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

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than one-third the thickness found elsewhere. The

aleurone cells are heavy-walled, essentially cube-

shaped, and free starch. The aleurone cells can vary

in thickness from 30 to 70 mm within a single kernel

and have thick (6–8 mm), double-layered cellulosic

walls.

Endosperm

0007 The starchy endosperm, excluding the aleurone layer,

is composed of three types of cells: peripheral, pris-

matic, and central. The peripheral cells are the last to

be initiated during grain filling and tend to be smaller

than the other endosperm cells (60 mm in diameter and

20–60 mm radially) and have thicker cell walls (8 mm).

Several rows of elongated prismatic cells are found

inside the peripheral cells. They extend inward to

about the center of the cheeks and are about

150–200 mm. The central cells are more irregular in

size and shape than are the other cells, and are located

inside the central endosperm. The central endosperm

cells are the first to be formed and have thinner walls

(2 mm). Cell wall thickness also appears to vary among

cultivars and between hard and soft wheat types. The

differences between hard and soft wheat may be the

result of selection: hard wheat (bread wheat) has been

selected for high water absorption. The endosperm

cell walls are composed of pentosans, other hemicel-

luloses, and beta-glucans, but not cellulose. The pen-

tosans in them absorb large amounts of water. The

endosperm cells are packed with starch granules

embedded in a protein matrix. Starch is the major

component of wheat endosperm, comprising approxi-

mately 64–75% of milled endosperm. Generally, the

starch granules in wheat are classified into two size

groups: large, lenticular (lens-shaped) A granules of

up to 40 mm across the flattened side, and small, spher-

ical B granules (2–8 mm in diameter), which are

formed later in the grain filling process.

0008In common bread wheat (Triticum aestivum) the

endosperm texture of grains varies both in texture

(a)

(b)

(c)

Length

Ventral side

Thickness

Hairs

Width

Crease

Dorsal side

Cheeks

NTTCCIHE

fig0001 Figure 1 Main morphological components of a wheat kernel. (a) Longitudinal section; (b) cross-section; (c) segment of bran layer

with aleurone and endosperm cells. E, epidermis; H, hypodermis; I, inner pericarp; C, cross cells; TC, tube cells; T, testa; N, nucellar

layer; A, aleurone layer; EN, endosperm; PE, peripheral cells of endosperm; PR, prismatic cells of endosperm; SE, scutellar

epithelium; S, scutellum; EM, embryo.

WHEAT/Grain Structure of Wheat and Wheat-based Products 6139

(hardness) and appearance (vitreousness). In general,

high-protein hard grains tend to be vitreous and low-

protein soft grain tend to be opaque. Some wheat

grains are vitreous or translucent in appearance,

while others are opaque, mealy, or floury. In vitreous

kernels, with no air spaces, light is diffracted at the

air–grain interface but then travels through the grain

without being diffracted again and again. With vitre-

ous kernels, the protein shrinks but remains intact,

giving a denser kernel. As expected, the presence of

air spaces within the endosperm makes the opaque

grain less dense. The air spaces are apparently formed

during drying of the grain. If grain is harvested imma-

ture and dried by freeze-drying, it becomes entirely

opaque. This shows that the vitreous character results

during intensive drying in the field. It is also well

known that vitreous grain wetted and dried in the

field, or for that matter in the laboratory, will lose

its vitreousness. In durum wheat (T. durum), which

is much harder than the common hard bread wheats,

a much larger number of broken starch granules

occur when the grain is fractured.

Germ

0009The wheat germ is composed of two major parts,

the embryonic axis (rudimentary root and shoot)

and scutellum, which functions as a storage organ.

The scutellum is adjacent to the endosperm and con-

tains the remaining two-thirds of the grain’s thiamin

content. The germ is quite high in vitamin E (total

tocopherol) and B-vitamins, and contains many

enzymes. It is relatively high in protein (25%), sugar

(18%), oil (16% of the embryonic axis and 32% of

the scetullum are oil) and ash (5%). The sugars are

mainly sucrose and raffinose. The wheat germ com-

prises 2.5–3.5% of the kernel. Recovery of the germs

Pericarp (fruit coat)

Outer

1. Epidermis (epicarp)

2. Hypodermis

3. Remnants of thin-walled cells

Inner

4. Intermediate cells

5. Cross cells

6. Tube cells

Wheat

kernel

(caryopsis)

1. Aleurone layer

2. Starchy endosperm

1. Epithelium

2. Parenchyma

3. Provascular tissues

Seed coat (testa, spermoderm, integument)

and pigment strand

Bran

Nucellar epidermis (hyaline layer,

perisperm) and nucellar projection

Seed Endosperm

Scutellum (cotyledon)

Embryonic axis

Epiblast

Primary root, covered

by coleorhiza

Secondary lateral

rootlets

Plumule, including

coleoptile

Germ

(embryo)

fig0002 Figure 2 Parts of the wheat kernel. Reproduced from Hoseney RC (1994) Structure of Cereals. In: Principles of Cereal Science and

Te c h n ol o g y , 2nd edn. American Association of Cereal Chemists with permission.

6140 WHEAT/Grain Structure of Wheat and Wheat-based Products

during the milling process is an important step

because of its value in the food and pharmaceutical

industries.

Techniques of Wheat Grain Structure

Study

0010 Many cereal scientists and technologists have success-

fully employed the three main branches of micro-

scopy – light (LM), scanning electron (SEM), and

transmission electron microscopy (TEM) – to study

the structure and composition of wheat and wheat-

based products. The objective of the studies varies

from gaining fundamental information on the accu-

mulation of cellular constituents in the developing

wheat grain to providing information which can im-

prove our understanding of differences in processing

ability or overall quality of wheat and wheat-based

products. Microscopic observation should always be

linked to other technological, chemical, or physical

data. This information will assist the microscopist to

select the most appropriate preparation techniques

and evaluate the nature and magnitude of any arti-

facts that may be created. In many cases a number of

microscopic techniques are used in order to gain the

maximum amount of information on the original

structure and composition of the product.

0011 LM and TEM have the advantage that many

staining techniques have been developed to assist rec-

ognition of constituents and provide data on chemical

composition. However, these techniques generally re-

quire the use of solvents during sample preparation

and these can give rise to artifacts (swelling, shrink-

ing, or leaching of soluble material). Few specific

stains are available for use in SEM, and constituents

are generally identified by their shape or location.

The advantage of SEM is that, in some cases, sample

preparation artifacts can be minimized. Recently,

X-ray techniques have been used to detect kernel

cracks (Figure 3).

Light Microscopy

0012 The main stages in the preparation of samples for

examination by LM are fixation, embedding,

sectioning, and staining. The aims of fixation are to

preserve samples from attack by enzymes or micro-

organisms, render some constituents insoluble, and

strengthen the sample to improve its structural integ-

rity during sectioning. The most commonly used

fixation is aqueous, buffered glutaraldehyde but

specialized fixation has been developed for specific

applications (e.g., fixation of lipid-rich samples).

Baked samples, which have been heat-fixed, may

not require chemical fixation. Embedding is used to

provide additional support during sectioning, and

commonly used embedding media are water and

aqueous gums for cryostat microtomy, synthetic

resins, or special waxes. Resins are generally used

where thinner sections are required and are cut

using glass knives. Cryostat microtomy and

sectioning of wax-embedded samples are carried out

using steel knives. Sections, supported on glass slides,

are then usually stained prior to mounting and

examination.

0013The stains commonly used for examination with

transmitted bright-field illumination are listed in

Table 1, together with their substrates. Fluorescence

microscopy has been widely used and may rely on

autofluorescence or the application of fluorescent

dyes. The use of fluorescent dyes coupled to specific

antibodies or lectins is rapidly expanding and several

are listed in Table 2. Other coupled antibody tech-

niques have been developed whereby colored reaction

products are produced. Polarized light can be used

to study external starch gelatinization or to provide

detail of cell wall structure.

Scanning Electron Microscopy

0014SEM has a greater depth of focus than LM and there-

fore it is not necessary to section samples prior to

examination. Samples are normally coated with a

thin layer of an electrically conducting material, usu-

ally gold, platinum, or carbon, prior to examination.

Samples of dry wheat (< 12% moisture) or similar

products only require air-drying over a desiccant

prior to coating. Samples which contain more mois-

ture, may be rapidly frozen prior to dehydration

using freeze-drying or critical-point drying. The

(a) (b)

fig0003Figure 3 X-ray images of the same wheat kernel with typical

radial cracks and transverse to the crease. (a) Front view; (b) side

view.

WHEAT/Grain Structure of Wheat and Wheat-based Products 6141

development of cold stages in SEM has obviated the

need for drying and this technique, together with

freeze-etching, has been successfully used to examine

those cereal foods in which moisture is high and

forms an integral part of the structure.

Transmission Electron Microscopy

0015 Samples for TEM require fixation and thin

sectioning. Initial fixation is usually with glutaralde-

hyde followed by fixation with osmium tetroxide.

The fixed tissue blocks are rinsed, dehydrated, and

embedded with a resin prior to sectioning. Heavy

metal salts are used to stain the sections and enhance

contrast; more recently, antibody staining has been

developed whereby the antigenic sites are located at

TEM level by the presence of colloidal gold particles,

which are coupled to the antibodies.

Image Analysis and Other Quantitative Techniques

0016 During the 1990s the value of all the above work

was enhanced by high-tech methods such as mag-

netic nuclear resonance, laser, and autoradiography

methods. More recently, the application of stereol-

ogy techniques and automatic image analysis of

grain have been used. Also, out-of-danger X-ray tech-

niques have been proven to be highly suitable for

visualization of inner cracks in kernels. Some internal

cracks of wheat endosperm can occur in the field,

even before harvesting, due to internal stresses

which are mainly induced by high moisture gradients.

The same process can be observed during intensive

wetting of dry wheat kernel in laboratory conditions.

In Figure 3 X-ray images of the same wheat kernel are

presented with typical radial cracks of endosperm.

X-ray detection makes it possible to identify the pos-

ition of cracks and also to quantify cracks inside the

kernel, and thus to evaluate the physical condition of

the grain endosperm.

Wheat-Based Products

0017The major processing stages to be considered are

milling, dough formation, and cooking. The structure

of the wheat grain has important implications in

the utilization of wheat-based products. This is

because millers could grind the grain more finely

than is now possible without damaging the starch –

a common milling problem. Most breeding programs

mainly emphasize grain yield, disease resistance, pro-

tein content, and quality while many other features

that affect grain quality are considered second or not

at all.

Milling

0018The aim of the milling processes is to remove the

endosperm of grain efficiently from the germ and

tbl0001 Table 1 List of stains commonly used for bright-field microscopy of wheat and wheat products

Substrate Stain Comments

Starch Periodic acid–Schiff (PAS) reaction Covalent, irreversible; may require aldehyde blockade if aldehyde-fixed

Iodine or potassium iodide Temporary stain only. Amylose stained blue-black; amylopectin stained

red-brown

Protein Fast green, Ponceau 2R, and

other anionic dyes

Ionic, reversible. Intensity influenced by pH and differentiation

Coupled antibody or lectin stains Potentially more specific for selected protein

Lipid Sudan IV or oil red O in 70% ethanol Temporary stain only; intensity influenced by differentiation

Cell walls Toluidine blue O Metachromatic stain. Lignified walls stained green; testa stained purple;

other cellulosic walls stained blue

Zinc-chlor-iodide Temporary stain only. Lignified walls unstained. Testa stained orange;

other cellulosic walls stained blue

Yeast Methylene blue, pH 46 Yeast stained blue; gluten unstained owing to low pH

tbl0002 Table 2 List of stains and conditions commonly used for fluorescent microscopy of wheat and wheat products

Substrate Stain Comments

Starch Periodic acid–Schiff (PAS) reaction More sensitive than bright-field PAS for thin sections

Protein Acid fuchsin More sensitive than bright-field stain for thin sections

Coupled antibody or lectin Potentially more specific

Lipid Aqueous phosphine 3R As aqueous solution, lipid distribution less likely to be altered compared to

Sudan dyes

Cell walls Autofluorescence Walls containing lignin or phenolic acids autofluoresce

Coupled lectin Specific for selected glycoprotein

6142 WHEAT/Grain Structure of Wheat and Wheat-based Products

surrounding bran so that the maximum yield of

white flour is achieved with the minimum contamin-

ation of nonendosperm material. Fluted rollers are

used to break open the grain and scrape the endo-

sperm from the bran; smooth rollers are used to

reduce the endosperm particles into flour. The mois-

ture content of cleaned wheat is adjusted to between

15% and 17% prior to being fed to the fluter. This

process is called conditioning and its aim is to facili-

tate separation of bran and endosperm, and to

toughen the bran, thereby reducing the amount of

ash in the flour.

0019 Conditioning can be completed in approximately

6 h for soft wheat, but hard wheat may require 24 h or

longer. The X-ray method has shown that added

water moves rapidly through the bran layers, but

may remain at the aleurone–endosperm interface for

several hours. The rate of water penetration through

the endosperm is dependent on protein content, ini-

tial moisture, and grain hardness. The air spaces that

are present in the endosperm of the soft wheat allow

water to move more rapidly and hence conditioning

time is less than that required for hard wheat.

0020 Of morphological factors which can influence the

yield of clean, white flour, the shape of grain and

the amount of endosperm within the grain are par-

ticularly important. Grain size has been shown to be

significant; larger grains have a higher potential flour

yield. Image analysis has been used to measure a large

number of morphological parameters, and extraction

has been shown to correlate with grain length

parameters. However, the manner in which these par-

ameters influence extraction is not clear and further

work is required to determine their potential as a

screening system at wheat intake.

0021In addition to the amount of endosperm contained

in the grain, the efficiency with which bran and

endosperm can be separated (‘bran clean-up’) is also

a major factor influencing flour yield. Because of

intrinsic differences in the structure of hard and soft

wheat, they must be milled differently. The cell struc-

ture of soft wheat is very weak and readily broken. In

addition, the endosperm of soft wheat appears to

adhere more strongly to the bran. Durum wheat is

designed to produce semolina, a granular material

analogous to the farina or flour middling of the

hard-wheat-milling process. Granular products such

as semolina and farina are used for pasta, baby foods,

and other specialty foods.

0022Endosperm hardness influences the manner in

which grains fragment, and this can also influence

flour yield. Grain hardness is, in turn, influenced by

moisture and this is another objective of the condi-

tioning process. The efficiency with which floury

endosperm is removed from overlying bran and the

degree to which bran is powdered or otherwise dam-

aged are both influenced by the manner in which

grain fractures during milling. If fracturing occurs at

the boundary between the endosperm cell wall and

cell contents (Figure 4), the endosperm is efficiently

fig0004 Figure 4 Scanning electron micrograph of subaleurone region of a mealy kernel of spring wheat var. Jara. Note that fracture has

taken place at the interface between cell contents and endosperm cell wall. Bar, 50 mm.

WHEAT/Grain Structure of Wheat and Wheat-based Products 6143

removed from the bran, but the bran is fractured into

small pieces.

0023 When the endosperm is fractured intracellularly

(Figure 5), bran clean-up is poor, but large pieces of

bran are produced. The manner in which the grain is

fractured is determined by both the inherent hardness

of wheat grain and its moisture content. It is thus

possible to optimize the relationship between grain

hardness, bran clean-up, and powdering by adjust-

ment of moisture level and conditioning time.

0024 Another difference between hard and soft wheat is

the point of the endosperm fragmentation. In hard

wheat it takes place mainly at the boundary between

adjacent cells as the contents of the cells are more

firmly bound together by a continuous matrix pro-

tein. Thus, hard wheat endosperm can be removed

more efficiently from bran as the shear forces

imparted by the fluted rollers are directed along the

boundary between adjacent endosperm cells towards

the bran (Figure 6).

0025 When they reach the bran, some of the forces are

deflected along the aleurone–endosperm interface,

facilitating separation of endosperm from bran. Of

course, as the kernel is reduced to flour size, the hard

wheat cell contents are also fractured. In soft wheat,

the discontinuities in the protein matrix allow endo-

sperm contents to break apart more easily and cleav-

age takes place intercellularly. Thus the shear forces

are dissipated within the endosperm and are not

redirected towards the bran.

fig0006Figure 6 Diagrammatic representation of the forces acting on

one endosperm cell of a wheat grain. Large arrow at the base of

the figure indicates the direction of the shear force caused by the

differential speed of the break rollers. In hard wheat the contents

of the cell behave as one unit and hence the cell is cleanly torn

from the overlying aleurone layer (indicated by the dashed line).

In soft wheat the shear force passes through the content of the

cell, as indicated by the dotted arrow. B, bran layer; A, aleurone

layer; W, endosperm cell wall; E, endosperm. Reproduced from

Wheat: Grain Structure of Wheat and Wheat-Based Products, En-

cyclopaedia of Food Science, Food Technology and Nutrition,

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

Press.

fig0005 Figure 5 Scanning electron micrograph of the subaleurone region of a mealy kernel of spring wheat var. Jara. Note that the fracture

is intercellular. Bar, 50 mm.

6144 WHEAT/Grain Structure of Wheat and Wheat-based Products

0026 Microscopy can also be used to study the mode of

action of different items of mill equipment and the

effect of processing variables on flour quality. Starch

damage is regulated by the amount of pressure ap-

plied by the smooth reduction rollers. The physically

damaged granules absorb more water and are more

susceptible to enzyme attack. A controlled level of

starch damage is required for bread flours. The higher

roller pressures produce thin flakes of endosperm and

microscopic examination has indicated that the ap-

pearance of the protein is altered. The protein has

been spread over the surface of the starch granules

and its functional properties are adversely affected so

that cohesive elastic gluten cannot be formed from the

protein in the flakes. The appearance of the starch

granules in flaked endosperm is usually similar to

that seen prior to flaking and the damage is only

apparent on hydration, when the granules absorb

water and rapidly swell. The amount and nature of

bran and wheat germ in stock or flour steams can be

determined using microscopy. Bran contamination is

important as it adversely influences flour color and

ash. Typical wheat has an ash content of about 1.5%.

However, the ash is not distributed evenly in the

grain. The inner endosperm may only contain 0.3%,

whereas the bran may contain 6%. Wheat germ has

less effect on flour color, but reduces gluten strength

and influences improved requirement in bread-

making.

Dough

0027 When water is added to flour and energy is imparted

to the dough by a mixer, the gliadin and glutenin

proteins interact to form elastic, cohesive gluten.

The mechanism and rate of gluten development

depend on the level of water addition and rate of

work input. In bread dough the level of water add-

ition (55–62%) is sufficient to cause the mixer ini-

tially to pull the gluten away from the starch granules

and form coarse, poorly connected masses (Figure 7).

As mixing proceeds, these masses are gradually

stretched out and become more interconnected and

eventually form a uniform, continuous, extensible

network, which surrounds the majority of the starch

granules in the dough (Figure 8).

0028 Such a gluten network gives dough a smooth exter-

nal appearance and prevents loss of carbon dioxide

produced by yeast during fermentation. This ultim-

ately results in a fine porous structure that is typical of

good-quality bread. If mixing is prolonged the protein

may become overdeveloped and flow over all starch

granules in a thin, veiling film. Carbon dioxide can

diffuse through the thin film and escape from the

dough, which ultimately results in a small, dense

loaf of bread. Overdevelopment only takes place if

rate of work input from the mixer is above the critical

level for that flour and dough system. If the rate of

work input is below the critical level, overdevelop-

ment does not occur and the protein matrix gradually

becomes more coarse and discontinuous. This phe-

nomenon has been termed ‘unmixing’. Much em-

phasis has been placed on the mixing process, but

fig0007Figure 7 Light micrograph of a cryostat section of an under-

developed bread dough. The stain was Ponceau 2R. Note the

coarse gluten masses (M) that do not surround the majority

of the starch granules (G). Bar, 60 mm. Reproduced from

Wheat: Grain Structure of Wheat and Wheat-Based Products, En-

cyclopaedia of Food Science, Food Technology and Nutrition,

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

Press.

fig0008Figure 8 Light micrograph of a cryostat section of an optimally

developed bread dough. The stain was Ponceau 2R. Note the

coarse gluten masses have become more interconnected and

form a continuous network (N) that surrounds the majority of the

starch granules (G). Bar, 60 mm. Reproduced from Wheat: Grain

Structure of Wheat and Wheat-Based Products, Encyclopaedia

of Food Science, Food Technology and Nutrition, Macrae R,

Robinson RK and Sadler MJ (eds), 1993, Academic Press.

WHEAT/Grain Structure of Wheat and Wheat-based Products 6145

the dough structure formed by mixing can be modi-

fied by the nature and timing of subsequent dough-

processing stages. Dough pieces are usually molded

into shape by passage through sheeting rollers. The

dough molder can further develop the gluten network

or unmix it. Which phenomenon occurs depends

on dough strength and the time between molding

and the previous processing stages. Generally, flour

from the hard wheat grains gives a more adequate

structure of dough for breads.

0029 In other types of dough the pattern of gluten devel-

opment is modified owing to lower levels of water

addition or the presence of other ingredients, particu-

larly high levels of fat. In flaky pastries or short, sweet

biscuits, a continuous gluten matrix is undesirable, as

it would give rise to a tough or hard end product.

0030 The level of water addition in noodle or pasta

dough is considerably less (30–33% addition) than

that in bread, and no protein pullback occurs during

mixing. The function of mixing in these processes is

to insure uniform distribution and hydration of ingre-

dients. The continuous gluten matrix is formed by

sheeting rollers in the case of noodles and by high-

pressure extrusion in the case of pasta.

Cooked Products

0031 One of the major changes that can take place during

the cooking of baked products is gelatinization of the

starch granules, and the extent to which this has

occurred is easily followed using polarized light mi-

croscopy. Starch gelatinization can also result in the

formation of more significant sample preparation ar-

tifacts than occur when samples containing ungelati-

nized starch are prepared using similar techniques. In

some products, the gelatinized starch granules absorb

a large amount of water, and freezing or dehydration

can give rise to artifacts. However, these can be min-

imized and appropriate allowance made when inter-

preting data. In expanded, extruded products the

gelatinized starch granules have a high capacity for

absorbing water and this gives rise to a high degree of

swelling if aqueous reagents are used during process-

ing for LM.

0032 For gelatinization to occur there must be an appro-

priate combination of heat and moisture and in many

baked products the latter can be insufficient. In bread,

only a narrow zone of starch granules in the crust still

shows some birefringence. However, in biscuits and

pastries that contain a high level of fat the majority of

the starch granules are frequently still birefringence

(i.e., not gelatinized). In some cooked cereal foods the

continuous phase, responsible for sample integrity, is

gelatinized starch (e.g., products made from batters

or from high-pressure cooker-extruders), whereas in

others (e.g., all breads, noodles, and pasta) it is

gluten. The nature of the continuous phase influences

raw material selection and the organoleptic proper-

ties of the end product.

0033Generally, flour from soft wheat grains gives a

structure of dough which is more adequate for

cooked products. Recent investigations have shown

that it is possible to identify the two molecular pro-

teins called puroindolines – pin A and pin B – which

correlate perfectly with wheat texture. This discovery

should help wheat breeders in the future to develop a

supersoft variety for making new kinds of cakes and

cookies.

See also: Microscopy: Light Microscopy and

Histochemical Methods; Scanning Electron Microscopy;

Transmission Electron Microscopy; Milling: Principles of

Milling; Types of Mill and Their Uses; Wheat: The Crop

Further Reading

American Association of Cereal Chemists (1990) Advances

in Cereal Science and Technology. St Paul, MN: Ameri-

can Association of Cereal Chemists.

Angold RE (1979) Cereals and bakery products. In:

Vaughan JG (ed.) Food Microscopy, pp. 75–

136.London: Academic Press.

Betchel DB (1990) Preparation of cereals and grain prod-

ucts for transmission electron microscopy. Food

Structure 9: 241–251.

Evers AD (1979) Cereal starches and proteins. In: Vaughan

JG (ed.) Food Microscopy, pp. 139–186. London:

Academic Press.

Fornal J, Jelin

ski T, Sadowska J and Quatrucci E (1999)

Comparison of endosperm microstructure of wheat and

durum wheat using digital image analysis. International

Agrophysics 13: 215–220.

Fulcher RG and Wong SI (1980) Inside cereals – a fluores-

cence microchemical view. In: Inglett GE and Munck L

(eds) Cereals for Food and Beverages, pp. 1–26. New

York: Academic Press.

Hoseney RC (ed.) (1990) Structure of cereals (I). In: Prin-

ciples of Cereal Sciences and Technology. St. Paul, Min-

nesota, USA: American Association of Cereal Chemists.

Niewczas J, Kudra T, Strumillo P and Wozniak W (2000)

Stress cracking as a measure of grain quality after ther-

mal drying. In: Mujumdar AS and Suvachittanont S (eds)

Development in Drying, vol. II, pp. 1–31. Bangkok,

Thailand: Kasertsart University Press.

O’Brien TP and McCully ME (1981) The Study of

Plant Structure. Principles and Selected Methods,

pp. 2.2–4.56. Melbourne: Termarcarphi.

Wozniak W, Grundas ST and Kocon J (1991) Qualitative

effect of moisture treatment of wheat grain by means

X-ray and SEM techniques. In: Kratochvil J (ed.)

Proceedings of the ICC Symposium, Prague, vol. 2,

pp. 494–499.

6146 WHEAT/Grain Structure of Wheat and Wheat-based Products

WHEY AND WHEY POWDERS

Contents

Production and Uses

Protein Concentrates and Fractions

Fermentation of Whey

Principles of Dialysis

Applications of Dialysis

Production and Uses

J G Zadow, Mount Martha, Victoria, Australia

Origins and Characteristics of Liquid

Whey

0001 For many years, whey, the byproduct of casein and

cheese manufacture, was treated as a waste product.

Most whey was disposed of by feeding to animals, or

running to waste in streams or on to the land. The

comparatively small amount that was used was

mainly directed at the animal-feed market. In the

past few decades, however, environmental pressures,

coupled with a recognition of the inherent value of

whey solids, have resulted in the development of pro-

cesses for the conversion of liquid whey into a range

of valuable food ingredients. Today, whey and whey

products are in considerable demand, both as replace-

ment ingredients for more expensive existing prod-

ucts and, more often, as functional and nutritional

ingredients in their own right. The use of whey prod-

ucts in the animal feed market is still very important,

but in many major markets, such as the USA, domes-

tic utilization of whey powder for human foods is

greater than that for animal feed. In Europe, however,

animal feed still remains the dominant use for whey

powder. This section reviews the source of wheys,

compositional factors affecting utilization, alterna-

tives for processing of whey properties of whey

powders, and their uses in the food industry.

0002 Little information is available on worldwide pro-

duction of whey. In 1987, it was reported that the

annual world production was about 110 million

tonnes, of which about 47 million tonnes were pro-

duced in Western Europe, 27 million tonnes in North

America, 30 million tonnes in Eastern Europe, and 8

million tonnes elsewhere; Australia produces about

1.8 million tonnes. Given the considerable increase in

cheese production worldwide since 1987, it is prob-

able that the current world production of whey is up

to 50% higher than the figure quoted above.

0003Therefore, two pressures exist on whey processors

to increase whey utilization. First, there is the rapid

increase in the total volume of whey produced, owing

to increasing cheese manufacture, and second, there is

much greater pressure from environmental regulatory

bodies to reduce disposal, and increase utilization of

whey. A detailed survey of whey utilization and dis-

posal practices was carried out in Australia in the

early 1990s, and many of the conclusions are prob-

ably applicable to other major whey-producing coun-

tries. In Australia, about 50% of the total whey

produced is used (an increase from 46% utilization

in the mid-1980s), with the remainder being disposed

mostly on to land. However, in spite of the consider-

able increase in whey production over the past

decade, the percentage of whey used is believed to

have remained approximately constant. In the 1980s

in Australia, membrane processing was a popular

means of whey utilization, representing about 25%

of the whey used. By the early 1990s, this level had

decreased to about 10% and is currently believed to

have fallen even lower. By far the greatest means of

whey utilization now is via whey powder manufac-

ture (of interest is the fact that, although membrane

processing represented only a comparatively small

amount of the whey processed, all of the permeate

produced was used).

0004It should be noted that the greatest disposal prob-

lems were encountered by factories that produced

only moderate amounts of whey. The major options

for disposal of whey, either through drying or through

membrane processing and permeate utilization, were

often too capital-intensive and scale-sensitive to be an

option for such companies. The development of

economic whey utilization techniques for such manu-

facturers would be of considerable benefit to the

industry.

Source and Composition of Wheys

0005Whey is the watery substance remaining after coagu-

lation of the casein in milk, either through the

WHEY AND WHEY POWDERS/Production and Uses 6147

addition of acid (as in casein manufacture) or through

the action of a protease such as chymosin (as in cheese

manufacture). Clearly, the composition of whey

varies considerably, depending on the source of the

milk and the manufacturing process involved. How-

ever, on average, whey contains about 65 g of solids

per kilogram, comprising about 50 g of lactose, 6 g of

protein, 6 g of ash, 2 g of nonprotein nitrogen, and

0.5 g of fat. Casein whey generally has a significantly

higher level of ash than cheese wheys.

0006 It is convenient to classify whey into three major

types:

0007 Sweet wheys: titratable acidity 0.10–0.2%, pH

typically 5.8–6.6. This category would include

wheys produced from chymosin-coagulated

cheeses with a low level of acidity.

0008 Medium acid wheys: titratable acidity 0.20–

0.40%; pH, typically 5.0–5.8. This class could

include whey from the manufacture of fresh acid

cheese such as ricotta or cottage cheeses.

0009 Acid wheys: titratable acidity greater than 0.40%,

pH less than 5.0. This class would include casein

wheys made by addition of mineral acids, and

some fresh acid cheese wheys.

A detailed composition of dried sweet and acid wheys

is shown in Table 1.

0010The following points are of particular relevance in

assessing options available for the processing of whey

streams:

0011Whey has a total solids of about 6.5%, i.e., it is a

fairly dilute product. Thus, to produce 1 kg of

whey powder requires the removal of about

twice as much water as does the production of

1 kg of milk powder. Removal of water is a costly

unit operation, and this factor alone mitigates

against many options for whey processing.

0012Of the total solids in whey, more than 75% is

lactose. The effective utilization of whey is there-

fore inextricably linked with the effective utiliza-

tion of lactose. Unfortunately, lactose is not a

commercially valuable sugar, as it is not particu-

larly soluble, and it is not particularly sweet

(Table 2). These factors limit the commercial

applications of whey solids considerably.

0013The proteins present in whey comprise about 50%

b-lactoglobulin, 25% a-lactalbumin, and 25%

other proteins. Whey proteins have a very high

nutritional profile, are rich in essential amino

acids, and can have excellent functional pro-

perties. Clearly, the whey proteins are the most

valuable components of whey, and most whey-

processing operations (e.g., ultrafiltration, manu-

facture of lactalbumin) therefore aim to increase

tbl0001 Table 1 Typical composition (%) of sweet and acid whey powders, and whey protein concentrates

Moisture Crude protein True protein Lactose Fat Ash

Sweet whey

3.2 12.9 74.4 1.1 8.4

Acid whey

3.5 11.7 73.4 0.5 10.8

35% WPC

4.6 36.2 29.7 46.5 2.1 7.8

50% WPC

4.3 52.1 40.9 30.9 3.7 6.4

65% WPC

4.2 63.0 59.4 21.1 5.6 3.9

80% WPC

4.0 81.0 75.0 3.5 7.2 3.1

Data from Posati LP and Orr ML (1976) Agriculture Handbook, No. 8^1. Washington, DC: US Department of Agriculture.

Data from Glover FA (1985) Ultrafiltration and Reverse Osmosis for the Dairy Industry. Technical Bulletin, No. 5. Reading, UK: National Institute for Research In

Dairying.

tbl0002 Table 2 Relative sweetness and solubility of lactose, sucrose, and some monosaccharides

Sugar Relative sweetness

Solubility (g per100 g of solution)



C 30



C 50



Sucrose 100 66 69 73

Lactose 16 13 20 30

D-Galactose 32 28 36 47

D-Glucose 74 40 54 70

D-Fructose 173 82 87

Data from Pazur JH (1970) Oligosaccharides. In: Pigman W, Horton D and Herp A (eds.) The Carbohydrates: Chemistry and Biochemistry, p. 69. New York:

Academic Press.

Data from Shah NO and Nickerson TA (1978) Functional properties of hydrolyzed lactose: solubility, viscosity and humectant properties. Journal of Food

Science 43: 1081.

6148 WHEY AND WHEY POWDERS/Production and Uses