development of methodology for assessing the heating performance of domestic microwave ovens

Development of methodology for assessing the heating

performance of domestic microwave ovens

C. James,* M. V. Swain, S. J. James & M. J. Swain

MAFF Advanced Fellowship in Food Process Engineering, FRPERC, University of Bristol, Churchill Building, Langford,

Bristol BS40 5DU, UK

(Received 10 June 2001; Accepted in revised form 3 May 2002)

Summary There is a need for standard methods of testing domestic microwave ovens that relate to

their reheating performance with chilled convenience meals. The investigations reported

here have produced a simple procedure for comparing the three most important reheating

characteristics of a domestic microwave oven: its �true� power; heating variability; and

repeatability. The tests are relatively simple to do, and require only a few additional items

of equipment to those that a laboratory performing existing output power tests would

already possess. Three identical tests are required: one with a liquid test material, one with

a solid and one with a combination of the two components. One additional stage is done

with the liquid test material. The data produced can be reduced to three numbers, which

are measures of the true power, variability and repeatability of the oven. A further simple

analysis, which weights the relative importance of each factor to the consumer, would

produce a single value for the oven’s relative reheating performance. Ovens with unusual

or extreme performance characteristics can therefore be identified easily.

Keywords Characterization, chilled convenience meals, power output, temperature variability, testing.

Introduction

Investigations in the last 10 years have revealed

considerable variability in the ability of different

models and types of domestic microwave oven to

reheat food (Burfoot et al., 1991). Studies have

also shown that Listeria monocytogenes can sur-

vive in some of the cooler areas of chilled foods

after reheating in a microwave oven (Walker et al.,

1991). They have also revealed a large degree of

non-repeatability during reheating.

A typical reheating instruction for a chilled food

would specify �reheating for 3.5 min in a 750 W

microwave oven�. Traditionally, reheating times

for ovens with different microwave powers were

arrived at by modifying the time to produce a

similar total energy input into the food. During

investigations by Burfoot et al. (1991), it became

clear that one of the reasons for the apparent

differences in heating effect between microwave

ovens was the lack of an agreed standard to define

power output. Companies producing ovens have

used up to ten different systems for defining power

output. Since September 1990, microwave oven

manufacturers have adopted a single standard for

measuring the power output for microwave ovens

to be sold in the UK. This standard, International

Electrotechnical Commission (IEC), 1988, uses a

1 L water load and the test is made using a cold

oven in defined ambient conditions.

Studies have clearly shown that power output of

a microwave oven is influenced by the �food� loadin the cavity. Investigations in the USA (O’Meara,

1989) looked at power output into load sizes

ranging from 50 to 2000 g. They showed that the

heating rate of a water load in a 625 W microwave

oven varied by a ratio of approximately 25:1,

depending on the exact cavity loading. Studies by

FRPERC (James et al., 1994) have shown that, in

addition to the effect of load, its position within

the cavity, the length of time the oven has been in*Correspondent: Fax: +44(0)117 928 9314;

e-mail: [email protected]

International Journal of Food Science and Technology 2002, 37, 879–892 879

� 2002 Blackwell Science Ltd

operation and the supply voltage also influence the

power produced. Other investigations (George

et al., 1992) have shown that not all ovens show

the same relationships. In some ovens there was

little change in power output with load. In other

ovens power output initially decreased as load size

was reduced then rose as the size was further

decreased. Complex systems have been developed

for understanding the underlying factors that can

generate complex or unpredictable heat patterns in

foodstuffs in microwave ovens and approaches for

modifying the food to control heating (Bows,

2000).

Many of the tests currently used to assess the

reheating of a foodstuff in microwave ovens are

either subjective, or use commercially produced

chilled meals (IEC, 1988; Burfoot et al., 1991;

George et al., 1994). Previous studies have clearly

shown that there is considerable variability

between individual packs of the same commercial

product (Swain et al., 1994). Consequently when

using commercial packs there is a considerable

degree of uncertainty whether differences in per-

formance between ovens are true differences and

not artefacts caused by pack-to-pack variability.

Additionally, it can be difficult to compare current

tests with those done in previous years as a result

of changes in formulation, and in some cases the

unavailability of some of the meals previously

used. The investigations reported here were

designed to provide a quantifiable test of the

reheating performance of domestic microwave

ovens.

The initial purpose of this work was to develop

a range of �foods� that are suitable for assessing thereheating characteristics of domestic microwave

ovens, i.e. basic �building blocks� that can be used

individually or in combination to simulate meals

of varying complexity, from a simple soup or

sauce to a multicomponent meal. It was essential

that the building blocks were as reproducible as

possible. Then any variation in reheating perform-

ance would be because of variation in the beha-

viour of the microwave oven and not variations in

the composition of the meal. Therefore, the first

stage of the project was to identify �chemical

equivalents� for the basic ingredients/building

blocks. Where this was not feasible, ingredients

were sought that, as far as possible were not

subject to biological or processing variations.

The next stage was to use practical trials to

evaluate the response of the ingredients and basic

building blocks to electromagnetic heating and

hence their suitability as test materials.

Having selected the most appropriate building

blocks, investigations were then done to determine

the best method for assembling reproducible �food�packs representing a range of ready meal type

products.

A method for deciding on the reheating time to

be used with a particular model of oven was also

developed. Only two pieces of data are available to

the purchaser of a domestic microwave oven in the

UK. One is the IEC power rating in Watts (IEC,

1988), which measures the amount of power

delivered by a cold oven into 1 kg of water. The

second is a category rating used in the UK, from A

to E, which relates to the amount of power

delivered into 350 g of water (George et al., 1992;

MAFF, 1992). The simple method developed to

calculate the reheating time was to multiply the

power delivered into a 350 g load by a constant

factor that produced an average final temperature

of approximately 70 �C in the ovens tested. In

practice this would translate into a set time for

each category.

The final part of the investigation was to

perform multireplicate reheating trials on the

simulated ready meals in a range of different

domestic microwave ovens. The resulting tem-

perature distributions after each trial were ana-

lysed to provide a measure of the heating

variability of each oven. Average temperatures

after reheating were calculated as a measure of the

relative energy supplied to the product. A second

estimate of the energy was obtained from a

measure of the liquid test material’s temperature

after stirring. The resulting temperature distribu-

tion data was also tabulated and analysed using

the standard deviation of the temperatures meas-

ured at set positions to assess the repeatability

after reheating.

Materials and method

The investigation was divided into the following

stages:

Stage 1: Identification of reproducible basic

ingredients/building blocks for the substitute

ready meals.

Heating performance testing of microwave ovens C. James et al.880

International Journal of Food Science and Technology 2002, 37, 879–892 � 2002 Blackwell Science Ltd

Stage 2: Practical trials to evaluate the response

of the ingredients and basic building blocks to

electromagnetic heating and hence their suitability

as a test material.

Stage 3: Determination of the best method for

assembling reproducible �food� packs representinga range of ready meal type products.

Stage 4: Multireplicate reheating trials using

substitute foods, made up to be representative of a

range of ready meals, in domestic microwave

ovens having a range of different characteristics.

Characteristics of the microwave ovens

Table 1 lists the six ovens used in the investigation,

and their characteristics. During each stage all six

ovens were tested, with each test being repeated at

least three times. All tests were performed using

cold ovens, i.e. all ovens were left to cool for a

minimum of 8 h before being used again.

Standard IEC-705 1000 g tests (IEC, 1988) were

made on the ovens to establish their characteris-

tics; standard UK 350 g tests (MAFF, 1992) were

used as part of the Stage 2 evaluations.

Stage 1: Identification of test materials

Sauce component

Initial investigations concentrated on developing a

simple homogeneous liquid food (sauce/soup) as a

test material.

The majority of microwaveable convenience

meals rely on an aqueous liquid component (sauce/

gravy/soup) as the main component for microwave

energy absorption and heat transfer. This is because

this component contains a high amount of water,

which has good microwave absorption. Liquids

also allow convective heat transfer, aiding the

dissipation of hot and cold spots.

Generally, sauces are starch based solutions

containing varying amounts of water, carbohy-

drates, proteins, lipids and minerals. As well as

being the main constituents of food, all these

components have an important influence on

microwave heating.

Initial studies determined composition and

identified ingredient sources. The composition of

the final sauce (Table 2) was chosen after com-

paring published values for soups and sauces (Paul

& Southgate, 1978).

Readily available sources of water, lipid, sugar

and salt were identified. Sodium caseinate was

identified as a suitable protein source, as it was

present as a stock item. It was decided that, to

simplify sourcing, the starch component would be

of natural origin, not a commercial modified

starch. This meant that the sauce would have to

be prepared by cooking up the ingredients, in

order to gelatinize the starch component, but

would be widely available. Cornflour was found to

be the most suitable.

The specific heat capacity of the sauce was

calculated using the simple equation published by

Singh & Heldman (1984).

Table 1 Characteristics of the six domestic microwave ovens used in the study

Oven

code

Oven make

and model

Cavity size

(m2)

Turn-table

type Combination Grill

Cavity

coating

Wave-guide

entry

I Matsui M180TC 0.029 Glass No No Yes Side

II Samsung M6135 0.018 Glass No No Yes Side

III Moulinex FMB 735A 0.027 Glass No No Yes Top

IV Belling MW820T 0.022 Glass No No Yes Side

V Philips Whirlpool M914 0.026 Glass No Yes No Side

VI Philips Whirlpool M902 0.026 Glass No No No Side

Table 2 Chemical composition and ingredients of the sauce

Chemical

component

Ingredient

used %

Amounts

needed for

2-kg batch (g)

Water Water 91.2 1824

Starch Cornflour 3.1 62

Lipid Household sunflower oil 2.1 42

Sugar Sucrose 1.9 38

Protein Sodium caseinate 1.2 24

Salt Cooking salt 0.5 10

Heating performance testing of microwave ovens C. James et al. 881

� 2002 Blackwell Science Ltd International Journal of Food Science and Technology 2002, 37, 879–892

Preparation of sauce

Ingredients and amounts are shown in Table 2.

The sauce was first prepared by combining

824 ± 0.1 g of cold water with the dry ingredients

and oil in an electric blender. This mixture was

added to 1000 ± 0.1 g of boiling water in a

saucepan and heated to 90 ± 2 �C, whilst stirringcontinually. The liquid was then strained and

poured into a suitable container, covered, and

chilled to 10 ± 1 �C by placing it in a tempera-

ture-controlled water bath overnight. The sauce

was always freshly prepared and used within 24 h.

Solid component

Investigations were done to determine a suitable

solid component. Solid components in chilled

meals are either of animal or vegetable origin.

Recognizing the inherent variability of meat and

fish samples, consideration was given to using a

structured protein such as soya protein, or

Quorn�. However, little is known about the

microwave properties of these products and their

continuing availability. A vegetable source seemed

to provide the most logical solution to the

problem.

Of the vegetable types commonly available,

potato was identified as the most suitable.

However, it was difficult to obtain accurately

cut shapes from raw potato and there was a

considerable degree of wastage. Potatoes have to

be pre-cooked if they are to represent commer-

cially produced products and the cooking

operation changes the water composition within

the potato. It was therefore decided to use a

reconstituted mashed potato. Mashed potato has

been previously used in many microwave oven

investigations and identified as being a homo-

geneous and reproducible test material (Burfoot

et al., 1991).

Preparation of mashed potato

Potato flake (Grade A, Larsen, ID, USA) was

combined with water at 70 �C in a food mixer

(mixing with a whisk attachment at 190 r.p.m.) at

a weight ratio of 14.3 : 85.7, respectively. The

temperature of mixture was reduced to 10 ± 1 �Cby either placing it in a refrigerator or a tem-

perature-controlled water bath. The mashed

potato was always freshly prepared and used

within 24 h.

Stage 2: Evaluation of suitability of ingredients

and basic building blocks as test materials

The solid component, i.e. mashed potato, was not

evaluated experimentally as its use in previous

investigations (Burfoot et al., 1991) was felt to

prove its suitability as a microwaveable test

material and its response to microwave heating.

The response of the sauce to electromagnetic

heating was evaluated by comparing it with water,

as a test material in UK 350 g tests on the six ovens.

All tests were performed in a controlled environ-

ment room running at 20 ± 2 �C. The power wassupplied to the ovens via a voltage stabilizer (VTR-

2500, Claude Lyons, Hoddesdon, Herts, UK)

providing an input voltage of 240 V ± 1%.

The test liquids were held in conical flasks in a

controlled water bath at 10 ± 0.5 �C. They were

removed from the water bath, stirred and the

temperature measured. Next, 350 ± 1 g of the

liquid was weighed into a cylindrical borosilicate

glass vessel. The vessel was placed in the centre of

the turntable of the microwave oven. The micro-

wave oven was then operated at maximum/full

power for a time calculated to produce a tempera-

ture rise of 10 ± 2 �C. Immediately after heating,

the vessel was removed from the oven, the liquid

stirred and the temperature measured using a fast

reaction T-type (copper-constantan, 0.4 mm diam-

eter) thermocouple placed on a stirring device.

Stage 3: Construction of multicompartment trays

Different ways of assembling �food� packs were

investigated. A twelve-compartment tray method

was considered to be the best option. In this

system, the position and volume of each compo-

nent in the meal could be controlled and the

temperature measuring sensors precisely posi-

tioned, enabling the determination of the posi-

tions of the hot and cold zones. In addition, the

use of a tray divided into a 3 · 4 matrix allowed

a simple analysis of the temperature distribution

in meals after microwave reheating. A compar-

ison of the temperatures attained in matching

compartments could be made, e.g. there are

four identical corners that could be compared

for evenness of heating, two identical inner

compartments each side of the centre line that

could be compared, etc.



Twelve compartment jigs to fit standard PET

(polyethylene terephthalate) ready meal trays

(BXL plastics 1182, Kunstaffwerke GmbH, Werk,

Dietenheim, Germany) were designed and con-

structed and installed in trays (Fig. 1). Jigs were

constructed from 3 mm thick PTFE plastic sheet-

ing. PTFE was selected for its ability to withstand

boiling temperatures and its low dielectric loss

properties. Each jig was manufactured using fric-

tion fitting without glue, which could affect the

microwave field. The jig was not a watertight fit so

that small gaps would allow liquid to filter through

from one compartment to another to reach a

constant level. This removed the need to carefully

fill each compartment to a set level or volume.

Stage 4: Multireplicate reheating trials

Part 1: 350 g water load heated by 10 ± 2 �Cin a twelve-compartment PET tray

Investigations were made to study the effect of

replacing the glass vessel as used in the standard

UK 350 g test, by the PET trays containing the

twelve-compartment jig. Load tests used the same

mass of water and heating times as for a UK 350 g

test, but with the following changes to the proce-

dure:

1 The 350 g of water was poured into the twelve-

compartment PET tray rather than a cylindrical

glass vessel.

2 At the end of the heating period, the door was

opened and a �temperature hedgehog� was

placed in the tray in the oven cavity to measure

the temperature within each of the twelve

compartments. The �temperature hedgehog�consisted of twelve K-type (chromel-alumel,

0.4 mm) thermocouples, each attached to a

vertical 3-mm diameter Tufnol rod held in place

by a polycarbonate sheet. The �hedgehog� waslocated on top of the water load so that the

thermocouples were in the centre of the com-

partments (plan view) and at a height of 14 mm

from the base of the tray (half the depth of

water). The thermocouples were allowed to

equilibrate thermally for 10 s before the data

was recorded using a data logging system

(Magus, Measurement Systems, Newbury,

UK).

3 After the compartment temperatures had been

recorded the tray was quickly removed from the

173 mm

41 mm

90 mm

140 mm

35 mm42 mm

41 mm

43 mm 43 mm

Figure 1 Schematic diagram of PET

ready meal tray with twelve-com-

partment jig inserted.



oven, the jig removed from the tray and the

temperature of the water measured using a fast

reaction T-type thermocouple placed on a

stirring device, as used in Stage 2.

Each test was replicated three times in each of

the six microwave ovens, and the results tabulated.

Part 2: 350 g load heated to a target mean

temperature of 70 �C in a twelve compartment

PET tray

These investigations were designed to provide data

on the temperature/power distribution within a

ready meal tray after heating to an average

temperature of approximately 70 �C. In addition,

these tests also determined whether the UK 350 g

test category remained constant during a longer,

more representative, heating time.

Tests similar to those in Stage 4: Part 1 were

done on the six ovens (three replicates). The

differences were:

1 Water, the sauce and mashed potato were used

as test materials. As well as single component

tests, tests were also made with mashed potato/

sauce combined loads. In this case, the two

components were arranged as shown in Fig. 2.

2 The heating time for each oven (t2) was calcula-

ted using the following equation, as temperature

increase rates were approximately linear:

t2 ¼ 70t1DT1

where t1 is the heating time used in Stage 4: Part

1, and DT1 is the mean temperature rise in stage

4: Part 1.

3 The �temperature hedgehog� was allowed to

equilibrate for 15 s instead of 10 s before

readings were noted, this being found to be

adequate for the higher temperatures.

4 In the case of water and sauce tests, after the

compartment temperatures had been recorded,

the tray was quickly removed from the oven,

the jig removed from the tray and the tempera-

ture of the liquid recorded using a fast reaction

T-type thermocouple placed on a stirring device

(as used in Stage 2). When the PET tray was

removed from the oven, it was placed on a

polystyrene mat to reduce heat loss from the

tray to the bench.

Results and discussion

Characteristics of the ovens

The results of standard IEC-705 and UK 350 g

tests on the six ovens are shown in Tables 3 and 4.

The range of microwave ovens selected for the

tests included ovens with UK 350 g power-outputs

ranging from 614 to 954 W. The measured output

power of ovens I and V exceeded the manufac-

turers E category rating; this has been indicated in

the tables by >E. Ovens II and III were declared

to be C and D category ovens, respectively, but

were B and >E when tested. Ovens IV and VI had

no declared category. Both ovens were found to

exceed the E category rating.

Stage 1: Identification of test materials

Sauce/soup component

It was found that, with careful preparation of the

sauce, a homogeneous liquid with characteristics

similar to that of a thick soup or sauce was

produced. Some separation of components was

encountered on storage, but stirring of the prod-

uct prior to testing seemed to alleviate any

problems.

The specific heat capacity of the sauce calcula-

ted using the Singh–Heldman equation was

4.115 kJ kg)1 �C)1, and this value was used in

calculating the power-outputs into the sauce.

Stage 2: Evaluation of suitability of ingredients

and basic building blocks as test materials

The results of the 350 g sauce load output power

tests performed on the six selected domestic

microwave ovens are shown in Table 4. For

comparison, the mean UK 350 g category, mean

Mashed potato

Sauce

Corner a Side b Side c Corner d

End e Centre f Centre g End h

Corner i Side j Side k Corner l

Figure 2 Arrangement of sauce and mashed potato in cells.



UK 350 g power-output (W) and standard devi-

ation (s.d.) of five replicates are also given for each

oven. Both tests were in identical glass vessels.

Figure 3 illustrates the relationship between the

use of the two test materials for measuring

microwave oven power-output. The best straight

line fit yielded the relationship: sauce output

power ¼ )76.091 + (1.1024 · water output

power), with a correlation coefficient of 0.989.

The repeatability of the sauce output power

tests, as measured by the standard deviations of

the five replicates, ranged from 7.8 to 16.8 W for

the six ovens, compared with 5.8–20.3 W for the

water tests. There was no consistent relationship

between the standard deviation of sauce output

power tests and the standard deviation of water

output power tests for the same oven. However,

overall, the repeatability of the two test methods

was very similar, with mean s.d. values of 11.9 W

for sauce and 13.2 W for water output powers.

The percentage differences between the water

and sauce output powers varied from )2.5 to

3.4%, with a mean difference of )1.0%. The results

indicate little difference between power-outputs

into either of the two solutions when measuring

power-output using the UK 350 g method.

Stage 3: Construction of compartment trays

The twelve-compartment trays were tested in a

range of ovens. No problems were encountered in

terms of localized over-heating, degradation, etc.

Stage 4: Multireplicate reheating trials

Part 1: 350 g water load heated by 10 ± 2 �Cin a twelve-compartment PET tray

A comparison of the results of the Stage 4: Part 1

PET tray test with the results of the UK 350 g

standard vessel load output power tests, performed

Table 4 Mean category, mean output power (W) and standard deviation (s.d.) of five replicated tests using two different

350 g loads in a cylindrical glass vessel (UK 350 g power-output test)

Oven

code

Water mean output

power (s.d.), W

Water

category

Sauce mean output

power (s.d.), W

Sauce

category

Water/sauce

difference, W

Water/sauce

difference, %

I 811.4 (8.1) >E 792.1 (8.0) E 19.3 2.4

II 613.6 (13.3) B 608.6 (9.7) B 5.0 0.8

III 954.4 (20.3) >E 968.7 (15.4) >E )14.4 )1.5

IV 828.7 (12.9) >E 839.3 (16.8) >E )10.7 )1.3

V 899.4 (5.8) >E 927.0 (13.6) >E )27.6 )3.1

VI 948.6 (19.0) >E 981.6 (7.8) >E )33.0 )3.5

Min 613.6 (5.8) 608.6 (7.8) )33.0 )3.5

Max 954.4 (20.3) 981.6 (16.8) 19.3 2.4

Range 340.8 (14.5) 373.0 (9.0) 52.3 5.9

Mean 842.7 (13.2) 852.9 (11.9) )10.2 )1.0

s.d. 126.9 (5.7) 140.7 (3.9) 19.7 2.3

The differences between the mean output powers are shown in Watts and as a percentage. The min, max, range, mean and s.d. at

the foot of the table refer to the columns above them.

Table 3 Results of IEC-705 1000 g and UK 350 g power-output tests

Oven

code

Declared IEC-705

power-output (W)

IEC mean output

power (s.d.), W

Declared UK

350 g category

UK 350 g mean

output power, W

True UK

350 g category

I 800 797.0 (8.5) E 811.4 (8.1) E

II 600 605.3 (6.7) C 613.6 (13.3) B

III 1000 1012.3 (8.0) D 954.4 (20.3) >E

IV 800 833.6 (13.5) – 828.7 (12.9) >E

V 1000 972.7 (16.4) E 899.4 (5.8) >E

VI 1000 1005.3 (20.9) – 948.6 (19.0) >E

s.d.: Standard deviation.



on the six selected domestic microwave ovens is

shown in Table 5. The percentage differences

between the two tests, measured power-outputs

varied from )1.3 to 7.9%, with a mean difference

of 1.8%. The results indicate little difference

between either assessment methods when measur-

ing power-output.

The repeatability of the Stage 4: Part 1 tem-

perature distribution data were judged by the

standard deviations calculated from the replicated

tests (data not shown). These standard deviations

varied from 0.0 �C (oven I, position Corner i) to

1.2 �C (oven IV, Corner a). These standard

deviations compared well with previous results

for thirty-six ovens (five replicates) where standard

deviations varied from 0.04 to 3.03 �C (Swain

et al., 1995).

1000900800700600

700

800

900

1000

UK 350 g output power (W) – Water

UK

350

g o

utpu

t po

wer

(W

) –

Sauc

e an

alog

ue

Figure 3 Plot of mean sauce output

power against mean water output

power for six ovens (error bars

± 1 s.d.).

Table 5 Mean category, mean output power (W) and standard deviation (s.d.) of replicated tests using a 350 g water load

in a cylindrical glass vessel (UK 350 g power test) and a PET tray (Stage 4: Part 1)

Oven

code

Water

category

Water mean output

power (s.d.), W

4: Part 1

category

4: Part 1 mean output

power (s.d.), W

UK/4: Part 1

difference, W

UK/4: Part 1

difference, %

I >E 811.4 (8.1) >E 809.9 (20.4) 1.6 0.2

II B 613.6 (13.3) B 604.5 (6.1) 9.1 1.5

III >E 954.4 (20.3) >E 967.2 (19.5) )12.8 )1.3

IV >E 828.7 (12.9) E 763.6 (15.4) 65.1 7.9

V >E 899.4 (5.8) >E 893.6 (10.0) 5.7 0.6

VI >E 948.6 (19.0) >E 931.0 (17.2) 17.6 1.9

Min 613.6 (5.8) 604.5 (6.1) )12.8 )1.3

Max 954.4 (20.3) 967.2 (20.4) 65.1 7.9

Range 340.8 (14.5) 362.7 (14.3) 77.9 9.2

Mean 842.7 (13.2) 828.3 (14.8) 14.4 1.8

s.d. 126.9 (5.7) 133.2 (5.6) 26.8 3.2

The differences between the mean output powers are shown in Watts and as a percentage. The min, max, range, mean and s.d. at

the foot of the table refer to the columns above them.



Part 2: 350 g load heated to a target mean

temperature of 70 �C in a twelve-compartment

plastic tray

Temperature distribution Table 6 shows a sum-

mary of the minimum, maximum and mean tem-

peratures recorded in each of the 12-compartments

using the temperature �hedgehog�, for the three

replicated tests.

The range of temperatures in the �mean� twelve-compartment tray was used as an indicator of the

potential for an oven to produce hot and cold

spots in a meal tray; the greater the range, the

greater the severity of hot and/or cold spots. The

minimum temperature range was 7.0 �C (oven VI,

water), the maximum temperature range 55.1 �C(oven II, mashed potato/sauce).

The most uniform temperature distributions

were found in trays containing water, correspond-

ing to overall mean �cold spot� and �hot spot�temperatures of 61.7 and 83.9 �C, respectively.

The least uniform temperatures overall were found

in the multicomponent trays containing mashed

potato and the sauce, with mean overall �cold spot�and �hot spot� temperatures of 36.7 and 91.8 �C,respectively.

The mean temperature distributions in the

twelve-compartment trays for each of the ovens,

in relation to the test materials, were calculated for

each oven, and each test material, and shown as

frequency distribution charts (Figs 4–9). Ovens V

and VI showed the smallest temperature distribu-

tions overall with all the test materials, and oven II

the largest.

Repeatability of temperature data The repeatabil-

ity of the Stage 4: Part 2 temperature distribution

data can be judged by the standard deviations

calculated from the replicated tests. These stand-

ard deviations varied from 0.0 �C (oven III, posi-

tion Side k, test material water) to 15.8 �C (oven

IV, End h, mashed potato).

Table 6 Summary of the minimum, maximum, range, mean, and standard deviation (s.d.) of mean temperatures (�C)measured in the 12-compartment trays taken from Stage 4: Part 2 data sheets for the six microwave ovens tested

Oven code Test material Minimum Maximum Range Mean s.d.

I Water 67.2 75.8 8.6 72.6 2.0

Sauce 56.3 92.2 35.9 75.4 10.8

Mashed potato 76.4 90.3 13.9 82.9 4.5

Mash/sauce 58.4 91.2 32.8 76.6 9.5

II Water 66.1 76.0 10.0 71.5 3.2

Sauce 45.9 91.9 46.1 72.5 16.3

Mashed potato 43.2 91.5 48.3 78.1 16.8

Mash/sauce 36.7 91.8 55.1 72.6 16.1

III Water 67.1 80.2 13.1 74.0 3.6

Sauce 50.9 91.3 40.4 75.2 13.4

Mashed potato 73.8 90.4 16.6 84.6 5.8

Mash/sauce 47.2 90.3 43.1 77.5 12.6

IV Water 67.1 76.2 9.1 71.2 2.8

Sauce 46.9 93.4 46.5 69.1 16.2

Mashed potato 55.9 92.8 36.8 79.1 12.7

Mash/sauce 49.2 90.4 41.2 71.2 14.2

V Water 61.7 77.1 15.4 73.0 3.9

Sauce 62.1 88.3 26.3 77.7 9.3

Mashed potato 77.1 88.8 11.7 84.1 3.5

Mash/sauce 60.2 91.1 30.9 79.6 9.7

VI Water 76.9 83.9 7.0 80.1 2.5

Sauce 66.8 87.4 20.5 76.7 7.1

Mashed potato 76.4 91.5 15.0 85.6 5.0

Mash/sauce 73.1 90.8 17.7 83.4 6.1

Overall Water 61.7 83.9 22.2 73.7 3.0

Sauce 45.9 93.4 47.5 74.4 12.2

Mash 43.2 92.8 49.6 82.4 8.1

Mash/sauce 36.7 91.8 55.1 76.8 11.4



Figure 5 Frequency distribution

chart of Stage 4: Part 2 tempera-

ture distribution in 12-compart-

ment trays in oven II.




ment trays in oven I.






ment trays in oven IV.




ment trays in oven III.






ment trays in oven V.




ment trays in oven VI.



In order to make an overall analysis of the

repeatability data, a mean of the 12-compartment

temperature standard deviations was calculated

for each oven, and each test material (Table 7).

These mean values varied from the most repeat-

able, 0.7 �C (oven V, water) to the least repeat-

able, 9.1 �C (oven VI, sauce). Of the test materials

themselves, the most repeatable was water, 1.0 �C,and the mash/sauce combination the least repeat-

able, 5.6 �C, with a combined overall mean

standard deviation value of 4.2 �C. The most

repeatable oven was oven II, 3.5 �C, and the least

repeatable oven was oven VI, 5.2 �C.

Stirred temperature There was no consistent rela-

tionship between an oven’s ability to heat the

sauce and its ability to heat a water load of the

same size, in the same container being heated for

the same time (Fig. 10). Some ovens were more

efficient at heating sauce, others water.

Conclusions

The initial stages of these studies showed that

substituting the sauce for water produced very

similar results in power output tests. The results

also demonstrated that a UK 350 g test using a

PET tray again gave very similar results to that

performed using the standard borosilicate cylin-

der. It can therefore be concluded that the UK

350 g rating is representative of the microwave

power delivered to a food under the test condi-

tions.

Heating a 350 g water load to approximately

70 �C and then repeating the procedure using the

same time but with the liquid food analogue

produced results that show the potential of the

analogue in characterizing oven performance.

Some ovens produced higher average tempera-

tures in water than the analogue, others lower.

This indicates that some models of oven are more

efficient at heating foodstuffs than other ovens.

The test can therefore be used to rank the

performance of the different ovens in terms of

delivered power and consequently heating effi-

ciency.

As would be expected, after a test using water,

very small temperature differences were measured

within the packs. However, much larger tempera-

ture differences were measured in the liquid food

analogue after similar reheating tests. More

importantly, the results revealed substantial dif-

ferences between different models of microwave

oven in terms of temperature variability after

reheating. The test can therefore be used to rank

the performance of the different ovens in terms of

reheating uniformity. Comparing the results from

replicate trials it can be clearly demonstrated that

some ovens perform more consistently than other

ovens. The test can therefore be used to rank the

Table 7 Summary of Stage 4: Part 2 mean standard deviations

Test

material

Oven I

(�C)

Oven II

(�C)

Oven III

(�C)

Oven IV

(�C)

Oven V

(�C)

Oven VI

(�C)

Overall

(�C)

Water 1.5 1.1 0.8 0.8 0.7 1.3 1.0

Sauce 5.2 3.2 3.8 3.2 4.7 9.1 4.9

Mashed potato 4.6 3.8 7.0 6.8 5.1 5.1 5.4

Mash/sauce 4.9 6.0 4.8 6.0 6.6 5.3 5.6

Mean 4.1 3.5 4.1 4.2 4.3 5.2 4.2

80757065605550

55

60

65

70

75

80

Water temperature (˚C)

Sauc

e te

mpe

ratu

re (

˚C)

Figure 10 Average stirred temperature in 350 g water

and sauce when both were heated under equivalent condi-

tions (error bars ± 1 s.d.).



performance of the different ovens in terms of

reheating repeatability.

Similar results were found when using the solid

food and the combination of liquid and solid food.

These tests therefore provide further information

that can be used to rank the performance of the

different ovens in terms of reheating uniformity

and repeatability.

It should be noted that these tests relate

specifically to an oven’s ability to reheat standard

chilled convenience meals in rectangular contain-

ers. While chilled convenience meals come in a

variety of different shaped containers, the majority

are of similar dimensions to those used. Trials

have not been done to assess the affect of different

packaging geometries or materials. Further

experiments will be need to be made to see if

these tests can be directly related to different

configurations.

Because all the trials are quantifiable, it is easy

to analyse the results to provide numerical

values for an individual oven’s performance.

The data produced in each test can be reduced

to three numbers that are measures of the true

power, variability and repeatability of the oven.

Further simple analysis, which weights the

relative importance of each factor to the con-

sumer, could produce a single value for the

oven’s relative reheating performance. Any sim-

ple spreadsheet program could reduce the 195

individual pieces of temperature data gathered in

the total procedure to a measure of oven

performance.

Acknowledgments

The authors would like to thank the Consumer

Association for funding this work.

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development of methodology for assessing the heating performance of domestic microwave ovens

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