PACKAGING TECHNOLOGY AND SCIENCE VOL 2 5-10 (1989)

The Role of Packaging in Achieving Microwave Browning and Crisping

Peter Harrison Waddingtons Cartons Limited, 40 Wakefield Road, Leeds, LS10 3TP, UK

Microwave interactive packaging is a rapidly expanding and evolving technology. This class of package contains heater elements (susceptors or receptors) which convert microwave energy into conventional heat, thereby providing useful browning and crisping functions. This paper discusses the principles of microwave interactive package design.

INTERACTIVE HEATER ELEMENT CONSTRUCTIONS

The prior art cited in US patent 4641005 describes a number of attempts to incorporate heating layers into containers. These approaches used a variety of materials to absorb microwaves and convert them to sensible heat. US patent 4641005 also describes a thin conductive film adhered to low-cost disposable fibrous substrates. Microwave interactive heater elements (suscep- tors or receptors) produced by Waddingtons Cartons Limited in the UK and James River Corporation in the USA employ this technology and are meeting with growing commercial accept- ance.

These heaters consist of vacuum-metallized heat-set polyester film, laminated to paper or board substrates. The film is metallized to a specific metal thickness. Since thicknesses in this range cannot be measured directly, the electrical conductivity and optical density are used to control both the level and the uniformity of the metal layer. The lamination process is also controlled to produce the desired product attri- butes.

Oriented polyester film provides a stable base for metallizing and a safe food contact surface. Paper or board substrates provide dimensional

089432 14/89/01 0 0 0 ~ 6 $ 0 5 . 0 0 0 1989 by John Wiley & Sons, Ltd.

stability during microwave heating. This com- bination of materials is specially suited to easy fabrication into a variety of package configura- tions.

HEATER INTERACTIVE CONFIGURATIONS

Microwave interactive heaters have been adapted to a variety of formats in order to achieve package design objectives. Board laminations can be cut and creased into very many shapes. However, the use of folding cartons, or cut and creased trays made from board laminations, is constrained by the potential for scorching during microwave heating where designs incorporate overlapping panels.

Board interactive heater laminations have been successfully pressed into formed containers such as round and rectangular tray shapes. This approach limits overlapping areas while achieving three-dimensional forms. Stripe metallized film laminations to board have also been produced to avoid these overlaps. This allows another degree of package design freedom by eliminating heating from linear segments of a package.

Received 25 Sepremher I988

6 P. HARRISON

The need to limit overlapping areas led to the development of paper-based interactive heater laminations. Lamination to paper provides a flexible substrate, useful for application to cut and creased board blanks via standard carton windowing equipment. This approach was used to produce the first successful seven-inch-diameter microwave interactive pizza trays.

Paper-based interactive heater laminations have also been converted into corrugated con- figurations. Corrugated microwave interactive heaters are used when a combination of micro- wave absorbance, elevation and thermal insula- tion is desired.

The need to apply unrestrained design capabil- ity led to commercialization of James River's Focus Qwik Crisp, a patent pending process, and product technology. Any pattern of microwave energy conversion can be incorporated into either paper or board laminations using this process. The importance of this achievement will become evident as other aspects of packaging and food interactions are discussed.

PACKAGE AND FOOD DESIGN CONSIDERATIONS

In order to achieve significant browning and crisping reactions, food surfaces must be raised above 150°C for a significant portion of the microwave heating cycle. Microwave interactive heater elements help to achieve this objective by absorbing part of the available microwave energy and converting it to conventional heat. When located in direct contact or close proximity to the food, significant conductive or radiant heat transfer occurs from the microwave interactive heater to the food. This heat transfer reverses the food surface temperature gradient normally associated with microwave heating, allowing crisping and browning reactions to proceed. A portion of the impinging microwave energy is transmitted through the microwave interactive heater, allowing direct dielectric coupling with the food load. Thus, the speed advantage of microwave heating is largely retained.

Figure 1 displays the desired thermal behaviour of microwave interactive heaters in a conductive heat transfer relationship with food. A fast temperature rise to a useful browning and crisping temperature is accomplished, but not at the expense of run-away heating. Temperature-

QWlK CRISP Performance First vs Second Use

"C

Elapsed Time (sec)

Figure 1. Thermal behaviour of microwave interactive heaters

limiting behaviour at 200°C is an important characteristic of useful microwave interactive heaters. These desirable responses are embodied in the First Use Curve. The Second Use Curve represents the behaviour of the same microwave interactive heater which has been allowed to cool to room temperature before a second exposure to microwave energy. The self-limiting ability of the microwave energy interactive heater is characte- rized by the reduced rate of temperature increase during this second heating cycle.

Figure 2 demonstrates that useful radiant heat transfer can occur between microwave interactive heaters and food products. Radiant heat transfer falls off with the square of the separation distance. Best results are generally obtained when the separation is less than 3mm.

Oven wattage has a dramatic effect on the ability to generate successful microwave heating results. Lower wattage ovens require longer

QWlK CRISP Performance Radiant Heat Transfer Effect

200 175 ]

25 l!!! 0 0 30 t8aosed 60 Time ( 5 ; 90

Se3o-ctor. d s o r ~ e -0mm --2mm -4mm -6mm

Figure 2. Radiant heat transfer effect

MICROWAVE INTERACTIVE PACKAGE DESIGN 7

cooking times. Generating high surface tempera- tures is a greater challenge in these ovens. As examples, Figures 3, 4 and 5 illustrate the effect of oven wattage on microwave popcorn quality in terms of volume, pop failures and time. Using microwave interactive heaters improves results at all wattages, but provides the greatest advantage over non-interactive packaging at the lowest wattages. Using microwave interactive heaters can thus minimize the negative effects of low wattage ovens by coupling a greater portion of the available microwave energy.

Figure 6 depicts the effect of starting tempera- ture on product performance. Identical pizzas were prepared on Receptor Topped trays. Time/ temperature profiles of the topping and the crusthnteractive heater interface were measured. Frozen and chilled starting conditions were compared. As Figure 6 shows, frozen starting conditions yield a better timehemperature re- sponse for maximum crust browning and crisping, i.e. the curves for crust and topping are at the greatest distance. The reason for this lies in the performance of the topping. Liquid water in the

QWlK CRISP vs Unenhanced Popping Performance

Popped Volume (ml) 2400 2100 1800 1500 1200 900 600 300

0 400 500 700 1100

Oven Wattage

Figure 3. Effect of oven wattage on microwave popcorn quality in terms of popped volume

QWlK CRISP vs Unenhanced Popping Performance

Pop Failures 180

150

120

90

60

30

0 400 500 700 1100

Oven Wattage

Figure 4. Effect of oven wattage on microwave popcorn quality in terms of pop failures

8

QWIK CRISP vs Unenhanced Popping Performance

P. HARRISON

Popping Time (sec) 300 270 240 210 180 150 120 90 60 30 0

400 500 700 1100 Oven Watlage

Figure 5. Effect of oven wattage on microwave popcorn quality in terms of popping time

topping absorbs microwave energy several thousand times better than ice. Thus, the topping rises more quickly in temperature from a thawed vs. frozen state. When the topping is a better energy absorber, less microwave energy is avail- able from the finite supply to interact with the interactive heating element and thus heat the crust. Constant competition for available micro- wave energy is a consideration to be kept in mind when designing microwave interactive heater- enhanced packages.

The relative proportion of food vs. interactive heater microwave energy absorbance can be manipulated in a variety of ways. Simply changing the food mass can increase or decrease its energy demand relative to the demand represented by

QWIK CRISP Performance Starting Temperature Effect

--Thawed crust -Thawed Topping -Frozen Crust -Frozen Topping

0 30 60 90 120 150 180 210 240 270 300 Elapsed Time (sec)

Figure 6. Effect of starting temperature on pizza perforrn- ance

the interactive heater. As a consequence, food product variability will contribute to variation in microwave heating results. Consistent results can only be achieved when food parameters such as mass, moisture content and moisture distribution are controlled. Product quality control is vitally important to the success of such microwave foods.

Optimizing food/package performance some- times requires exploring food serving sizehnterac- tive heater area ratios for the best balance of energy absorbance by the food and the packag- ing. As an example a 'shadow' effect exists for large-diameter pizzas. Here, the geometrical relationships between the oven cavity, the pack- age and the pizza limit impingement of micro- waves on the interactive heater at the pizza's centre. This leads to a soft soggy result in the centre of the pizza.

Focused pattern interactive heaters represent another way to effect the food/package absorb- ance balance. Figure 7 illustrates the effect of focus vs. stripe interactive heater configurations on pizza crust temperatures. A stripe configura- tion presents a greater heater area to the available microwave energy, thus reducing the absorbance at the crusthnteractive heater interface. Pattern interactive heater elements limit the heater area to useful product contact locations. Also, it is easier for the consumer to handle a pattern interactive heater package when the microwave oven cycle is completed.

Use of microwave reflecting shields is another way of achieving desired heating patterns. Shields

MICROWAVE INTERACTIVE PACKAGE DESIGN 9

200 - 175 - 150 -

50

QWlK CRISP Performance Focus vs Stripe Heating

OC

- lbpphg-SMelded - Topping-No Shield

200 ---_____ 1 7 5 1 __----- 150 /'---- - -

7 5 : t 50 25

50 25

- CruSt-Shielded -- CrusI-No Shield

-Focus - --Stripe

0 30 60 90 120 150 180 210 240 270 300

8.5 ounce pizzas Elapsed T h e (see)

Figure 7. Effect of focus vs. stripe interactive heater configurations on pizza crust temperatures

have been used successfully to redirect energy for maximum absorbance in interactive heaters. Figure 8 demonstrates the effect on crust temper- ature of shielding a pizza's topping from direct microwave absorbance. More microwave energy is available for the interactive heater and a faster temperature rise is obtained, which yields a crisper product. Aluminium foil has been the material of choice for commercial shielding applications. Unfortunately, foil laminations can be damaged during the package distribution cycle. Damaged foil laminates offer potential sites for arcing when exposed to microwave energy of sufficient intensity. Consumers should be advised not to use damaged foil shields nor to place two foil shielded packages near each other or near the walls of their microwave ovens.

Figure 9 reveals the surprising topping temper- ature responses in the same shielded vs. un- shielded pizza package example. This is attribut-

QWIK CRISP Performance Effect of Shielding

able to steam trapped by the shielding cover. Venting the cover with suitably small holes allows an intermediate temperature result to be achieved. Avoiding elevated boiling points due to trapped steam can be very important. Most bread-based foods suffer from undesirable soggi- ness unless the package is adequately vented.

Strategic elevation of the interactive heater from the reflecting ground plane (i.e. the distance above the floor of the oven), located beneath the oven floor, can also dramatically alter the balance of microwave energy absorbance within a food/ package system. Figure 10 illustrates the rela- tionship of interactive heater elevation and relative absorbance by the pizza topping, and the major change which occurs at a height of 5 4 c m . This translates into an ideal elevation above the oven floor of about 2.5cm. Capitalizing on this relationship was the basis for the first commercial seven-inch-diameter microwave interactive pizza

QWIK CRISP Performance Absorbance vs Elevation

Power Fraclion Absorbed 1 .o

."a. Pizza Topping -3 QWlK CRISP n -'-Pizza

0.2 n i

Elevation (em) at 5OoC

Figure 10. Interactive heater elevation vs. relative absorb- ance by pizza topping, at 50°C

10

0.5 -

0.4 -

0.3 -

0.2 - 0.1 -

0 . 0 ,

P. HARRISON

- ntlel of Twbot - QWlK CRISP

I I I I I i I I I I I t I I I I I I I , I

tray. Elevating the pizza crust from the oven floor increased microwave absorbance by the interac- tive heater and reduced absorbance by the topping without the use of an expensive foil reflecting shield.

Figure 11 depicts the same pizzdpackage absorbance relationship at a higher temperature. Changing food properties as a function of temperature have altered the balance of micro- wave energy absorbance by the interactive heater and the pizza topping.

QWlK CRISP Performance Absorbance vs Elevation

QWlK CRISP Performance Absorbance vs Food Thickness

Power Fraction Absorbed

0.4 -QWIK CRISP - mt of EJrbOI

0.1 ::I: 0.0 0 .2 .4 .6 .8 1 1.2 1.4 1.6 1.8 2

Fillet of Turbot TMckness (cm) Elevated 3cm at 50%'

Figure 13. Microwave absorbance vs. thickness of fillet of turbot

Power Fraclion Absorbed

0.8 -Pizza Topping -0WIK CRISP - -Pizza Crust

0.4 0.3

---------_c_____---- , , 7 7

0 1 2 3 4 5 6 7 8 9 10 Elevatlon (cm)

at 100°C

Figure 11. Interactive heater elevation vs. relative absorb- ance by pizza topping, at 100°C

QWlK CRISP Performance Elevation Effect

Different foods have different microwave absorbance properties. Thus, a package elevation which produces optimum pizza results does not produce the same effect to a similar degree for other foods, such as fish. Illustrating this fact, Figure 12 depicts the relationship between micro- wave absorbance and elevation above the ground plane for a fillet of turbot resting on an interactive heater; strong absorbance by the fish is evident at all elevations.

Changing food dimensions can have a strong effect on the balance of microwave absorption. Figure 13 shows how the interactive heater microwave absorbance can be increased by changing the thickness of the fillet of turbot.

CONCLUSION

Microwave interactive packages and food interact intimately in order to produce satisfying results. Designing convenient, economical, optimum per- forming combinations of food and microwave interactive packaging is a complicated process. Experience indicates that the most successful product introductions results when simultaneous formulation of the food and design of the package occur via a cooperative approach between food manufacturer and carton producer.