrolyte is characterized by its molecular

weight, the nature of the functional group, and the

charge density. An important consideration in choos-

ing a polyelectrolyte for a desired process is its poten-

tial as a coagulant (by destabilization of the colloid

via neutralization) and as a flocculant (by interparti-

cle bridging). The pH is also an important parameter

to be considered when selecting a polyelectrolyte for a

particular application. Sensitivity to pH occurs with

cationic polymers in which quaternary ammonium

groups are dominant, and with anionic polymers

containing sulfonic acid groups. Flocculants with

carboxyl or amine groups confer strong pH depend-

ence. The toxicity of polyacrylamides is usually low,

usually lower than 0.05%, and results mainly from

the presence of free acrylamide.

Coagulants

0009The reagents most widely used in coagulation pro-

cesses are polyvalent cations, such as Al(III) or Fe(III)

or Fe(II), available as chlorides or sulfates in crystal-

line forms. Iron salts generally offer advantages over

aluminum salts, and Fe(III) is also often superior to

Fe(II) because it has a wider pH range of action.

Other Flocculants

0010Clay materials, such as bentonite or montmorillonite,

activated carbon or activated silica are sometimes

used to improve the filtration or sedimentation of

suspended solids.

(a)

(b)

(c)

Polyethylenoimine

Polyethylene oxide

Polystyrene sulphonate

−

Polyacrylic acid

−

Polyacrylamide

Polydiallyldimethylammonium

CH CH CH

CCH

fig0002Figure 2 Characteristics of main groups of polymeric floc-

culants: (a) cationic polyelectrolytes; (b) anionic polyelec-

trolytes; (c) nonionic polymers. Reproduced from Flocculation,

Encyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic

Press.

2532 FLOCCULATION

Practical Aspects

Selection of Polyelectrolyte

0011 Factors which affect the choice of the particular floc-

culant or coagulant are the nature of the impurities

and the particle size of suspended solids. Selection

and dosage required can be determined by laboratory

testing of the settling rate, clarity, and volume of

sedimented mud. Laboratory testing does not exactly

reproduce plant conditions and should be considered

as a prelude to plant trials.

Preparation of Polyelectrolyte Solution

0012 Most polyelectrolytes used as flocculants are avail-

able as a low dusting powder, and have to be distrib-

uted for use as a colloidal sol of concentration

0.05–0.5%. This limitation is due to the high viscos-

ity of more concentrated dispersions (0.1–0.2 Pa s for

0.5%). The casual addition of a polyelectrolyte solid

to water results in a jelly-like structure, which is

useless as a flocculating agent. Efficient dispersion

is achieved by use of an Educator–Venturi system,

providing a homogeneous solution. Long-term stor-

age or temperatures greater than 60



C result in a loss

of effectiveness caused by depolymerization.

Point of Addition

0013 The point of addition of a polyelectrolyte is of utmost

importance. The flocculant should be added at a

point which allows for uniform mixing with the

slurry, but it should not be subjected to excessive

turbulence, which can disrupt the floc. It is also

necessary to arrange for the introduction of the floc-

culant – as dilute as possible to ensure efficient distri-

bution – at a point which allows time for particle

collisions to take place prior to the solid removal

stage. In practice, there are two systems which

achieve the correct conditions for flocculation: gently

agitated tanks, and flocculation chambers.

Removal of Flocculated Solids

0014The two principal techniques for removal of the solid

formed – flotation or sedimentation – are employed,

depending on the kind of flocs formed. A light mater-

ial can be subjected to air treatment for flotation;

flocs that tend to settle are generally separated by

sedimentation (see Figure 3). Filtration is rarely used

– mostly in water treatments, with sand or carbon

filters, or rotary vacuum filters for carbonated raw

beet juice.

Applications

Waste Water Treatment

0015Industrial waste water is frequently a complex solids–

colloidal system, with soluble compounds of organic

or inorganic origin.

0016Physicochemical treatments of industrial waste

water can achieve good results where biological pro-

cesses are inapplicable, e.g., with nonbiodegradable

materials, toxic discharges, or removal of minerals

and color. This is cheaper in terms of capital cost,

more easily controlled and less space-consuming

than biological treatment, but higher in terms of

operational costs.

0017Adsorptive precipitation and flocculation are

the main processes for coagulating colloidal particles

to form larger aggregates. The principal reagents

used are lime, ferric or aluminum sulfate, and

polyelectrolytes.

0018The low-molecular-weight polymers have the ad-

vantage, over the inorganic coagulants, that they do

not increase the salts’ level, but they are often rela-

tively uneconomical. The salts Al(III) or Fe(III) are

Air

(a)

Foam

Vacuum

(b) (c)

fig0003 Figure 3 Equipment for removal of solids formed in flocculation: (a) flotation machine; (b) decanter; (c) vacuum rotary filter.

Reproduced from Flocculation, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ

(eds), 1993, Academic Press.

FLOCCULATION 2533

commonly used first to promote coagulation; then, a

high-molecular-weight polyelectrolyte is introduced

to the coagulated solids. Highly dispersed solids

need either polyvalent ions or low-molecular-weight

polyelectrolytes. With the larger particles, high-

molecular-weight polymers are more effective; for

examples of applications, see Table 1. The floccula-

tion permits the recovery of protein from waste water

coming from the food industry, as well as from bypro-

ducts, e.g., agglomeration of whey proteins with

carboxy-methylcellulose, or protein precipitation by

amide polyelectrolytes in waste water of the potato

industry. (See Water Supplies: Water Treatment.)

Raw Sugar Process

0019 Raw sugar juices contain considerable amounts of

colloidal materials which have to be removed in clari-

fication. There are two groups of colloid: constituents

of the beet cells transferred to the sol form during

extraction, and those that originate as a result of

condensation, e.g., melanine, melanoidine or invert

or saccharose decomposition products. The process

of removing colloidal substances is essential, as their

presence in thin or dense juices, delays the sedimenta-

tion and filtration process, and favors foaming and

the increase of coloration, and sugar loss during crys-

tallization.

0020 Lime acts as a primary coagulant, precipitating

colloidal and colored matter. A radical improvement

in the process of clarification of sugar juice was

brought about by the application of flocculants. The

application of sodium alginate or modified starch at

dosage rates of 10–50 p.p.m. was not very satisfac-

tory. Significant progress was made with the intro-

duction of synthetic polyacrylamides. In general,

anionic flocculants are applicable for the treatment

of the raw juice from cane or beet. The flocculant

dosage rates range from 0.8 to 2 p.p.m., and experi-

ence has shown that it is better to dilute the flocculant

with raw juice instead of water. The favorable effect

of this flocculant introduction was evident in the

way the juice with sediment flowed to the filter or

decanter.

0021A proper application is likely to increase the size of

the colloidal particles by several times, which facili-

tates the plate and frame filter press function, and more

compact filter cakes than normal are obtained. Poly-

electrolytes are also recommended in sugar refining,

and the addition to carbonated sugar juice improves

both the clarification and filtration processes. (See

Sugar: Refining of Sugarbeet and Sugarcane.)

Technology of Leaf-protein Concentrates

0022Leaf-protein concentrate (LPC) is one product

resulting from green crop fractionation that provides

the animal feed industry or food industry with a

protein. Green crop fractionation offers the possibil-

ity of a greater total biological efficiency in the pro-

duction of protein than can be achieved by other

systems. It is possible to envisage several systems

defined by the products of the process, which have

as one of their products LPC in a wet or dried form.

Freshly expressed juice from macerated green plants

contains small amounts of fiber, minerals and pro-

teinaceous material of leaf origin, chloroplasts or

fragments thereof, and about 90% water. Removal

of the protein fractions is possible through the desta-

bilization of the colloidally suspended particles

(Figure 4).

0023The flocculation of the chloroplastic function is

carried out in chambers under light mechanical

motion (100 r.p.m.) by introducing the flocculant

directly to the green juice which was alkalized with

sodium hydroxide to pH 7–8.5. Coagulation occurs

almost instantaneously to produce macroflocs that

settle down and make subsequent filtration easy.

0024The effectiveness of flocculation depends on the

ionic character of the polyelectrolyte, as well as its

tbl0001 Table 1 Application of flocculation in industrial waste water treatment

Industry Pollution parameters Flocculant Benefits Removalof solids

Processing of poultry,

slaughterhouse waste

Fats, grease, blood Al(III) and polyelectrolyte Reduction of total

solids

Dissolved air

flotation

Fish or shrimp processing Fats, protein Chitosan or anionic

polyelectrolyte

Reduction of total

solids

Dissolved air

flotation

Pulp and paper Lignin, color Cationic polyamines Color removal Sedimentation

Fellmongering Hair and skin products

sulfide, ferrous salts

Fe(III) and anionic

polyelectrolyte

Reduction of solids Sedimentation

Vegetable and fruit

processing

Carbohydrates, protein Lignosulfonic acid Reduction of BOD Dissolved air

flotation

Starch – gluten Protein, carbohydrates Hexametaphosphate or

synthetic polymers

Reduction of BOD Dissolved air

flotation

BOD, biochemical oxygen demand.

2534 FLOCCULATION

charge density and concentration, on the acidity of

the plant juice, and on the species of plant. Satisfac-

tory results, in terms of both quantity and concen-

trates and protein content, were obtained by adding

150–300 mg of Magnafloc LT-27 (Allied Colloids,

UK) or Prodefloc C/1 (Prodeco, Italy) per liter of

green alfalfa juice. From 100 kg of fresh plants, the

process yields 1.6 kg of dry LPC containing 56% of

protein.

0025Cationic and anionic polyacrylamides were applied

on a laboratory scale for the flocculation of green

juice from the following plants: leaves of sugar beet,

clover, buckwheat, vetch, sunflower, and tobacco.

The chemical composition of products obtained is

similar to that obtained by means of the thermal

coagulation method called Pro-Xan.

0026The application of flocculants in the processing of

LPC allows a decrease in the energy consumption and

yields proteins which are not denatured thermally.

See also: Filtration of Liquids; Sugar: Refining of

Sugarbeet and Sugarcane; Water Supplies: Water

Treatment

Further Reading

Eilbeck WJ and Mattock G (1987) Chemical Processes in

Waste Water Treatment. Chichester, UK: Ellis Horwood.

Fiorentini R, Pachetti G and Galoppini G (1978) Attualita e

Prospettive del Frazionamento umido dei Foraggi. Pisa:

Universita di Pisa.

Ives KJ (1978) The Scientific Basis of Flocculation. Alphen

aan Rin, The Netherlands: Sijthoff and Noordhoff.

McGinnis RA (1981) Beet Sugar Technology. Fort Collins,

CO: Beet Sugar Development Foundation.

FLOUR

Contents

Roller Milling Operations

Analysis of Wheat Flours

Dietary Importance

Roller Milling Operations

J E Dexter, Canadian Grain Commission, Winnipeg,

Manitoba, Canada

A K Sarkar, Canadian International Grains Institute,

Winnipeg, Manitoba, Canada

History of Flour Milling

0001 The origins of wheat milling date back thousands of

years to the dawn of civilization. Milling technology

gradually advanced from the mortar and pestle, used

by primitive cultures about 10 000–12 000 years ago,

to the invention of the millstone in Roman times.

Millstones remained in vogue until the introduction

of the purifier, plansifter, and the horizontal roller

mill in Europe during the latter part of the nineteenth

century. The introduction of improved equipment and

the concept of gradual reduction of wheat into flour

allowed vast improvements in the degree of refinement

of flour. Milling equipment has undergone continual

improvement since then, but the basic concept of

modern roller milling has remained unchanged.

Green plant

Homogenizing

Extraction

Juice

Total

coagulation

thermal

acidity

polyelectrolytes

Whole

protein

concentrate

(Feed

monogastric)

Chloroplastic

protein

concentrate

(Green LPC)

(Feed

monogastric)

Cytoplasmatic

protein

concentrate

(White LPC)

(Human use)

Fractional

coagulation

thermal

acidity

polyelectrolytes

Animal

feeds

(Polygastric)

Food fiber

production

Fiber

fig0004 Figure 4 Protocol for obtaining protein fractions from extracted

juice. LPC, leaf-protein concentrate. Reproduced from Floccula-

tion, Encyclopaedia of Food Science, Food Technology and Nutrition,

Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic

Press.

FLOUR/Roller Milling Operations 2535

Structure and Composition of the Wheat

Kernel and Flour

0002 The main constituents of the wheat kernel are bran,

germ, and endosperm (Figure 1). The endosperm

includes the outer aleurone layer, which is removed

with the bran during flour milling, and the starchy

endosperm. The germ contains the embryo, and a

transport organ, the scutellum.

0003 High-quality flour originates from the starchy

endosperm. The starchy endosperm comprises about

85% of the wheat kernel. Therefore, it is theoretically

possible to convert about 85% of wheat into high-

quality flour, free of bran and germ contamination. In

practice, the crease, which runs along the length of

the grain and reaches almost to the middle of the

grain, makes perfect separation of the endosperm

impossible.

0004 The constituents of the wheat kernels have very

different compositions (Table 1), so that the extent

of contamination of the flour by bran and germ alters

flour composition and flour functionality. The

starchy endosperm contains a low level of minerals

compared to other kernel constituents. Accordingly,

flour ash (mineral matter left behind after inciner-

ation of flour) is widely used as a flour refinement

index; the low ash content is indicative of a highly

refined flour containing almost pure endosperm. The

endosperm contains proteins, known as gluten, which

give wheat flour dough the viscoelastic properties

essential for high-volume bread and firm pasta.

0005Wheat kernel hardness has a wide range. Durum

wheat is very hard, whereas common wheat ranges

from hard to soft. Endosperm structure differs

for wheat of different hardness. The endosperm of

Hairs of

brush

Endosperm

Aleurone cell

Nucellar tissue

Seed coat (testa)

Tube cells

Cross cells

Hypodermis

Epidermis

Bran

Germ

Crease

fig0001 Figure 1 (see color plate 66) The structure of the wheat kernel sectioned longitudinally (left) and transversely (right). The bran layer

is comprised of six layers. The aleurone layer, considered part of the endosperm, is removed with bran during roller milling. The

crease, which runs the full length of the kernel, prevents fully efficient separation of bran from endosperm.

tbl0001Table 1 Approximate composition of common wheat and its

constituents

Wheat Bran

Endosperm Germ

Proportion of kernel (%) 100 13 85 2

Protein content (%) 13 14 12 40

Ash content (%) 1.5 7 0.4 5

Thiamin (mg kg

1

) 3 5 0.4 100

Nicotinic acid (mg kg

1

) 33 200 7 40

Taken from various sources. Expressed on 13% moisture basis.

Includes the aleurone layer.

2536 FLOUR/Roller Milling Operations

hard wheat is compact, with starch granules rigidly

held within a protein network. The endosperm of soft

wheat is more porous.

0006 In general, as the proportion of wheat converted

into flour (flour extraction rate or flour yield) in-

creases, contamination by bran and germ increases.

As a result the nutritional value of the flour improves

as extraction rate increases, but processing quality

and shelf-life diminish. The proteins in germ and bran

are nongluten proteins that contribute to nutritional

value, but do not contribute to processing properties.

The germ contains low-molecular-weight sulfhydryl

compounds that weaken dough properties by reduc-

ing disulfide cross-links, which are key elements of

gluten polymer structure. The germ also contains oils

and enzymes that reduce storage stability by promot-

ing rancidity. The germ and bran are both rich in

nutritionally important vitamins. The bran contains

most of the fiber in the kernel, also an important

nutritional consideration. However, as the level of

bran in flour increases, flour dough functionality is

reduced, and flour becomes darker, detracting from

end-product appearance.

The Flour Milling Process

0007 Wheat selection, preparation of wheat for milling,

and milling technique all play a role in determining

flour quality and milling profitability. The remainder

of this article describes a modern roller milling oper-

ation, and explains how the various stages in the

milling process contribute to the yield and quality of

final milled products.

Wheat Selection

0008 The milling characteristics and processing quality of

wheat are dependent on physical condition and inher-

ent quality traits. Wheat of intrinsically good quality

can be rendered of poor processing quality by envir-

onmental damage or improper storage. Different

classes of wheat, and wheat from different origins,

have different quality traits. High-protein hard

common wheat is ideal for producing bread flour.

Low-protein soft common wheat is preferred for

producing cake and biscuit flour. Durum wheat is

preferred for pasta products.

0009 In order to meet customer requirements, millers

specify wheat type and minimum quality standards

that must be met by suppliers. Differences in wheat

hardness among wheat classes require alterations in

conditioning and milling technique to achieve opti-

mum results. When blends of different wheat types

are milled, it is preferable to condition each type

separately, but convenience, cost, and storage limita-

tions often require blending prior to conditioning.

Wheat Reception and Storage

0010Wheat may be received at mills by truck, rail, or

ship (Figure 2). Incoming wheat is weighed, and a

sample is analyzed prior to storage. The content of

foreign material and damaged grains is determined.

Analyses such as weight per unit volume (test

weight), protein content, and moisture content are

performed. This determines whether the incoming

wheat is within specification. Wheat is placed in a

bin with other wheat of comparable physical condi-

tion and similar end-use potential to allow quality

characteristics to be preserved, and used to the

best advantage. In most cases, the wheat passes

through a magnet to remove ferrous metals, and it

may be given a cursory cleaning prior to elevation to

storage bins.

Wheat Cleaning

0011Wheat must be cleaned prior to milling to remove

foreign material such as stones, metal, unthreshed

grain, badly damaged kernels, and foreign seeds that

either adversely affect flour quality or pose a hazard

to milling equipment.

0012Figure 2 shows a typical wheat-cleaning scheme.

The wheat first passes through a magnet, and then

proceeds to a grain separator which uses sieves and

aspiration to remove impurities. Large impurities,

such as maize, soybeans, and unthreshed wheat are

removed by the top screen; small impurities, such as

small and broken kernels and weed seeds, pass

through the bottom sieve. As wheat falls off the end

of the bottom sieve, it is aspirated to remove chaff,

dust, and shriveled kernels.

0013Next, the wheat goes to a destoner where impur-

ities such as mud balls and small stones comparable

in size to wheat kernels, but denser, are removed.

A destoner is an oscillating inclined metal screen

through which an upward air current is passed.

Wheat floats on an air cushion and moves down the

slope. Denser materials make more contact with the

screen, are propelled up the slope by the screen oscil-

lations, and are collected separately.

0014The wheat now passes through disk separators

to remove remaining impurities that differ in

length to wheat kernels, but are of similar density

and cross-section. Disk separators comprise disks

revolving in a vertical plane through the wheat.

The surface of the disks has numerous pockets

which pick up particles that are short enough to

lodge in the pockets. There are generally two units.

The first is designed to pick up wheat while rejecting

kernels longer than wheat, such as oats and barley.

The second picks up small seeds and rejects wheat

kernels.

FLOUR/Roller Milling Operations 2537