E484 JOURNAL OF FOOD SCIENCE—Vol. 70, Nr. 8, 2005Published on Web 10/6/2005

© 2005 Institute of Food TechnologistsFurther reproduction without permission is prohibited

E: Food Engineering & Physical Properties

JFS E: Food Engineering and Physical Properties

Effect of Near-infrared Radiationand Jet Impingement Heat Transferon Crust Formation of BreadE.E.M. OE.E.M. OE.E.M. OE.E.M. OE.E.M. OLSSONLSSONLSSONLSSONLSSON, A.C. , A.C. , A.C. , A.C. , A.C. TTTTTRÄGÅRDHRÄGÅRDHRÄGÅRDHRÄGÅRDHRÄGÅRDH, , , , , ANDANDANDANDAND L.M. A L.M. A L.M. A L.M. A L.M. AHRNÉHRNÉHRNÉHRNÉHRNÉ

ABSTRAABSTRAABSTRAABSTRAABSTRACTCTCTCTCT: R: R: R: R: Rapid heat trapid heat trapid heat trapid heat trapid heat transfer methods can be used to speed up the baking pransfer methods can be used to speed up the baking pransfer methods can be used to speed up the baking pransfer methods can be used to speed up the baking pransfer methods can be used to speed up the baking process and crocess and crocess and crocess and crocess and create new preate new preate new preate new preate new productoductoductoductoductprprprprproperoperoperoperopertiestiestiestiesties. . . . . This study invThis study invThis study invThis study invThis study investigates the effect of air jet impingement and infrestigates the effect of air jet impingement and infrestigates the effect of air jet impingement and infrestigates the effect of air jet impingement and infrestigates the effect of air jet impingement and infrararararared red red red red radiation (alone or in combina-adiation (alone or in combina-adiation (alone or in combina-adiation (alone or in combina-adiation (alone or in combina-tion) on crtion) on crtion) on crtion) on crtion) on crust forust forust forust forust formation of parmation of parmation of parmation of parmation of par-baked baguettes dur-baked baguettes dur-baked baguettes dur-baked baguettes dur-baked baguettes during post-baking. ing post-baking. ing post-baking. ing post-baking. ing post-baking. The parThe parThe parThe parThe parameters invameters invameters invameters invameters investigated arestigated arestigated arestigated arestigated are cre cre cre cre crust colorust colorust colorust colorust color,,,,,crcrcrcrcrust thicknessust thicknessust thicknessust thicknessust thickness, total water loss, total water loss, total water loss, total water loss, total water loss, and heating time, and heating time, and heating time, and heating time, and heating time. . . . . The rThe rThe rThe rThe results shoesults shoesults shoesults shoesults show that infrw that infrw that infrw that infrw that infrararararared red red red red radiation and jet impingement,adiation and jet impingement,adiation and jet impingement,adiation and jet impingement,adiation and jet impingement,as comparas comparas comparas comparas compared with heating in a conved with heating in a conved with heating in a conved with heating in a conved with heating in a conventional household oentional household oentional household oentional household oentional household ovvvvven, incren, incren, incren, incren, increased the reased the reased the reased the reased the rate of color devate of color devate of color devate of color devate of color development of the crelopment of the crelopment of the crelopment of the crelopment of the crustustustustustand shorand shorand shorand shorand shortened the heating timetened the heating timetened the heating timetened the heating timetened the heating time. . . . . The fastest color devThe fastest color devThe fastest color devThe fastest color devThe fastest color development was obtained belopment was obtained belopment was obtained belopment was obtained belopment was obtained by combining infry combining infry combining infry combining infry combining infrararararared and im-ed and im-ed and im-ed and im-ed and im-pingement heating. pingement heating. pingement heating. pingement heating. pingement heating. The water loss rThe water loss rThe water loss rThe water loss rThe water loss rate was incrate was incrate was incrate was incrate was increased due to the high heat treased due to the high heat treased due to the high heat treased due to the high heat treased due to the high heat transfer ransfer ransfer ransfer ransfer rateateateateate, but the total water loss, but the total water loss, but the total water loss, but the total water loss, but the total water losswas rwas rwas rwas rwas reduced because of the shoreduced because of the shoreduced because of the shoreduced because of the shoreduced because of the shorter heating timeter heating timeter heating timeter heating timeter heating time. C. C. C. C. Crrrrrust thickness was most dependent on heating time and crust thickness was most dependent on heating time and crust thickness was most dependent on heating time and crust thickness was most dependent on heating time and crust thickness was most dependent on heating time and crustustustustusttempertempertempertempertemperaturaturaturaturatureeeee. I. I. I. I. In genern genern genern genern general, the cral, the cral, the cral, the cral, the crust was thinner for infrust was thinner for infrust was thinner for infrust was thinner for infrust was thinner for infrararararared-heated baguettesed-heated baguettesed-heated baguettesed-heated baguettesed-heated baguettes.....

KKKKKeyworeyworeyworeyworeywords: jet impingement heat trds: jet impingement heat trds: jet impingement heat trds: jet impingement heat trds: jet impingement heat transferansferansferansferansfer, infr, infr, infr, infr, infrararararared red red red red radiation, bradiation, bradiation, bradiation, bradiation, bread, color devead, color devead, color devead, color devead, color development, crelopment, crelopment, crelopment, crelopment, crust forust forust forust forust formationmationmationmationmation

Introduction

Bread is traditionally baked in conventional ovens using heatedair with little or no air recirculation. The heat transfer in such

ovens is low, and interest in alternative ways of heating is increasing.Infrared radiation and hot air impingement are 2 rapid heatingmethods that can be used to achieve high rates of heat transfer andshort baking times.

Par-baking (or partially baking) is a method of bread manufac-turing that involves 2 stages of baking with an intermediate freezingstep (Vulicevic and others 2004). The first stage of par-baking createsa rigid bread structure with minimal crust color. The bread is fro-zen immediately after par-baking. The par-baked bread is a semi-finished product, which can be delivered and post-baked at retaillocations to provide fresh-baked bread. The color and flavor devel-opment and the crust formation are attained during post-baking.Baking involves simultaneous heat and mass transport. The heat istransported from the surface into the bread, and as the temperaturerises, moisture evaporates. The crust thickness increases as thesurfaces gets dehydrated, and the browning and aroma reactionsstart as the temperature rises in the crust.

Infrared energy is thermal radiation that is located between vis-ible light and microwaves in the electromagnetic spectra; the wave-length span is about 0.8 to 1000 �m. Near-infrared waves have awavelength peak of 0.8 to 1.4 �m (Sakai and Hanzawa 1994). The ad-vantage of infrared heat treatment in the food industry is the effec-tive and instant heat transfer and the penetration properties. Thepenetration depth is short compared with microwave and high-frequency waves, but shorter wavelengths provide deeper penetra-tion into the food than longer wavelengths (Dagerskog and Öster-ström 1979). The incident radiation is reflected, transmitted, or

absorbed. The amount of absorption depends on the food propertiesand on the wavelength of the infrared radiation. Optical propertiesof bread have been determined by Skjöldebrand and others (1988)and Dagerskog and Österström (1979). Near-infrared radiation hasthe advantage over conventional baking of decreased baking timeand lower weight loss (Skjöldebrand and Andersson 1987). Bakingwith infrared heating results in bread with a thinner crust and softercrumb (Skjöldebrand and Andersson 1989).

Impingement heating is directed hot air jets of high velocity thatimpinge orthogonally on a surface. The jets disrupt the stagnant veloc-ity boundary layer between the surface and the bulk flow. Higher ve-locity results in a thinner boundary layer and a higher degree of tur-bulence, which enhances the heat transfer. Several reviews and surveysexist for impingement heat transfer (Downs and James 1987; Jambu-nathan and others 1992). Impingement in the food industry has beenreviewed by Ovadia and Walker (1998). The characteristics of impinge-ment ovens versus conventional ovens and the quality of impinge-ment-baked products over conventional-oven-baked products havebeen studied by Ovadia and Walker (1997) and Li and Walker (1996).They reported benefits such as reduced baking time, lower oven tem-peratures, and reduced moisture loss. Bread baked in impingementovens has higher moisture content, which results in softer crumb andincreased shelf life (Yin and Walker 1995).

Impingement has been successfully combined with other heatingtechniques (Walker and Li 1993; Li and Walker 1996; Ovadia andWalker 1998). Datta and Ni (2002) reported the application of com-bined infrared, microwave, and hot air heating for food products.A combined mid-infrared and hot air dryer has been developed byUmesh Hebbar and others (2004). Infrared and hot air were com-bined and resulted in an efficient drying process.

The objective of this study was to investigate the effect of air im-pingement and infrared heating (alone or in combination) on colorand crust development during the post-baking of par-baked ba-guettes. Results were compared with baking in a conventional house-hold oven. The characteristics investigated were crust color, crustthickness, and water loss during heating.

MS 20050021 Submitted 1/11/05, Revised 4/18/05, Accepted 7/1/05. AuthorsOlsson and Ahrné are with SIK – The Swedish Inst. for Food and Biotech-nology, P.O. Box 5401, SE-402 29 Gothenburg, Sweden. Author Trägårdh iswith Dept. of Food Technology, Engineering and Nutrition, Lund Univ.,Lund, Sweden. Direct inquiries to author Olsson (E-mail: eo@sik.se).

Vol. 70, Nr. 8, 2005—JOURNAL OF FOOD SCIENCE E485URLs and E-mail addresses are active links at www.ift.org

E: Fo

od En

ginee

ring &

Phys

ical P

rope

rties

Effect on crust formation of bread . . .

Materials and Methods

The baguetteThe baguetteThe baguetteThe baguetteThe baguetteA standard recipe for white bread was used: 62% wheat flour, 34%

water, 3% yeast, 0.6% salt, and 0.6% improver (Lecimax 2000, Nord-falksbakels, Sweden). The bread was formed as a small baguette andprebaked in a rack oven at 155 °C for 12 min. Prebaking created abaguette with a ready-baked crumb and a thin, colorless crust. Thelength of the baguette was about 12 cm and the diameter was 40 to50 mm after baking. The baguettes were immediately frozen andstored in a –40 °C freezer. Before post-baking, the baguettes werethawed in +30 °C and 80% relative humidity for 1 to 2 h.

The infrThe infrThe infrThe infrThe infrararararared-impingement combination oed-impingement combination oed-impingement combination oed-impingement combination oed-impingement combination ovvvvvenenenenenThe combination oven (Ircon Drying Systems AB, Vänersborg,

Sweden) is composed of a cassette with 2 near-infrared radiators, 1on each side of a slot nozzle (Figure 1). The radiators consist of aquartz tube filled with halogen gas and a tungsten thread. The max-imum color temperature is over 2100 °C, which corresponds to awavelength peak of 1.2 �m. To create the impinging air jet, a fan anda heater are connected to the oven. The maximum average velocityof the jet exit is about 7.5 m/s (determined by a vane probe anemom-eter) and the maximum temperature is >500 °C. The maximum powerof the infrared radiators (100%) and the impinging jet (7.5 m/s) wereestimated in the center of the oven using a black-painted copperplate (50×50×10 cm, with a thermocouple in the center) and lumpedsystem analysis, described in Singh and Heldman (2001). The methodgives an approximate value of the heat transfer to the surface. The ap-parent radiative heat transfer was found to be about 40 kW/(m2°C),and the apparent heat transfer coefficient was about 140 W/m2°C fora jet with a velocity of 7.5 m/s. The width of the slot nozzle (d) is 10mm, and the distance from the nozzle to the surface of the baguette(H) is 50 mm, which gives an H/d distance of 5. The dimensions of theoven are 500×450×330 mm and the baguettes rest on a stationary trayin the middle of the oven.

The post-baking experThe post-baking experThe post-baking experThe post-baking experThe post-baking experimentsimentsimentsimentsimentsA screening test was performed for infrared heating, impingement

heating, and simultaneous infrared/impingement heating following3 full factorial designs with a triple center point (Figure 2). The con-sidered factors were infrared power and heating time for infraredheating; air temperature, air velocity and heating time for impinge-ment heating; and infrared power, air temperature (maximum airvelocity of the jet) and heating time for combined infrared and im-pingement heating. The investigated responses were color, crustthickness, and water loss. The experimental design program Modde5.0 (Umetrics, Umeå, Sweden) was used to generate experimentaldesign and analyze the result. After the screening test, additionalexperiments (Table 1) were performed to better understand thedevelopment of color, crust, and water loss during heating. Post-baking in a conventional household oven (no air-recirculation) wasalso made for comparison.

During heating, the temperature was measured every second in4 locations: in the oven chamber, in the nozzle, in the center of thebaguette, and on top of the baguette. Very thin thermocouples (0.07-mm dia) were carefully placed in the baguette surface to measurethe temperature of the crust at different depths. The accurate loca-tion of the thermocouples was measured after post-baking using adigital caliper. Copper-constantan thermocouples of type T (Pen-tronics, Sweden), a logger, and a computer were used to record thetemperatures.

The post-baking prThe post-baking prThe post-baking prThe post-baking prThe post-baking product measuroduct measuroduct measuroduct measuroduct measurementsementsementsementsementsAfter post-baking, the color was measured on top of the baguette

with a Minolta CR-10 camera and the L*a*b* color space analysismethod, where L* is the lightness variable and a* and b* are thechromaticity coordinates. The result was reported in the form of acolor difference, dE*:

where dL*, da*, and db* are the differences in the L*, a*, and b*values between the sample and the reference (a white ceramic platewith target L = 93.4, a = -1.8, and b = 4.4).

The thickness of the crust was measured using a digital caliper.The crust was obtained by carefully separating the entire soft crumbfrom the hard crust. Thus, the crust measurements may also includethe part of the crust that is dried, but does not have any color. Tominimize experimental errors, the crust color and thickness weremeasured 3 times on the top of each baguette (and averaged) by thesame person and in the same order. The measurements were tak-en as nearly as possible at the same time after post-baking.

The total water loss of the bread (g water lost/g initial weight) wasdetermined by measuring the weight before and after heating. Theintensity of heat treatment was calculated from the area under thecrust temperature profile curve, giving a measurement of the exter-nal heat required to obtain a certain color, crust thickness, or wa-ter loss.

Results and Discussion

HHHHHeat treat treat treat treat transfer and temperansfer and temperansfer and temperansfer and temperansfer and temperaturaturaturaturature pre pre pre pre profilesofilesofilesofilesofilesInfrared and impingement heating generated different rates of

heat transfer, as shown by the temperature profiles in the crust(about 0.5 to 2.5 mm from the top) and in the crumb during post-baking (Figure 3). For clarity purpose and better comparison betweenthe different heating techniques, only the first 2 min of heating ispresented. For radiative heat transfer, the maximum power (100%)was about 40 kW/m2, causing the crust temperature to increase at aconstant rate. The convective heating achieved by the impinging jetFigure 1—The infrared-impingement combination oven

d

E486 JOURNAL OF FOOD SCIENCE—Vol. 70, Nr. 8, 2005 URLs and E-mail addresses are active links at www.ift.org

E: Food Engineering & Physical Properties

Effect on crust formation of bread . . .

is dependent on air temperature and velocity. The apparent heattransfer coefficient was estimated to be about 140 W/(m2°C) for avelocity of 7.5 m/s. The initial heat transfer rate to the surface washigh because of a large temperature difference between the initialsurface temperature and the jet temperature. In this study, thebaguettes were placed in the oven before turning on the heating ofthe air. The initial heating-up period of the air reduced the heattransfer compared with if preheated air was used.

High convective heat transfer caused high dehydration of thesurface, which made the temperature increase above 100 °C quickly.For a jet of 7.5 m/s and 250 °C, a temperature difference of about100 °C gave a heat flux of about 14 kW/m2. A lower temperature(180 °C) of the jet reduced the initial temperature difference and thecorresponding heat flux (a temperature difference of 60 °C gave about8 kW/m2). As the surface temperature approached the air tempera-ture of the jet, the heat transfer rate was reduced. The temperaturedifference was about 30 °C when the crust temperature reached itsmaximum temperature, giving a heat flux of about 4 to 5 kW/m2.The crust temperature in convective heating cannot exceed the tem-perature of the surrounding air, as it can in the case of radiativeheating. In case of combined infrared and impingement, the tem-perature profile increased rapidly as in the case of impingementalone, but the maximum crust temperature is not limited by the airtemperature. In a household oven, the crust temperature increasedslowly toward the oven temperature due to the low heat transfer.

The purposes of post-baking are color and flavor development inthe crust region, but also to reverse starch staling by reaching 70 °Cto 80 °C in the crumb. This temperature was reached at differenttimes depending on the heating technique and process conditions.A crumb temperature of 70 °C was reached fastest for infrared heat-ing, and longer times were necessary for impingement-baked andhousehold oven–baked products. In some cases, when the heat trans-

fer rate was high and the heating time short, the crumb did not reach70 °C. The temperature profiles in the crust for the combined infra-red/impingement case and the impingement alone case were sim-ilar, but the crumb temperature was reached earlier in the case ofcombined heating. The penetration properties of the infrared radi-ation are probably responsible for the earlier temperature rise in thecrumb.

SSSSSignificant prignificant prignificant prignificant prignificant process parocess parocess parocess parocess parametersametersametersametersametersA screening test was made using full factorial design for infrared,

impingement, and combined infrared and impingement heating toidentify the most important factors on the responses. A Partial LeastSquare (PLS) model was used to develop a relationship of the re-sponses to the factors. The goodness factor of the fit, R2 that is thepercent of the variation of the response explained by the model isshown in Figure 4. The model fitted well the data for all responses(R2 > 0.9), except for color development in infrared heating (R2 <0.9). The most important parameters that influence crust color,thickness, and water loss can be seen in Figure 4. A positive barrepresents a positive response, that is, a parameter (infrared power,heating time, and so forth) that increases the response (the crustcolor, the crust thickness, or the water loss) and a negative bar rep-resents a negative response.

For infrared-heated baguettes, infrared power and heating timewere both important parameters for color development, crust thick-ness, and water loss. For color development, infrared power hadgreater significance than heating time, whereas for crust thicknessand water loss, heating time had more effect than infrared power. Theair temperature in impingement heating was the most significantparameter affecting color development. Crust thickness and waterloss were most affected by temperature, but heating time and airvelocity also had an effect. Air velocity was found to play a less sig-

Table 1—Experimental conditions used in the additional experiments

IR Impingement IR/impingement Household oven

IR (%) Time (min) T (°C) v (m/s) time (min) IR (%) Ta (°C) Time (min) T (°C) Time (min)

50 2.25 180 7.5 1 100 180 3 180 550 4 180 7.5 2 100 250 1 180 10

100 1 250 7.5 2 — — — 250 2.5100 2.25 — — — — — — 250 5— — — — — — — — 250 10

aUsing maximal air velocity.

Figure 2—The factorial experimental design plan. IR = infrared power; time = heating time; T = air temperature of impingingjet; v = air velocity of impinging jet. *Using maximal air velocity.

V

Vol. 70, Nr. 8, 2005—JOURNAL OF FOOD SCIENCE E487URLs and E-mail addresses are active links at www.ift.org

E: Fo

od En

ginee

ring &

Phys

ical P

rope

rties

Effect on crust formation of bread . . .

nificant role than expected, probably due to the generally low velocitycompared with high air temperature. The small effect of air veloci-ty was considered when the selection of the conditions in the addi-tional experiments was made; for that reason, only experiments atmaximal velocity were done. Color development for baguettes heatedby simultaneous infrared and impingement was most strongly affect-ed by air temperature and heating time. Heating time dominated incrust formation and water loss. Infrared power had a small but pos-itive effect for all 3 responses (color, crust thickness, and water loss).

WWWWWater lossater lossater lossater lossater lossThe changes in the surface of the bread during post-baking, in

addition to color and flavor development, have some similaritieswith a drying process; the temperature increases as water evapo-rates and the crust is formed. The water loss is strongly related tothe temperature in the crust because the temperature increases asthe crust dehydrates. The total amount of water loss dependedmostly on the heating time and the air temperature (Figure 4). Fig-ure 5 shows that heating with infrared radiation resulted in lowestwater loss rate and lowest total water loss. The temperature in thecrust of infrared-heated baguettes increased slowly (Figure 3).Combined infrared and impingement had the highest rate of wa-ter loss, but because of the low baking time, the total water loss wasreduced. Impingement and household oven baking incurred aboutthe same water loss rate, but because the heating time was shorterin impingement, the total water loss was less than with conventional

heating. The water loss is also dependent on the humidity in thebaking chamber, but not studied here.

The reduction of total moisture loss in impingement has beenreported by several authors (Walker and Li 1993; Yin and Walker 1995;Ovadia and Walker 1997). Wählby and Skjöldebrand (2002) found thatthe water loss rate in jet impingement baking of buns was greaterthan baking in a hot air oven (with recirculation) at the same air tem-perature, and that the water loss increased with temperature. Re-duced weight loss with infrared radiation has been reported bySkjöldebrand and Andersson (1987).

The effect of maximum temperature in the crust and the intensityof heat treatment applied on the total water loss for the differentheating techniques are shown in Figure 6. The intensity of heattreatment was defined as the area under the crust temperatureprofile curve (Figure 3). The water loss in infrared heating was lowerthan with the other methods, but increased rapidly with higher crusttemperature. The dependency on maximum temperature in the crustdecreased for the different methods in the following order: infrared,impingement, simultaneous infrared and impingement, and house-hold oven (Figure 6a). The total water loss as a function of the inten-sity of heat treatment is shown in Figure 6b. Infrared heating by it-self and in combination with impingement had a lower total waterloss compared with the other methods.

CCCCColor devolor devolor devolor devolor developmentelopmentelopmentelopmentelopmentSurface browning is a result of the temperature rise and the wa-

Figure 3—Temperature profilesduring the 1st 2 min of heating,in jet exit, oven chamber andcrust and crumb of the ba-guette for the different heatingtechniques

E488 JOURNAL OF FOOD SCIENCE—Vol. 70, Nr. 8, 2005 URLs and E-mail addresses are active links at www.ift.org

E: Food Engineering & Physical Properties

Effect on crust formation of bread . . .

ter loss in the crust. The temperature in the crust increased rapidlyas the surface dried out, and the browning reactions started at about130 °C to 140 °C. Surface browning is caused by Maillard reactions,the complex reactions between sugars and proteins on the surface,which also are responsible for the bread’s aroma. A thin, non-coloredcrust (dE* = 23) was obtained by pre-baking the baguettes. A widecolor range, from light brown (dE* = 30) to dark brown (dE* = 55), wasdefined as acceptable after post-baking. Color development on thesurface increased with surface temperature and heating time. Rapidheat transfer methods such as infrared radiation and hot air impinge-ment produced a desirable color on the surface faster than with theconventional household oven (Figure 7).

The temperature profiles in the crust are closely connected to colordevelopment. In infrared-heated baguettes, the temperature increasein the crust in the beginning was slow compared with impingement(Figure 3). It can be coupled to the lag effect in the color developmentprofiles in Figure 7. The time to reach the temperature where theMaillard reactions start was longer, but when that temperature was

reached, the color development was rapid. Infrared heating was, ingeneral, faster than air impingement for color development, andsimultaneous infrared and impingement heating was the most rapid.The temperature of the crust sets the final color on the surface. Jetimpingement heating and heating in a conventional oven at the sameair temperature resulted in baguettes with the same final color on thesurface, but the impingement-baked baguettes reached the color ina shorter time. Higher velocity of the air jet increased the rate of colordevelopment.

Full power (100%) infrared heating combined with an air jet ofmaximum velocity (7.5 m/s) and a temperature of 250 °C increasedthe rate of color development compared with infrared heating alone.However, lowering the air temperature of the jet to 180 °C reduced theheat transfer and the color development rate (Figure 7). The heattransfer from the air jet was low compared with the infrared radia-tion. The impinging jet therefore had a cooling effect as the temper-ature of the air jet was lower than the temperature of the bread sur-face. For optimal heat transfer, this effect must be considered when

Figure 4—The effect of design factors on color development, crust thickness, and water loss. Normalized coefficients on they-axis (the coefficients are centered and scaled to unit variance and normalized by dividing with the standard deviation of theresponse). IR = infrared power; time = heating time; T = air temperature of impinging jet; v = air velocity of impinging jet.

Figure 5—Water loss forthe investigated heatingtechniques: higherheating (a) and lowerheating (b). HO = house-hold oven; IM = impinge-ment; IM/IR = simulta-neous impingement andinfrared; IR = infrared.

Vol. 70, Nr. 8, 2005—JOURNAL OF FOOD SCIENCE E489URLs and E-mail addresses are active links at www.ift.org

E: Fo

od En

ginee

ring &

Phys

ical P

rope

rties

Effect on crust formation of bread . . .

designing equipments and process conditions that combine infra-red and air impingement heating.

Color development was dependent on the maximum tempera-ture in the surface (Figure 8a). After the browning had started (atabout 130 °C), the final color increased linearly with the maximumtemperature on the surface, a result also reported by Skjöldebrandand Andersson (1989). The intensity of heat treatment (area underthe crust temperature profile curve) is a measurement of the exter-nal heat transported to the crust. The color increased with an increasein intensity of heat treatment for all methods, but with different rates

(Figure 8b). For the same final color, the conventional household ovenand the impingement technique required larger amounts of heattreatment than infrared heating and the combination of infrared andimpingement heating.

In post-baking, the temperature of the crumb is also important.If the color development is too rapid, it is possible to burn the crustbefore the crumb reached the desirable temperature. For refresh-ment reasons, the crumb temperature should reach about 70 °C to80 °C. However, if this temperature is not reached during heating, thetemperature in the crumb can continue to increase after the heating

Figure 7—Colordevelopment onthe surface of thebaguette: higherheating (a) andlower heating (b).HO = householdoven; IM =impingement; IM/IR = simultaneousimpingement andinfrared; IR =infrared.

Figure 8—Color development onthe surface of the baguette as afunction of maximum tempera-ture in the crust (a) and intensityof heat treatment (b). HO =household oven; IM = impinge-ment; IM/IR = simultaneousimpingement and infrared; IR =infrared.

(104°Cs)

Figure 6—The total water lossas a function of maximaltemperature in the crust (a)and intensity of heat treatment(b). HO = household oven; IM =impingement; IM/IR = simulta-neous impingement andinfrared; IR = infrared.

(104°Cs)

E490 JOURNAL OF FOOD SCIENCE—Vol. 70, Nr. 8, 2005 URLs and E-mail addresses are active links at www.ift.org

E: Food Engineering & Physical Properties

Effect on crust formation of bread . . .

is stopped because of the large temperature gradient between thecrust and crumb. If the crumb temperature is reached a long timebefore the desired color on the surface, the baguette may become dry.

CCCCCrrrrrust thicknessust thicknessust thicknessust thicknessust thicknessThe crust of the prebaked baguette was thin, about 0.3 to 0.5 mm.

During post-baking, as the drying zone moves toward the interior ofthe bread, the crust gets thicker. High heat transfer rates and hightemperatures in the crust are connected to a high dehydration rate,as discussed in previous paragraphs. The heating time was foundto be important for crust formation for all the heating techniques;in impingement, the crust temperature was also significant (Figure4). The increase of crust thickness with time can be related to theloss of moisture by comparing Figure 5 and 9; a process resulting ina thick crust also typically had a high dehydration rate. However,infrared heating had a higher rate of crust formation than water loss,relative to the other methods.

A convective method, such as impingement generated largercrust thickness than infrared radiation. The rate of crust formation(and water loss) was highest for the combined infrared and im-pingement heating method, followed by heating with impingementonly (Figure 9). In general, a rapid heat transfer method required lesstime than a conventional heating method for the desired color tobe reached, resulting in a thinner final crust.

Infrared heating with low power (50%) had initially a slow tem-perature increase in the surface (not shown), a low water loss rate(Figure 5b), and a low color development rate (Figure 7b). The crustwas very thin for the first minutes of heating. The water loss and crust

thickness increased after reaching the same heating time (Figure 5band 9b). The crust formation for infrared was slow for short times, butincreased rapidly as the temperature of the surface rose (Figure 9b).The crust thickness increased fast (after the lag period) for infraredheating, but because the heating time was short, the final crust wasthin. The same result was observed by Skjöldebrand and Andersson(1989). For conventional heating, the crust thickness increased slowlybecause of the low heat transfer and low moisture transport, but thefinal thickness was large because of the long heating time.

The crust thickness increased with higher temperatures in thecrust at about the same rate for all the heating techniques. In Figure10a, the crust thickness is plotted as a function of maximum temper-ature in the crust. Wählby and Skjöldebrand (2002) showed that thecrust thickness became larger when higher air temperatures wereused. In Figure 10b, the estimated intensity of heat treatment washigh for the conventional household oven and for impingement witha high temperature. For infrared and the combination of infrared andimpingement, the heat requirements were lower.

Conclusions

Infrared radiation and air jet impingement are rapid heating meth-ods that increase the rate of color development and shorten the to-

tal heating time. In this study, the heat flux was higher for infraredheating than impingement. The different heating techniques causeddifferent temperature profiles in the crust. Initially, the temperaturein the infrared-heated crust increased slowly, while the temperaturein impingement-heated crust increased rapidly until the tempera-ture of the surface came close to the air temperature. The crumb

Figure 9—Crustformation for theinvestigated heatingtechniques: higherheating (a) and lowerheating (b). The errorbars indicates thevariation of 3 measure-ments. HO = householdoven; IM = impinge-ment; IM/IR = simulta-neous impingementand infrared; IR =infrared.

Figure 10—Crust thickness asa function of maximal tem-perature in the crust (a) andintensity of heat treatment (b).HO = household oven; IM =impingement, IM/IR = simulta-neous impingement andinfrared; IR = infrared.

(104°Cs)

Vol. 70, Nr. 8, 2005—JOURNAL OF FOOD SCIENCE E491URLs and E-mail addresses are active links at www.ift.org

E: Fo

od En

ginee

ring &

Phys

ical P

rope

rties

Effect on crust formation of bread . . .

temperature rose to a high temperature earlier in case of infraredheating, most likely due to the penetration properties of the infraredradiation.

The high heat transfer rates in impingement resulted in high masstransfer rates, but because the total heating time was short, the to-tal amount of water loss was reduced as compared with heating in aconventional household oven. Lower total water loss was also gen-erally observed for infrared- than impingement-heated bread. Thecrust temperature was central for color development. Infrared heat-ing caused slow initial color development, but after a lag period,when the crust temperature had reached a sufficiently temperature,the color development was rapid. In general, higher color develop-ment was observed for infrared heating than impingement heating,and both were higher than for conventional heating. Infrared in com-bination with impingement heating was found to increase the col-or development rate and shorten the heating time when the air jethad sufficiently high temperature and velocity. When the air temper-ature and the velocity were not high enough, the combination hada cooling effect and reduced the heat transfer. Crust thickness wasmost dependent on heating time but also on the crust temperature.A short heating time resulted in a thin crust. The thickness of thecrust, for the same final surface color, was less for infrared-heatedbaguettes than baguettes heated by impingement or in a convention-al household oven.

By combining impingement and infrared heating, it is possibleto control the heat and mass transfer processes and consequentlyachieve the aimed bread crust characteristics. The results from thisstudy will be useful for designing baking processes, based on theproduct target crust and crumb characteristics.

AcknowledgmentsThe Swedish Knowledge Foundation is acknowledged for financialsupport of E Olsson’s Ph.D.-project and Ircon Drying System AB inVänersborg; Sweden is acknowledged for financial support and formanufacturing of the infrared-impingement combination oven. Thisarticle also has been supported by the European Commission, Pri-

ority 5 on Food Quality and Safety (Contract nr FOOD-CT-2003-506820 Specific Targeted Project), “Heat-generated food toxicants –identification, characterization and risk minimization.” This publi-cation reflects the author’s views and not necessarily those of the EC.The information in this document is provided as is and no guaran-tee or warranty is given that the information is fit for any particularpurpose. The user thereof uses the information at its sole risk andliability.

ReferencesDagerskog M, Österström L. 1979. Infra-red radiation for food processing. I. A study

of the fundamental properties of infra-red radiation. Lebensm Wiss Technol12:237–42.

Datta AK, Ni H. 2002. Infrared and hot-air-assisted microwave heating of foods forcontrol of surface moisture. J Food Eng 51:355–64.

Downs SJ, James EH. 1987. Jet impingement heat transfer—a literature survey. NewYork, N.Y.: ASME Paper nr 87-HT-35:1-11.

Jambunathan K, Lai E, Moss MA, Button BL. 1992. A review of heat transfer data forsingle circular jet impingement. Int J Heat Fluid Flow 13(2):106–15.

Li A, Walker CE. 1996. Cake baking in conventional, impingement and hybrid ov-ens. J Food Sci 61(1):188–91,197.

Ovadia DZ, Walker CE. 1997. Opportunities for impingement technology in the bak-ing and allied industries. Am Inst Baking Technol Bull 19:1–8.

Ovadia DZ, Walker CE. 1998. Impingement in food processing. Food Technol52(4):46–50.

Sakai N, Hanzawa T. 1994. Applications and advances in far-infrared heating in Ja-pan. Trends Food Sci Technol 5:357–62.

Singh RP, Heldman DR. 2001. Introduction to food engineering. 3rd ed. London:Academic Press. 659 p

Skjöldebrand C, Ellbjär C, Andersson CG, Eriksson TS. 1988. Optical properties ofbread in the near-infrared range. J Food Eng 8:129-139.

Skjöldebrand C, Andersson CG. 1987. Baking using short wave infra-red radiation.In: Morton ID, editor. Proceedings from Cereals in a European Context. 1986,Bournemouth; Chichester, England: 1st European Conference on Food Science andTechnology. U.K.: Ellis Horwood. p 364–76.

Skjöldebrand C, Andersson C. 1989. A comparison of infrared bread baking and con-ventional baking. J Microwave Power EE 24(2):91–101.

Umesh Hebbar H, Vishwanthan KH, Ramesh MN. 2004. Development of combinedinfrared and hot air dryer for vegetables. J Food Eng 65:557–63.

Vulicevic IR, Abdel-Aal E-SM, Mittal GS, Lu X. 2004. Quality and storage life of par-baked frozen breads. Lebensm Wiss Technol 37:205–13.

Walker CE, Li A. 1993. Impingement oven technology-Part III: Combining impinge-ment with microwaves. Am Inst Baking Technol Bull 15:1–6.

Wählby U, Skjöldebrand C. 2002. Reheating characteristics of crust formed on buns,and crust formation. J Food Eng 53:177–84.

Yin Y, Walker CE. 1995. A quality comparison of breads baked by conventional ver-sus nonconventional ovens: a review. J Sci Food Agric 67:283–91.