effects of crust constraints on bread expansion and co2 release

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Journal of Food Engineering 80 (2007) 1302–1311

Effects of crust constraints on bread expansion and CO2 release

Lu Zhang a,b,*, T. Lucas a, C. Doursat c, D. Flick c, M. Wagner a,c

a Cemagref, Food Processing Technology Research Unit, 17 Avenue de Cucille, 35044 Rennes Cedex, Franceb Department of Chemical and Materials Engineering, The University of Auckland, PB 92019 Auckland, New Zealand

c JRU Genial, Cemagref-ENSIA-INAPG-INRA: ENSIA, 1 avenue des Olympiades, F-91744 Massy, INAPG, 16 rue Claude Bernard, F-75006 Paris, France

Received 7 August 2006; received in revised form 4 October 2006; accepted 5 October 2006Available online 28 November 2006

Abstract

Artificial covers were designed and used to study the effects of crust constraints on bread expansion and gas release during baking.The results were interpreted by comparison with the results of simulations using a mathematical model of baking. The cover levels wereset at 45 mm, 50 mm and 55 mm, as the maximum height reached by the dough without cover under the conditions studied was 60 mm.MRI images were used to evaluate the local porosity of the crumb. Porosity kinetics showed that different cover levels resulted in den-sification within the crumb structure: the later the dough encountered the cover, the deeper the dense crumb formed in the loaf. Thekinetics of CO2 release were monitored with an infrared detector and the results showed that the lower the cover level the shorter theinduction time. The predominant role of crust setting in the CO2 release response was demonstrated.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Crust; Constraint; Pressure; Porosity; CO2 release; Dough; Bread; Baking; Oven rise; MRI; Cell bubble; Wall; Membrane

1. Introduction

The original dough used for producing bread is mixedand kneaded to enclose gas cells, which originally are fromthe air and subdivided into small-sized gas cells. Duringyeast fermentation, these gas cells are filled mainly withCO2. During baking, the dough expands further so thatthe volume increases by one-third, which is called oven rise(Dobraszczyk, 2004). The predominantly liquid dough isalso transformed into predominantly elastic bread crumb.Crumb is developed to the centre of the dough with pene-trating heat, going through a complex progression of phys-ical, chemical and biochemical changes, and finally thefoam structure with separate, closed gas cells is trans-formed into a sponge structure with interconnected cells(Dobraszczyk, 2004). The combined mechanisms leadingto the expansion of gas cells and the dough/crumb transi-

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

doi:10.1016/j.jfoodeng.2006.10.008

* Corresponding author. Address: Department of Chemical and Mate-rials Engineering, The University of Auckland, PB 92019 Auckland, NewZealand.

E-mail addresses: [email protected], [email protected] (L.Zhang).

tion as analysed in this study are applied to all typical ofthe baking process.

At the beginning of baking, yeast continues CO2 pro-duction until it is inactivated by heat at a temperature ofabout 55 �C. The dissolved CO2 and water in the liquidphase dough desolubilises in gas cells with increasing tem-perature (Wiggins, 1998). More molecules of gas arereleased into the cells and, depending on the extensibilityof the cell membranes, either the volume or the internalpressure of the cell tends to increase according to theGay–Lussac law (Therdthai et al., 2002).

It is generally accepted that one reason for the cessationof dough expansion during baking is the resistance of thedough to extension (Bloksma, 1990). The closed cell mem-branes in the dough may resist expansion, depending ontheir rheological properties (elasticity and viscosity). Starchgelatinisation in the cell membranes occurs when the tem-perature exceeds 65 �C (Bloksma, 1990), in turn increasingdough viscosity and impairing the extensibility of thedough (Thorvaldsson and Skjoldebrand, 1998). As a finalresult, the pressure in closed gas cells increases, and the cellmembranes may then rupture under increasing constraints.

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L. Zhang et al. / Journal of Food Engineering 80 (2007) 1302–1311 1303

As a consequence, the gas molecules will exchange betweenadjacent cells and ultimately be transported to the outsideof the dough, resulting in a loss of gas and presumably lim-ited capacity for expansion. Some studies have focused onmonitoring the CO2 release as a key element for betterunderstanding of the cell rupturing during baking (Danielsand Fisher, 1976; He and Hoseney, 1991; Lucas et al., inpress).

Crust formation is another factor limiting the overalland possible local expansion of the dough/bread. As bak-ing proceeds, the layer where most of the evaporation takesplace moves below the dough surface and the crust starts toform (Therdthai et al., 2002; Wiggins, 1998). Surface dry-ing during baking has been studied (Zanoni et al., 1994;Lostie et al., 2002) and the results have suggested thatthe crust could restrict the water vapour flow from the coreto the surface of the dough. However, the latter authorsconsidered that the crust acted as resistance to both heatand mass transfer during the baking of sponge cake.

The constraints exerted by the dry, rigid crust on thecrumb structure during baking have rarely been studied(Hayman et al., 1998) in breads baked with conventionaland electrical resistance ovens (ERO), the latter preventingthe formation of any crust on the external surface of bread.The results suggested that the formation of a crust on thedough during baking produced a coarser crumb structurein the final baked bread. As a possible explanation, theauthors suggested that crust formation restricted theexpansion of the gas cells in the unbaked portion ofthe dough and resulted in increased internal pressure,imposing additional stress on the cell membranes. If thegas cell membranes could not withstand this increase inpressure, the cell membranes would rupture at an earlystage and the cells would coalesce, resulting in a non-uni-form, coarse crumb structure. By imposing varying degreesof pressure on the dough surface at different stages duringbaking, the same authors also found that with a certainpressure load (300 Pa), the temperature range over whichthe gas cell membranes of a wheat flour dough were mostsusceptible to coalescing was 60–70 �C, the temperatureat which starch starts to gelatinise in common doughs,inducing an increase in dough viscosity. It must be empha-sized that these conclusions have not been drawn fromdynamic observations during baking, but deduced fromobservations of the final crumb structure combined withthe development of ingenious experimental protocols.

Recent advances have been proposed in continuous,non-invasive monitoring of bread baking (Hong, Yan,Otterburn, & McCarthy, 1996; Wagner, Loubat, et al., sub-mitted for publication; Whitworth & Alava, 2004) based onuse of MRI (magnetic resonance imaging) and X-raytomography. Only local porosity (volumetric gas fraction)and large-sized gas cells can be monitored since the obser-vation scale is about one square millimetre. Local crumbdensification has been reported to differ with different airtemperatures (130 �C versus 182 �C) and was assumed tobe related to an earlier and thicker crust at 182 �C (Wag-

ner, Quellec, Trystram, & Lucas, submitted for publica-tion). Theoretical approaches have also highlighted therole of the crust as a constraint on the crumb during bakingand as the origin of local crumb densification (Datta et al.,2007; Jefferson et al., 2006; Wagner et al., submitted forpublication-a; Zhang and Datta, 2006).

The aim of the present study was to characterise non-invasively and dynamically the role of the crust as a con-straint on the local expansion of bread during baking. Afabric cover placed at different heights over the fermenteddough was expected to generate varying constraints onthe top dough surface and mimic earlier setting of the crust.Obviously at this stage, the entire process of crust settingwas not well controlled and neither the drying processnor the biochemical changes taking place in the top layersof the dough were assessed. A quantitative method basedon MRI images was used to monitor local expansion anddensification during baking (Wagner, Quellec, et al., sub-mitted for publication). The release of CO2 was monitoredin parallel as an additional response variable. To help tounderstand the effects of the top constraints describedabove, the experimental results were compared to simula-tions using a 1D mathematical model of baking (Wagneret al., submitted for publication-b).

2. Materials and methods

2.1. Dough samples

Dough samples were prepared as previously described(Wagner, Loubat, et al., submitted for publication), with300 g wheat flour (12.5% gluten), 225 g water, 6 g sucrose,6 g salt, 9 g compressed yeast and 6 g rape seed oil toobtain a final water content of dough at 49.6 ± 1.4%.The final temperature of the dough was 26 ± 1 �C. Doughportions of 200 ± 10 g were placed in rectangular glassmoulds of constant dimensions at the base (250 � 50 mm)but of varying heights (45, 50 or 55 mm). The internal sur-faces of the mould were previously coated with Teflon toreduce adhesion. For MRI measurements (see the corre-sponding section), microcapsules containing paraffin oilcovered with reticulated gelatin with thermal resistanceup to 120 �C (Microcapsules-Technologies, Puiseaux,France) were incorporated into the dough as follows (Wag-ner, Quellec, et al., submitted for publication): an initialdough layer of about 3 mm was deposited at the bottomof the mould and five strips of agglomerated microcapsules(2 � 10 mm) were deposited over the dough layer uni-formly distributed over the mould width. The operationwas repeated four times and mould filling (up to about200 g) was completed in less than 10 min. It was checkedthat the sequential filling of the mould with thin layers ofdough did not significantly affect the local expansion,which was mainly due to the high hydration of the dough(Wagner, Quellec, et al., submitted for publication). Thedough was proved for 60 ± 10 min in a fermentation cham-ber with water-saturated air (>90%) at 27 �C to obtain a

1304 L. Zhang et al. / Journal of Food Engineering 80 (2007) 1302–1311

height of 40 mm, which was approximately 3 times higherthan the original dough.

2.2. Baking procedure

Just before baking, the lateral surfaces of the mouldwere insulated externally with a ceramic layer and thewoven glass fibre cover was stretched on the top. Ceramicinsulation ensured that heat transfer was 1D along the ver-tical axis (y-axis) during baking and mass transfer wasexpected to behave similarly (Wagner, Loubat, et al., sub-mitted for publication). In fact this experimental configura-tion was consistent with the mechanisms on which thebaking model was based and simulations were comparedwith the experimental results for further interpretation.

The general idea was to set up a cover on the top of themould to stop the overall expansion at different final heights(40, 50, 55 mm) and to generate various levels of constrainton the top dough surface, referred to below as cover 45,cover 50 and cover 55. Without any obstacle to expansion,the dough was shown to expand up to 60 ± 2 mm (Wagner,Quellec, et al., submitted for publication). The cover wasdesigned to be permeable to gases, in order to provide min-imum bias during the baking process except for the con-straint effect. A woven cloth made of glass fibres (300 g/m2, Rovine, France) with high thermal tolerance (200 �C)was selected for this study. Each piece of cloth (300 �70 mm) was stretched over the top of the mould and sealedon the surrounding ceramic layer (Fig. 1).

The dough was then baked in a convection oven for45 min at an air temperature of 182 �C (±3 �C) and meanair speed of 3 m/s (Wagner, Loubat, et al., submitted forpublication). MRI and gas release measurements were per-formed at the entrancy of the dough in the oven.

2.3. MRI measurement and image analysis using

microcapsules in dough

MRI images were acquired using a SIEMENS OPEN0.2T imager with a Spin Echo sequence, with the followingparameters: echo time (TE) 8 ms; repetition time (TR)420 ms; slice thickness 10 mm; size of the field of view(FOV) 64 � 128 mm; matrix size 128 � 128; acquisitiontime 47 s; 1 acquisition. The sequence was launched every30 s from t = 0 to 45 min of baking. Oil microcapsules pro-viding distinguishable signals were introduced betweendough layers to monitor the structure changes in the doughduring baking by MRI (see above); since oil has a lower

250 mm

45 m

m

40 m

m

50 m

m

250 mm

45 m

m

40 m

m

Top ceramic

50 m

m

Tissue cover B

Fig. 1. Side view of the mould with the cloth cover s

relaxation rate than the dough under study, the microcap-sules showed up as spots of hyper signal (Wagner, Quellec,et al., submitted for publication). The spatial distributionof microcapsules in the dough section after the proving stepis shown in Fig. 2. The position of the microcapsules in thedough may change during baking (Fig. 2) and it wastracked on the MRI images using ImageJ manual tracking(J1.3.1.3_03, Wayne Rasband, National Institute Health,USA). Five dough regions, each delimited by two rows ofmicrocapsule strips, were defined as the ‘bottom surface’,‘bottom middle’, ‘top middle’ and ‘top surface’ regions(Fig. 2). It must be emphasized that the initial thicknessof each region could not be accurately reproduced fromone experiment to another. The variations in mean poros-ity in each dough region ðD��Þ were calculated as follows:

D�� ¼ ��ðtÞ � ��ð0Þ ð1Þ

D�� ¼ V gðtÞV totðtÞ

� V gð0ÞV totð0Þ

ð2Þ

D�� ¼ jD�yðtÞ � D�yð0ÞjD�yðtÞ ð1� ��ð0ÞÞ ð3Þ

where ��ðtÞ was the mean porosity of the dough region attime t, ��ð0Þ the initial porosity of the dough region after fer-mentation (estimated at 0.71), D�y the mean thickness (fol-lowing the y-axis of the dough region D�y ¼ Dy1þDy2þDy1

3Þ,Vg

the volume of the gas phase in the dough region and Vtot

the total volume of the dough region.For comparison between simulated and experimental

porosity, the simulated mean porosity was calculated inthe same regions as in the experiment as follows:

�� ¼P�ðtÞ � DyiðtÞP

DyiðtÞð4Þ

the dough being divided i 2 [1,n] into elementary volumesor meshes of thickness Dyi. The simulated porosity wasfinally expressed using Eq. (1) as for experimental porosity.

2.4. CO2 release

CO2 (ppm in volume) concentrations in the oven weremeasured with a multi-gas analyser equipped with an infra-red detector (type 1302, Bruel & Kjaer, Denmark). TheCO2 concentration measured was converted into gas con-centration released CCO2

release

� �by taking into account the vol-

ume of continuous air replacement in the oven (Lucaset al., in press). To compare experimental and simulatedvalues, the mass of CO2 released per surface unit ðmCO2

Þ

55 m

m55

mm

Dough after provingottom ceramic

tretched at different heights (45, 50 and 55 mm).

0 0- 25mm +25mm - 25mm +25mm

40 m

m

Bottomsurface region

Bottommiddle region

Top middle region

Top surface region

y(0)y1(t)

y2(t) y3(t)y(t)

m

Bottommiddle region

Top middle region

Top surface region

Δ

_Δ

Before baking(t=0) During baking(t)

Fig. 2. Distribution of microcapsules in the dough and calculation of porosity.


was expressed as a function of CO2 concentrationCOCO2

release

� �in the oven

mCO2¼

COCO2releaseV ovenqCO2

Sexch

ð5Þ

where qCO2was the density of CO2 (1.79 g/L) at 455 K, the

oven volume (119.2 L),and dough surface free for mass ex-change (250 � 50 � 10�6 m2).

2.5. Model of baking

The experimental results were initially interpreted withthe aid of a previously published 1D model of baking(Wagner et al., submitted for publication-a). The modelwas first compared with a set of experimental data acquiredin the same conditions as described above, but with freeexpansion (total water loss, temperature at six positions,local water contents from dry matter on samples or byMRI, CO2 release, local and total expansion by MRI).As there was good agreement, all the parameters used inthe model for this first comparison were therefore retained.Only the principal features of the model are reported belowand the interested reader is referred to this previous studyfor further details.

1. Dough was considered as a deformable porous medium.The gaseous phase was described in terms of porosity(m3 of gas per m3 of dough) and the mechanisms takingplace at the gas cell scale were not finely described. How-ever the heterogeneous development of the gaseousphase through the dough/crumb was taken into account,according to the heterogeneous heat and mass transport.

2. The opening of the gas cells, marking the transitionfrom the foam formed by the dough to the sponge-likestructure typical of the crumb, was taken into accountby adding a convective transport of gases (DarcyLaw). Cell opening was determined by a simple temper-ature criterion (TKg

) set at 50 �C. Under such conditions,baking was started with a closed-cell structure and the

exchange of gases between gas cells and the consecutivechanges in internal pressure occurred through the doughas local temperature reached (TKg

).3. Three modes of water transport were taken into

account: diffusion in the liquid state through the volumefraction occupied by crumb membranes; convectivetransport under a total pressure gradient (Darcy law)and transport by the evaporation–condensation–diffu-sion mechanism (De Vries et al., 1989). Vaporisationor condensation of water was evaluated at each doughmembrane/cell interface, including the top dough sur-face, to verify local liquid/vapour equilibrium accordingto water activity.

4. In the model, the dough layers near the surface becamedry and rigidified when the temperature reached a givenvalue (Tg) due to starch gelatinisation and protein coag-ulation. However, with 1D resolution these mechanismsdid not produce any mechanical constraint on the over-all expansion. Such constraint was thus generated byforcing the total height to a constant after a given bak-ing time.

5. Expansion of the dough during baking resulted from thebalance of forces: (i) the pressure force being determinedby the Gay–Lussac law, (ii) the viscous force exerted bythe cell membrane and strengthened by the starch gela-tinisation as temperature increased, (iii) constraint fromthe top crust once it became rigid enough or from thecloth cover

P tot � r ¼ P atm þ fc ð6Þ

where Ptot is the total pressure in the bubble, r the vis-cous stress from the dough/crumb film fc and the equiv-alent stress exerted by the rigid crust or the cloth coverfor the dough not exceeding a given total height zmax.

Although more sudden than the real process of crust set-ting, the model reproduced fairly well the evolution of totalheight with baking time (Wagner et al., submitted for pub-lication-b). Changes in viscosity are discussed as factors


controlling cell expansion. The onset temperature of starchgelatinisation was selected for this change, so that starchgelatinisation and increase in viscosity were considered assynonymous, although the underlying biochemical mecha-nisms may be more complex. As a first approximation, asudden increase in viscosity was selected for the model,with the viscosity passing from 4.5 � 102 to 4.5 � 106 m2/s at Tg = 65 �C.

3. Results and discussion

3.1. Porosity

Experimental and simulated porosity values during bak-ing are compared in Fig. 3. For covers 45 and 50, theexpansion process in the bottom (surface and middle) lay-ers were well reproduced by the baking simulator, both inthe dynamics and the final porosity value (less than 2% dif-ference). For cover 55, the kinetics in expansion were not

Δε

Δε

Δε

-0.6

-0.55-0.5

-0.45

-0.4

-0.35-0.3

-0.25

-0.2

-0.15-0.1

-0.05

0

0.050.1

0.15

0 2 4 6 8 10 12 14

bottom surface

bottom middle

top middle

top surface

a(1)

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8 10 1 4

bottom surfacebottom middletop middletop surface

a(2)

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8 10 12 14


a(3)

-0.6

-0.55-0.5

-0.45

-0.4

-0.35-0.3

-0.25

-0.2

-0.15-0.1

-0.05

0

0.050.1

0.15

0 2 4 6 8 10 12 14

bottom surface

bottom middle

top middle

top surface

a(1)

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8 10 1 4

time (min)

time (min)

time (min)


a(2)

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8 10 12 14


a(3)

Fig. 3. Mean porosity (D�) monitored dynamically by MRI (a) and simulated (55 mm. The four regions (top surface, top middle, bottom middle and boexperimental and simulated values comparable in this condition only.

perfectly reproduced, although the final values agreedwithin 2%.

Local compression was also observed, with porositysometimes decreasing below the initial value (negative val-ues in Fig. 3). Compression phenomena were associatedwith the dough touching the cover; this is illustrated inFig. 3 at 1.5, 3.5 and 5 min for covers 45, 50 and 55, respec-tively. After the dough touched the cover at 50 mm forinstance, the bottom surface layer kept expanding until6 min of baking and the bottom middle layer expandedeven further (until 10 min). Considering the total volumewas limited at 3.5 min of baking by touching the cover,the continued expansion of these layers resulted in the com-pression of the top surface and middle layers (alreadyexpanded at the beginning of baking). All these results indi-cate that local expansion of the dough continued even afterthe overall expansion ceased, and that such local expansionwas not uniform through the dough but was accompaniedby local compression. Although the porosity in the com-

-0.6-0.55

-0.5-0.45

-0.4-0.35

-0.3-0.25

-0.2-0.15

-0.1-0.05

00.05

0.10.15

bottom surface

bottom middle

top middle

top surface

b(1)Δ

εΔ

εΔ

ε

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8 10 12 14


b(2)

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8 10 1 4


b(3)

-0.6-0.55

-0.5-0.45

-0.4-0.35

-0.3-0.25

-0.2-0.15

-0.1-0.05

00.05

0.10.15

0 2 4 6 8 10 12 14

bottom surface

bottom middle

top middle

top surface

b(1)

time (min)

time (min)

time (min)

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8 10 12 14


b(2)

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8 10 12 14


b(3)

b) over 14 min of baking. (1) Cover 45 mm, (2) cover 50 mm and (3) coverttom surface) are specified to a given experimental condition, making


pressed region varied between simulated and experimentalresults by 4–10%, it must be emphasized that the sameregion(s) were compressed in both the simulation and theexperiment. In particular, the most intense compressionwas observed in the top surface layer for covers 45 and50, but in the top middle layer for cover 55. This will befurther discussed below.

It was not our initial intention to reproduce exactly theexperimental results observed in these three conditions withthe baking simulator. Model parameters suitable for simu-lation in the case of free expansion (overall expansion stop-ping naturally at 60 mm) (Wagner et al., submitted forpublication-b) were kept the same for simulations with acover. In particular, resistance to heat transfer at the topsurface was not affected although the cover material actedas an additional thermal barrier. A slight difference in thedough reaching the cover was observed between the exper-iment and the simulation. This was attributed to the pres-ence of the cover which undoubtedly affected the kineticsof expansion/compression. As discussed below, local tem-perature in the dough strongly determines the expansion/compression process. Because of the cover, the temperatureincreases slowly mainly at the top surface layer, which mayexplain the differences observed in these layers (compress-ing layers): the higher the cover, the deeper the layer likelyto be affected by the temperature slow down. Again thegreatest differences were observed for cover 55.

The viscous character of the dough membranes retainedfor the baking model in this study could also partly explainthe difference between experimental and simulated poros-ity. Taking into account the viscoelastic character of thedough membrane would undoubtedly have distributedthe compression forces over a larger area; such limitationcan be particularly evaluated with cover 45 where the biasinduced on heat transfer by the cover was less. Obviously,using local data for model validation highlights all theimperfections in the parameters selected and the physicalmechanisms. Given all these limitations, we accepted at thisstage of the work general agreement between experimental

Fig. 4. Simulated porosity (left), temperature and pressure (right) profiles in do(3.5 and 4 min) of baking. Horizontal lines delimited 5 regions (noted 1 to 5)

and simulated trends. Evaluating the rheological propertiesof dough will be part of a future study. Given the fairlygood agreement obtained at this stage, simulated resultswill be further analysed to explore the mechanisms govern-ing the interactive expansion and compression processes ona local scale. In particular, key factors or rules will besought to explain why a given region is compressed whileanother expands. Such analysis will be carried out for cover50 first, and continued with covers 45 and 55 for validationand completion purposes.

Expansion in the baking model depends on the balancebetween pressure and viscous forces, which themselvesdepend on the state of the cell membranes: if starch is gel-atinised, the viscosity and the viscous force are high; if thecell membrane is ruptured, gases may escape and the pres-sure lowers. As shown in Fig. 4, when the dough touchedthe cover, three status combinations were encounteredfor the dough membranes: cells opened and rigidified(where the dough temperature had reached TKg

and waseven higher than Tg), cells opened but were not rigidified(TKg

< T < g) and cells neither opened nor rigidified(TKg

< T < g). Because of the almost symmetric heat flowat the top and bottom surfaces of the dough, these threestates defined five new regions in the dough (Fig. 4).

The compressed layer was located in the area wheredough cells had already opened and connected to the airoutside but had not rigidified, i.e. region 2 beneath thecover in Fig. 4. At that point in time starch gelatinisationhad not started and dough cells were compressible. Fromthe pressure profile, it can also be seen that the surface lay-ers exhibited lower pressure levels than the rest of thedough. This can be explained by the open structure in theselayers (T > TKg

), and connection to the dough surface per-mitting the gas release to the dough outside: with less gas,the pressure dropped, tending to equilibrate with the atmo-spheric pressure, and the increase in volume was sloweddown. This surface layer thus appeared as the region wherethe pressure and viscous forces were the lowest. Two addi-tional forces were being exerted on both sides of this

ugh with cover 50 mm before (3 min) and after the dough reached the coverof different states of dough membranes as detailed in the left figure.

Fig. 6. Simulated porosity and temperature profiles at 9 and 10 min ofbaking with cover 50 mm.


region, one from the crust (impeding any upward motion)and the other from the expanding dough centre and bot-tom. This region was thus squeezed as a result of externalforces exceeding internal forces.

Another consequence was the expansion of the middle-bottom and bottom surface regions at a constant total vol-ume (regions 3 and 4 in Fig. 4). In region 3, whereT < TKg

< Tg, the cells remained intact. As the temperaturecontinued to increase, more gas molecules were vaporisedinto these cells from the liquid dough membranes. Thiswas accompanied by an increase in the cell volume(Fig. 4, left) and in internal pressure (Fig. 4, right) accord-ing to the Gay–Lussac law.

Given the present oven, heat transfer occurred at the topand bottom of the mould, and TKg

was first reached in thetop and bottom layers. Although the cell membranes inregion 4 (Fig. 4) became permeable to gases early, gaseswere not released because the cell membranes were stillclosed at the dough center: in 1D geometry the dough coreacted as a barrier blocking the gas pipe (Wagner et al., sub-mitted for publication-b). This explains why the bottomlayer (region 4) was not compressed despite presenting anopen, unrigidified structure like region 2, and why itsexpansion was even enhanced since the accumulation ofgases induced high levels of pressure (Fig. 4, right) (Wagneret al., submitted for publication-b).

Finally, high viscous forces (T > Tg), as in regions 1 and5, prevented any change in porosity, either in expansion orcompression.

With prolonged baking (i.e. 1–2.5 min after the doughtouched the cover), the dough temperature increased fromtop and bottom surfaces towards the core of the dough(Fig. 5). The areas where the temperature was higher thanTg became thicker and thicker at the top and bottom(regions 1 and 5 of increasing thickness in Fig. 5). Simi-larly, the compressible area with TKg

< T < Tg (region 2)

Fig. 5. Simulated porosity (left), temperature and pressure (right) profiles bedefined in Fig. 4(left) of which thickness and location changged with baking timexpanding region.)

moved down towards the centre (Fig. 5). The coreexpanded until the cells opened, and even after, becauseof gases accumulated in the bottom part in the 1D geome-try (Wagner et al., submitted for publication-b). When thewhole dough exceeded Tg, i.e. after 9 min of baking(Fig. 6), the total dough became rigidified and gases werepermeable, and the viscous forces were great enough tostop the local process of expansion (and compression).

With a lower cover level (cover 45), the compressed areamoved to the very top surface layer of the dough (Fig. 7)and the compressed area moved closer to the core with ahigher cover level (cover 55); the higher the cover level,the later the constraint, allowing a thicker dough layer togelatinise, rigidify somehow and resist compression. This

tween 4 and 6 min with cover 50 mm. (Double arrows delimit the regione; 1 & 5: region of stabilized porosity; 2: region under compression; 3 & 4:

Fig. 7. Simulated porosity profile at the end of baking (45 min) fordifferent cover levels. (Double arrows delimiting the regions have beencompressed with reference to the initial porosity and show the displace-ment of the compressed region to the core.)


indicates that local expansion and compression also resultfrom competition between the rate at which heat is trans-ported through the dough and the instant at which thetop constraint (from the non-deformable ‘crust’) sets. Bothchange the forces and thus the force balance driving furtherexpansion or compression. These results suggest that notonly formula but also process variables could be used forlocal control and orientation of the final aerated structureof the crumb.

Heat transport was defined as a key factor for the presentbaking model since the underlying mechanisms of expan-sion (mechanical properties of dough membranes includingthe increase in viscosity and the resistance to rupture) wereonly governed by a temperature criterion. As emphasized inSection 2, this is a simplified view although temperaturemay play a key role in practice: again, starch gelatinisationis temperature-dependent and induces a pressure increase in

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 5 10 15 20 25 30 35 40 45

time (min)

MC

O2

(kg

/m2)

cover 45 simulated resultscover 50 simulated resultscover 55 simulated results

MC

O2

(kg

/m2)

Fig. 8. Mass of CO2 released per surface area during bread baking. Relative confor the continuous air replacement in the oven space (Lucas et al., in press): s

cells which may be a major element in membrane break-down. Surprisingly, the use of such a simple criterion gavesimulated trends. in local porosity which were consistentwith the experiment, especially the displacement of the com-pressed area to the dough core (Fig. 3).

3.2. CO2 release

Fig. 8 presents the experimental CO2 release with differ-ent cover levels. The general trend was with three distinctstages, i.e. an induction period followed by a sudden, linearincrease in CO2 release, and ending with a plateau zone, thelower the cover level, the shorter the induction perioddefined from the beginning of baking to the suddenincrease in CO2 release marking a breakdown (Fig. 8, left).This trend was fairly well correlated with the simulatedresults, although more clearly marked (Fig. 8, right); thispoint is discussed further below.

Further analysis of simulations (Fig. 9) showed that theend of the induction period corresponded to an increase ininternal pressure, which itself resulted from the setting ofthe top constraint (that represented by the cover). It isworth noting here that in real baking conditions (in theabsence of any cover), crust setting will also result in a con-straint on the top dough surface, provided that it occursbefore all regions have completed expansion. Again, whenan early constraint was applied to the top dough surface,the total volume of dough was limited. Similarly, the bub-bles at the core were still closed (T < TKg

) and the rise in gastemperature and the vaporization of new gas moleculesfrom the liquid dough resulted in higher pressures insidethe dough. This increased the total pressure gradient (aspreviously illustrated in Figs. 4 and 5) and may have drivengases from the already opened structure to the outsideaccording to the equation of Darcy’s Law,

_mCO2¼ �

X CO2vap

X g

� Kg

mg

� oP tot

ozð7Þ

where X CO2vapand are the mass fractions of CO2 and total

gases in the dough bubbles, respectively (kg kg�1 of dry

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

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0.09

0 5 10 15 20 25 30 35 40 45

time (min)

cover 45 experimental results

cover 50 experimental results

cover 55 experimental

centration in the oven due to release by bread during baking and correctedimulated (left) and experimental (right) results for different cover levels.

Fig. 9. Kinetics of pressure (at core), total height and CO2 release duringbaking at cover 50 mm.

Fig. 10. Simulated CO2 release with early and late bubble opening withnatural formation of the crust (cover 60 mm).


matter), mg is the kinematical viscosity of the dough(m2 s�1), Kg is the permeability coefficient of the dough/crumb structure, Ptot is the total pressure in the doughand z is the position along the vertical axis (y-axis) accord-ing to the 1D model. Finally, it must be emphasized thatthe constraint on the top dough surface was applied sud-denly in the baking model (see Section 2), whereas in realbaking conditions it sets progressively with the drying ofthe top dough layers. This probably explains why the differ-ences in the induction period were even more clearly sepa-rated than in the three simulated conditions (Fig. 8).

More generally, two mechanisms are necessary to observeCO2 release according to Eq. (7): (i) the breakdown of cellmembranes and the formation of an open porous structureconnected to the oven atmosphere, and (ii) the developmentof an internal pressure gradient. Simulations obtained withthe baking model were compared with early and late cellopening (Fig. 10). Surprisingly, the induction period wasalmost the same when the cell opening occurred atTKg

= 50 �C and when the structure was open from thebeginning of baking (TKg

= 30 �C). This implies that theCO2 induction period is not always related to the occurrenceof rupture of cell membranes. In fact, bubbles expandedalmost freely at early baking times, because of low viscousforces and the low resistance of cell membranes to expan-sion. The pressure gradient was therefore low (even less thana few Pascals) and this mainly limited CO2 release, accordingto Darcy’s law. This view seems to be consistent with the lowpressure levels reported in the literature either at the end ofproving or at the very beginning of baking (Hibberd andParker, 1976; Sommier et al., 2005). These results thus sug-gested a new interpretation of previously reported findings.The release of CO2 has usually been reported to be associ-ated with the progressive opening of cell membranes (Dan-iels and Fisher, 1976; He and Hoseney, 1991; Lucas et al.,in press). From the above conclusions, the start of CO2

release cannot be attributed to the opening of cell mem-

branes alone but rather to combined occurrences. CO2

release was previously proposed for studying the effects ofadded fat on the capacity of cell membranes to resist rupture(Daniels and Fisher, 1976). However, the addition of fat alsoaffects the drying rate at the surface and the mechanicalproperties of the crust. Such effects should also be consideredwhen interpreting the induction period in CO2 release. Thelower release of CO2 obtained at lower baking temperatures(Lucas et al., in press) should also be reinterpreted andrelated to the delayed formation of the bread crust in addi-tion to a direct temperature effect of CO2 dissolution.

4. Conclusion

A simple device in the form of an artificial cover wasfirst designed for this study. A temperature-resistant clothmade of glass fibres was stretched over a mould containingthe dough and permitted to form an early constraint on thetop surface of the dough to mimic crust formation. Thematerial allowed gas exchange between the loaf and theoven and the baking process was little disturbed in thisrespect. The induction period between simulated andexperimental CO2 release was very similar, suggesting thatthe cover had little effect on mass transfer between thedough and the oven. On the other hand, some differencesin porosity were attributed to the cover acting as a thermalbarrier and affecting local expansion of cells.

The influence of the cover height on the development ofthe crumb was monitored dynamically by MRI, which is anovel approach compared to previous attempts on sectionsof final baked loaves (Hayman et al., 1998). The presentresults provide an intermediate scale (1 � 1 mm2) of localporosity and complement previous scales related to bubblecoalescence reported by Hayman et al. (1998). In our study,the experimental results showed that the time at which theconstraint was applied could affect both the overall andthe local expansion of the dough during baking. Althoughthe overall volume was kept constant, dough continued to


expand locally. Some regions expanded to the detriment ofmechanically weak areas in which the porosity decreased.These results with varying levels of cover, thus generatingvarious levels of constraint, confirmed the role played bythe crust in the crumb structure first proposed by Wagner,Quellec, et al. (submitted for publication). The general trendsin expansion were consistent between the experiment and thesimulation, and in view of this fairly good agreement, themechanisms underlying local compression and expansioncould be further studied with the baking model (see below).

Although the position of the area to be compressed chan-ged with the cover level, analysis of the simulated datashowed that the same characteristics applied for the areato be compressed: the cells were already open but the cellmembranes were not yet rigidified. By extending the investi-gation to different experimental conditions, the presentresults validate the mechanisms of compression first pro-posed by Wagner et al. (submitted for publication-a). Finallya new finding of the present study was the competitionbetween the heating up of the crumb and the crust settingas a factor affecting the local porous structure: the earlierthe crust formation, the closer the compression region tothe top surface of the dough/bread. This suggests that bettercontrol of these two processes would provide freedom forlocal orientation of the growth of bubbles in the crumb.

Fairly good agreement between experimental and simu-lated findings also demonstrated that considering a viscousliquid for dough during baking and a temperature criterionfor changes in the mechanical behavior of dough mem-branes (loss of extensibility and resistance to rupture) weresufficient to roughly reproduce the local expansion processin dough. Finally, the slight discrepancies observed indynamics of CO2 release and porosity change betweenexperimental and simulated results indicates that morework needs to be done to fully validate the model sinceso many variables were involved in the process (e.g. differ-ent mass and heat transfer pattern combined with dehy-drating polymerization might have occurred). In anycase, as another such important variable, pressure shouldreceive further attention in future studies on the bakingprocess. More experimental devices, as for instance thosepublished by Sommier et al. (2005), should be developedfor this purpose. Our results also suggest that CO2 releaseshould be monitored and analysed as an indicator of theextent of internal pressure.

Acknowledgements

This study was supported by grants from the FrenchMinistry of Research (RARE-Canal-salve, 2002–2005,Nos. 01 P0838-01 P 0839-01 P 0840) and from the RegionalCouncil of Brittany.

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effects of crust constraints on bread expansion and co2 release

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