Effect of Power Output Reduction of Domestic Microwave

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Effect of power output reduction of domestic microwave ovensafter continuous (intermittent) use on food temperature after reheating

M.J. Swain * , S.J. James, M.V.L. SwainFood Refrigeration and Process Engineering Research Centre, University of Bristol, Churchill Building, Langford, Bristol BS40 5DU, UK

Received 11 January 2007; received in revised form 1 March 2007; accepted 11 March 2007Available online 21 March 2007

Abstract

The domestic microwave oven is now commonplace in the home and is often used to reheat chilled ready meals. Previous studies bythe authors have demonstrated that the power output of such ovens can drop signicantly within the rst few minutes of use. This studyhas demonstrated that this power output reduction can have a signicant affect on the expected nal food temperatures after heating. Areproducible procedure was used to carry out heating tests of simulated chilled ready meals in 16 different models of domestic microwaveovens. All of the test loads had lower minimum measured temperatures (mean of three replicates) after heating in the hot ovens (ovensthat had been in use for 15 min prior to the heating test) compared to when they were heated for the same time in the cold ovens (ovensthat had not been used for P 6 h). Minimum measured temperatures fell from a mean of 70.3 C in the cold ovens to mean of 61.3 C(range 50.367.6 C) in the hot ovens. In 12 out of the 16 ovens (75%) these reductions in minimum temperature were statistically sig-nicant ( P 6 0.05). There is an increased risk in terms of the potential for survival of bacteria such as Listeria monocytogenes whenreheating food products to these lower temperatures. It was evident that at the minimum temperatures attained in the food simulant testloads heated in all the hot microwave ovens, the heat treatment would be insufficient to produce a 10 6 reduction of L. monocytogeneseven allowing for an acceptable standing time. 2007 Elsevier Ltd. All rights reserved.

Keywords: Microwave ovens; Microwave heating; Power output; Food temperature; Food safety

1. Introduction

The use of domestic microwave ovens has become com-monplace and the market for prepared microwaveablefoods continues to grow rapidly. All retail food productsthat are intended for microwave heating should provide

on-pack reheating instructions relating the heating timeto the power output of a range of domestic microwaveovens. These instructions aim to provide the consumer witha nal product that is acceptable in terms of both foodsafety and quality.

Guidelines for verifying microwave reheating instruc-tions prepared by the UK Microwave Working Group(Richardson & Gordon, 1997 ) advise that a minimum of

four different ovens, with a specied range of measuredpower outputs, should be used. Therefore, it is importantto be aware of the power output characteristics of micro-wave ovens or more importantly their potential effect onproduct temperatures after reheating.

The guidelines also state that products must achieve a

minimum temperature of 70C for 2 min or an equivalenttime and temperature combination. This reheating sche-

dule is found in a number of advisory documents concern-ing the safe heating of food and makes use of data on theheat resistance of Listeria monocytogenes provided byGaze, Brown, Gaskell, and Banks (1989) . A series of inves-tigations funded by the UK Ministry of Agriculture, Fish-eries and Food (MAFF) in the early 1990s revealed theconsiderable variability in the performance of domesticmicrowave ovens (Burfoot et al., 1991; Swain, Foster,Philips, & James, 1994) and the potential for survival of

0260-8774/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2007.03.013

* Corresponding author. Fax: +44 (0)117 928 9314.E-mail address: [email protected] (M.J. Swain).

www.elsevier.com/locate/jfoodeng

Available online at www.sciencedirect.com

Journal of Food Engineering 87 (2008) 1115
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L. monocytogenes when reheating food products ( Walker,Bows, Richardson, & Banks, 1991 ). Further investigationsstudied the effect on microwave oven power output of arange of factors including; size of load, position of load,variation in mains supply voltage and continuous (inter-mittent) use. James et al. (1994) reported that there was a

signicant reduction in the power output of domesticmicrowave ovens after successively heating water loadsfrom 10 to 20 C over 90 or 30 min periods. The power out-put of all ovens fell with heating time, with most of the falloccurring over the rst 15 min of heating. Power outputswere almost constant after 30 min; on average about 13%below the initial values. However, large variations in powerdecline (820%) were found with different ovens used for30 min. Prior to this there was much anecdotal evidencethat the power output of a microwave oven decreased astheir components became hot but no published informa-tion was readily available on the magnitude of the effect.Decat, Woulters, and Kretzschmar (1993) revealed in theirinvestigations that the efficiency of microwave ovens fellwith increased heating times of water loads. However, thiswas only demonstrated at 1, 3 and 5 min heating times andwith the water reaching different and higher end-point tem-peratures than in a standard test. A more recent study bythe authors ( Swain, Ferron, Pinto Coelho, & Swain,2005) has conrmed that the power output of ve recentmicrowave ovens (on UK market 2003/2004) also fell sig-nicantly during the rst 30 min (mean reduction 17.3%)of continuous use and indicated the need for studies tosee how this affects the temperatures achieved when theovens are used to reheat foods.

The number of households in the Great Britain havinguse of a microwave oven has risen from 62% in 1993, to89% in 2003 (The Office for National Statistics, 2004 ). Withthis and the substantial growth in the chilled ready mealsmarket (70% increase in market value from 1999 to 2003;Chilled Food Association, 2005 ) the likelihood of the ovenbeing used sequentially to heat several food products ismuch greater. Therefore, if the power output of a micro-wave oven falls signicantly as its components heat up,there is going to be an increased risk of under-heating thelater meals. It would also conrm the increased riskinvolved in using such a domestic microwave oven in retailor catering outlets where food is being reheated frequently.

The aim of this study was to measure the effect of poweroutput reduction of domestic microwave ovens after con-tinuous (intermittent) use on the nal temperature of foodafter reheating.

2. Materials and method

Tests were carried out using 16 domestic microwaveovens purchased in the UK ( Table 1 ). The ovens weremicrowave only, microwave and grill or microwave andforced convection combination models with manufactur-ers stated International Electrotechnical Commission

(IEC) 60705 power outputs ranging from 700 to 1000 W,

UK heating categories of either D or E, cavity volumesranging from 17 to 28 l and all had glass turntables. In theUK microwave ovens are labelled by the manufacturerwith a letter (AE) which corresponds to the power outputmeasured in to a 350 g water load, which is similar to themass of a single portion ready meal (e.g. A for a poweroutput between 500 and 560 W, up to E for a power out-put between 741 and 800 W). Microwaveable food packsprovide heating guidelines referring to these heating cate-gories, e.g. heat for 6 min in a category B microwave oven

or 5 min in a category D microwave oven

.The tests were based on the test procedure described bySwain, Spinassou, and Swain (in press) using a 350 g foodsimulant test load with similar microwave heating charac-teristics to a slow heating chilled convenience meal(Swain, Russell, Clarke, & Swain, 2004 ). The food simulantmaterial was composed of TX151 powder, a hydrophilicpolymer (Weatherford, Aberdeen, Scotland), potable waterat 20 C and salt in a TX151:NaCl:water ratio of 22.2:0.7:77.1 by weight. A reproducible food simulantwas used to avoid the inherent biological variability of food.

The rst task was to determine the correct heating timefor the food simulant test load when using each cold oven(i.e. an oven that had not been used for a minimum of 6 h).This was dened as the heating time required for the chilledtest load (equalised to 5 1 C throughout) to reach aminimum measured temperature of 70 2 C at 1 min postheating. To commence each run a new chilled test load wasremoved from the refrigerator, weighed and the speciedlid placed on it. The sample was then placed in the centreof the oven turntable with the long edge parallel to theoven front and the oven started at full power. The timetaken from opening the refrigerator to starting the micro-wave oven heating was 30 1 s. At the end of heating,

the oven was stopped, the test load removed and the nal

Table 1Manufacturers stated power output, UK heating category, oven type andheating time determined to heat the food simulant test load

Oven Power output IEC60705 (W)

Heatingcategory

Oven type Heatingtime (s)

A 700 D Microwave only 345B 900 E Microwave only 230C 800 E Microwave only 345D 900 E Combination 300E 900 E Combination 350F 1000 E Combination 235G 850 E Microwave only 280H 800 E Microwave only 245I 800 E Microwave + Grill 345J 800 E Microwave + Grill 315K 800 E Microwave + Grill 370L 700 D Microwave only 360M 900 E Microwave only 240N 900 E Microwave only 270O 800 E Microwave only 300P 900 E Microwave only a 310

a

Built-in model.

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weight recorded. The test load temperature was then mea-sured using the 39-point temperature hedgehog, ensuringthat the insertion was 30 1 s after the oven had beenstopped. If the minimum measured temperature was notwithin the specied tolerance, the oven was left to coolbefore repeating the test with an appropriately adjusted

heating time and a fresh test load. Once a satisfactory heat-ing time had been achieved, the test was replicated a furthertwo times using that time, recording the test load tempera-tures and weights.

The second task was to replicate a hot microwave oven(i.e. an oven that had been in near continuous use for15 min). In order to this, a cylindrical borosilicate glasscontainer (external diameter 190 mm, height 90 mm, thick-ness 3 mm, Pyrex, Barloworld Scientic, Stone, UK) lledwith 1.7 l of potable water at 20 C was placed in the centreof the turntable and heated for 5 min on full power. After5 min the warm load was quickly removed and replacedwith another lled container and heated for 5 min on fullpower. This cycle was repeated once more until the ovenhad been in use for a total of 15 min.

The third task was to repeat the food simulant heatingtest, but using the hot microwave oven in place of thecold oven immediately after the last water container wasremoved from the 15 min pre-heating task. The heatingtime remained the same as that used for each cold oventest. The hot oven test was replicated a further two times,ensuring that each oven was allowed to cool for at least 6 hbefore the 15 min pre-heating task.

All tests were carried out in an air conditioned labora-tory controlled to 20 2 C. Power to the microwave

ovens was supplied via a constant voltage stabiliser (TS-3B RMS, Claude Lyons, Waltham Cross, UK) and a var-iable transformer (Regavolt 715-G2PE, Claude Lyons,Waltham Cross, UK) providing an input voltage of 230 V 1%.

Statistical analysis ( t-Test) was performed on the mini-mum test load temperatures and test load weight lossesafter heating to determine if there were signicant differ-ences in the means.

3. Results and discussion

The heating time (s) determined in the rst task to heat thechilled test load (equalised to 5 1 C throughout) to reacha minimum measured temperature of 70 2 C at 1 minpost heating is provided in Table 1 and ranged from a min-imum of 230 s (oven B) to a maximum of 370 s (oven K).

Table 2 shows minimum measured temperatures ( C) inthe food simulant test load after heating for the time shownin Table 1 for each cold microwave oven (unused forP 6 h) and hot microwave oven (previously used to heatthree water loads on full power for 5 min each). Each valueis the mean of three replicated trials, with the standarddeviation shown in parentheses.

Fig. 1 shows a plot of the minimum measured tempera-

tures ( C) in the food simulant test load after heating in the

cold and hot microwave ovens, ranked in descending orderof hot oven minimum temperature (mean of three repli-cates, error bars show 1 s.d.). It can be seen that all of the test loads had lower minimum measured temperatures(mean of three replicates) after heating in the hot ovenscompared to when they were heated for the same time inthe cold ovens, falling from a mean of 70.3 C in the coldovens to mean of 61.3 C (range 50.3 C (oven K) to67.6 C (oven H)) in the hot ovens. In nine (ovens A, B,C, D, E, F, I, J, K, L, M and N) out of the 16 ovens(75%) these reductions in minimum temperature were sta-tistically signicant. However, it could be argued that there

Table 2Minimum measured temperatures ( C) in the food simulant test load afterheating in cold (unused for P 6 h) and hot (used for 15 min) microwaveovens

Oven Minimum temperature coldoven ( C)

Minimum temperature hotoven ( C)

A 70.0 (3.33)a 57.2 (5.52)b

B 69.5 (1.63)a 59.7 (1.37)bC 69.7 (1.77)a 62.2 (1.16)b

D 70.8 (0.42)a 63.2 (0.32)b

E 69.7 (1.79)a 62.0 (2.61)b

F 71.4 (0.79)a 58.2 (1.68)b

G 70.2 (3.09)a 67.1 (1.54)a

H 71.0 (1.58)a 67.6 (1.82)a

I 70.5 (1.88)a 56.7 (2.48)b

J 69.5 (0.74)a 65.8 (1.71)b

K 70.4 (5.14)a 50.3 (1.59)b

L 70.3 (5.33)a 56.3 (4.01)b

M 69.8 (2.51)a 62.9 (0.76)b

N 71.1 (0.67)a 65.4 (2.18)b

O 69.5 (8.96)a 58.8 (8.43)a

P 71.3 (4.35)a 67.2 (2.64)a

Values in the same row with different superscripts are signicantly different(P 6 0.05). Mean of three replicates, standard deviation shown inparentheses.

40

45

50

55

60

65

70

75

80

H P G J N D M C E B O F A I L K

Oven code

M i n i m u m

t e m p e r a

t u r e

( C )

Cold oven

Hot oven

Fig. 1. Minimum measured temperatures ( C) in the food simulant testload after heating in cold (unused for P 6 h) and hot (used for 15 min)microwave ovens, ranked in descending order of hot oven minimum

temperature. Mean of three replicates, error bars show 1 s.d.

M.J. Swain et al. / Journal of Food Engineering 87 (2008) 1115 13


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is an increased risk in terms of the potential for survival of bacteria such as L. monocytogenes when reheating foodproducts to these lower temperatures, regardless of theirstatistical signicance. Based on data provided by Gazeet al. (1989) it is estimated that the necessary heat processto produce a 10 6 reduction of L. monocytogenes (one of the

most heat resistant vegetative pathogens) is 70 C for 2 minor equivalent. At 68 C the equivalent heat treatment timeincreases to 3 min 42 s, at 65 C it is 9 min 18 s and at 60 Cit has risen to 43 min 29 s. Therefore, it is evident that atthe minimum temperatures attained in the food simulanttest loads heated in all the hot microwave ovens the heattreatment would be insufficient to produce a 10 6 reductionof L. monocytogenes even allowing for an acceptable stand-ing time.

It was also noted that even when using a cold microwaveoven to heat the food simulant some of the modelsappeared more variable than others. For example, comparethe s.d. of the minimum measured temperatures for oven O(8.96) to that of oven D (0.42). This indicates that evenwhen using a cold oven the minimum temperature of foodheated in oven O could be well below that acceptable,based on guidelines used for safe reheating of chilled readymeals. This is conrmed by the lowest measured minimumtemperature of 60.8 C in one of the replicates heated in acold oven O. In contrast the most reproducible cold micro-wave (oven D) achieved a lowest measured minimum tem-perature of 70.6 C.

Fig. 2 shows the weight loss (%) after heating the testloads in cold and hot ovens. In all cases the weight lossfrom the food simulant heated in the hot microwave ovens

was signicantly less (Table 3 ), which conrmed that thehot ovens heating performance had changed when com-pared to the cold oven. Weight loss ranged from a mini-mum of 2.6% to a maximum of 13.2% in the cold ovensand from 1.2% to 9.9% in the hot ovens. These values also

indicate that there is a large difference between the perfor-mance characteristics of domestic microwave ovens thatthe consumer would be unaware of when considering pur-chasing the different models.

4. Conclusions

Previous studies carried out by the authors highlightedthat domestic microwave ovens suffer from a reduction in

power output during the rst 30 min of continuous use.However, it was not demonstrated as to what the magni-tude of any such effect was on the temperature of foodheated in a hot microwave oven compared to one thathad not undergone a period of continuous use (i.e. a coldmicrowave oven).

On the basis of the this study, it can be concludedthat in 12 of the 16 microwave ovens (75%) there wasa signicant reduction ( P 6 0.05) in the minimum mea-sured food simulant test load temperature after heatingin a hot oven (in use for 15 min) compared to when theywere heated for the same time in a cold oven (not in usefor P 6 h). However, in all hot microwave ovens, mini-mum test load temperatures were recorded which werebelow those required to produce an acceptable temper-ature/time treatment to meet safe reheating guidelines,based on the reduction of L. monocytogenes to anacceptable level.

The authors believe that many users, including con-sumers, caterers and those using domestic microwaveovens for research purposes, would benet from agreater awareness and understanding of how the rapidreduction in power output in the rst 15 min of useaffects the expected heating performance and in somecases compromise the assumed level of microbial safety

provided by food reheating.

0

2

4

6

8

10

12

14

E J K C D I O G N L A H P F B M

Oven code

W e i g

h t l o s s

( % )

Cold oven

Hot oven

Fig. 2. Weight loss (%) after heating food simulant heat test load in coldand hot microwave ovens, ranked in descending order of cold oven

weight loss. Mean of three replicates, error bars show 1 s.d.

Table 3Weight loss (%) after heating food simulant test load in cold and hotmicrowave ovens

Oven Weight loss cold oven (%) Weight loss hot oven (%)

A 4.7 (0.08)a 2.7 (0.33)b

B 2.6 (0.36)a 1.2 (0.10)b

C 8.8 (0.11)a 5.0 (0.16)b

D 8.7 (0.26)a 5.6 (0.10)bE 13.2 (0.08)a 9.9 (0.14)b

F 3.8 (0.14)a 1.5 (0.13)b

G 5.6 (0.06)a 4.6 (0.20)b

H 4.4 (0.21)a 3.3 (0.19)b

I 8.4 (0.08)a 4.2 (0.13)b

J 10.2 (0.28)a 8.0 (0.16)b

K 9.8 (0.61)a 7.8 (0.47)b

L 5.0 (0.15)a 2.9 (0.11)b

M 2.6 (0.35)a 1.5 (0.11)b

N 5.5 (0.40)a 3.8 (0.59)b

O 5.8 (0.36)a 4.5 (0.12)b

P 4.4 (0.21)a 2.8 (0.08)b

Values in the same row with different superscripts are signicantly different

(P 6 0.05). Mean of three replicates, standard deviation shown inparentheses.

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References

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Chilled Food Association. (2005). Chilled foods market data spreadsheet .

[Internet document] URL; http://www.chilledfood.org/downloads/UK%20Chilled%20Food%20Market%201999-2003.xls Accessed 01/03/2005.

Decat, G., Woulters, G., & Kretzschmar, J. G. (1993). Energy efficiency of domestic microwave ovens. In Proceedings of the fourth international conference on microwave and high frequency (p. L1). Go teborg, Sweden.September 2830.

Gaze, J. E., Brown, G. D., Gaskell, D. E., & Banks, J. G. (1989). Heatresistance of Listeria monocytogenes in homogenates of chicken, beef steak and carrot. Food Microbiology, 6 , 251259.

James, S. J., Foster, A. M., Phillips, I. C., Wilkins, T. J., Swain, M. J., &Burfoot, D. (1994). Effects on microwave power-output of size of load,continuous (intermittent) use, position of load and variation in mainssupply voltage. Microwave Science Series Report No. 11 . London:MAFF Publications.

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Swain, M. J., Ferron, S., Pinto Coelho, A. I., & Swain, M. V. L. (2005).Effect of continuous (intermittent) use on the power output of domestic microwave ovens. International Journal of Food Science and Technology, 40 , 15.

Swain, M. J., Spinassou, A., & Swain, M. V. L. (in press). A test procedureto characterise the heating performance of domestic microwave ovens.International Journal of Food Science and Technology .

The Office for National Statistics. (2004). Consumer durables, central heating and cars: 19722003: General household survey 2003 . [Internetdocument] URL; http://www.statistics.gov.uk/StatBase/Expodata/Spreadsheets/D8744.xls Accessed 01/03/2005.

Walker, S. J., Bows, J., Richardson, P., & Banks, J. G. (1991). Listeriasurvival in chilled retail products: effects of recommended microwavecooking. Microwave Science Series Report No. 1 . London: MAFFPublications.

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http://www.chilledfood.org/downloads/UK%20Chilled%20Food%20Market%201999-2003.xlshttp://www.chilledfood.org/downloads/UK%20Chilled%20Food%20Market%201999-2003.xlshttp://www.statistics.gov.uk/StatBase/Expodata/Spreadsheets/D8744.xlshttp://www.statistics.gov.uk/StatBase/Expodata/Spreadsheets/D8744.xlshttp://www.statistics.gov.uk/StatBase/Expodata/Spreadsheets/D8744.xlshttp://www.statistics.gov.uk/StatBase/Expodata/Spreadsheets/D8744.xlshttp://www.chilledfood.org/downloads/UK%20Chilled%20Food%20Market%201999-2003.xlshttp://www.chilledfood.org/downloads/UK%20Chilled%20Food%20Market%201999-2003.xls

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Effect of Power Output Reduction of Domestic Microwave