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

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but also allows xerophilic species to develop slowly.

At such hydration levels, a slight increase in water

content, due for example to water vapor migration or

to insect respiration in a limited pocket, can be suffi-

cient to cause a significant acceleration in microbio-

logical activity, leading to the formation of ‘hot spots’

in the bulk.

0016 In hot spots, molds grow actively, producing

further heat and water, leading to moisture content

which is eventually sufficient to permit the growth of

yeasts and bacteria. Grain dust and small particles

accumulate at the center of the pile of grain in bulk

silos, because grain tends to separate into heavier and

lighter components when poured into a bin. Such

accumulations of dust and broken kernels greatly

increase the risk of hot spot formation by molds

and insects, because they reduce the circulation of

dry or cool air through the grain and promote the

accumulation of heat and moisture. In such situ-

ations, it becomes urgent to cool and dry the grain

by forced ventilation in order to avoid further prob-

lems throughout the bulk of the silo. (See Spoilage:

Bacterial Spoilage; Yeasts in Spoilage.)

Role of Insects in Stored Grains

0017Among animal pests (i.e., mites, insects, rodents, and

birds), insects cause by far the most damage to stored

cereal. They are also the most feared because, even if

they do not produce clearly toxic substances or strong

off-flavors as do molds, their presence depreciates the

value of grain even in cases of light infestations.

Insects secrete some metabolic residues like quinones

or uric acid that sometimes produce objectionable

smells and tastes. They may also transmit pathogenic

bacteria to the grain or leave fragments (including

feces) in flours and other cereal products. Most con-

sumers are strongly repelled by this filth, and one can

say that, due to their contamination, insects ruin

much more grain than they consume. (See Insect

Pests: Insects and Related Pests; Problems Caused by

Insects and Mites.)

0018Infestation of grain by insects is a constant phenom-

enon and several species are particularly well adapted

to the grain ecosystem. They exist permanently in

machines and buildings for crop reception and must

always be regarded as a permanent, potential pest

danger, leading to sytematic grain treatments by chem-

ical contact insecticides or by fumigation.

0019Storage insects belong mainly to the orders Coleop-

tera and Lepidoptera. Most are adapted forms of crop-

destroying species. The most injurious species belong

to genera Sitophilus, Rhyzopertha, Trogoderma, Sito-

troga, Cryptolestes,andTribolium, and are very flex-

ible and adaptable with only minimal nutritive

requirements: for example, they tolerate movements

of grain during intersilo transfer or temperature fluc-

tuations induced by heating and ventilation. Most

important species have a classical development cycle:

the females lay fertilized eggs which lead to worm-like

larvae, and then to adults through successive stages

and a final metamorphosis via a pupal stage.

0020Adults usually live for several months, sometimes

more than a year, depending on storage conditions.

fig0001 Figure 1 Scanning electron micrographs of Eurotium chevalieri,

a xerophilic species common on stored grains: (A) 1400, round

conidia on the surface of the kernel; (B) 280, conidial heads,

responsible for vegetative multiplication and dissemination).

Courtesy of INRA, LMTC, Nantes.

1016 CEREALS/Bulk Storage of Grain

Insects do not like low temperatures and cannot survive

subzero temperatures for more than weeks. For this

reason, damage caused by insects is generally only a

secondary problem in cold or temperate climates.

0021 Like molds, insects are aerobic organisms, only

able to ‘burn’ with oxygen the dry matter they eat,

producing carbon dioxide, heat, and water. They can

initiate hot spots in dry grains, but such dry heating

generally remains below 40



C, which is the lethal

temperature for most grain-infesting insects. By con-

trast, heating due to aerobic microorganisms can

exceed 65–70



C and can even lead to autocombus-

tion of the bulk, when the moisture content becomes

sufficient for the development of thermophilic fer-

mentative bacteria and, subsequently, the occurrence

of exothermic chemical reactions.

0022 Despite their absolute requirement for oxygen,

insects are able to stop breathing for several hours,

days, and even weeks, depending on the development

stage. However, they are also very sensitive to high

concentrations of carbon dioxide and this is why it is

possible to control insect infestations with efficiency

by the use of modified atmospheres. (See Controlled-

atmosphere Storage: Applications for Bulk Storage of

Foodstuffs.)

0023 They are remarkably tolerant to dryness, and com-

monly develop in grains with 11–14% water content

(wet basis) but cereal weevils cannot multiply below

9% moisture content and adults cannot survive long

in such dry grains. On the other hand, flour beetles

seem to be able to develop in completely dry flour. At

least some storage insects seem capable of reutilizing

their own metabolic water (produced by respiration,

as with molds) when surviving in extremely dry grains

(below 6% moisture content).

Moisture Migration in Silos

0024 In silos, elevators, and other installations for storage

and transportation, the temperature fluctuates as a

consequence of solar heating and daily or seasonal

fluctuations in ambient temperature. The outermost

layers of grain are most affected by such temperature

variations. In a dark-green metallic silo, the wall

temperature on the sunny side can frequently change

from 15 to 80



C on a sunny summer day in northern

Europe. This produces measurable temperature

changes in the outermost 30–40 cm of grain, and

large oscillations occurring in the first 5–10 cm.

Such fluctuations are less pronounced in concrete

silos.

0025 When heated, grain loses water vapor and dehy-

drates. Due to convective movements of the intergra-

nular atmosphere, the lost water vapor is transported

to the coldest zones in the bulk where it condenses,

increasing the water content of the cold grain. The

main consequence is always an increase in microbial

activity.

0026In nonhermetic silos, when the intergranular

atmosphere is not controlled, oxygen is always avail-

able, at least in the outer layers, for respiration of

molds, which in turn produce heat and moisture and

an acceleration of microbial activity, which may cause

carbon dioxide to accumulate deeper in the bulk. This

chain reaction always occurs, everywhere in the world,

under all climatic conditions. The main difference from

one place to another is the speed and intensity of this

general phenomenon but, whatever the initial moisture

content of the grain, sunshine will more or less rapidly

destroy the stored cereals it previously allowed to pro-

duce. (See Oxidation of Food Components.)

0027The only way this process can be slowed is by

removing heat and moisture using forced ventilation

of the bulk, or by transfer of the grain from one bin to

another to mix the hot and dry grain with cold and

moist grain from other parts of the silo. Underground

storage under airtight conditions, where temperature

fluctuations and moisture migration are minimized,

provides an interesting alternative to bin storage,

especially when germinative capacity is not the main

characteristic to be preserved.

Losses Due to Rodents

0028Many species of small rodents cause grain losses

throughout the world. Rats and mice, which are

found everywhere, are probably the most important,

causing damage mainly at farms where grain is stored

in loose structures. Control measures against rats and

mice include traps, baits containing rodenticides, and

fumigation similar to that used against insects. In

many cases, it is important to know which kind of

rodent is concerned in order to take its feeding pref-

erences into account for poisoning, or to find the best

way to trap it. Rodent control in larger grain stores

requires a sound concrete construction that is able to

resist rat gnawing, and the silo must be maintained in

good condition. (See Fumigants.)

Biochemical Changes During Storage

0029For dry grains, i.e., with moisture contents permitting

only very slow development of molds, slight changes

in biochemical composition can be observed over

several months. The lipid fraction of the grain deteri-

orates most rapidly during open-air storage under the

influence of microbial enzymes, leading to the

accumulation of fatty acids. Unsaturated acids can

be further oxidized by the oxygen in air to produce

volatile compounds with rancid odors and off-

CEREALS/Bulk Storage of Grain 1017

flavors. When moisture contents exceed 15–16%

(wet basis), amylases from both grain and micro-

organisms start to hydrolyze starch into dextrins

and maltose, resulting in losses of dry matter and

a deterioration in quality. Very small changes

seem to occur in the protein fraction of stored grains

when molds cannot develop significantly in a few

months, i.e., below 15% moisture content. However,

the aggregative properties of some proteins may be

modified after several years of storage, leading to a

decrease in the water-soluble fraction. This may result

in slightly lowered digestibility or, for wheat, in losses

in bread-making value.

0030 In order to verify good storage of cereal grains,

several simple laboratory examinations can be per-

formed. Arguably the most sensitive is the germina-

tive capacity of the grain, which clearly indicates the

presence or absence of mold growth during storage in

the open air. Fat acidity is another good criterion

which correlates well with the metabolic activity of

storage fungi and the quality of the grain after a long

period of storage.

0031 In practice, in business transactions, the most im-

portant criterion is to insure the absence of any live

insect. In most cases, it is too often still necessary to

fumigate the grain or treat it with insecticide before

dispatching from the elevator. Modern consumers are

more and more suspicious of chemical residues in

food and the coming years will probably see a revi-

sion of the risk assessment of chemical residues and of

the real benefit of insect destruction in grains.

See also: Controlled-atmosphere Storage: Applications

for Bulk Storage of Foodstuffs; Fumigants; Insect Pests:

Insects and Related Pests; Problems Caused by Insects

and Mites; Mycotoxins: Occurrence and Determination;

Oxidation of Food Components; Spoilage: Bacterial

Spoilage; Molds in Spoilage; Yeasts in Spoilage; Storage

Stability: Mechanisms of Degradation; Parameters

Affecting Storage Stability; Water Activity: Effect on Food

Stability

Further Reading

Calderon M and Barkai-Golan R (eds) (1990) Food Preser-

vation by Modified Atmospheres. Boca Raton: CRC Press.

Christensen CM (ed.) (1982) Storage of Cereal Grains and

their Products, 3rd edn. St Paul: American Association

of Cereal Chemists.

Multon JL (ed.) (1988) Preservation and Storage of Grains,

Seeds and their By-Products. Paris: Lavoisier.

Highley E, Wright EJ, Banks HJ and Champ BR (eds)

(1994) Stored products protection. Proceedings of

the 6th International Working Conference on Stored-

products Protection. Canberra: CAN International.

Handling of Grain for Storage

D Richard-Molard, INRA, Nantes, France

Background

0001The conservation of cereal grains is of utmost import-

ance for the survival of human populations.

0002Different technologies have been used to conserve

grain, such as storage in waterproof or airtight buried

silos.

0003In the sixteenth century, in Europe, the first texts

explaining the importance of drying, ventilation, or

modified atmospheres for the good conservation of

grains appeared, but the scientific bases explaining

the reasons for these practices were absent.

0004Today, with the development of scientific knowl-

edge on the grain ecosystem, and progress in the fields

of building materials, sensors, and measurement

techniques, grain storage technologies have been

mastered.

0005However, with continuous, and increasing, require-

ments of technological and sanitary quality, particu-

larly at the present time, increasing world trade of

grains, and the resulting increased commercial com-

petition mean that this area of industry is very dy-

namic and responsive to new innovative technologies.

Storage Facilities

0006Storage facilities may take many forms, from a simple

pile of unprotected grain on the ground in a farm

building to the most sophisticated terminal elevator

designed to handle grain for long-distance export-

ation. At the farm level, grain may be stored tempor-

arily in small concrete or round steel bins of small

capacity (25–100 tonnes), which receive the crop

immediately after harvest and before transport to a

better long-term storage facility or in order to wait for

the best market situation.

0007Grain may also be taken from the farm directly to a

collecting center, a small storage installation with sev-

eral small bins providing an average capacity of about

1000 tonnes. This does not generally have machinery

for grain cleaning or drying, since the grain is usually

transferred soon after to an elevator (Figure 1).

0008Country elevators (5000–50 000 tonnes), so called

because they are filled with grain elevated into them

by rolling belts with buckets, receive grain from the

individual producer or from small collecting centers.

Their capacity is adapted to the seasonal production

of the area, and their main role is to keep crops in

good condition before and during storage, and to

1018 CEREALS/Handling of Grain for Storage

reload it into trucks or rail cars for transportation to

terminal silos, export elevators, or industrial users.

0009 There is no general rule for determining the best

dimensions of a farm bin or a country elevator. Usu-

ally, the cost of storage increases sharply as the size of

the silo decreases, so there is a tendency to build large

structures. However, the grains are managed in

multiple silos, where grains of different grades or

water contents can be separated easily.

0010 The terminal elevators are generally located close

to trade centers and/or transportation terminals, such

as harbors (Figure 2). Terminal elevators commonly

offer storage capacities of 5000–500 000 tonnes in

the USA or northern Europe.

0011 Equipment for grain handling and control is often

basic at the farm level, with only very simple devices

for grain elevation and ventilation. By contrast, coun-

try and terminal elevators have handling capacities

adapted to their transportation facilities and usually

have sufficient capacity for peak load handling during

the short period just following harvest. Today, most

are fully equipped with remote control and automa-

tion, so that grain can be automatically transferred

from one silo to another or to ships and railway cars.

Weighing, cleaning, sampling, and even several grain

sample tests are now electronically controlled.

0012 When possible, gravity discharge from grain bins is

ideal, but the cost of the corresponding structures

and equipment is high. Consequently, flat storage of

cereals is also commonly carried out in many situ-

ations, both at the farm level and in terminal eleva-

tors, to provide additional storage capacities when

necessary. Such flat-bottomed silos are emptied

through a central outlet, where a rotating screw-

conveyor draws the grain away beneath the floor.

Many other sophisticated devices have been de-

veloped for moving the grain using air blown through

metallic networks of ducts below the bins, or with

suction blowers as back-up devices.

Quality Maintenance During Storage

0013When the farmer brings freshly harvested cereals to

the elevator, several tests are performed to determine

the appropriate treatment for safe storage, such as

cleaning, dust removal to avoid very dangerous dust

explosions, drying, cooling, chemical treatments

against insects, etc.

0014As previously emphasized, the major parameter to

be controlled is water content. Even with mature dry

grain harvested at about 12–13% moisture content,

serious problems will be encountered within a few

months if moisture cannot be properly managed to

prevent the heat and moisture transfer that can occur

in bulk during storage and transportation. It is, there-

fore, important to store grains with a sufficiently low

initial moisture content to prevent mold development

and biochemical changes, and to maintain this low

temperature as constant as possible everywhere in the

silo throughout the storage period. This can be

achieved by artificially drying grains with excessive

humidity at harvest, like maize in Europe, and by

periodic forced ventilation during storage to remove

Field

Collecting

center

Country elevator

Harbor elevator

fig0001 Figure 1 Grain collection, from field to terminal elevator, a long trip for the grain.

CEREALS/Handling of Grain for Storage 1019

excess water where condensation has occurred.

(See Storage Stability: Mechanisms of Degradation;

Parameters Affecting Storage Stability.)

Moisture Levels for Safe Storage

0015 Determination of the correct moisture level for safe

storage of a given crop in a given situation and for a

given storage time is difficult. Very general and theor-

etical rules can be given, but one should bear in mind

that a large mass of cereal grain is never homoge-

neous in temperature, moisture, apparent density,

thermal conductivity, etc., and so continuous moni-

toring with modern techniques, such as silothermo-

metry, and periodical sample examination is highly

recommended in every situation.

0016 The thermodynamic activity of water, which repre-

sents the availability of water in the grain, is the most

important physical parameter governing grain stabil-

ity during storage, determining both biochemical

changes and microbial growth. The availability can

be estimated (and measured) by the equilibrium rela-

tive humidity (ERH) of the intergranular air in equi-

librium with the grain. A nonlinear relation relates

ERH to moisture content, as shown in Figure 3 for

wheat and maize. This curve is named a ‘sorption

isotherm’ and depends on both the average tempera-

ture and the biochemical composition of the grain. It

differs from one cereal to another, but its relationship

to the level of deterioration by different causes, such

as microorganisms or enzymes, remains constant to a

first approximation. Wheat stored at 16% moisture

content is in equilibrium with an ERH of about 80%

and will undergo the same kind of degradation at the

same rate as maize with 15% moisture content,

which also has an ERH of 80% at the same tempera-

ture. As the temperature increases, the sorption iso-

therm is shifted to the left, i.e., at the same water

content, there is a higher ERH, and spoilage is faster.

(See Water Activity: Effect on Food Stability.)

0017Silothermometry is a sophisticated technique, util-

izing thermocouples placed in the silo able to detect

slight variations in grain temperature, which indicate

the beginning of deterioration through an aerobic

process, resulting from a localized increase in humid-

ity. Modern sensors can detect temperature changes

fig0002 Figure 2 (see color plate 24) Harbor elevator in Rouen, France, with a total storage capacity of 300 000 tonnes. This is able to load

grains into vessels at a rate of 5000 tonnes h

1

. Photo courtesy of J. Pfeiffer.

Water content

(wet basis)

Maize

Wheat

Bacteria

Yeasts

Molds

25 50 75

E.R.H

fig0003Figure 3 Sorption isotherms for wheat and maize at 20



C and

zones of activity for the main causes of grain degradation during

storage (the wider the zone representing a cause of spoilage, the

more intense and rapid the spoilage).

1020 CEREALS/Handling of Grain for Storage

of less than 0.5



C. In most situations, this gives

sufficient sensitivity and overcomes the problem of

the very low thermal conductivity of cereal grains,

which allows temperature differences to be transmit-

ted only over short distances in the bulk. With this

method it is also possible to determine the best

moment for drying and ensure the completion of

grain cooling by nocturnal ventilation.

Drying Methods and Alternatives

0018 The use of adequately dried grains for storage in silos

is a mandatory condition (Figure 3). Many types of

driers, such as full-bin driers, layer driers, continnous-

flow driers, etc., are presently used on farms or in

country elevators, but all utilize the same basic

principle: heated dry air is blown through the grain

to remove excess water. Figure 4 represents a farm bin

equipped with a low-heat drier capable of drying

grains with moisture contents of less than 20% (wet

basis). (See Drying: Theory of Air-drying.)

0019 When large quantities of moist grain are to be dried

rapidly, high-temperature drying is necessary. This

gives a higher extractive capacity but also can cause

thermal degradation of the grain, if used improperly,

as often happens with maize, which is sometimes

harvested with a water content as high as 35% (wet

basis) in Europe. If the air for drying is too hot, the

functional proteins are denatured, and the value of

the crop is decreased. However, high drying tempera-

tures destroy all forms of insects in the bulk. (See

Drying: Hygiene.)

0020 When possible, a combination of different drying

and cooling practices often gives the best technical and

economic results. The essential principle is to decrease

the moisture content rapidly to a level, thus decreasing

the rate of mold development, i.e., to about 18%

moisture in a first step, then to decrease the residual

humidity to 15–16% by forced aeration when the

ambient air is cool and dry enough to be efficient.

Natural Drying Methods

0021 Alternative natural methods for grain drying have

often been used, and at least two must be mentioned.

The first, used mostly for maize, uses gentle drying by

the wind of cobs stored outside in cribs or other forms

of mesh silos. This is satisfactory, provided that the

temperature remains sufficiently low to inhibit the

growth of such toxinogenic fungi as Fusarium species.

0022 The second type of natural drying method uses

solar driers. These have been tested in efforts to save

energy, but their use can be recommended only in

regions where sufficient energy is provided by the

sun to achieve drying in a time short enough to

avoid spoilage by microorganisms. Unless drying to

80–82% ERH can be achieved in less than 7–10 days,

the grain will certainly be damaged. (See Drying:

Drying Using Natural Radiation.)

Modified-atmosphere Storage

0023When cereals are not intended for uses requiring par-

ticular properties, such as a high germination ability

for malt production or functional properties of pro-

teins for breadmaking, it is possible to store them

with a higher water content than usual, provided the

oxygen is removed from the system.

0024Underground storage and storage in anoxic atmos-

pheres in hermetically sealed silos, or silos continu-

ously flushed with nitrogen, for example, are quite

possible. For example, it was recently shown that the

nutritional value of wheat for animal feeding could be

successfully maintained over several months by

storing it, after grinding, in airtight conditions at

21% water content and 15–20



C. Storage of wet

(35% water content or more) maize for pig feed in

hermetic silos is now popular in several countries.

Because of the important demand for intergranular

oxygen at such a high humidity, especially through

respiration of yeasts and lactic acid bacteria, the con-

centration of available oxygen remains extremely low

in the silo. The growth of molds is inhibited and, at

least until the silos are nearly empty, that is to say

when free oxygen again appears and concentrations

of carbon dioxide are decreasing, no mycotoxins can

be produced. With such hermetic silos, it is not neces-

sary to grind the humid grain in order to produce

silage, and the commodity can be handled more easily

than with traditional practices. (See Controlled-

atmosphere Storage: Applications for Bulk Storage

of Foodstuffs; Spoilage: Molds in Spoilage.)

Air exhaust

Wet

grain

Perforated

floor

Drying zone

Dry grain

Fan

Unloading auger

fig0004Figure 4 Farm bin for low-temperature drying.

CEREALS/Handling of Grain for Storage 1021

Control of Insects

0025 Storage of dry or wet grains under modified atmos-

pheres is an excellent way to kill insects, and modern

techniques for storage of dry wheat under high con-

centrations of carbon dioxide have yielded very good

results in Australia in recent years. Most often, if all

living insects are to be killed, chemicals need to be

used when infestations are detected in a silo or a ship.

(See Insect Pests: Insects and Related Pests; Problems

Caused by Insects and Mites.)

0026 Insecticides can be divided into two main classes:

contact insecticides, which kill insects and prevent

reinfestation owing to their remanent effect, and

fumigants, which destroy insects without leaving

any significant residues. Organochlorine insecticides,

which belong to the first class, were extensively used

in the past but are now forbidden because the toxicity

of their residues is very high for humans.

0027 Pyrethroids like deltamethrin, permethrin bifen-

thrin, and organophospates like malathion, dichlor-

vos or methyl pirimifos, are also contact insecticides,

used at concentrations of about 4–8 g of active sub-

stance per tonne of grain. They are very fast-acting

and highly toxic by contact and ingestion, but their

residues necessitate careful consideration of the

number and timing of treatments. Their relative inef-

ficiency against the hidden forms of insect (eggs,

larvae) that live within the kernels should also be

mentioned. (See Pesticides and Herbicides: Residue

Determination.)

0028 The only current fumigant is hydrogen phosphide,

which is used to rapidly kill all live insect stages,

including adults. It is used in airtight structures

equipped with systems that allow the gas to be intro-

duced and removed safely. (See Fumigants.)

0029 Apart from modified-atmosphere storage under ni-

trogen or carbon dioxide, only persistent insecticides

can provide good long-term protection for grain. It is

then of primary importance to ensure that there is no

residue in byproducts like flour and brans used for

human and animal food. At the present time, there is

a general tendency to decrease the concentrations

of toxic substances used against insect infestations.

A possible approach would be to combine the use

of carbon dioxide with an active substance, such

as methyl bromide or hydrogen phosphide, which

would permit a considerable reduction in the dose

of fumigant used because of synergism with carbon

dioxide, which increases the penetration of the active

substance in the respiring stages.

The Future

0030All over the world, safe storage of cereal grains is a

vital but expensive activity. Too many cereal grains

are still used in the food industry despite the poor

quality resulting from inappropriate storage condi-

tions. Current scientific and technical knowledge in

this domain is sufficient to answer the main ques-

tions about handling and storage of grain, but new

trends are appearing: consumer demand for safe

products, i.e., free of chemical residues, is growing

fast. This probably means that the use of contact

insecticides and even fumigants should be replaced

by more acceptable techniques like physical tech-

niques in the coming years.

0031Another trend seems to be a clear demand for iden-

tified grains, nongenetically modified grains, and

grains from a precise geographic origin or a well-spe-

cified technological quality, for example. Such an evo-

lution would certainly introduce very important

changes in the way in which the grains are harvested,

collected, stored, and distributed. Loading speeds as

high as 5000 tonnes per hour are often reached today

in harbor elevators, but this is not the most convenient

technology if small bulks of 50–100 tonnes of well-

certified grains are to be prepared.

See also: Controlled-atmosphere Storage: Applications

for Bulk Storage of Foodstuffs; Drying: Theory of Air-

drying; Drying Using Natural Radiation; Hygiene;

Fumigants; Insect Pests: Insects and Related Pests;

Problems Caused by Insects and Mites; Storage

Stability: Mechanisms of Degradation; Parameters

Affecting Storage Stability; Water Activity: Effect on Food

Stability

Further Reading

Christensen CM (ed.) (1982) Storage of Cereal Grains and

their Products, 3rd edn. St. Paul, MN: American Associ-

ation of Cereal Chemists.

Navarro S and Donahaye E (eds) (1993) Controlled atmos-

phere and fumigation in grain storages. In: Proceedings

of the International CAF Conference, Winnipeg,

Canada, June 1992. Jerusalem: Caapit Press.

Richard-Molard D (1990) Conservation of humid grains in

controlled atmosphere storage. In: Calderon M and

BarkaiGolan R (eds) Food Preservation by Controlled

Atmosphere, pp. 57–82. Boca Raton, FL: CRC Press.

1022 CEREALS/Handling of Grain for Storage

Breakfast Cereals

R S Kadan, United States Department of Agriculture,

New Orleans, LA, USA

E F Caldwell, American Association of Cereal

Chemists, St Paul, MN, USA

Introduction

0001 The US breakfast cereals (BC) industry, in its history

of over 110 years, has emerged as an important seg-

ment of the food industry, with annual sales of about

$9.5 billion (9.5  10

). This remarkable progress has

been achieved through a combination of knowledge

gained in human nutrition, innovative progress in

cereal processing technology, and imaginative mar-

keting. The chief driving force has been the continued

market for new, convenient, nutritious, and fun foods

for a discriminating, affluent, and mobile society. Any

large grocery store in the USA, Canada, or the UK is

likely to stock 100 or more different BC. In the UK,

annual sales are about £1 billion (1 10

). The rela-

tively lower penetration of BC into other European

and other developing countries increased significantly

during the 1990s, and this is forecast to continue over

the next decades.

0002 BC have been defined as processed grains for human

consumption. The major grains used in the manufac-

turing of BC are corn, rice, wheat, oats, and barley.

Cereals can be divided into those that are ready-to-eat

(RTE) or ready-to-cook, also known as hot cereals

(HC). Alternative breakfast products based on cereal

grains include cereal bars, toaster pastries, waffles,

French toast, and bagels. RTE cereals are frequently

made from mixtures of one or several grain compon-

ents with other ingredients; they require extensive

processing, are usually fortified with vitamins and

minerals, and are specially packaged to protect their

flavor, texture, and nutrition during storage. On the

other hand, HC are typically made from a single grain

component by using relatively simple processing and

packaging technologies. The difference between the

RTE and HC products is reflected in the price at the

consumer level. The typical single serving (28–35 g or

1 oz) of RTE cereal costs about $0.20 ($0.08–0.40) in

the USA and represents about 90% of the business. A

typical 28–35-g or 1-oz serving of HC costs from

about $0.06 to $0.24, for bulk-packaged product to

instant single-serving pouches, respectively or up to

$0.30 for instant farina.

0003 Annual sales in the USA of alternative breakfast

products amounted to over $2 billion (2  10

)by

1999, but the per capita annual consumption of RTE

cereals had still increased from 3.8 to 5.0 kg

(8.3–11.0 lb) since 1970. On the other hand, HC

per capita annual consumption had declined from 1.1

to less than 0.6 kg (2.4–1.2 lb) over the same period.

The latest per capita annual consumption in UK

was 5.9 kg (13.0 lb) and 0.6 kg (1.4 lb) for RTE and

HC respectively. (See Barley; Oats; Rice; Wheat:

The Crop.)

Methods of Manufacture

Ready-to-eat Cereals

0004RTE breakfast cereals are primarily grain formula-

tions, suitable for human consumption without

further cooking. Manufacturing technology varies

according to type of cereal.

0005Flaked cereals Flaked RTE breakfast cereals can be

divided into two groups: flakes made from whole

grains or parts of the whole grains, and flakes made

from more finely ground materials that must be first

extruded into pellets that can then be rolled into

appropriate-size flakes.

0006The whole grains (wheat and rice) or major com-

ponents (such as grits from degermed yellow maize or

corn) are cooked with flavorings such as sugar, salt,

and malt; they are then dried and tempered to a firm

but slightly plastic state, flaked by passing between

rolls, and toasted or dried to a final specified moisture

content. Cooking of grits or whole grains for trad-

itional flakes is usually done in batches. The moisture

content of the cooked mass at the end of cooking is

usually about 28%. The cooked material should

not be mushy, soft, or sticky. After cooking, the

mass of material is cooled, dried at about 120



and tempered (held) for several hours at a final equili-

brated moisture content of 10–18%, depending upon

the grains. The tempering process is important, not

only to allow equilibration of moisture within the

cooked grains or pellets, but also to allow sufficient

starch recrystallization or retrogradation to provide a

suitable texture for flaking (or for shredding, as

discussed next).

0007Flaked RTE cereals can also be made from

extruded pellets rather than cooked grits or whole

grains. A variety of floury or finely ground grain

products such as whole wheat or oat flour can be

mixed with sugar, salt, and malt syrup or other

flavoring and coloring ingredients to form a dough.

This is then extruded to form pellets about the same

size as cooked grits or whole grains, the cooking

having taken place in the extruder. Pellet moisture is

in the range of 18–24%. Conditioning or tempering

prior to flaking may or may not be required.

0008Whether consisting of cooked grits or whole grains

or of extruded pellets, the tempered material is then

CEREALS/Breakfast Cereals 1023

flaked. Tremendous pressures are necessary to flatten

the prepared material into thin flakes, which trad-

itionally have been toasted by keeping them sus-

pended in hot air between 270



C and 330



C for

about 90 s. Some properties of the finished flakes

can be modified by a heating or steaming step imme-

diately before flaking (e.g., infrared heating immedi-

ately before flaking will increase the tendency of

flakes to crinkle rather than remain flat), while

other properties, such as blistering, color, crispness,

tenderness, and flavor of the final product, are influ-

enced by toasting conditions. The moisture content of

the finished product is about 1–3%.

0009 Puffed cereals The predominant grains used for

making puffed cereals are wheat and rice. The milled

rice used for puffing is either short- or medium-grain

white rice. Such rice usually requires no pretreatment

other than the hull and bran removal that occurs

during milling or pearling. Wheat, on the other

hand, is usually decorticated (bran removed), either

by pretreatment with concentrated brine solution or

by pearling. In the pearling operation, the grain is

passed through a revolving cylinder inside which are

mounted silicon carbide or carborundum stones.

0010 Usually hard wheat, preferably durum, is used for

puffing. A grain flour mixture can also be formed into

desired shapes by an appropriate extrusion process

and dried to a moisture content of 9–12% for subse-

quent gun-puffing.

0011 Puffing guns are vessels capable of holding very high

temperature and high-pressure steam. The critical fea-

ture of any puffing gun vessel or barrel is a quick

opening lid that seals the vessel. Operation of a simple

single-shot gun consists of placing the grain in the

vessel, closing the lid and raising the pressure and tem-

perature in the vessel to about 1380 kPa (200 psi). The

quick-opening lid is then activated, causing rapid pres-

sure drop, which in turn instantaneously causes mois-

ture in the grain to ‘flash off’ as steam, and the kernels

of grain or preformed pellets to expand to as much as

16 times their original volume. The guns can also be

multiple-shot automatic or continuous in operation.

0012 Another group of BC is made by oven puffing.

Specially prepared rice or corn grains or their mix-

tures are oven-heated to 290–340



C in order to puff

them. Puffed rice cereal sold in the USA can be either

oven-puffed (e.g., Rice Krispies, prepared from short-

grain rice) or gun-puffed (e.g., Puffed Rice). Whether

gun- or oven-puffed, the grains are then screened to

remove any unpuffed kernels, loosened bran, fine

dust and broken kernels, and the product is heated

to lower the final moisture to 1–3%. Processes such

as coating or vitamin enrichment can be carried out

during this drying operation.

0013Shredded cereals Shredded wheat, one of the oldest

mass-produced breakfast cereals, is still an important

product. Soft white wheat is the primary grain used.

Other varieties of wheat, as well as other cereals such

as rice, corn, and oats or mixtures thereof, can also be

used to make shredded products. The grains are

cleaned, cooked in boiling water, partially dried, and

tempered. The shredding operation consists of pass-

ing the tempered grain between two rolls. One roll is

grooved and the other is smooth. Strong forces in the

range of 7000 kPa (1000 psi) along the roll length are

often required to maintain the rolls in tight contact.

The tempered kernels are squeezed into the grooves of

the roll and emerge as strands roughly 1 mm in diam-

eter. These strands are accumulated in multiple layers

below multiple pairs of rolls, and formed into biscuits

or bite-sized pieces. A refinement of the shredding

process is the use of a roll with added cross-grooves

to provide lateral strands that can tie the shreds to-

gether in a coherent sheet. Mixtures of ground cereal

products with sugar, salt, malt, and flavoring can be

cooked by extrusion, formed into pellets, tempered,

and then shredded in the same way as cooked whole

grain kernels.

0014The formed pieces are oven-dried and baked to the

desired color, texture, and moisture. The baking pro-

cedure is done in a specialized manner to impart fluffy

texture, particularly for products containing rice or

corn. Other minor ingredients, such as flavorings,

nutrients, or antioxidants, can also be added prior

to drying and baking.

0015Granola cereals The main grain used to make gran-

ola cereals is either regular or quick-cooking rolled

oats. The cereal products are mixed with other de-

sired ingredients such as nut pieces, coconut, corn

syrup, brown sugar, honey, malt extract, dried milk,

dried fruits, and/or vegetable oil. Liquid and dry

ingredients are prepared separately, then mixed

together and spread in a uniform layer on the band

of a continuous oven. Baking temperatures are in the

range of 150–220



C, until the product is uniformly

toasted to a light brown color and the moisture is

reduced to about 3%. After baking, the product is

broken into small pieces and packaged.

0016Extruded expanded cereals Successful application

of extrusion technology for producing cereal prod-

ucts from grain flours or meal mixtures by formulat-

ing, continuous preconditioning, extrusion cooking,

and forming has been an important achievement of

the BC processing industry. The extruders can be

single-screw, twin-screw, low-shear, high-shear, and

combinations of these or other features. Extrusion

technology was developed to produce flaked, puffed,

1024 CEREALS/Breakfast Cereals

shredded, and expanded breakfast cereals. For ex-

truded expanded cereals, cereal flour mixtures having

about 25–30% moisture are preconditioned, cooked,

and extruded. Perforations in the die and design of the

cut-off knives at the end of the extruder, as well as

temperature, pressure, time, and shear in the extruder

barrel, all influence the shape, size, and texture of the

final product. The sudden change from high pressure

and temperature to ambient conditions as the product

exits the extruder imparts the necessary expansion,

texture, and other characteristics. The extruded prod-

ucts may be coated with solutions of vitamins and

minerals, and with sweetening and flavoring mater-

ials, and are then oven-dried to desired moisture levels.

Hot Cereals

0017 HC are products of a single grain or a simple mixture

that require cooking or heating in water before con-

sumption. The main such products are rolled oats and

related oat products, farina and other wheat frac-

tions, and corn grits.

0018 After cleaning, oat kernels are dehulled by an

impact process. The resulting groats are steamed,

primarily to deactivate lipolytic enzymes, then rolled,

dried, and packaged. Quick-cooking rolled oats are

made by steel-cutting groats into four or five pieces

before the steaming and flaking process. Instant oat-

meal is made by subjecting groats to a special process

that results in rapid cooking of the flakes. Incorpor-

ation of 0.1–1.0% of an edible gum during processing

helps to achieve this objective. Distribution of the

gum and necessary salt without damaging the extra

thin flakes is accomplished by portion-control pack-

aging. Such instant products require only the addition

of boiling water, stirring, and a short standing time to

make the product ready for consumption. The indus-

try has successfully incorporated dried fruits, nuts,

and other flavorings to make instant oatmeal prod-

ucts more attractive to consumers.

0019 Wheat farina is another important HC. In the USA,

farina and enriched farina have to meet respective

legal standards of identity. Farina is basically wheat

endosperm, preferably obtained from hard red spring

or winter wheat, the granules of which stay intact

during cooking at home. Farina is normally obtained

by drawing off chunks of endosperm during milling

of wheat into bread flour. In the UK it can also be

called semolina, but in the USA that term is reserved

for a similar product from durum wheat intended for

macaroni and other pasta products. Farina is heated

at around 60



C for 15 min before packaging, for

flavor and to head off infestation. Quick and instant

farina products are prepared by the use of disodium

phosphate, enzymes, and other treatments. Similarly,

wheat germ, bran, malted barley, cocoa, and other

flavorings can be mixed into farina to enhance the

consumer appeal.

0020Other important HC products include wheat prod-

ucts such as cracked wheat and bulgur, and corn grits.

Grits and bulgur are generally used as cereal accom-

paniment to other meal menu components such as

eggs, sausage, meat and related preparations, whereas

cracked wheat products are cooked for breakfast.

Grits usage is mostly in southern parts of the USA,

garnished with butter and salt rather than with sugar

and milk as in traditional breakfast cereals. Grits are

produced by dry milling of white degermed corn and

can be fortified with vitamins and minerals. Bulgur or

parboiled wheat is one of the oldest cereal-based

foods and has been consumed for centuries in Turkey,

Syria, Jordan, Lebanon, and Egypt. To prepare it,

common hard or soft wheat varieties are soaked in

water and heated to about 90



C for 2–3 h. The

cooked wheat is then cooled, dried, moistened, peeled

(optional), redried, cleaned and sized. Bulgur has a

good shelf-life because cooking destroys insects

present in harvested crops.

Significance in the Diet and Fortification

0021BC are one of the most highly fortified foods in both

vitamins and minerals. One serving typically can

supply 25% of the daily values of most vitamins and

trace minerals as these are defined in the USA for

labeling purposes. A few BC supply 100% of all

established daily values, making them as much nutri-

tional supplements as breakfast foods. The cereal

industry justifies these practices on the basis that

they fulfill a genuine need to improve the overall

nutritional needs of consumers. Food consumption

surveys have shown that some of the US population

had inadequate intakes of several vitamins and

minerals, particularly vitamin A, folic acid, calcium,

and iron, and possibly vitamin B

, zinc, and magne-

sium. (See Calcium: Physiology; Food Fortification;

Iron: Physiology; Magnesium; Retinol: Physiology;

Vitamin B6: Properties and Determination; Zinc:

Physiology.)

0022The practice of fortification of cereal foods started

in 1941, when the US Food and Drug Administration

(FDA) established a standard of identity for enriched

flour to include the addition of thiamin, niacin, and

iron. Riboflavin was subsequently included, and add-

ition of calcium and vitamin D was optional. Such

recommendations developed after extensive discus-

sions by the various committees of the American

Medical Association, hearings sponsored by the

FDA, and the Food and Nutrition Board. As a result

of these meetings, criteria for fortification of foods

were developed:

CEREALS/Breakfast Cereals 1025