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Journal of Food Engineering 78 (2007) 1382–1387

Transport and related properties of breads bakedusing various heating modes

Gulum Sumnu a,*, Ashim K. Datta b, Serpil Sahin a, S. Ozge Keskin a, Vineet Rakesh b

a Department of Food Engineering, Middle East Technical University, 06531 Ankara, Turkeyb Department of Biological and Environmental Engineering, Cornell University, Ithaca, USA

Received 29 July 2005; accepted 6 January 2006Available online 28 February 2006

Abstract

Physical properties of breads during baking were measured using three different heating modes of microwave plus infrared (MIR),microwave plus jet impingement (MJET) and jet impingement (JET) in two different commercially available microwave combinationovens. Breads baked in JET oven were significantly different from the breads baked in other ovens with respect to their specific volumeand moisture content. Transient values of dielectric constant, dielectric loss factor, specific bulk volume, porosity, thermal conductivityand moisture content were determined. For all heating modes, thermal conductivity and dielectric properties of breads decreased sharplywithin the first 2–3 min of baking and then remained constant. Regression equations were developed to relate these properties to mois-ture content and porosity changes during baking.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Baking; Bread; Infrared; Microwave; Transport properties

1. Introduction

Air jet impingement is a form of convection heat transferin which high velocity air is forced against the food product.Air impingement systems have been used in food processingoperations such as drying, baking and freezing (Borquez,Wolf, Koller, & Speiss, 1999; Li & Walker, 1996; Lujan-Acosta, Moreira, & SeyedYagoobi, 1997; Midden, 1995;Moreira, 2001; Nitin & Karwe, 2001; Ovadia & Walker,1998). Considerable reduction in process times andimprovement in product quality can be obtained using thismethod (Wahlby, Skjoldebrand, & Junker, 2000). Impinge-ment, which directs hot air jets at baking items, improvesoven efficiency and accelerates baking, achieving the desiredchanges with more rapid, convective heat transfer.

Using microwaves in food processing is another alterna-tive. Besides the several benefits of microwave heating in

0260-8774/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2006.01.010

* Corresponding author. Tel.: +90 312 2105628; fax: +90 312 2101270.E-mail address: gulum@metu.edu.tr (G. Sumnu).

terms of speed and convenience, it has some disadvantages,such as high moisture loss, soggy and gummy texture andlack of browning. Combining microwaves with othermodes of heating is a solution developed to overcome theseproblems and to provide more control on the moisturetransport while simultaneously increasing the speed ofheating. Combination of microwave with hot air jetimpingement or infrared heating, leads to a much higherrate of heating than microwave heating alone. Infraredand hot air modes of heating are complementary to micro-wave heating, since they can act at different times and dif-ferent spatial locations relative to microwave heating,which allows selective heating. This complementary natureallows combining them to increase the spatial uniformity ofheating while simultaneously increasing the overall rate ofheating (Datta, Greedipalli, & Almeida, 2005).

Microwaves have been combined with jet impingementso as to produce brown and crisp crust in microwave bakedproducts (Walker & Li, 1993). It was possible to achievebrowning in microwave baked cakes by the help of jetair impingement (Li & Walker, 1996). Cakes baked in

Table 1Processing conditions used for different heating modes

Modes of heating Baking condition Baking time

JET 200 �C Up to 8 minMJET 200 �C + 20% microwave power Up to 6.5 minMIR 70% IR + 20% microwave power Up to 8 min

G. Sumnu et al. / Journal of Food Engineering 78 (2007) 1382–1387 1383

microwave–jet impingement oven had acceptable color butthe lowest volume and the firmest texture.

Combining microwaves with near infrared heating is arecent development in microwave baking. The studies onmicrowave–near infrared combination baking are aboutthe investigation of the effect of this heating method onquality (texture, volume and color) of breads (Demirekler,Sumnu, & Sahin, 2004; Keskin, Sumnu, & Sahin, 2004) andcakes (Sumnu, Sahin, & Sevimli, 2005). Breads baked inhalogen lamp–microwave combination oven had compara-ble quality with conventionally baked ones in terms ofcolor, textural characteristics, specific volume and porosity(Demirekler et al., 2004). Cakes baked in infrared–micro-wave combination oven had similar color and firmness val-ues with conventionally baked ones (Sumnu et al., 2005).

Dielectric and thermal properties are the physical prop-erties of food that affect microwave heating. Studies ondielectric and thermal properties of baked products willbe helpful in understanding the heating patterns duringmicrowave baking of foods. There is no dielectric data inliterature during baking of breads in microwave and micro-wave assisted ovens. In addition, very little published infor-mation is available concerning thermal properties of breadduring baking. The studies in literature are about determi-nation of thermal conductivity of bread during conven-tional baking (Bakshi & Yoon, 1984; Unklesbay,Unklesbay, Nahaisi, & Krause, 1981). Bakshi and Yoon(1984) observed that thermal conductivity of bread samplesfirst decreased and then stayed constant during baking.According to the study of Unklesbay et al. (1981) thermalconductivity of breads slightly decreased during conven-tional baking.

In literature, there is no data of thermal conductivitydata of breads during baking in jet impingement (JET),microwave–jet impingement (MJET) and microwave–infrared (MIR) ovens. Dielectric properties of breadsbaked in MJET and MIR ovens are also missing in litera-ture. Therefore, the major objective of this study is todetermine and compare transport properties of breadsbaked in different heating modes which will be helpful toanalyze heat transfer during baking. It is also aimed torelate dielectric and thermal conductivity with moisturecontent and porosity of breads during baking.

2. Methodology

2.1. Dough preparation

Bread flour containing 12.4% dry gluten, 10% moistureand 0.57% ash was used in the study. According to farino-graph analysis, flour has a water absorption of 64.5%,development time of 3 min and stability of 10.7 min. Thecomposition of the prepared dough was on flour weightbasis; 100% flour, 8% sugar, 6% milk powder, 2% salt,3% dry yeast, 8% margarine, 60% water. Dough containing500 g flour was prepared by using straight dough method(Cauvain & Young, 1999).

First, all the dry ingredients were mixed. Milk powderand dry yeast was dissolved in water at 24 ± 1 �C sepa-rately. Margarine was melted and added to the dry ingredi-ents in liquid phase together with dissolved yeast and milkpowder. All the ingredients were mixed by a mixer (KitchenAid, KSM90WH, Greenville, OH, USA) for 3 min. Mixerhas a bowl volume of 41

2quart and dough hook was used in

mixing. After complete mixing of the dough, it was placedinto the incubator at 30 �C. The total duration of the fer-mentation was 105 min. After the first 70 min, the doughwas taken out of the incubator, punched and placed intothe incubator again. A second punch took place after35 min. Then, the dough was divided into 50 g pieces afterfermentation. Each piece was shaped and placed into theincubator for proofing for 20 min under the same incuba-tion conditions.

2.2. Jet impingement baking (JET)

JET baking was performed in a commercial electricaloven (Thermador, 3000 Double Jet DirectTM Oven,CJ302UB, USA) by using high-speed convection heat.The processing conditions used in the oven are shown inTable 1. The velocity of air was 10 m/s and air jets areintroduced from both the top and the bottom. The ovenwas preheated to the set temperature before placing thedough samples into it. Four breads were baked at a time.

2.3. Microwave–impingement combination baking (MJET)

MJET baking which combines the convection mode andmicrowave mode was performed in the same JET ovenmentioned above by combining high-speed convection heatwith microwaves. The frequency of the oven was2450 MHz. The maximum power of the microwaves is750 W. The prepared dough samples were baked in thisoven by using the conditions given in Table 1. The velocityof air was 10 m/s. Microwaves are introduced from the topand air jets are introduced from both top and bottom. Theoven was preheated to the set temperature before placingthe dough samples into it. Four breads were baked at atime.

2.4. Microwave–infrared combination baking (MIR)

MIR baking which combines microwave and IR heatingwas performed in combination oven (Advantium ovenTM,General Electric Company, Louisville, KY, USA). The fre-quency of the oven was 2450 MHz. The maximum power

1384 G. Sumnu et al. / Journal of Food Engineering 78 (2007) 1382–1387

of the microwaves is 950 W. There was a rotary table in theoven to improve heating uniformity of samples. Halogenlamps at the top and bottom were operated at the samepower during halogen lamp baking. There are two halogenlamps on the oven ceiling and one on the oven floor, powerlevel of each being 1500 W. Four breads were baked at theconditions shown in Table 1.

2.5. Temperature measurement

Fiber optic temperature probes were placed at the centerof the roll and temperature readings were obtained using aFISO real time measurement system (FISO Technologies,Inc., Quebec, Canada).

2.6. Moisture content

Moisture content of samples was obtained by drying thesamples in an oven at 105 �C until constant weight wasreached.

2.7. Specific bulk volume and porosity

Specific bulk volume of bread (Vb) was determined bythe rape seed displacement method (AACC, 1988). Specifictrue solid volume (Vt) of the same bread was also deter-mined by rape seed method after mechanically compactingthe bread to exclude all the pores. Total porosity was cal-culated from the following equation:

e ¼ V b � V t

V b

ð1Þ

2.8. Measurement of bread dimensions

Before baking, each roll is in the shape of a semi-spherewith a diameter of 5.9 cm and height of 4.3 cm. The dimen-sions were measured using a digital micrometer (Mitutoyo,Japan). In the case of baked samples breads were cut verti-cally into two halves and images were obtained by a scanner(HP Scanjet 5470C, USA) with a resolution of 300 dpi.Height and diameter of bread samples were measured byanalyzing the images using Image J software (Abramoff,Magelhaes, & Ram, 2004). In images using bars with knownlengths, pixel values were converted into centimeters.

2.9. Thermal conductivity

Thermal conductivity of dough and bread samplesbaked for different times at different ovens were measuredat 23 �C using a Thermal Properties Analyzer (ModelKD2, Decagon Devices, Inc., Pullman, WA, USA). Thisinstrument consists of a hand-held readout and a single-needle sensor made of stainless steel and having a lengthand diameter of 60 mm and 1.28 mm, respectively. Theneedle contains both a heating element and a thermistor.The principle of measurement of thermal properties using

this instrument is based on the dissipation of heat from aline heat source given a known voltage (3 V). After baking,breads were covered with stretch film, kept in a plastic baguntil measurement. The breads were put on top of eachother from base to base whenever the height of a singlebread was lower than the probe length, as was the case dur-ing initial baking periods.

2.10. Dielectric properties

The open-ended coaxial probe method was used to mea-sure dielectric properties of dough and bread samples. Thedielectric measurement system includes HP85070 openended coaxial high temperature probe (Agilent Technolo-gies, Inc., Palo Alto, CA, USA) and a S parameter networkanalyzer (Agilent 8722ES, Agilent Technologies, Inc., PaloAlto, CA, USA). The system was calibrated by measure-ments on air, short and distilled water. Measurements weremade at 2450 MHz.

Dielectric properties of dough and bread samples weremeasured at 23 �C. Breads baked in MIR and MJET ovenswere taken out of the oven, covered with stretch film andkept in a plastic bag until measurement. Breads were slicedin the radial direction to have a thickness of 20 mm byusing a razor blade for measuring dielectric properties ofcrumb. Probe was contacted to the cut bread surface atthe central region and four different positions away fromthe center. Data obtained from near the center and awayfrom the center were analyzed separately. For measuringproperties of crusts, the same from four breads baked atidentical conditions were removed and put together as lay-ers. As the probe was placed on the layers, they were com-pressed slightly just before making a measurement.

2.11. Statistical analysis

Multiple regression analysis was performed to fit sec-ond-order models to dependent variables, by using MinitabRelease 13.0 Software. Different dough prepared on threedifferent days were baked at the same conditions and twoof the breads baked at the same time were used for eachanalysis. Thus, six replications were used for each analysis.

3. Results and discussion

3.1. Temperature and moisture changes during baking

Figs. 1 and 2 show the temperature and moisturechanges during a typical baking period in the threeovens—MIR, MJET and JET. Temperatures were thehighest for the MIR, followed by MJET and JET. Additionof microwaves to jet impingement heating increases thetemperatures significantly. The cycling of temperaturesseen in Fig. 1 for MIR and MJET is due to the on–offcycling of the microwaves.

Fig. 2 shows that the moisture loss occurs primarily in thelater stages of baking when the temperatures have reached

JET

MJET

MIR

0

10

20

30

40

50

60

70

80

90

100

110

0 1 2 3 4 5 6 7 8Time (min)

Tem

pera

ture

(˚C

)

Fig. 1. Transient temperature near the center of breads during baking indifferent heating modes.

MIR

JET

MJET

30

33

36

39

42

0 2 4 6 8

Time (min)

Moi

stur

e co

nten

t (%

)

Fig. 2. Transient moisture content of breads during baking in differentheating modes.

MIR

JET

MJET

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8

Time (min)

Spe

cific

bul

k vo

lum

e (m

L/g)

Fig. 3. Transient specific volume of breads during baking in differentheating modes.

MIR

MJET

Initialdough

JET

3.0

3.5

4.0

4.5

5.0

5.5

6.0

5.5 6.0 6.5 7.0 7.5 8.0

Diameter (cm)

Hei

ght (

cm)

Fig. 4. History of dimension changes in bread during baking usingdifferent heating modes. The baking times for MJET are 0, 2, 3.5 and6.5 min, for JET are 0, 2, 3.5, 6 and 8 min, and for MIR are 0, 2, 3, 4, 6and 8 min, respectively. Error bars represent range of height and diameterin replicates.

G. Sumnu et al. / Journal of Food Engineering 78 (2007) 1382–1387 1385

high enough. In the initial stages of baking there was no sig-nificant difference between moisture losses in three differentheating modes. At the end of baking, MIR heating lead tohigher moisture loss compared to MJET heating, consistentwith higher temperatures during the MIR heating. MIRcombination heating was previously shown to increasemoisture loss of breads as compared to conventional baking(Keskin et al., 2004; Sumnu et al., 2005). The effect of theaddition of microwaves can be seen between the curves forMJET and JET heating. In addition to higher temperaturefor MJET, internal heating also increases internal pressureleading to pressure driven flow and further accelerates mois-ture loss. The least moisture loss (highest final moisture con-tent) occurred in the breads baked in JET.

3.2. Specific bulk volume of bread

Fig. 3 shows that the most of the increase in the specificvolume (a measure of oven rise) of breads occur during thefirst 3–4 min of heating for all three heating modes. Fig. 4shows the history of dimension changes during baking pro-cess. The sudden drop in specific volume (Fig. 3) of breadsbaked in MIR oven at the early time reflects difficulty inmeasurement due to the collapse of the dough as soon asit was removed from the oven. For this reason we feel thatthe actual volume during the first 2 min of MIR heating is

likely to be significantly higher. This difficulty in measure-ment goes away toward the latter part of heating when asomewhat rigid structure has formed and collapse is nolonger an issue. Breads baked in JET oven had the highestvolume. Volume in MJET heating is lower compared toJET because the faster heating in MJET does not providesufficient time for gas generation. The final volume ofbreads baked in MIR oven is 9.1% lower than that of bakedin MJET because of the faster heating of the MIR and alsobecause MIR has low surface temperature such that therigid outer crust does not develop that would allow thegas generated in the dough to expand the volume (insteadthe gas is released). This also explains the lower height ofbreads (Fig. 4) baked in MIR than breads baked in othermodes of heating. With a lower height, the breads spreadin the radial direction. The low final volume of bread bakedin MIR oven is also contributed by its high moisture loss.

3.3. Porosity of bread

Fig. 5 shows porosity data of breads baked in three dif-ferent modes of heating. Porosity change during baking

MIR

MJET

JET

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0 2 4 6 8

Time (min)

Por

osity

Fig. 5. Transient porosity of breads during baking in different heatingmodes.

MJETMIR

0

5

10

15

20

25

0 2 4 6 8Time (min)

Die

lect

ric c

onst

ant

MJETMIR

0

2

4

6

8

10

12

0 2 4 6 8Time (min)

Loss

fact

or

(a)

(b)

Fig. 6. Transient dielectric constant (a) and loss factor (b) of breadsmeasured at the central region of crumb during baking in different heatingmodes.

Table 2Estimated constants of predicted model of dielectric properties of breadduring baking at different heating modes

Constants Dielectric constant (e0) Loss factor (e00)

MIR MJET MIR MJET

a �2858.48 �3108.26 �1423.38 �1648.01b 3086.69 3173.75 1535.44 1684.09c 7496.32 8066.14 3727.03 4270.67d �8078.34 �8223.60 �4015.56 �4362.20r2 0.910 0.999 0.904 0.998

Model equation is Y = a + be + cM + deM, where Y is dielectric constantor loss factor, e is porosity and M is moisture content.

1386 G. Sumnu et al. / Journal of Food Engineering 78 (2007) 1382–1387

follows the trend in specific bulk volume during baking forvarious modes of heating. As expected, breads baked inJET had the highest porosity since they had the highest vol-ume. Breads baked in MJET has porosity in between thebreads baked in MIR and JET. Porosity of breads bakedby JET and MJET increased during early baking and thenremained constant after 2 min of baking. The decrease inthe initial stages of baking in MIR oven is a result of col-lapse of bread as soon as it is removed from the oven, asmentioned earlier.

3.4. Dielectric properties of bread

Dielectric properties of the breads are shown in Fig. 6for the central region of the crumb. Both dielectric constantand dielectric loss factor of bread crumb decreased rapidlyaround 2–3 min and were relatively unchanged for the restof the time. This sharp decrease in dielectric properties withtime can be attributed to the increase in porosity (increasein specific volume of breads) at the initial stages of baking,as illustrated in Figs. 3 and 5. Samples taken from thecrumb but away from the center also showed the sametrend (data not shown to improve clarity) with their finalvalues slightly lower than the values in the central regionof the crumb, which can be attributed to the slightly lowermoisture contents at these outer locations.

For the crust at the end of heating, dielectric constants(6.53 and 6.44 for MIR and MJET, respectively) and lossfactors (1.75 and 1.78 for MIR and MJET, respectively)are significantly higher than those of the crumb portion.This can be attributed to the crust being less porous (higherbulk density) than the crumb. Increase in dielectric proper-ties with bulk density is well documented in the literature(Calay, Newborough, Probert, & Calay, 1995; Nelson,1983).

No significant difference was noticed between the combi-nation heating modes MIR and MJET in how the dielectricproperties measured at the central region of the crumb varyduring heating (Fig. 6a and b). Measured properties of thecrust, as noted earlier in this section, also show similartrend between the heating modes.

As dielectric properties of foods are known to be a func-tion of specific volume (density) and moisture content,regression equations were developed to correlate data ofdielectric properties with moisture content and specific vol-ume (Table 2). The coefficients of determination valueswere quite high in both of the baking methods showingthat dielectric properties during baking were primarilyaffected by the decrease in moisture content and increasein porosity.

3.5. Thermal conductivity of bread

Variation in the thermal conductivity of bread with bak-ing time is shown in Fig. 7. Thermal conductivity of breads

MIR

JET

MJET

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 2 4 6 8

Time (min)

The

rmal

con

duct

ivity

(W

/m˚C

)

Fig. 7. Transient thermal conductivity of breads during baking in differentheating modes.

Table 3Estimated constants of predicted model for thermal conductivity of breadduring baking using different heating modes

Constants MIR MJET JET

a �45.043 9.76 128.715b 48.901 �9.92 �129.615c 116.615 �21.09 �318.685d �126.198 21.70 321.206r2 0.951 0.999 0.940

Model equation is Y = a + be + cM + deM where Y is thermal conduc-tivity, e is porosity and M is moisture content.

G. Sumnu et al. / Journal of Food Engineering 78 (2007) 1382–1387 1387

decreased sharply during initial stages of baking andremained constant afterward. As discussed in the dielectricproperties section, the sharp decrease of thermal conductiv-ity can be attributed to an increase in specific volume(Fig. 3) of bread as more air is entrapped. Such decreasehas been noted in the literature (Bakshi & Yoon, 1984).Regression equations determined to correlate thermal con-ductivity with moisture content and porosity of breads aregiven in Table 3. Like for dielectric properties, high valuesof regression coefficient imply that thermal conductivitychange can be explained primarily due to the changes inmoisture content and porosity.

4. Conclusions

Transient changes in several physical properties ofbreads during baking using three different novel heatingmodes were obtained. Breads baked in MIR (microwaveplus infrared) oven had significantly lower specific volume,porosity and moisture content as compared to the breadsbaked using the heating modes of JET (jet impingement)and MJET (microwave plus jet impingement) for the powerlevels and air temperatures used. Variations in dielectricproperties and thermal conductivity were not significantlydifferent for the various heating modes. Dielectric proper-ties of the crust portion of breads were found to be higherthan the crumb portion. Regression equations were

developed to relate dielectric properties and thermal con-ductivity as a function of moisture content and porosity.The physical property data obtained in this study will pro-vide quantitative information on baking processes usingnovel heating modes and are critical for mathematicalmodeling studies to optimize these processes.

Acknowledgment

This research was supported by the TUBITAK-NSFJoint Research Project.

References

AACC (1988). Approved methods of the AACC. Method 74-09. St. Paul,MN: American Association of Cereal Chemists.

Abramoff, M. D., Magelhaes, P. J., & Ram, S. J. (2004). Image processingwith image J. Biophotonics International, 11, 36–42.

Bakshi, A. S., & Yoon, J. (1984). Thermophysical properties of bread rollsduring baking. Lebensmittel-Wissenschaft und Technologie, 17, 90–93.

Borquez, R., Wolf, W., Koller, W. D., & Speiss, W. E. L. (1999).Impinging jet drying of pressed fish cake. Journal of Food Engineering,

40, 113–120.Calay, R. K., Newborough, M., Probert, D., & Calay, P. S. (1995).

Predictive equations for dielectric properties of foods. International

Journal of Food Science and Technology, 29, 699–713.Cauvain, S. P., & Young, L. S. (1999). Technology of bread baking (pp. 22–

24). Maryland: Aspen Publishers.Datta, A. K., Greedipalli, S. S. R., & Almeida, M. F. (2005). Microwave

combination heating. Food Technology, 59, 36–40.Demirekler, P., Sumnu, G., & Sahin, S. (2004). Optimization of bread

baking in halogen lamp–microwave combination oven by responsesurface methodology. European Food Research and Technology, 219,341–347.

Keskin, S. O., Sumnu, G., & Sahin, S. (2004). Bread baking in halogenlamp–microwave combination baking. Food Research International,

37, 489–495.Li, A., & Walker, C. E. (1996). Cake baking in conventional, impingement

and hybrid ovens. Journal of Food Science, 61, 188–191, 197.Lujan-Acosta, J., Moreira, R. G., & SeyedYagoobi, J. (1997). Air-

impingement drying of tortilla chips. Drying Technology, 15, 881–897.Midden, T. M. (1995). Impingement air baking for snack foods. Cereal

Foods World, 40, 532–535.Moreira, R. G. (2001). Impingement drying of foods using hot air and

superheated steam. Journal of Food Engineering, 49, 291–295.Nelson, S. (1983). Density dependence of dielectric properties of partic-

ulate materials. Transactions of ASAE, 26, 1823–1825, 1829.Nitin, N., & Karwe, M. V. (2001). Heat transfer coefficient for cookie

shaped objects in a hot air jet impingement oven. Journal of Food

Engineering, 24, 51–69.Ovadia, D. Z., & Walker, C. E. (1998). Impingement in food processing.

Food Technology, 52, 46–50.Sumnu, G., Sahin, S., & Sevimli, M. (2005). Microwave, infrared and

infrared–microwave combination baking of cakes. Journal of Food

Engineering, 71, 150–155.Unklesbay, N., Unklesbay, K., Nahaisi, M., & Krause, G. (1981).

Thermal conductivity of white bread during convective heat process-ing. Journal of Food Science, 47, 249–253.

Walker, C. E., & Li, A. (1993). Impingement oven technology—Part III:Combining impingement with microwave. American Institute of Baking

Technology Bulletin, 15, 1–6.Wahlby, U., Skjoldebrand, C., & Junker, E. (2000). Impact of impinge-

ment on cooking time and food quality. Journal of Food Engineering,

43, 179–187.