Yam, Kit L. (ed.). The Wiley encyclopedia of packaging technology

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Borden Global Packaging, UK, has recently announced

the availability of their version of foamed CPET under the

trade name Ovenex Lite. Trays made of Ovenex Lite

contain 25–33% less material than their solid counter-

parts. Commercial availability in stock sizes are available

from Borden in mid-1996. Borden uses their own process

for foaming CPET for which a patent is pending.

Thermoset Polyester. Thermoset polyester plates were

the ﬁrst commercial application of dual-ovenable materi-

als when used for frozen meals in the mid-1980s. The

compound used is an unsaturated polyester that is highly

ﬁlled with minerals such as talc and calcium carbonate,

along with glass ﬁbers and catalyst materials to produce

the chemical reaction to convert the compound into mate-

rial that is irreversibly set. The polyester compound is

mixed and extruded to form logs that are cut to the proper

size and weight for the ﬁnished container. The material is

placed into a heated mold in a hydraulic press that closes

the mold. The pressure causes the material to ﬂow into the

shape of the mold, and the heat cures the compound while

under pressure into the ﬁnished and irreversible material.

Typically, the container must be sanded to remove any

ﬂashing around the edges and often is run through a

conveyor oven for a postbake cycle to drive off any residual

uncured compound, and washed to remove any dust from

sanding. This process produces containers that are very

strong and stiff, even at high temperatures (4251F), and

are heavy with a china-like feel and appearance. Unlike

thermoforming from a sheet of material with consistent

thickness, thus producing containers having essentially

the same material thickness throughout, compression

molding permits the ﬁnished containers to have varying

degrees of thickness throughout the dimension to add

strength or design features where desired. Because of

the amount of material used, the multiple steps in the

manufacturing process, and the relatively low output per

machine cycle for compression molding presses, the price

for a thermoset polyester plate is proportionally higher

than competitive thermoformed plastic materials that can

be produced at much higher rates with less material and

cellulose-based containers that have a lower material cost.

Nylon 6/6. Mineral-ﬁlled nylon (or polyamides) is a

dual-ovenable material that today is no longer commer-

cially available. Developed by DuPont Canada during

the mid-1980s, mineral-ﬁlled nylon was also converted

by DuPont Canada by injection molding into containers.

Mineral-ﬁlled nylon plates have a higher temperature

resistance and stiffness (5001F) than does CPET and were

priced between CPET and thermoset polyester containers.

Although the material was successfully introduced com-

mercially, the use was limited because of the higher cost

than CPET; also, the hydroscopic nature of ﬁlled nylon

plates sometimes caused performance problems in the

microwave oven. Nylon has good resistance to oils and

fats; however, within the moist operating environment of

the microwave oven, some of the water present in the food

would be absorbed into the nylon plate, resulting in a loss of

dimensional stability. Nylon plates’ strength was in the

conventional oven, where unlike some competitive thermo-

plastic materials, it retained its rigidity.

Nylon plates were injection-molded, a process that

generally has higher tooling costs and higher operating

costs when compared with thermoforming comparable

unit volumes.

Polyetherimide. Polyetherimide (PEI) is a high-tem-

perature thermoplastic resin available from General Elec-

tric Plastics under the trade name Ultem.

PEI provides a dual-ovenable material for applications

of about 3501F. PEI is available only in injection-molding

grades, and with its high price per pound, it is generally

better suited for multiple-use versus single-use applica-

tions. PEI also has good chemical and stain resistance,

allowing for it to effectively be cycled through commercial

dishwashing systems.

PEI has been used for plates and containers in institu-

tional feeding programs and airline meals.

Polysulfone. Polysulfone (PSO) is an amorphous ther-

moplastic with good rigidity and toughness. PSO is

capable of withstanding oven temperatures below 3251F,

making it well suited to low-temperature and microwave

applications. PSO is available in transparent form, lend-

ing its use in appliances as a replacement for glass and

other multiple-use applications.

Because of its high price, PSO has not been used in

single-use applications.

Liquid-Crystal Polymer. Liquid-crystal polymers (LCPs)

offer very good high-temperature resistance up to about

5001F in some grades. LCP can be injection-molded and is

generally pigmented from its natural beige color (see also

Liquid crystalline polymers).

LCP is transparent to microwave energy; however,

the high price of this resin limits its use to specialized

applications. At one time, LCP was in commercial use as

dual-ovenable cookware marketed by Tupperware.

Aluminum. Prior

to the explosion

of microwavable foods

during the mid-1980s, aluminum trays dominated the

prepared- and frozen-meals market as well as food-service

applications such as school-lunch programs. As a package

material for use only in conventional ovens, aluminum

was ideal. It was capable of withstanding very high

temperatures for long times—certainly exceeding the

temperature limitations of the food, was usually available

at attractive prices, and could be run at a high speed on

packaging equipment.

During the early stages of the microwave oven, arcing

occurred when metal objects were placed in the oven

cavity during operation, sometimes disabling the unit.

This was largely corrected when the electronics were

improved so that energy could not be reﬂected back into

the magnetron. Even though it was safe for metal objects

to be used in the microwave oven, consumers did not want

to take the risk.

In the mid-1980s, Alcoa developed a plastic-coated

aluminum tray that was formed without the typical

wrinkled corners. The vinyl/epoxy coating was often

758 MICROWAVABLE PACKAGING AND DUAL-OVENABLE MATERIALS

pigmented to appear more like plastic and allowed the

tray to work in both conventional and microwave ovens.

Performance in the microwave oven is different from that

of plastic- or cellulose-based materials since the aluminum

shields microwave energy, often leading to longer heating

times than for similar trays of competitive materials. This

lends the design of the tray shape to be shallow to lessen

the amount of microwave energy shielded. Coated alumi-

num foil trays were used commercially for pot pies where

they were able to maintain high ﬁlling line speeds.

Polycarbonate. Polycarbonate (PC) is an amorphous

thermoplastic resin that is capable of withstanding tem-

peratures above 4001F. PC can be injection-molded, blow-

molded, and thermoformed.

PC has been used in applications for multiple-use

products such as microwavable cookware. PC is virtually

unbreakable, making it a good replacement for glass. PC

was used in a commercial package during the late 1980s

for Stouffer’s dual-ovenable meals. The structure used

was thermoformed from a three-layer coextruded sheet;

however, because of its high price per pound, it was

replaced by competitive materials that offered acceptable

performance for a much lower cost.

MICROWAVE-ONLY MATERIALS

Polypropylene. During the late 1980s, polypropylene

grew as a microwave material because of its ﬂexibility

and relatively low cost. Polypropylene (PP) use in packa-

ging was bolstered by the rapid growth of shelf-stable

meals such as Lunch Buckets (registered trademark) and

the like, in which ethyl vinyl alcohol (EVOH) is used to

improve the oxygen-barrier properties required to safely

preserve the cooked food at ambient temperatures.

Microwavable containers of PP can be thermoformed or

injection-molded, generally in two different types: homo-

polymer PP and random copolymer PP. Homopolymer PP

is produced using propylene monomer without the addi-

tion of other monomers. Random copolymer PP is similar

in polymeric structure to homopolymer PP but also in-

cludes the random addition of ethylene to a polypropylene

chain as it grows. Random copolymer PP gains some

molecular orientation, providing certain advantages over

homopolymer PP such as improved impact strength and

much better clarity.

Homopolymer PP can also be ﬁlled with minerals such

as talc or calcium carbonate at levels of 20–40%. Filled

homopolymer PP will have a slightly higher end-use

temperature and greater stiffness than its unﬁlled form.

Polyphenylene Oxide/Polystyrene. Polystyrene (PS) by

itself does not have a sufﬁciently high-temperature resis-

tance (about 1801F), but when blended with polypheny-

lene oxide (PPO), the temperature-resistance properties

are increased depending on the ratio of PS to PPO. For

temperature resistance in the range of 212–2301F, a blend

of 25% PPO and 75% PS is recommended.

PPO has a low resin ﬂow and is therefore difﬁcult

to form; however, when it is blended with PS, the

ﬂow characteristics and processing requirements are

improved. PPO/PS can be thermoformed on equipment

used for PS forming with only minor modiﬁcations. It is

important to have accurate blending during the extrusion

process; therefore, a high-intensity mixing screw is re-

quired. Additionally, PPO/PS is able to be foamed (much

like expanded polystyrene) by extruding with tandem

extruder systems and blowing agents used for polystyrene

such as pentane.

PPO is available from General Electric Plastics under

the trade name Noryl.

Polyethylene. High-density polyethylene (HDPE) is an

acceptable thermoplastic resin for some microwave appli-

cations. HDPE starts to lose its rigidity at temperatures

above 2001F, resulting in distortion of the tray or con-

tainer, so care must be given in selecting this material for

food products that will not exceed this temperature. This

means that foods that have a high fat or oil content or

those that generate steam will not be good candidates for

HDPE. Generally, applications for HDPE are for foods

that do not have a long heating cycle and have a homo-

genous texture to balance heating throughout the food,

thereby eliminating hot spots.

The advantages of HDPE are its relatively low cost in

comparison with other resins, its processing ease, and its

good impact properties at frozen temperatures. HDPE for

food trays is most commonly thermoformed but also can be

injection-molded (see also Polyethylene, high density).

Glass. Although glass usage as a packaging material

has declined steadily, it is a material that is able to

withstand the rigors of microwave heating. Because of

the advantages plastic has over glass in consumer safety,

transportation costs, and design ﬂexibility, there are

very few applications where glass is selected as a packa-

ging material because of its ability to be used in the

microwave.

BIBLIOGRAPHY

H. A. Rubbright and N. O. Davis, The Microwave Decade, Packa-

ging Strategies, West Chester, PA, 1989, p. 6.

MICROWAVEABLE FOODS PACKAGING

KIT L. YAM

Department of Food Science,

Rutgers University, New

Brunswick, New Jersey

INTRODUCTION

While microwaveable food products have been quite pop-

ular in the past two decades, there still remain many

challenges for the developer and manufacturer. Besides

convenience, other factors such as taste and texture are

MICROWAVEABLE FOODS PACKAGING 759

also important to the consumer. Quite often, the consumer

perceives food heated by the microwave oven as not

tasting as good as that heated by the conventional oven.

This is because the heating mechanisms of food in the

microwave oven and conventional oven are indeed quite

different. To develop successful microwaveable food pro-

ducts, the developer must have a good understanding of

the microwave heating of food, as well as careful consid-

erations of the package design and consumer expectation.

BASICS OF MICROWAVE HEATING OF FOOD

Microwave heating of food is a complex process that

requires a good understanding of several relevant disci-

plines: electromagnetism, food engineering, food chemis-

try, food packaging, and food microbiology. It is beyond the

scope of this article to provide detailed descriptions of the

many aspects of this complex process. Instead, this article

is aimed at acquainting the reader with the basic working

knowledge most relevant to the microwave heating of

foods. More general information can be found from refer-

ences in the literature (1–3).

Microwaves

Microwaves are short electromagnetic waves located in

the portion of electromagnetic wave spectrum between

radio waves and visible light. The energy is delivered in

the form of propagating sine waves with an electric ﬁeld

and a magnetic ﬁeld orthogonal to each other. Microwaves

are relatively harmless to humans because they are a form

of non-ionizing radiation, unlike the much more powerful

ionizing radiation (such as X ray or gamma ray) that can

damage the cells of living tissue. Microwaves are used in

daily applications such as cooking, radar detection, tele-

communications, and so on.

Most microwave ovens for food applications operate at

two frequencies. The household microwave oven operates

at 2450 MHz (2.45 10

cycles per second), and the in-

dustrial microwave oven operates at 915 MHz (9.15 10

cycles per second). The wavelengths associated with those

frequencies are 0.122 and 0.382 m, respectively, when the

microwaves are assumed to travel at the speed of light

(3 10

m/s). Microwaves travel at approximately the

speed of light in air, but they travel at a lower speed

inside a food material. The relationship between fre-

quency and wavelength is expressed by the equation

v =fl, where v is velocity (m/s), f is frequency (Hz), and l

is wavelength (m) of an electromagnetic wave.

There are three possible modes of interaction when

microwaves impinge upon a material: absorption of

microwaves by the material, reﬂection of microwaves

by the material, and transmission of microwaves through

the material. The material may be a food or a packaging

material. The food must absorb a portion of the microwave

energy in order for heating to occur. Most foods do not

reﬂect microwaves, and thus all the remaining unab-

sorbed microwave energy is transmitted. Some packaging

materials, such as susceptors, absorb microwave energy

and become hot. Metals, such as aluminum foils, reﬂect

microwaves. Paper, plastics, and glass are transparent to

microwaves. To optimize the microwave heating of food, it

is necessary to consider the reﬂection, absorption, and

transmission of microwaves by the food and the package.

In the microwave oven, microwaves are generated by

an electronic vacuum tube known as a magnetron. The

microwaves then travel through a hollow metal tube

(called a ‘‘waveguide’’) to the oven cavity. To improve the

heating uniformity, the microwave oven is often equipped

with a stirrer or a turntable. The stirrer is a fanlike set of

spinning metal blades used to scatter the microwaves and

disperses them evenly within the oven. The turntable

rotates the food during the microwave process. The his-

tory, features, standardization, and safety matters relat-

ing to the microwave oven are discussed by Decareau (1).

Microwave/Heat Conversion

Microwave energy is not heat energy. In order for micro-

waves to heat food, they must ﬁrst be converted to heat.

There are two mechanisms by which this energy conver-

sion can occur: dipole rotation and ionic polarization. The

two mechanisms are quite similar, except the ﬁrst involves

mobile dipoles while the second involves mobile ions. Both

dipoles and ions interact only with the electric ﬁeld, not

the magnetic ﬁeld.

Figure 1a illustrates the dipole rotation mechanism of a

polar molecule. In the presence of an electrical ﬁeld, the

polar molecule behaves like a microscopic magnet, which

attempts to align with the ﬁeld by rotating around its axis.

As the polarity of the electric ﬁeld changes, the direction of

rotation also changes. The molecule thus absorbs micro-

wave energy by rotating back and forth billions of times at

the frequency of microwaves. Since the molecule is often

bound to other molecules, the rotating action also causes it

to rub against those other molecules. The rubbing action

disrupts the bonds between the molecules, which in turn

causes friction and heat dissipation.

(a) Di

ole rotation

++++

-- ------

++++

-- ------

(b) Ionic

olarization

++++

-- ------

Figure 1. Microwave/heat conversion mechan-

isms. The dashed lines denote the alternating

electric ﬁeld at the frequency of microwaves. (a)

A dipole rotates back and forth. At high frequen-

cies (such as 2450 MHz), there is not sufﬁcient

time for the dipole to rotate 1801, and thus the

actual rotation angle is much smaller. (b) A

positive ion and a negative ion oscillate in an

alternating electric ﬁeld.

760 MICROWAVEABLE FOODS PACKAGING

The water molecule is the most abundant polar molecule

in food. The water molecules in liquid water are quite

mobile, and they readily absorb microwave energy and

dissipate it as heat through dipolar rotation. In contrast,

the water molecules in ice are much less mobile due to the

conﬁned crystal structure, and they do not absorb micro-

wave well. The distribution of moisture and the state of

water (liquid water or ice) are often two critical factors that

determine the behavior of microwave heating of foods .

Figure 1b illustrates the ionic polarization of a positive

ion and a negative ion in solution. In the presence of an

electric ﬁeld, the ions move in the direction of the ﬁeld. As

the polarity of the electric ﬁeld changes, the ions move

in the opposition direction. The ions absorb microwave

energy by oscillating themselves at the frequency of

microwaves. The oscillating action in turn causes heat

dissipation through friction. The common ions in food are

those from salts such as sodium chloride. Since ions are

less abundant than water molecules in most foods, ionic

polarization often plays a less important role than dipole

rotation.

Dielectric Properties

While dipole rotation and ionic polarization provide a

qualitative understanding of the microwave/heat conver-

sion mechanisms, the dielectric properties provide a

quantitative characterization of the interactions between

microwave electromagnetic energy and food. The dielec-

tric properties, along with thermal and other physical

properties, determine the heating behavior of the food in

the microwave oven.

An important dielectric property is dielectric loss factor

(euu), which indicates the ability of the food to dissipate

electrical energy. The term ‘‘loss’’ refers to the loss of

energy in the form of heat by the food. It is useful to

remember that a material with a high euu value (also

known as a lossy material) heats well, while a material

with a low euu value heats poorly in the microwave oven.

The dielectric loss factor is related to two other dielectric

properties by the equation

tan d ¼ euu=eu ð1Þ

where tan d is loss tangent and eu is dielectric constant.

The dielectric properties (eu and ev ) are functions of

frequency, temperature, moisture content, and salt con-

tent. Values of dielectric properties for foods and other

materials can be found in the literature (4–6). Examples

of ev and e u values at 2450 MHz are shown in Table 1.

Although the literature values can be used as guidelines,

actual measurements are often required because of the

variability of composition of the materials.

The dielectric properties provide a quick indication of

how well a material heats in the microwave oven. For

example, the euu value of water (12.48) at 25 1C is several

orders of magnitude higher than that of ice (0.0029) at

12 1C. This means that water heats far better than ice in

the microwave oven. Ice is almost transparent to micro-

waves because its molecules are tightly bound and do not

rotate easily through the mechanism of dipolar rotation.

The dramatic increase in ev value is also observed when

ice changes to water—that is, during the thawing of frozen

foods

including beef,

potato, and spinach (Table 1). Plas-

tics and paper have low euu values because they are almost

transparent to microwaves.

Penetration Depth

The speed of microwave heating is due to the deep

penetration of microwaves into the food, and the dielectric

properties can be used to determine the extent of penetra-

tion. When microwaves strike a food surface, they arrive

with some initial power level. As microwaves penetrate

the food, their power is attenuated since some of their

energy is absorbed by the food. The term penetration

depth (D

) is deﬁned as the depth at which the microwave

power level is reduced to 36.8% (or 1/e) of its initial value.

The meaning of penetration depth is further illustrated

in Figure 2. At the ﬁrst D

, 36.8% of the initial power

remains, while 63.2% of the power is absorbed. At the

second D

, (0.368)

= 13.5% remains and 86.5% is ab-

sorbed. At the third D

, (0.368)

= 5.0% remains and 95%

is absorbed. The penetration depth depends on the com-

position of material, the frequency of microwaves, and

temperature.

Table 1. Dielectric Constant (eu), Dielectric Loss Factor

(ev), and Penetration Depth (D

) of Various Foods at

2450 MHz

Food or Other Material eu ev D

(cm)

Ice (12 1C) 3.2 0.0029 1203

Water (1.5 1C) 80.5 25.0 0.71

Water (25 1C) 78 12.48 1.38

Water (75 1C) 60.5 39.93 0.40

0.1 M NaCl (25 1C) 75.5 18.1 0.94

Fat and oil (average) 2.5 0.15 20.6

Raw beef (15 1C) 5.0 0.75 5.83

Raw beef (25 1C) 40 12 1.04

Roast beef (23 1C) 28 5.6 1.85

Boiled potatoes (15 1C) 4.5 0.9 4.6

Boiled potatoes (23 1C) 38 11.4 1.07

Boiled spinach (15 1C) 13 6.5 1.11

Boiled spinach (23 1C) 34 27.2 0.45

Polyethylene 2.3 0.003 986

Paper 2.7 0.15 21.4

Metal N 00

Free space 1 0 N

1st

2nd

3rd

100%

Power remains

Power absorbed

36.8%

63.2%

13.5%

86.5%

Microwaves

Food surface

5.0%

95.0%

Figure 2. Power levels at various penetration depths.

MICROWAVEABLE FOODS PACKAGING 761

Typical values of D

for various materials at 2450 MHz

are also shown in Table 1. As mentioned earlier, liquid

water absorbs microwaves far better than ice. The D

water at 25 1C is 1.38 cm, but D

of ice at 12 1Cis

1203 cm! Frozen foods have longer penetration depths

than unfrozen foods. For example, the D

values for frozen

beef and unfrozen beef are 5.83 cm and 1.04 cm,

respectively.

CHALLENGES IN MICROWAVE HEATING OF FROZEN

FOOD

While microwave heating offers the beneﬁts of speed

cooking and convenience, it also presents many technical

challenges to the food scientist or technologist. Those

challenges arise from the need to deal with the many

variables relating to the food, package, and microwave

oven. For the food, there are variables of food composition,

shape, size, speciﬁc heat, density, dielectric properties,

and thermal conductivity. For the package, there are

variables of shape, size, and properties of packaging

material. For the microwave oven, there are variables

relating to the design of the oven. A related and more

important challenge is to solve the problems of the con-

sumer. From the consumer’s point of view, the most

noticeable problems are those associated with nonuniform

heating, lack of browning and crisping, and variation in

microwave ovens.

Nonuniform Heating

Nonuniform heating is a major problem associated with

microwave heating. The problem is especially noticeable

for frozen food. It is not uncommon for a frozen food heated

in a microwave oven to boil around the edges while the

center remains frozen. The problem is caused by the

differences in microwave energy absorption of liquid water

and ice.

In frozen foods, the water molecules on the surface are

relatively free to move compared to the water molecules

inside the food. When a frozen food is microwaved, heating

begins at the surface where the water molecules are more

ready to absorb microwave energy. This causes the adja-

cent ice crystals to melt and the surface temperature to

rise, while the inside temperature is still little unaffected.

As more liquid water is available, the heating of the

surface becomes more rapidly. This can lead to ‘‘runaway

heating,’’ in which heating is excessive at the surface

while the inside is still frozen. To minimize runaway

heating during thawing, microwave energy should be

delivered at a slow rate, which allows more time for heat

to conduct from the surface to the inside.

Irregular shape of the food can also cause nonuniform

heating. The thin parts tend to overcook, while the thick

parts tend to undercook. This situation also occurs in

conventional cooking but is less pronounced because the

cooking is slower. Another cause of nonuniform heating is

that different foods have different dielectric and thermal

properties. When a microwave meal consists of two or

more items, it is possible that the items heat at different

rates. For example, when microwave heating a frozen

meal consisting of meat and vegetable, the vegetable often

becomes overheated and dried out before the meat reaches

the serving temperature.

Lack of Browning and Crisping

Another problem is that, unlike the conventional oven, the

microwave oven is not able to produce foods that are

brown and crisp. This is because the heating mechanisms

of the conventional oven and the microwave oven are quite

different.

In the conventional oven, the food is heated by hot air

in the oven; and if the heating element is not shielded, the

food is also heated by radiated heat. Heating is concen-

trated on the food surface by means of heat convection and

radiation. The inside of the food is also heated, at a slowly

rate, by means of heat conduction. The heating causes the

moisture on the food surface to evaporate rapidly, and

later it causes browning and crisping to occur. Although

the moisture inside the food tends to migrate to the

surface, the rate is not sufﬁciently fast to prevent brown-

ing and crisping. As a result, the food surface becomes

brown and crispy while its inside remains moist and soft.

In the microwave oven, there is no hot air, and heating

is mostly due to the interaction between microwaves and

water. Microwave heating is not concentrated on the food

surface, but it is distributed within the food depending on

the penetration depth. The heating on the food surface is

no longer sufﬁciently intense to cause browning and

crisping. Unless the food is microwaved for a long time

to remove all or most of the water in the food (which is not

desirable because the food quality may no longer be

acceptable), browning and crisping either do not occur at

all or are inadequate.

Browning formulations have been developed for var-

ious meat and dough products (1). Commercial steak

sauces, barbecue sauces, soy sauces, and so on, are

brushed on meat before microwave heating. Reusable

browning dishes are also available for browning food

surfaces in the microwave oven. Most of the commercial

browning dishes are made of glass–ceramic substrate with

tin oxide coating on the underside. The packaging indus-

try has also developed a disposable browning and crisping

material, known as susceptor, discussed later in this

article.

Variation in Microwave Ovens

Yet another problem is the large variation of performance

in different microwave ovens. Microwave ovens are avail-

able in different powers, oven cavity sizes, with or without

a turntable, with or without a stirrer (to distribute micro-

waves more evenly in the oven). Consequently, different

microwave ovens may produce greatly different results,

even if the same cooking instructions are used. To accom-

modate the differences, the food manufacturer can only

place vague microwave heating instructions on their

packages. For example, a package may contain vague

instructions such as ‘‘heat between 4 and 8 minutes,

depending

on the microwa

ve oven.’’

762 MICROWAVEABLE FOODS PACKAGING

Meeting the Challenges

There is no easy solution to deal with the complex process

of microwave heating. In developing a microwavable food

product, the scientist or technologist has to rely on

the somewhat useful but incomplete scientiﬁc knowledge

described in the previous sections, as well as trial-and-

error or empirical methods.

From the discipline point of view, there are three

approaches to deal with the challenges. The ﬁrst is the

food chemist’s approach, in which food ingredients are

modiﬁed and browning formulations are added to make

the food more microwaveable. The second is the packaging

engineer’s approach, in which the package is modiﬁed to

enhance the performance of microwave heating. The third

is the microwave engineer’s approach, in which new and

useful features are added to the microwave oven. Ideally,

these approaches should be integrated into a system to

deliver the highest quality of microwaveable foods to the

consumer.

Many microwaveable food products have failed in the

past because of lack of performance or high cost. Good

technical and marketing tools are essential for developing

better-tasting microwaveable products, without increas-

ing the cost or decreasing the effectiveness of cooking.

Although the food manufacturer and the packaging sup-

plier have been working together to develop microwave-

able products, there has been relatively little collaboration

between them and the oven manufacturer. There is a need

to have all parties (including also the academia) work

more closely together to bring about innovations that can

deliver better microwaveable products to the consumer.

MICROWAVEABLE PACKAGING

The primary functions of the package are to contain,

protect, and sell the product. A general discussion on the

packaging of frozen foods is presented in Chapter 6. If the

package is used to hold the food during microwave heating

(as is the case for many microwaveable frozen meals), the

interactions between microwave and the package must

also be considered. Since the package can transmit, re-

ﬂect, or absorb microwaves, it can also greatly inﬂuence

the microwave heating behavior. The package may act

‘‘passively’’ by simply transmitting microwaves. The pack-

age may also act ‘‘actively’’ by reﬂecting and absorbing

microwaves in a manner that the power distribution of

microwaves and the surface temperature of package are

modiﬁed. To optimize microwave heating, it is necessary to

properly balance these three microwave/package interac-

tions (transmission, reﬂection, and absorption) to optimize

the heating of the food. Microwaveable packaging materi-

als may be classiﬁed into microwave transparent materi-

als, microwave reﬂective materials, and microwave

absorbent materials.

Microwave Transparent Materials

A microwaveable package must be wholly or partly trans-

parent to microwaves. The most common microwave

transparent materials are paper and plastics. Although

glass is also transparent to microwaves, it is seldom used

to package frozen food.

Plastic-coated paperboard trays are popular for micro-

wavable frozen meals, mainly because of their low cost.

The trays combine the rigidity of the paperboard and the

chemical resistance of the plastic. The inside of the trays is

either extrusion-coated with a resin or adhesive-lami-

nated with a plastic ﬁlm. For microwave-only applications,

plastics such as low-density polyethylene (LDPE), high-

density polyethylene (HDPE), and polypropylene (PP) are

used. For dual oven applications (i.e., usable in both

microwave and conventional ovens), polyethylene ter-

ephthalate (PET) is used because of its high temperature

stability (up to about 200 1C).

Molded pulp trays are another common paper product.

The containers are dual ovenable and can be molded into

several compartments. They are stronger and can carry

more load than the paperboard containers.

Thermoformed plastic trays are also common heating

containers for microwaveable frozen foods. LDPE trays

are suitable for light microwave heating because the trays

tend to distort at temperatures as low as 75 1C. PP trays

have a distortion temperature of about 110 1C. Homopoly-

mer PP trays are brittle at low temperatures and can

crack during distribution and handling at freezer tem-

peratures. Copolymer PP trays have somewhat improved

low-temperature durability. Crystallized PET (CPET)

trays are the most widely used plastic trays for micro-

waveable frozen meals. The CPET trays are functional in

the temperature range from 40 to 220 1C. Thus the trays

can withstand not only the low temperatures encountered

in distribution and handling, but also the temperatures in

conventional oven (i.e., the trays are dual ovenable).

Microwave Reﬂective Materials

Aluminum foil, aluminum/plastic laminate, and alumi-

num/plastic/paperboard laminate are the most common

microwave reﬂective materials. Since these materials do

not allow the transmission of microwave, they are also

known as microwave shielding materials.

Aluminum is often used to selectively shield micro-

waves from certain areas of a food (Figure 3). For example,

a multicomponent meal may consist of food items that

heat at different rates in the microwave oven. The more

microwave-sensitive

food item(s) can

be shielded so that

the entire meal can be heated more even.

Aluminum is also used as an electromagnetic ﬁeld

modiﬁer to redirect microwave energy in a manner to

optimize the heating performance (10). Aluminum can

intensify the microwave energy locally or redirect it to

places in the package that otherwise would receive rela-

tively little direct microwave exposure. This approach has

been used to redirect microwave energy from the edges to

the center for frozen food products such as lasagna.

When aluminum foils are used in the microwave oven,

precautions are necessary to prevent arcing, which can

occur between foil packages and the oven walls, between

two packages, across tears, wrinkles, and so on. Arcing can

be prevented by following several simple design rules (11).

For example, any foil components should be receded from

MICROWAVEABLE FOODS PACKAGING 763

the edge of the package to avoid arcing with the oven

walls. In additional to following those rules, it is also

necessary to thoroughly test the package/product to en-

sure that the package is safe to use.

Microwave Absorbent Materials

Microwave absorbent materials used for food packaging

are commonly known as susceptors. The major purpose of

susceptors is to generate surface heating to mimic the

browning and crisping ability of the conventional oven.

Although many types of susceptors have been invented

(12), the only commercially available type is the metal-

lized ﬁlm susceptor (Figure 4). This type of susceptor

consists of a metallized polyethylene terephthalate ﬁlm

laminated to a thin paperboard. The metal layer is a very

thin (less than 100 angstroms), discontinuous layer of

aluminum, which is responsible for generating localized

resistance heating when exposed to microwaves. The

heating can cause the susceptor to reach surface tempera-

tures over 200 1C within seconds.

Susceptors have been used for products such as pizza,

French fries, wafﬂes, frozen hot pies, and popcorn. Sus-

ceptors are available in the forms of ﬂat pads, sleeves, and

pouches. The ﬂat pads are suitable for products (such

as pizza) that require heating only on one surface. The

sleeves and pouches are suitable for heating on multiple

surfaces (9). Susceptors are also available in various

patterns, in which portions of the metallized layer are

deactivated (10). The patterns are designed to provide

more control of heating. A company uses a printed check-

erboard pattern to generate various levels of heating

based on the size of the check.

There is a public concern of migration of mobile com-

pounds from the susceptor to the food, because the sus-

ceptor can reach high temperature. The FDA has issued

voluntary guidelines regarding the safe use of the suscep-

tor for the packaging to follow.

CONCLUDING REMARKS

While microwaveable foods and meals have become an

integral part of our lifestyle, improvements are still

needed to continue to justify their place in the freezer

case. Although microwaveable products can provide the

consumer with convenience, they often fail to impress the

consumer with taste and texture. There are many techni-

cal and economical challenges for developing new and

improved microwaveable frozen products. To meet those

challenges, the industry (food manufacturer, packaging

supplier, and oven manufacturer) and the academia

should work more closely together—to innovate and de-

velop better microwaveable food products that are more

tasty, healthy, and convenient to use.

BIBLIOGRAPHY

1. R. V. Decareau, Microwave Foods: New Product Development,

Food & Nutrition Press, Trumbull, Connecticut, 1992.

2. C. R. Bufﬂer, Microwave Cooking and Processing: Engineer-

ing Fundamentals for the Food Scientist, Van Nostrand

Reinhold, New York, 1993.

3. A. K. Datta and R. C. Anantheswaran, Handbook of Micro-

wave Technology for Food Applications, Marcel Dekker, New

York, 2001.

4. N. E. Bengtsson and P. O. Risman, ‘‘Dielectric Properties of

Foods at 3 GHz as Determined by a Cavity Perturbation

Technique II. Measurements of Food Materials,’’ J. Microwave

Power 6(2), 107–123 (1971).

5. S. O. Nelson, ‘‘Electrical Properties of Agricultural Products

(A Critical Review),’’ Trans. ASAE 16(2), 384–400 (1973).

6. A. K. Datta, E. Sun, and A. Solis, ‘‘Food Dielectric Property

Data and Their Composition-Based Prediction’’ in M A. Rao

and S. S. H. Rizvi. eds., Engineering Properties of Foods,

Marcel Dekker, New York, 1995, pp. 457–494.

7. C. Saltiel, A. K. Datta, ‘‘Heat and Mass Transfer in Microwave

Processing,’’ Adv. Heat Transfer 32, 000–000 (1998).

8. K. G. Ayappa, Modeling Transport Processes During Micro-

wave Heating: A Review,’’ Revi. Chem. Eng. 13(2), 1–68

(1997).

Figure 3. Trays with selectively shielded areas. (Courtesy of

Graphic Packaging Inc.)

Figure 4. Metallized ﬁlm susceptors. (Courtesy of Graphic Pack-

aging Inc.)

764 MICROWAVEABLE FOODS PACKAGING

9. J. R. Quick, J. L. Alexander, C. C. Lai, S. A. Matthews, D. J.

Wenzel, Tube from Microwave Susceptor Package, U.S. Patent

No. 5,180,894, January 19, 1993.

10. T. H. Bohrer and R. K. Brown, ‘‘Packaging Techniques for

Microwaveable Foods’’ in A. K. Datta and R. C. Ananthes-

waran, eds., Handbook of Microwave Technology for Food

Applications, Marcel Dekker, New York, 2001, pp. 397–469.

11. A. Russell, ‘‘Design Considerations for Success in Microwave

Active Packaging Development’’ in Conference Proceedings,

Future-Pak 99, George O. Schroeder Associates Inc., 1999.

12. G. L. Robertson, ‘‘Packaging of Microwavable Foods’’ in Food

Packaging Principles and Practice, Marcel Dekker, New York,

1992, pp. 409–430.

MIGRATION FROM FOOD CONTACT

MATERIALS

MANFRED OSSBERGER

FABES Forschungs-GmbH

INTRODUCTION

Food and packaging are closely tied together. Food packa-

ging material requires optimization of its properties to

control physical and chemical interaction between food

and packaging. In general, food consists of numerous

ingredients with in many cases low chemical stability.

Therefore, food has to be packed to protect it against

spoilage, insects, micro-organisms, light, and other envir-

onmental impacts like humidity or oxygen, which could all

lead to a decrease of the food quality. Protection against

unauthorized access and information about the packaged

foodstuff are some other important functions of packaging.

There are a variety of plastics with different properties

composed of several polymers and containing additives

that are likely to migrate used to fulﬁll the speciﬁc

packaging requirements. It is important to realize that

not only the food quality is time dependent, but also the

food packaging is not at ‘‘steady state.’’ After production

the packaging mass transfer processes starts immediately.

Two diffusion-controlled processes have to be considered:

. Permeation (lat. permeare) refers to the process,

where chemical substances diffuse from the environ-

ment into the food or vice versa.

. Migration (lat. migrare) means the diffusion of sub-

stances from the packaging into the food or from the

food into the packaging.

In addition, chemical reactions in the packaging or food or

between them are also possible and have to be taken into

account.

PLASTIC MONOLAYER MATERIALS IN CONTACT WITH

FOODSTUFF

Diffusion (kinetics) and partitioning (thermodynamics)

constitute the most important interactions:

The driving force of diffusion comprises different che-

mical potentials between two adjacent media, which was

ﬁrst mentioned and investigated by Adolf Fick (1829–

1901), who postulated the following laws:

Fick’s 1st law: J ¼D

ð1Þ

where J expresses the ﬂux (or ‘‘current density’’ of parti-

cles), c the concentration of particles, and x the distance

for particles travel. The ﬂux J expresses the amount of

matter passing through a unit area of a ﬁlm in unit time.

Fick’s 1st law is the solution of Fick’s 2nd law under

steady state conditions @c=@t = 0 as in the case of permea-

tion, e.g., of gases like oxygen or water vapor through

packaging materials (1, 2).

Fick’s 2nd law:

¼ D

ð2Þ

where t expresses the time and D the diffusion coefﬁcient.

Fick’s 2nd law describes the diffusion of migrants, e.g., in

the two-phase system of packaging and food in the direc-

tion of the concentration gradient.

Most solutions of the second diffusion equation for

systems in contact with liquid are derived from analogous

mathematical solutions of the heat conductance equation

(standard works by Carslaw and Jaeger, Crank) (3, 4).

These analytical solutions are obtained by using simplify-

ing assumptions. For example, one get the expression

F; t

ﬃﬃﬃ

P;0

ﬃﬃﬃﬃﬃﬃﬃﬃ

ð3Þ

where m

F,t

is the amount of the migrated compound at

time t, A the contact area, c

P,0

(wt/wt) the initial concen-

tration of the migrant in the polymer, r

the density of the

polymer, D

the diffusion coefﬁcient of the migrant in the

polymer, and t the contact time. There it is assumed that

the solubility of the migrant in the liquid is very high, the

thickness of the packaging inﬁnite, and the migration far

from equilibrium.

This expression overestimates migration as the system

approaches equilibrium (mass transfer greater than ap-

proximately 60%). Therefore, more general solutions have

to be deﬁned to characterize the migration process (see

next equation):

F; t

¼ c

P;0

1 þa



 1 

n¼1

2a 1 þ aðÞ

1 þa þ a

exp D



ð4Þ

with a ¼

P;F

; K

P;F

¼ c

P;N

F; N

and tan q

¼aq

where in addition to constants deﬁned for Equation (3),

is the density of the liquid food, d

is the thickness of

MIGRATION FROM FOOD CONTACT MATERIALS 765

the polymer, V

and V

the volume of the polymer and the

liquid food, K

P,F

the partition coefﬁcient, and q

the

positive roots of the trigonometric identity. K

P, F

is deﬁned

as the ratio of the migrant concentration in the polymer

and the liquid food at equilibrium. Using this expression,

overestimation is limited and the migration can be esti-

mated at equilibrium; there a ﬁnite volume of the polymer

is assumed.

Regarding Equation (4), the total amount of material is

transported away from the contact layer into the foodstuff

at the beginning of migration. Thus, the concentration in

the contact layer goes to zero. Whereas the diffusion rate

of the migrants in the packaging material is the rate-

determining step, the partition coefﬁcient K

P,F

(and a,

respectively) plays an increasing role with longer migra-

tion times. Especially for K

P,F

c1(a{1), there is only very

low migration into the food.

If the foodstuff is viscous or solid, one gets a different

solution of Fick’s 2

law since the diffusion coefﬁcient of

the migrant in the solid food has to be taken into account:

F; t

ﬃﬃﬃ

P;0

1 þb

ﬃﬃﬃﬃﬃﬃﬃﬃ

ð5Þ

with b ¼

P;F

ﬃﬃﬃﬃﬃﬃﬃ

where D

is the diffusion coefﬁcient of the migrant in the

food. For D

and K

P,F

r 1, equation (3) results.

In order to use equation (4), the two coefﬁcients D

and

P,F

are needed. As the values of both coefﬁcients can vary

over orders of magnitudes and may not be readily avail-

able, one has to assume ‘‘worst-case’’ scenarios like good

solubility of the migrant in the liquid food or simulant (i.e.,

P,F

= 1). In order to estimate upper limits of D

, the

following semi-empirical formula was developed (5–7):

¼ D

exp A

 0:1351M

2=3

þ 0:003M



10454



ð6Þ

with A

¼ A



where D

is 10

/s, A

the material speciﬁc constant,

the relative molar mass of the migrant (Dalton), and T

the temperature (Kelvin). Values of the constant t and Au

for certain polymers are given in Table 1. The equation

can be used for M

r4000 Dalton. As regulation asks for

safety margins and thus ‘‘worst-case’’ Au

values for mod-

eling migration to avoid underestimation, it is possible to

match the parameter A

to yield a ‘‘worst-case’’ value, Au*

(see Table 1). This leads to corresponding D*

instead of

the real D

. In equation (6), A

rA*

= Au*

t/T, the para-

meter A*

should now be regarded as an ‘‘upper-bound’’

conductance of the polymer.

Values for A

are obtained by performing kinetic stu-

dies, measuring migration after, e.g., 1, 2, 4, 8, and 10 days

(see example; Figure 1). By comparison of the experimen-

tal data with calculated data (software programes, please

see last section), one can calculate D

and K

P,F

For the polyoleﬁnes, another semi-empirical diffusion

model has been developed by Limm and Holliﬁeld (8):

¼ D

exp aM

1=2

 KM

1=3



ð7Þ

with values for D

, a, and K given in Table 2.

A basic conclusion of both Fickus laws is that if a

concentration gradient is present, diffusion takes place

immediately. The velocity of mass transport is subject to

the diffusion coefﬁcient D, the kinetic dimension of the

diffusion process, which is a function of temperature. D for

a migrant with medium molecular weight varies from ca.

Table 1. Values for worst case A

(Au

), mean A

(



)

and the parameter s for estimating diffusion coefﬁcients

in polymers (eq. 6)

Polymer Au*



(max) Au

(min) t

LDPE 11.7 10.0 11 7.0 0

HDPE 13.2 10.0 12.6 5.0 1577

PP 12.4 9.4 12.9 6.2 1577

PET 6.35 2.2 7.2 4.3 1577

PEN 3.7 0.34 3.8 5.5 1577

PS 0.7 2.8 0 6.5 0

HIPS 0.1 2.7 0 6.2 0

PA(6,6) 1.9 1.54 2.3 7.7 0

100

120

024681012

Irganox 1010 - 80°C

Calc.

conc. (μg/dm

)

Time (days)

Figure 1.

766 MIGRATION FROM FOOD CONTACT MATERIALS

5

/s in solutions to 10

18

/s in PET-materials.

The quantity of migrants in the other layer(s) can be

further dictated by thermodynamics, which is expressed

by the partition coefﬁcient K. Consequently, if the diffu-

sion rate is high, but the migrant is not soluble in the

other media, there will be only low mass transport from

the origin to the adjacent layer(s). This coefﬁcient is

strongly inﬂuenced by the solubility of the migrant in

the contacting media and varies roughly from K

P,F

= 1 for

well solubility to K

P,F

= 1000 for bad solubility. In addition

to estimations for diffusion coefﬁcients, procedures exist to

estimate the partition coefﬁcients (9).

In summary, the quantity of migration is strongly

controlled by the diffusion coefﬁcient (D) and partition

coefﬁcient K, which are affected by the identity of the

packaging material and its chemical structure (- D, K ),

chemical structure and polarity of the migrant (- K),

molecular weight of the migrant (- D), concentration of

the migrant in the packaging (- D), kind of packed food

(water, acid, alcoholic, fatty, dry) (- D, K) and if there is

an interaction between food and packaging (- D), volume

of the packed food (- K), and time and temperature

conditions (- D, K) at which the packed food is stored.

The preceding discussion holds true only if the diffu-

sion and partition coefﬁcients are constant while diffusion

takes place and if the packaging is inert against the

packed foodstuff. Otherwise if D and K vary with time or

swelling is observed, diffusion can no longer be described

by Fick’s 2

law (‘‘non-Fickian processes’’). In addition, at

very low migrant concentrations, the migration rate may

no longer be proportional to its concentration in the

polymer. Considering paper and board, physical interac-

tions and mass transportation are controlled by sorption

processes, what makes estimation more intricate.

MULTILAYER PLASTIC MATERIALS IN CONTACT WITH

FOODSTUFF

In the past, packaging was generally made of monolayer

materials like polyethylenes (PE), polypropylenes (PP),

polyamides (PA), polystyrenes (PS), polyethylene ter-

ephthalates (PET), paper and board, aluminium, cera-

mics, and glass. As each material has unique useful

properties, several materials are now combined into multi-

layer materials. In addition, such multilayer systems

consist of lacquers, coatings, printing, and/or adhesive

layers. Migration processes may originate from all differ-

ent layers, each with different physical properties (like D

and K, vapor impermeability). As a consequence, migra-

tion processes are more complicated and the use of numer-

ical models and algorithms is sometimes necessary (see

the section ‘‘Measurement Procedures for Quantiﬁcation

of Migrants’’), especially if there are analytical problems.

MIGRANTS

The use of glass as a food contact material is advantageous

as there is no migration possible. In contrast, as men-

tioned, migration in plastics has to be considered. A

common feature of all polymer materials is their backbone

of natural or synthetic macromolecules composed of hun-

dreds of repetitions of small chemical units (monomers)

that are connected via covalent bonds. Compared with

pure natural polymers, e.g., proteins, synthetic polymers

are polydisperse; i.e., the polymer is a mixture of

molecules with various chain lengths and molar masses.

Up to ca. 2% of low-molecular-weight molecules with

o 10

g/mol account for the polymer, which are of

possible toxicological interest. All other polymer com-

pounds are immobile due to their high molecular weight.

Hence, there is a trend to switch from monomeric

additives to polymeric additives. The disadvantage of

this is the lower reactivity, and thus, higher concentra-

tions of polymeric additives are needed.

Actually many more substances are incorporated into

plastics that the producer and end user has to pay atten-

tion to. In addition to the residual amounts of initiators

(e.g., peroxide compounds) and metal oxide catalysts,

there are a huge amount of additives, such as antioxi-

dants, plasticisers, antistatics, lubricants, and pigments,

added to the polymer for better processing, protection, or

appearance of the polymer, which are all possible mi-

grants. Some common examples for each additive group

are given in Table 3.

Beside that not-intended-additives (NIAS) such as

transformation products and contamination from plastic

and additive production are additional sources of

migrants.

As stated in the Introduction, migration is possible in

two directions, from the packaging into the food and from

the food into the packaging. Therefore, many substances

(and ﬂavors) in food are also likely to migrate (e.g.,

Limonene, which is contained in soft drinks, migrates

into plastic packaging). This is also true considering

permeation. ‘‘Flavors’’ or gases can permeate from the

environment into the food and thus have to be taken into

account. It should also be stressed that sometimes migra-

tion of, e.g., antistatics and lubricants is intended and

crucial for certain applications.

MEASUREMENT PROCEDURES FOR QUANTIFICATION

OF MIGRANTS

The purpose of migration testing is to ensure the compli-

ance of food packaging by detection and quantiﬁcation of

migrants. Since migration and not extraction is to be

measured, interactions changing the mechanical and dif-

fusion properties of the package should be minimized

between food contact material and foodstuff.

Table 2. Values for D

, a and K in eq. (7).

Polymer ln(D

) a K

LDPE 4.16 0.555 1140.5

HDPE 0.90 0.819 1760.7

PP 2.10 0.597 1335.7

MIGRATION FROM FOOD CONTACT MATERIALS 767