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

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0029 The light, open texture of baked flour products is

due to the presence of gas or air cells in the prebaked

dough or batter. During heating the gases expand; this

expansion is aided by the generation of additional gas

from chemical leavening agents (e.g., sodium bicar-

bonate) and from an increase in the vapor pressure of

the water present. Finally, the liquid matrix is heat-set

by denaturation of proteins and starch gelatinization.

In the case of popcorn the impervious grain coat acts

as the wall of pressure vessel. As the grain is heated

the water vapor pressure increases until the wall

ruptures. The vapor then flashes off, opening out

the endosperm which then heat-sets.

0030 A crisp outer surface is often associated with dry

cooking. The temperature in excess of 100



C results

in moisture being lost from the surface.

Color and Flavor Changes

0031 With the exception of added colors sometimes used in

domestic cooking (e.g., saffron or cochineal), two

types of color change result from domestic cooking:

modification of natural pigments present in the raw

materials, and browning reactions. Most natural

colors tend to be relatively unstable when cooked.

In addition to these chemical reactions, in wet

cooking loss of natural color may result from leaching

of water-soluble pigments into the cooking medium.

0032 Metalloporphyrin colors are found in foods of both

animal and plant origin. Myoglobin, the pigment

found in muscle, undergoes color change from red-

purple to brown when heated; this is a result of cer-

tain amino acids in the globin protein constituent of

the molecule becoming coordinated with the central

iron atom. Chlorophylls are also prone to lose their

color when heated in acid conditions. The change

from bright green to a greyish green is the result of

demineralization of the central magnesium atom

from the molecule. (See Colorants (Colourants):

Properties and Determination of Natural Pigments.)

0033 Sensitivity to pH is exhibited by other natural pig-

ments. Anthocyanins, for example, change from red

(in acid conditions), through colorless (when neutral)

to blue (when alkaline). This can result in color changes

when heat disrupts cells and allows these water-soluble

pigments to mix with the cooking water.

0034 Browning reactions tend to occur at low levels of

available water. They are therefore common in dry

cooking where the surface layers may dry out.

Caramelization occurs when concentrated sugars are

heated, particularly in the presence of acids or alkalis.

Caramelization is of importance in both sugar and

flour confectionery. The most famous of these reac-

tions is Maillard browning, which involves the reac-

tion between carbonyl groups (as found in reducing

sugars) and amino groups. The reaction gives rise to

a variety of brown compounds and characteristic

aromas. Simple mixtures of one amino acid and one

reducing sugar have been shown to produce distinct-

ive aromas, reminiscent of particular foods (e.g., glu-

cose and cysteine heated at 180



C for 30 s smells of

puffed wheat, yet after 3 min it smells of overroasted

meat). (See Browning: Nonenzymatic.)

0035Generally speaking, volatile components are lost

during cooking, and this can result in a loss of the

uncooked flavor. Since domestic cooking is carried

out by the end user, loss of volatiles is not completely

wasted and may act as an appetizer.

0036Sustained heating of dry surfaces can result in car-

bonization, usually accompanied by the generation of

smoke. In such circumstances the surface browns,

then blackens as it burns; such products are usually

regarded as spoiled.

Nutritional Changes

0037Although the act of cooking involves raising the tem-

perature of the food, the term is frequently undifferen-

tiated from other food preparation procedures, many

of which commonly precede cooking. Peeling and trim-

ming are two such operations which can lead to sub-

stantial loss of available nutrients, by cutting unsightly

portions off the food and throwing them away!

0038Nutrient loss during cooking is attributable to two

basic routes: thermally induced chemical reactions,

and leaching of nutrients into the cooking medium.

0039Many nutrients are thermally unstable and when

heated their concentration falls exponentially with

time. Obviously, different nutrients have their own

rates of destruction. The most sensitive vitamins are

ascorbic acid (vitamin C) and folic acid, both of

which can be completely destroyed by domestic

cooking. Of the essential amino acids, lysine is the

least stable to heat and up to 40% may be lost by

domestic cooking practices. In general, rapid cooking

methods, using high temperature for short times, or

microwaves cause less nutrient destruction than long-

duration, low-temperature cooking methods such as

stewing. (See Amino Acids: Properties and Occur-

rence; Ascorbic Acid: Properties and Determination;

Folic Acid: Properties and Determination.) High-

temperature short time processes such as frying

can result in similar levels of vitamin C, as in raw

potatoes.

0040In addition to thermal decomposition, nutrients

can be lost by reacting with each other. For example,

proteins will participate in Maillard reactions, par-

ticularly when E-amino groups are present.

0041Sodium bicarbonate is sometimes added to vege-

tables for its softening effect. Unfortunately, its

1626 COOKING/Domestic Techniques

addition leads to the destruction of vitamin C, as well

as chemically modifying proteins, lowering their

biological value.

0042 Cooking processes are not entirely detrimental

from a nutritional point of view. For example, frying

can result in an uptake of oil, resulting in an increased

energy density in foods.

Safety Aspects

0043 A common misconception is that domestic cooking is

performed purely to improve the gastronomic experi-

ence at the detriment of the food’s biological value. In

fact, domestic cooking can make safe products

which would otherwise contain harmful or toxic

components.

0044 Harmful components in food arise in the form of

naturally present toxins (e.g., cyanides in kidney

beans and cassava), or compounds that interfere

with digestion, effectively making the food less nutri-

tious (e.g., trypsin inhibitors found in many legumes).

The presence of pathogenic microorganisms may lead

to infections, while other microorganisms produce

toxins, both of which result in food poisoning. (See

Hemagglutinins (Haemagglutinins); Plant Antinutri-

tional Factors: Detoxification; Trypsin Inhibitors.)

0045 Cooking is effective at destroying or removing

many of these harmful components. However, some

toxins are relatively heat-stable (e.g., the toxin pro-

duced by Staphylococcus aureus) and may not be

destroyed by the temperatures and times incurred

during domestic cooking. It is of course likely that

untrained domestic cooks will be oblivious to the

risks involved in eating particular foods, or to the

neutralizing effect of cooking. A further risk arises

from cross-contamination of cooked food by raw

ingredients, resulting in a potential for fresh growth

of pathogenic microorganisms.

See also: Amino Acids: Properties and Occurrence;

Ascorbic Acid: Properties and Determination;

Browning: Nonenzymatic; Carbohydrates: Interactions

with Other Food Components; Colorants (Colourants):

Properties and Determination of Natural Pigments; Folic

Acid: Properties and Determination; Hemagglutinins

(Haemagglutinins); Plant Antinutritional Factors:

Detoxification; Protein: Functional Properties;

Interactions and Reactions Involved in Food Processing;

Starch: Structure, Properties, and Determination;

Trypsin Inhibitors

Further Reading

McGee H (1991) On Food and Cooking: The Science and

Lore of the Kitchen, 2nd edn. London: Harper Collins.

Domestic Use of Microwave

Ovens

T Ohlsson, SIK – The Swedish Institute for Food and

Biotechnology, Gothenburg, Sweden

Microwave Ovens

0001Towards the end of the 1970s, sales of domestic

microwave ovens started to grow rapidly in Japan, a

couple of years later in the USA, and at the end of the

1980s also in western Europe. The domestic micro-

wave oven industry has now grown into a world-wide

multibillion dollar business, with about 24 million

ovens being sold annually. On many markets, the

microwave oven is the home appliance with the

largest sales volume. It is estimated that there are

about 225 million microwave ovens in the homes in

Japan, the USA and western Europe. User studies

have shown that it is predominantly the affluent,

‘middle-aged parents with kids’ families that own

microwave ovens.

0002This population of microwave oven owners repre-

sents a large potential market for food products that

can be microwave-heated. The food industry has

quickly responded to these market possibilities. The

number of new product introductions for the micro-

wave food segment has grown rapidly on many

markets; e.g. in the USA during the middle and later

part of the 1980s.

0003Microwave and microwaveable food today consti-

tute an important segment of the food market in

Japan, the USA and, increasingly, western Europe.

The microwave food segment comprises not only

prepared food items, such as meals and side dishes,

but also products specially made for microwave

heating, the most striking example being the micro-

wave popcorn bag, developed by Golden Valley

Microwave Foods in the early 1980s. It is today a

billion-dollar market, alone. The presence of micro-

wave ovens in the homes affects the preparation or

cooking instruction for more or less all packed food

products, even those not traditionally associated with

microwave heating.

0004The growing number of microwave ovens and

microwave foods has also greatly influenced the food

packaging industry. Requirements of microwave com-

patibility have greatly changed the way in which the

food industry selects food packaging. For prepared

foods, microwave-transparent plastic and paper-

board packaging has grown at the expense of alumi-

num packaging. A vast number of packaging designs,

specific to microwave heating, have been developed.

COOKING/Domestic Use of Microwave Ovens 1627

The most striking example is the so-called susceptor

package, in which an extremely thin metallized

film on a PET film is heated by the microwaves to

temperatures sufficient to crisp or brown pizza or

pie crusts, or to pop popcorn.

0005 The food technology differences between micro-

wave and conventional heating are manifested by

differences in development of food flavor and aroma

and of food texture and appearance. Thus, modified

recipes and special ‘microwave-adapted’ food ingre-

dients have been developed by the industry to over-

come at least in part some of the quality problems of

microwave-heated foods.

0006 This very substantial development of foods, pack-

aging, and ingredients for microwave heating has

given an increased general knowledge about micro-

wave heating in the industry. Much of this knowledge

is empirical, but a clear trend towards more scientific-

ally based R&D has been seen in later years.

Heating Mechanism

What are Microwaves and How Do They Heat

Foods?

0007 Microwaves are electromagnetic waves in the fre-

quency range from 300 to 300 000 MHz, correspond-

ing to wavelengths from 1 m to 1 mm. For domestic

food applications we are, however, limited to the

ISM (Industrial, Scientific, Medical) band of 2450

+ 50 MHz.

0008 The heating of foods by microwave energy is

accomplished both by the absorption of microwave

energy by rotation of the dipolar water molecules and

by translation of the ionic components of the food.

This energy is converted into heat. Thus, both the

water content and the dissolved ion content (often

salt) are dominating factors in the microwave heating

of foods. When the dipolar water molecule is sub-

jected to a microwave field that rapidly changes

direction, the dipole tries to align itself with the field

direction. There is a time lag, as some response time

is required for the water molecule to overcome

the inertia and the intermolecular forces in the

water. The electric field thus provides energy for

the water molecule to rotate into alignment. The

energy is then lost to the random thermal motion of

the water and equivalent to a temperature rise.

0009 The energy-transfer mechanism will be efficient

only if the time between the changes of direction of

the electric field is so short that the dipolar molecule

aggregates can barely follow the changes. If the time

is long (frequency low), the alignment will be good

and the energy transfer low. If the time is short

(frequency high), the aggregates will not move much

between field reversals, and the energy-transfer rate

will again be low. Since the statistical number of

water molecules that are bound together by hydrogen

bonding will decrease with increasing temperature, so

the inertia and energy release will be reduced. As the

frequency in dielectric heating equipment will be

maintained constant and below the energy transfer

efficiency optimum, the efficiency will decrease with

increasing temperature.

0010Hydrated ions, such as sodium and chloride from

table salt, try to move in the direction of the electrical

field, and an electric resistance heating effect is

accomplished. The ions are surrounded by water

molecules and will, in the movement, transfer energy

randomly to the water molecules. These are more

mobile at higher temperature and not so tightly

bound to ions. The ions can then move more freely

and absorb and dissipate more energy. The conduct-

ive heating due to dissolved ions increases with

increasing temperature.

0011When a microwave impinges on a food surface,

some of it will be reflected, and some will be trans-

mitted and penetrate into the food and gradually be

absorbed, while generating heat. Materials can be

classified into reflecting, absorbing, and transparent,

according to their interaction with the dielectric field.

Reflecting materials are mostly metals, where the

electric field creates surface currents that only pene-

trate a few microns into the material. Transparent

materials, however, do not absorb the energy to any

significant extent. Glass and most plastic materials

are typical examples of this. Materials that will

absorb microwave energy, according to the heating

mechanism explained above, are those containing

polar constituents, predominantly water.

0012The penetration depth (as calculated from dielec-

tric data) for water and three food materials is shown

in Figure 1 as a function of temperature. It can be seen

that microwave penetration in water rapidly increases

with temperature. For beef, the penetration depth is

very high in the frozen state, rapidly decreasing during

defrosting and leveling out with further temperature

increases, whereas the penetration depth continues to

decrease with rising temperature for salty ham.

0013Frozen foods show much lower dielectric proper-

ties as compared with thawed foods. Most of the

water in frozen foods is present as ice crystals inside

the food, and pure ice has very low dielectric proper-

ties. However, approximately 10% of the water

remains unfrozen as a strong salt solution in the

food, thus explaining why frozen foods absorb micro-

wave energy. There is a marked jump in dielectric loss

in the melting region, which partly explains the ten-

dency towards so-called run-away heating in micro-

wave thawing of foods, where already thawed parts

1628 COOKING/Domestic Use of Microwave Ovens

absorb most of the available microwave energy

because of their higher loss factor.

0014 For nonfrozen foods, dielectric loss tends to

decrease with increasing temperature, more pro-

nounced so, the higher the water content and the

lower the ionic content. This will tend to slow down

runaway heating in thawing such foods. Salty foods,

however, demonstrate a pronounced increase in di-

electric loss with increasing temperature due to con-

ductive losses. Since most foods contain some ionic

material, the reduction in dipolar absorption with

increasing temperature tends to be partly balanced

by increasing conductive losses.

0015 The actual temperature rise in microwave heating

depends not only on the energy released in the food

but also on its thermal properties, as apparent from

the following energy balance:

Pt ¼ mc

T or Pt ¼ mH,

where P is power in watts; t is time in seconds; c

specific heat; m is mass in grams; DT is temperature

increase in degrees Celsius; and DH is the change in

enthalpy corresponding to the temperature rise.This

means that low microwave absorption can still lead to

a considerable temperature rise in foods of low

specific heat, such as fats and oils.

0016 The release of microwave energy in foods is largely

dependent on both the microwave oven field and the

microwave penetration pattern in the particular food

material. The field distribution is a function of both

the type of microwave oven and the manner of intro-

ducing the microwaves into it, as well as the type,

shape, and distribution of the food inside.

0017In a rectangular slab of food of even thickness, the

power level will gradually decrease inwards to an

insignificant level if the slab thickness is large, in

comparison with the calculated penetration depth.

0018For layered materials of different dielectric proper-

ties, microwaves will be reflected and refracted also at

the interface between these materials, for example

between meat and an outer layer of fat. Depending

on the thickness and dielectric properties of the layers,

standing wave patterns may develop, with maxima

and minima. The result can be that the fat layer will

be overheated, in spite of its lower dielectric loss

factor. A contributing factor will then be the much

lower specific heat of the fat material.

0019In a slab with sharp corners and edges protruding

into the microwave field, energy concentrations will

occur there, causing selective heating, especially at

the corners. A sharp edge or corner will act as an

antenna and attract more energy than surrounding

areas.

0020When microwaves impinge on to a food surface at

different angles, part of the energy is reflected, part

refracted. Refracted energy will be partially

absorbed. Most of the remaining energy will be re-

flected back at the other food surface and so on.

Depending on the food geometry, the result can be

focusing of energy to certain areas, which may be part

of the explanation for concentration heating effects.

0021For spherical or cylindrical shapes, the result can be

a concentration of energy to the center of the food,

depending both on the food diameter and on its

dielectric properties. This central heating effect

occurs for diameters approximately one to three

times the penetration depth in the material. For cylin-

ders, concentration effects occur when the electrical

field is parallel to the cylinder axis. The effect is

stronger for foods with high values for its dielectric

properties. At 2450 MHz, central heating usually

happens at diameters between 25 and 55 mm, a

common food size.

Heating Effects

Microbial Effects

0022The basis for the application of microwaves to food is

the thermal heating effects caused by a combination

of dipole rotation and electrical resistance heating.

Over the years, there has been considerable discus-

sion about the possibility of additional, nonthermal

or per se, effects, from the exposure of the food to

microwave fields.

0 20406080100

Temperature (8C)

Penetration depth (mm)

Water

Peas

Beef

Ham

fig0001 Figure 1 Penetration depth of foods at 2450 MHz.

COOKING/Domestic Use of Microwave Ovens 1629

0023 In what ways, if any, will the effects on micro-

organisms of microwave heating differ from that of

conventional heating methods?

0024 The problems of insufficient microbiological in-

activation in microwave reheating of chilled foods

were highlighted by a study on survival of Listeria

in ready meals heated according to recommended

cooking instructions in the UK.

0025 The study gave rise to much concern about the

microbiological safety of foods heated in microwave

ovens. The requirements that all parts of the food

must reach 70



C have been made clear to all parties

involved, as a result of the UK microwave oven safety

concerns.

0026 It had already been claimed in the early 1960s by

some researchers that microwave heating may have

special, nonthermal effects on microorganisms, per-

mitting sterility to be reached at considerably lower

temperatures and with shorter treatment times than

by other means of heat processing.

0027 A number of comprehensive studies on micro-

organisms have been reported, which demonstrate

the absence of any other effects than purely thermal.

0028 The conclusion reached in 1989 by an IFT expert

panel on Food Safety and Nutrition was that it is

generally recognized that microwave interactions are

solely due to thermal effects.

0029 In 1989, results were published on exposure of

milk, claiming the formation of unacceptable levels

of d-proline and cis-hydroxyproline, involving a risk

of brain damage in infants. However, the experi-

mental conditions were subject to grave doubts, and

efforts by other research groups to duplicate his

results have been futile, even under the very excessive

microwave heating conditions used.

Nutritional Effects

0030 There are several good reviews available on the nutri-

tional effects of microwave exposure. The general

conclusion is that no significant nutritional differ-

ences exist between foods prepared conventionally

or by microwave heating.

0031 As regards the nutritional quality after comparable

degrees of conventional and microwave heating, it

was concluded that protein quality is retained in

microwave heating of animal foods and improved in

vegetables. Since no crust is formed in microwave

cooking, the availability of amino acids, especially

of lysine, is high. The content of fat in fatty details

of meat and pork may decrease. Minerals and vita-

mins such as potassium and vitamins B

and C, are

better retained in microwave heating as the cooking is

done in a minimum amount of water – provided

excessive heating is avoided.

Sensory Effects

0032Microwave-heated foods demonstrate more or less

pronounced differences in appearance, flavor, and

texture from conventionally heated foods. This is a

result of the differences in temperature, moisture, and

pressure gradients already discussed, and the related

changes in structure and binding properties. Absence

of surface color can be avoided only by combining

microwave heating with other surface heating by hot

air or by the use of susceptors.

0033In conventional cooking, a combination of enzym-

atic and chemical reactions is given time to develop.

These reactions cause textural changes such as

softening, firming, and tenderizing, and the forma-

tion and/or breakdown of flavor substances to give

the combined flavor that is traditionally regarded as

the normal or optimal. The absence of surface drying

and Maillard browning reactions in microwave

heating will also mean the absence of flavor notes

that are very important to meat and bread.

The Microwave Oven

0034It is estimated that there are more than 225 million

microwave ovens in homes in the industrialized world

today. Penetration into households has reached above

100% in the USA, Japan, and Australia and above

80% in the UK and the Nordic countries in Europe.

There are hundreds of models on the market in a wide

range of design, control features, sophistication, and

price. The basic oven shape and components are

illustrated in Figure 2.

0035Alternating current from the mains is transformed

to around 4 kV and rectified to direct current in the

power pack. This is fed to the magnetron tube, in

which the electric energy is transformed into micro-

wave energy. This, in turn, is transferred through a

waveguide to the rectangular oven space, the so-

called microwave cavity. The cavity is a closed metal

box, designed to prevent any external leakage of

microwave energy. It is fitted with a door with micro-

wave seals and often has a microwave tight metal

screen for visibility of the oven interior during

heating. The food to be heated is positioned on a

bottom plate of microwave transparent material or

on a rotating plate, or turntable.

0036Differences can be considerable between the

multitude of oven makes and models on the market

today, both in generated microwave power and in

field distribution. Commonly, microwave output

power lies between 500 and 1000 W for domestic

ovens. Control features range from a simple timer

and a start–stop button to weight and temperature

sensors and microchip-based programing. The

1630 COOKING/Domestic Use of Microwave Ovens

dimensions of domestic ovens are normally

30 30 25 cm, dimensions that are of comparable

magnitude to the wavelength in air at 2450 MHz

(12 cm), which is the only frequency in use today

for domestic microwave ovens. There is a growing

trend to combine microwaves with a conventional

heat source, such as hot air convection heating.

Similar combination ovens already enjoy a substan-

tial share of the domestic oven market on the

European continent.

0037 Energy is fed into the cavity by means of some kind

of waveguide and antenna arrangement or opening

slot in the roof or upper side wall of the cavity. The

arrangement is designed to distribute the energy effi-

ciently and evenly inside the cavity to enable rapid

and uniform heating of a wide range of food loads.

0038 In the oven space, a limited number of standing

wave patterns can be generated as a result of multiple

reflections at the metal cavity walls, usually not more

than three to five different patterns. This means that

the field is not entirely evenly distributed in the three

dimensions, but demonstrates a pattern of maxima

and minima. At the metal surfaces, the field strength

is always zero. The field pattern is continuously modi-

fied by using a mode stirrer, a rotating propeller-like

device, that will help a number of the possible modes

to exist in the oven for certain time intervals. Alterna-

tively, field differences can be compensated for by

moving the food through the field on a rotating turn-

table at the bottom of the oven space. Most ovens use

a combination of direct microwave radiation from

the in-feed region and a multimode standing wave

pattern in the oven.

0039In addition, ovens are often designed to contain

devices that to some extent direct microwaves into

the central area of the oven shelf, where foods are

placed for heating. These so-called confined modes

between the food and the oven bottom have been

shown recently to be of great importance for develop-

ing ovens with more uniform heating patterns. The

major reason for this is a promotion of bottom heating

through promotion of the propagation of waves with

an exceptionally long horizontal wavelength.

0040When food material is introduced into the oven

cavity, the food composition, geometry of the food

and package, and the positioning of the food in the

oven will affect the field distribution compared with

that in an empty oven.

0041Computer simulations of the microwave field and

food temperature distribution have been used to

evaluate the influence of the large number of product

and oven factors since 1971, when a one-dimensional

temperature profile simulation was presented. In

recent years, thanks to the vast improvements in

computer calculation power, numerical simulations

of the three-dimensional microwave field pattern are

possible, even on PCs. Both the FEM (Finite Element

Method) and the FDTD (Finite Diference Time

Domain) method is used. The FDTD method offers

advantages of lower computer requirements and the

simplicity of understanding the method and of ‘build-

ing’ models. However, the uniform grid (element) size

Viewing

screen

Door

Cooling fan

Air flow

Filament

transformer

Power

transformer

Capacitor

Diode

Magnetron

Exhaust air

Waveguide

Stirrer

Exhaust air

Feed box

fig0002 Figure 2 The microwave oven.

COOKING/Domestic Use of Microwave Ovens 1631

gives unnecessarily large matrices. The simulations

give primarily the normalized electric field distribu-

tion. The translation to temperature patterns will also

require modeling of the power dissipation of the

oven.

0042 Computer simulations will be an important field

for research in the future, for improving the under-

standing of the complex interaction of the variables

influencing the microwave field during heating, as

well as for the optimization of oven design and micro-

wave foods.

0043 The rate of heating for a single food component in

a microwave oven is primarily a function of the avail-

able microwave power, but the sample weight and the

dielectric properties of the sample will to some extent

affect this available power. Normally, available

power will diminish with decreasing sample weight.

Another factor that influences the available oven

power is the line voltage.

0044 The recognized standard procedure today for

determining oven power output is the International

Electrotechnical Commission (IEC) standard, which

is based on heating a 1000 g water load, from 10 to



C under precisely controlled conditions. The IEC

has also issued an internationally recognized standard

performance test for microwave ovens.

Microwave Foods and Packaging

0045 Taking into account that microwave ovens are

designed to give as even a field distribution as possible

in the loaded oven with minimal corner and edge

overheating, then food composition, geometry, and

positioning as well as packaging will constitute the

main remaining factors that determine the heating

performance and resulting food quality.

0046 For slab-shaped foods, the product thickness

should be even, and preferably limited to less than

2.5 times its microwave penetration depth. Rounded

edges and corners will limit preferential corner and

edge heating tendencies. The temperature variations

over large, flat surfaces of food, in terms of hot and

cold spots, are normally larger than the in-depth tem-

perature variation, and may have a greater influence

on the overall heating results.

0047 When heating food components of different dielec-

tric and thermal properties together, such as in meals

on a tray or a plate with different compartments, care

must be taken in selecting the shape and relative

arrangement of the components so as to optimize the

heating uniformity. In this context, advantage can

be taken of the central focusing tendencies of rounded

(cylindrical or semispherical) geometry for thick food

samples of limited penetration depth. Food compon-

ents with a high dielectric loss, such as meat stew, can

be partially shielded by components of lower loss

(such as mashed potato) to even out the temperature

distribution. Material with a low dielectric loss is less

susceptible to corner and edge overheating, etc. By

adjusting the thickness of the material with a low

dielectric loss, the formation of variable standing

wave patterns inside the food can be kept to a

minimum.

0048In microwave heating, packaging is an integral part

of the food product. Packaging composition, shape,

and incorporated active functions all contribute to

the heating result, in direct interaction with the com-

position, properties, geometry, and positioning of the

food inside.

0049The packaging material should permit tempera-

tures of at least 120



C without affecting the handling

stability or food protective properties. For low-

moisture foods high in oil or sugar, the temperature

stability may have to be extended up to 150



C, be-

cause these foods will easily overheat in a microwave

field. To permit reheating also in a conventional oven

(dual ovenable), a temperature resistance of up to

200



C is required.

0050The main packaging alternatives are:

0051polymer-coated paperboard;

0052mono- or multilayer polymers, laminated or

coextruded;

0053aluminum foil or foil- and polymer-laminated board;

0054glass.

Of the polymer materials in use, PP (polypropylene)

will meet the minimum temperature stability require-

ment, whereas APET (polyester) can be used up to

200–250



0055The ideal food package shape is round or oval with

vertical sides and rounded edges to limit sharp

corners and edges to a minimum. Rectangular con-

tainers may be required for practical reasons but

should have rounded corners and edges to reduce

preferential heating tendencies.

0056Metals reflect microwaves completely, and food

areas close to a metal surface will not be micro-

wave-heated at all. Also, the presence of metal in the

package or other metal objects in the microwave

oven will markedly influence the microwave field

distribution.

0057Previously, recommendations have been to avoid

metal trays or foil laminated packages in microwave

heating, because they were likely to cause damage to

the magnetron, due to excessive reflection of energy.

Since modern magnetrons are considerably less sensi-

tive, aluminum containers, with transparent covers

may now be used safely, provided they are not

positioned so close ( 2 mm) to the oven wall that

electrical discharges or sparks can occur.

1632 COOKING/Domestic Use of Microwave Ovens

0058 The metal changes the mode pattern in the oven

and thus influences the heating result. This may even

be improved compared with heating in a transparent

package. Because microwaves can now penetrate

only from the top and not from the sides, corner and

edge overheating is eliminated, even to the extent that

these parts may remain cooler than the central parts

of the food.

0059 The product thickness will be limited to about half

the penetration depth of the food material, to permit

sufficient heating of the bottom layer. The property

of metal to completely reflect microwaves can be

utilized in the design of packages, which minimize

heating differences between food components of dif-

ferent physical properties, shape or thickness. A pat-

tern of aluminum foil can be applied on the package

to screen areas that would otherwise overheat, but

permit full penetration into areas that would other-

wise remain cooler.

0060 The use of susceptors was briefly mentioned earlier

in this article. Susceptors, or receptors as they have

also been called, are extremely thin films of metal,

vacuum-deposited on PET film with paperboard

backing. At a metal film thickness of only 25–50 nm,

part of the impinging microwave energy is transmit-

ted, part reflected, and a substantial part absorbed

generating heat and leading to high temperature in

the metal film. When such a susceptor film is applied

a few millimeters from the food surface, the generated

heat will transfer rapidly to the food and heat the

food surface. The surface temperature reached will

be sufficient to reverse the temperature and moisture

profile in the food and to crisp and brown the food

surface, like in conventional grilling. Susceptor tem-

peratures will usually not exceed 115–130



C when in

close contact with the food, but may otherwise reach

temperatures up to and above 250



C. At such high

temperatures, there will be some risk of migration of

substances into the food from the plastic film and

paperboard backing. Therefore, susceptors are being

developed that have a built-in temperature-limiting

function.

0061 There has been much concern for migration of

hazardous chemical components from susceptor

packaging material. Extensive research in the USA,

UK, Germany, The Netherlands, and Sweden have

demonstrated, however, that microwave heating itself

does not increase migration rates.

Future Outlook

0062 The market for domestic microwave ovens will

continue to grow world-wide, even though a near-

saturation point has already been reached in a few

countries, like the USA and Japan, where all house-

holds already have one or more ovens.

0063The heating performance and consistency of micro-

wave ovens will improve gradually as a result of the

ongoing international standardization of perform-

ance and output power testing methods. At the same

time, the growth in fundamental knowledge on field

distribution and the interactions between oven design,

microwave field, and food will lead to improved oven

performance.

0064Packaging development continues in materials,

shape, shielding, susceptors, and other forms of active

packaging, in combination with a product develop-

ment that increasingly takes into account dielectric

and thermal properties of foods and their interaction

with the microwave field.

0065There will also be important improvements in oven

components, such as solid-state components, which

will substantially lower both oven weight, size, and

production costs. In Europe, at least, the proportion

of combination ovens, combining microwaves with

heat by convection or radiation, will continue to grow.

See also: Browning: Nonenzymatic; Packaging:

Packaging of Liquids; Packaging of Solids

Further Reading

Buffler CR (1993) Microwave cooking and processing. In:

Engineering Fundamentals for the Food Scientist. New

York: AVI, Van Nostrand Reinhold.

Cross GA and Fung DYC (1982) The effect of microwaves

on nutrient value of foods. CRC Critical Reviews in

Food Science and Nutrition 16: 355–381.

Jauchem J (1991) Alleged health effects of electromagnetic

fields: Misconceptions in the scientific literature. Journal

of Microwave Power and Electromagnetic Energy 26(3):

189–195.

Mudgett R (1985) Dielectric properties of foods. In: Decar-

eau R (ed.) Microwaves in the Food Processing Industry.

London: Academic Press, pp. 15–37.

Mudgett RE (1989) Microwave food processing. A scien-

tific status summary by the IFT’s expert panel on food

safety and nutrition. Food Technology 43: 117–126.

Ohlsson T (1983) Fundamentals of microwave cooking.

Microwave World 4(2): 4–9.

Risman PO (1993) Microwave food hazards. Microwave

World 14: 21–23.

Sacharow S and Schiffmann RF (1992) Microwave Pack-

aging. Leatherhead, UK: Pira International.

Sundberg M (1998) Analysis of industrial microwave

ovens. Doctorate thesis, Department of Microwave

Technology, Chalmers University of Technology and

SIK – The Swedish Institute for Food and Biotechnology,

Gothenburg.

Walker J (1987) The secret of a microwave oven’srapid

cooking is disclosed. Scientific American 258(2): 98–102.

COOKING/Domestic Use of Microwave Ovens 1633

COPPER

Contents

Properties and Determination

Physiology

Properties and Determination

R Barbera

, R Farre

and M J Lagarda, University of

Valencia, Burjassot, Spain

Physical and Chemical Properties

0001 Copper is a tough, soft, ductile, reddish metal, with

an atomic number of 29, a relative atomic mass of

63.54 amu, an electronic configuration 1s

and a relative density of 8.95. The

melting and boiling points are 1083 and 2595



respectively.

0002 Copper is the 25th most abundant element in the

earth’s crust, comprising about 0.01%, and has two

natural isotopes,

Cu and

Cu, with isotopic abun-

dances of 68.94 and 31.06%, respectively. In total, 11

isotopes are known. Copper is found in its native state

and also as a component of sulfides, oxides, and

carbonates. Copper is used in alloys such as brasses

and is completely miscible with gold (Au). It can be

only superficially oxidized in air, sometimes acquiring

a green coating of hydroxo carbonate and hydroxo

sulfate.

0003 The principal states of copper oxidation are þ1and

þ2. However, Cu(I) is not stable in solution, since the

disproportionation reaction has an equilibrium con-

stant of 1.1 10

1 mol

1

. The complex formation

can stabilize Cu(I) with the complex formed

with cyanide presenting a high stability. Most Cu(I)

compounds are fairly easily oxidized to Cu(II) com-

pounds, but further oxidation to Cu(III) is more

difficult.

0004 The most common oxidation state, especially in

aqueous solution, is þ2. In diluted acid solution,

Cu(II) precipitates as the sulfide (solubility product

35

mol

2

) with hydrogen sulfide or ammonium

sulfide. Copper dissolves readily in HNO

and H

in the presence of O

, and it is soluble in

OH, ammonium carbonate, or KCN solution.

Most Cu(II) salts dissolve readily in water.

0005 Cu(I) compounds are diamagnetic and, except

where color results from the anion or charge-transfer

band, colorless. Cu(II) complexes and compounds

are blue or green, except for red or brown species.

The blue or green colors are due to the presence of an

absorption band in the 600–900-nm region of the

spectrum.

Determination in Foods and Beverages

0006The copper content in foods and beverages is in the

order of micrograms per gram (mgg

1

0007Atomic absorption spectrometry (AAS) is probably

the most commonly used technique of the wide range

available for the analysis of copper in foods

and beverages. When the contents are sufficiently

high, flame atomic absorption spectrometry (FAAS)

is used, and when they are low, electrothermal atomic

absorption spectrometry (ETAAS) is employed.

Whenever possible, FAAS is the technique of choice,

since it is less time consuming and less sensitive to

interference (background absorption).

0008The widespread distribution of copper is a source

of problems of contamination, and as a result, special

care must be taken when dealing with samples con-

taining very low levels, as occurs with some foods,

bearing in mind that the lower the concentration, the

higher the risk of contamination. Wherever possible,

plastic instruments should be used to prepare bio-

logical samples whose copper contents are low. In

order to prevent contamination of samples, all the

glass- and plastic-ware used must be thoroughly

washed with diluted nitric acid. Blanks should be

used to check the absence of contamination. To

ensure the quality of the results, certified reference

materials of a similar nature to the material being

analyzed should be used. This also allows any suspi-

cion of contamination to be eliminated.

Sample Digestion

0009Samples of most types of food and beverage require a

prior preparation in order to convert the sample into

solution before analysis. The choice of the procedure

depends on the element to be determined, the com-

plexity of the matrix, and the analytical technique to

be applied.

1634 COPPER/Properties and Determination

0010 This preparation can be omitted in some samples,

especially in beverages, such as tap water or wine,

where, in the latter, a simple dilution of the sample

can be sufficient. Nevertheless, in most food and

beverage samples, the destruction of the organic

matter is necessary to a greater or lesser degree,

depending on the technique of analysis to be applied.

0011 The sample preparation is a critical step in trace-

element analysis. The classic standard methods for

organic-matter destruction are: wet digestion by

conductive heating (hot plate, digestion block) or

dry ashing (muffle furnace) at a defined temperature.

Wet-digestion methods are generally more rapid than

dry digestion methods. The disadvantages are that

only fairly small samples can be used and also

that the digests normally have to be heavily diluted

before the analysis, resulting in poor detection limits.

These techniques usually use mineral acids, oxidants,

and ashing aids. Wet digestions can be carried out

with a concentrated acid or mixture of acids, in

open or closed systems. Since 1975, microwave-

assisted sample digestion has been increasingly used

for sample preparation in trace-element determin-

ation in food samples.

0012 The advantages of microwave digestion over con-

ventional sample preparation techniques are: a reduc-

tion in the time needed for the sample preparation and

also of the amounts of acids used for digestion,

and the possibility of controlling pressure/tempera-

ture, automation, and flexibility in sample prepar-

ation. Two types of microwave digestion systems are

now available for wet digestions: closed-vessel micro-

wave and open-vessel focused microwave.

0013 Each of these techniques offers specific advantages.

Closed systems can be used to prepare samples under

controlled digestion conditions with a minimal volume

of acid and a reduced risk of contamination. Nitric,

hydrochloric, and hydrofluoric acids and their

mixtures are the most widely used with closed digestion

vessels. Nitric acid is usually added due to its antioxi-

dant properties, and hydrofluoric acid is used when

siliceous matter is present. The addition of hydrogen

peroxide to nitric acid is also recommended.

0014 Automated focused open-vessel microwave systems

operate at ambient pressure and offer flexibility in

digestion conditions, with vessels and reflux columns

available in borosilicate glass, Teflon perfluoralkoxy,

and quartz. These systems can be used to prepare

samples sequentially with minimal attended operation,

since digestion reagents are added automatically in

programmed volumes and dispensing rates. To minim-

ize analyte losses, a ramped, multistage heating pro-

gram can be used in addition to reflux columns with the

digestion vessels. When compared with the closed

vessel system, the open-vessel system can process

large samples, which leads to lower detection limits.

High-pressure and high-temperature conditions are not

required to obtain a complete digestion when an instru-

mental technique such as ETAAS is used for trace-

element analysis.

0015Precautions should be adopted in all wet-digestion

procedures when strong acids and strong oxidizing

agents are used. The formation of explosive nitro

compounds has been reported when fat-rich samples

have been digested with nitric acid. Whichever wet

digestion procedure is applied, the digests are left to

cool and diluted with water.

0016Dry digestion or ashing procedures are also widely

used to destroy organic matter. The main problems

with these procedures are the possible losses of ana-

lyte by volatilization or by retention on crucible walls

during dry ashing. Although several studies strongly

indicate that ashing at temperatures up to 450



Cis

free of losses by volatilization or retention, it is rec-

ommended that the temperature be increased grad-

ually up to 400–500



C and maintained until white

ashes are obtained. Dry ashing is time-consuming,

usually taking a day or more, although it requires

very little attention from the analyst. The resulting

ash can be dissolved in a small amount of solvent, and

contamination through reagents is scarce.

0017To eliminate potential losses and/or to speed up the

ashing procedure, ashing aids and matrix modifiers

can be used. This, however, always increases the risk

of contamination and worsens detection limits. Ashes

are usually dissolved in diluted HCl or HNO

and it is

sometimes necessary to heat them with concentrated

HNO

and HClO

until no more fumes are produced,

in order to complete the destruction of the organic

matter. In some cases, organic-matter destruction can

be replaced by acid extractions of the food product.

Hydrochloric acid, nitric acid, or a mixture is used as

the extractant, and the use of an ultrasonic bath

facilitates the extraction.

0018Some analytical techniques that are useful for

copper determination, such as ETAAS, do not require

organic-matter destruction in some kinds of food.

Nevertheless, even when this is not the case, a com-

plete digestion, minimizing residual carbon content,

is not required. The suppression of the digestion step

will reduce the time needed for the analysis and the

risk of losses and contaminations. Organic-matter

destruction and dilution or dissolution of the residue

can be replaced by:

0019direct injection of the sample (water) or after the

appropriate dilution (wine);

0020dissolution of the sample in an adequate solvent,

e.g., butter in the mixture composed of butylamin/

water/tetrahydrofuran (48/12/40);

COPPER/Properties and Determination 1635