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Journal of Cereal Science 52 (2010) 161e169 Contents lists available at ScienceDirect Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs Dough/crumb transition during French bread baking J. Rouillé 1, H. Chiron, P. Colonna, G. Della Valle*, D. Lourdin INRA, UR 1268 Biopolymères Interactions and Assemblages (BIA), BP 71267, 44316 Nantes Cedex 3, France a r t i c l e i n f o a b s t r a c t Article history: Received 10 April 2009 Received in revised form 21 April 2010 Accepted 28 April 2010 The modifications occurring during dough to crumb (D/C) transition of French bread (350 g) were studied in an instrumented pilot-scale oven for doughs with different contents of minor components, soluble, lipids and puroindolines. Internal temperature measurements showed that, for most compositions, complete D/C transition occurred between 55 and 70  C, after 5 min of baking, and coincided with maximum loaf expansion. Differential scanning calorimetry (DSC) in excess of water performed on samples taken during baking (3 and 5 min) showed that starch gelatinization and melting developed continuously during D/C transition for various contents of the soluble fraction in dough. Dynamic thermomechanical analysis (DMA) on dough showed that dough stiffened between 60 and 70  C, as seen by the increase of elastic modulus E0 by more than one decade, for all dough compositions. Relating these changes to the results of baking experiments, D/C transition was assigned first to gluten reticulation and, to a lesser extent, to continuous starch granule swelling. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Starch Gluten Dynamic mechanical analysis (DMA) Baking 1. Introduction Baking is the final stage of the breadmaking process, in order to obtain an expanded product satisfying the consumer. The final exposure to high temperatures in an oven chamber with controlled ambient humidity is essentially to give bread its main characteristics and to fix them, i.e. open gas cell structure surrounded by a crusty envelop, characteristic of French bread. Expansion, volume increase due to bubble growth, and dough/crumb transition, i.e. setting of the matrix, are the most outstanding macroscopic phenomena (Zghal and Scanlon, 2001). Transient changes in several physical properties of breads during baking have been investigated to a great extent (Mondal and Datta, 2008) and allow phenomenological and/or mathematical models for dough expansion during baking. Heat and mass transfer phenomena are taking place simultaneously during bread baking which cause physical, chemical and structural modifications. There are four phases in the transport process, i.e. solid, liquid water, water vapour and CO2. To know the baking process in detail it is necessary to know all these processes together with the effect of temperature on dough macromolecular components. Wang and Sun (1999), using dynamic mechanical analysis (DMA), found that the thermal expansion of the flourewater * Corresponding author. Tel.: þ33 2 40 67 50 00; fax: þ33 2 40 67 50 05. E-mail address: dellaval@nantes.inra.fr (G. Della Valle). 1 Present address: Moulins Soufflet, BP 12, 91100 Corbeil, France. 0733-5210/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2010.04.008 system (from 50 to 70% water, wb) included four stages corresponding to gas thermal expansion (between 25 and 60  C), starch gelatinization and gluten reticulation (60e100  C), vapour pressure expansion (100e120  C) and structure fixation (>120  C). These values are not completely in agreement with those of Singh and Battacharya (2005) who found a maximum expansion at around 80  C. On a macroscopic scale, oven rise has been assessed continuously by Zanoni et al. (1993), who measured bread height in tin moulds, and at a local scale by Wagner et al. (2008), using Magnetic Resonance Imaging, both in order to ascertain numerical models for bread expansion. During baking, total moisture loss is about 20%, on a wet basis, and it is essentially localised in the crust. Moisture content slightly increases at the dough centre for a temperature of about 70 þ/ 5  C, as shown by measurement of local water content using a fibre-optic NIR instrument, (Thorvaldsson and Skjöldebrand, 1998) or simply by samples removed during baking (Purlis and Salvadori, 2009; Wagner et al., 2007). In these works, this local increase was reported to be caused by opening up of the pore structure and related to the dough/crumb transition. In these moisture intervals (25e50% on a total wet basis), both gelatinization of starch and denaturation of proteins may occur in the range of 60e100  C and contribute to the setting of the matrix, i.e. the change from dough to crumb (Bloksma, 1990; Wang and Sun, 1999). Gelatinization is strongly water dependent. In excess of water, starch gelatinization occurs at T ¼ 62  C for wheat flour, 5  C higher than for wheat starch but with the same enthalpy value (11 J/g) (Chevallier et al., 2000). Simulating dough (MC ¼ 45% wb) baking 162 J. Rouillé et al. / Journal of Cereal Science 52 (2010) 161e169 using DSC, Fessas and Schiraldi (2000) found that 70% of starch was gelatinized at this temperature, likely due to the competition for available water. Gluten proteins, which confer to dough its unique viscoelastic properties, are modified by temperature. Heating promotes network formation through association of the polymers by hydrophobic interactions and disulfide bonds (Tsiami et al., 1997; Weegels et al.,1994). According to Hoseney et al. (1986), gluten starts to cross-link in bread at 74  C. For a moisture content of about 40%, gluten experiences structured flow between 27 and 93  C, before networking through cross-linking reactions (Toufeili et al., 2002). A lot of studies have tried to determine the effects of heating on various gluten fractions, from HMW glutenins to LMW gliadins. Schofield et al. (1983) found a loss of solubility of glutenins (65% wb) and gliadins at 50  C and 70  C, respectively. Stathopoulos et al. (2006) found that the increased elasticity of various gluten fractions previously heated at 90  C is more significant for gluten with high glutenin content. Because of the high thermal and moisture sensitivities of the two major components of flour (i.e. starch and gluten), rheological properties of dough are also modified during heating (Bloksma, 1990). Up to 60  C, apparent viscosity values decrease with temperature, like in many materials, with no significant structural change. Then dough viscosity increases markedly above 60  C mainly not only because of starch gelatinization, but also because of gluten cross-linking, leading to transition from a viscous dough into an elastic crumb. Finally, various works have shown that dynamic thermomechanical analysis could be useful to study those modifications during processing. Rolee and Lemeste (1999) ascertained the viscosity increase of starch suspension at intermediate moisture content. Angioloni and Dalla Rossa (2005) showed that the effect of salt addition on dough thermomechanical behaviour depends on the mixing process. Singh and Battacharya (2005) associated the different stages of modulus development to the cell growth and opening in doughs, and DMA experiments of Stathopoulos et al. (2008) suggested that the decrease of tan d around 60  C gave the possibility to relate gluten molecular characteristics to breadmaking quality. The objective of this study was to ascertain the main dough modifications at a macromolecular scale (changes in starch and gluten) and relate them to those occurring at a macroscopic scale (dough/crumb transition, loaf expansion) during baking. For this purpose, experiments at a pilot scale on an instrumented baking oven were coupled to thermal and rheological analyses at a lab scale. Dough composition was changed by modifying the soluble fraction and lipid/puroindolines ratio because our previous studies have shown their importance to crumb cellular structure, dough bubble size and rheology (Rouille et al., 2005a,b,c). 2. Experimental 2.1. Ingredients The flour was a standard commercial mixture, CNS, with no additives from Moulins Soufflet (Pantin, 93, France) with 10.5% proteins (db), 13.80% water (wb), 0.56% ash (db), W ¼ 194 J and P/ L ¼ 0.56 from Chopin alveograph (biaxial extension test). Baker yeast is fresh compressed Saccharomyces cerevisiae (70% water, wb), provided by DSM bakery ingredients (Roissy 91, France). 2.2. Fractionation and dough preparation Defatting native flour CNS (def CNS) and water fractionation (based on centrifugation) into soluble (F2) and insoluble (F1) fractions was carried out according to preceding work (Rouille et al., 2005a), where their detailed composition was also given. The soluble fraction (F2) mainly contained low molecular weight sugars and soluble proteins (puroindolines). Puroindolines (Pin) extracted from the same flour were also added to (defatted or not) flour (þ0.1% on wet flour basis). Dough pieces were then prepared from various flours: native CNS, defatted CNS, insoluble þ 2 soluble fractions (F1 þ 2F2), CNS þ 0.1%Pin. Flour (100), distilled water (63% on flour wet basis), salt NaCl (2.2% wb) and yeast (2.5% wb) were mixed to form a dough in a bakery mixer. When using only the insoluble fraction, F1 was stored for 12 h at 20  C and at 75% relative humidity (RH), in order to favour water absorption and repartition during mixing. This step increased moisture content of F1 up to about 14% (wb). Before use, accurate moisture contents were determined after drying samples for 2 h at 130  C. Water addition was calculated in order to reach the same dough hydration at the end of mixing, i.e. 45.6  0.5% (wb). According to a French standard breadmaking procedure (AFNOR, 2002), mixing time was 4 min at 40 rpm followed by 17 min at 80 rpm. Salt was added 5 min before the end of mixing. Once mixed, dough was gently removed from the mixer, rounded by hand and allowed to rest at 27  C, 75% RH. After 20 min of rest, dough was fractionated into 350 g pieces and put into a mechanical molder for rolling and stretching. Dough cylinders (length 25 cm, diameter 4 cm) were then rested at 27  C, 75% RH for 90 min. Before baking, blade cuts, or scars, were made on the top of the dough pieces, in order to orientate dough growth during oven spring and produce final scars, which is characteristics of French bread. 2.3. Baking Baking was performed in a pilot-scale oven (115 L, 10.5 kW) and in a standard bakery oven (4  300 L, 21.1 kW), both from Bongard (F67, Wolfisheim). As described in detail before (Sommier et al., 2005), the pilot-scale oven has been instrumented with a frame of type K thermocouples (Ø 800 mm) vertically inserted in the dough to prevent them from any motion during oven rise, to measure precisely the temperature (þ/ 0.15  C) at different locations in the dough: 0 (bottom crust), 1, 2, and 3 cm distant from the hearth. An image acquisition system (digital video camera, SONY Media Vision) allowed the recording of images of dough rise, as described in detail before (Sommier et al., 2005). Simple thresholding of the front image allowed evaluation of dough rise through the determination of dough perimeter and area, discarding dimension variation in the orthogonal direction of the loaf image. Hearth and vault temperatures were set to 265  C and 235  C, respectively, in the pilot-scale oven, and 250  C and 245  C, respectively, in the standard bakery oven, oven air temperature being close to 182  C for both in these baking conditions. These temperatures were chosen to give similar baked product in terms of volume, texture and colour in both ovens. Two dough pieces baked in the standard bakery oven were removed at two different baking times (3 and 5 min) and immediately cut in the transversal axis. A picture was acquired with a digital camera, and dough and crumb samples were carefully taken and instantaneously frozen in liquid nitrogen for further DSC analysis. Complete baking lasted for 27 min. Volume and mass were measured 90 min after the end of baking on two different breads. Volume was estimated by rapeseed displacement. 2.4. Starch gelatinization and melting Differential scanning calorimetry (DSC) experiments on dough and crumb were performed on a DSC 121 (SETARAM, F69-Caluire). Samples of dough and crumb taken, respectively, at (0, 3 min) and (3, 5 min) during baking, were immediately frozen in liquid nitrogen. They were thawed for z30 min at T ¼ 20  C before analysis. About 30 mg of sample was precisely weighed and then put in a stainless steel pan, and water was added (>100 mg) before J. Rouillé et al. / Journal of Cereal Science 52 (2010) 161e169 163 Fig. 1. Temperature evolution in bread dough during baking (a) for CNS at three different locations: close to the hearth (,), at 1cm (6), 2 cm (B) and 3 cm () from the hearth, and (b) for two different breads, at 2 cm from hearth: CNS and reconstituted doughs (,) and F1 dough (B). Arrows indicate times at which breads were sliced for pictures and DSC analysis. sealing. The reference cell contained e 112 mg of water (Cp water ¼ 4183 J kg 1  C 1). Two scans were carried out from 5 to 180  C at the same heating rate of 3  C/min for all experiments. Thermograms showed an endothermic peak in the range of 55e65  C depending on dough composition and thermal history of the sample. The temperature peak value was taken as the gelatinization temperature. Percentage of gelatinization was calculated from the ratio of the peak area DH for the sample taken at baking time t (3 or 5 min) to peak surface of the sample of the same composition at the initial time t0 as defined by the simple relation:   DH0 %G ¼ DHt : DH0 2.5. Dynamic thermomechanical analysis (DMA) The unyeasted dough sample was prepared in a 2-g mixograph (National, Lincoln, USA). For 100 g of flour, 2.2 g of salt and 63 mL of water were added and mixed for 6 min (total dough development for CNS dough), so that final moisture content was close to 45% (wb). The gluten fraction was extracted from high rotating speed centrifugation (60 000g, 1 h, 4  C) of flourewater dough (200 g flour þ 140 mL distilled water mixed for 2 min at 40 rpm and for 3 min at 80 rpm), as suggested by Larsson and Eliasson (1996). When preparing gluten dough, 1 g of distilled water was added to 1 g of gluten (MC w 55%, wb). Samples were stored for 15 min at 4  C before DMA measurement. A DMTA Mk IV (Rheometric Scientific, USA) was used in the compression mode at a frequency of 1 Hz with a deformation amplitude of 0.1%. This value is in the range of linear viscoelasticity, any deviation from this domain during heating is discarded and supposed to be the same for all doughs tested. A cylindrical sample of dough (z0.9 g) was placed between the two plates (z17 mm). Initial sample thickness is fixed at 3.2 mm. Sample was carefully coated with silicon grease in order to prevent moisture loss. To maintain contact between plates and sample during temperature sweep, a constant static force, supposed to be negligible compared to modulus values, was fixed at 10 2 N. Measurement sampling rate was 1 measurement/10 s. Temperature was increased from 25 to 180  C, at 10  C/min, a heating rate supposed to be close to the 164 J. Rouillé et al. / Journal of Cereal Science 52 (2010) 161e169 Fig. 2. Increase of dough section as a function of baking time. CNS dough (C) average of two replicates (error bars width is 3 SD), F1 (A), F1 þ 2F2 (B), CNS þ 0,1%Pin (6), defatted CNS (,) doughs. internal heat rate of baking dough, in order to perform measurements in kinetic conditions close to those undergone during baking. However, for this high heating rate (10  C/min), the real temperature Tsample was measured inside the dough by inserting a thermocouple. It was found that the sample did not equilibrate instantaneously with oven ambience and that: Tsample ¼ 0:0029  ðToven Þ2 þ0:0421  Toven   þ 22:9 r 2 ¼ 0; 99 for Toven in the interval [25,160  C], which was further used for representing the variations of modulus with sample temperature. It Fig. 3. Photos of sliced bread (a) for CNS at three baking times (3, 5 and 10 min), showing thermocouple location (A, B, C, D at 0, 1, 2, and 3 cm from the hearth, respectively) and (b) after 3 min baking of different doughs. J. Rouillé et al. / Journal of Cereal Science 52 (2010) 161e169 165 Fig. 4. DSC thermograms of dough and crumb (a) in excess of water, taken at different baking times and locations (from black to light grey curves): t0, 3 min dough, 3 min crumb and 5 min crumb (A and B marks: integration limits, C: onset temperature, D: gelatinization temperature) and (b) for starch melting in CNS, F1 and F1 þ 2F2 doughs (45% water, wb) (D: gelatinization peak, Tm is the melting temperature, and double arrow is for temperature interval of dough/crumb transition). was also checked that if the heating rate finally reached a constant value of 6.5  C/min, at Tsample ¼ 100  C (Toven ¼ 150  C). Data were collected by a computer program (Orchestrator Rheometric Scientific, USA), which calculated the dynamic properties E0 , E00 and tan d ¼ E00 /E0 , where E0 is the storage or elastic modulus and E00 is the dissipative or viscous modulus. At least 2 replicates were made for each dough composition. 3. Results and discussion 3.1. Macroscopic changes during baking of reconstituted dough The final moisture content was 31% (total wet basis) for all breads, and their density was 0.2 g cm 3, except for F1 bread for which it was 0.3 g cm 3. These values were lower than those measured by Rouille et al. (2005a) for smaller breads made from 80 g dough pieces, in the range of 0.22e0.38 g cm 3. This difference was likely due to the increased size of bread, increasing the crumb/crust ratio, because crust had a larger density than crumb. Temperature measurements showed that the closer to the hearth, the quicker the temperature increased at the beginning of baking (Fig. 1a). Conduction from the hearth was very important, as shown by the rapid increase of temperature, from 27 to 95  C in less than 3 min, at the bottom of the loaf. Temperature at the dough centre (3 cm from the hearth) displayed the classical sigmoid shape with an inflexion point at 55  C, with a heating rate of about 17  C/ min. The internal temperature gradient, from the coldest region (centre of the dough) to the sides of the loaf, decreased with time. Its maximum order of magnitude was 20  C/cm. For all locations, at least 1 cm from the dough limit, the temperature reached z100  C at the same time (13 min). The result of heating rate confirms those obtained in the preceding work (Sommier et al., 2005) and it is also of the same order of magnitude as those measured by Patel et al. (2005) for various ovens and bread sizes. This value is logically 166 J. Rouillé et al. / Journal of Cereal Science 52 (2010) 161e169 higher than the one reported by Thorvaldsson and Skjöldebrand (1998), since dough was put in baking tins, and with Purlis and Salvadori (2009), likely due to the lower value of oven temperature in their case (T < 220  C). Another source of difference may come from the lower density of French bread dough, after proofing, before introduction into the oven which led to larger heating rate, as shown by Fig. 1b, which compared the evolutions of temperature at 2 cm from the hearth for CNS and F1 doughs. Temperature kinetics measured in the other doughs were not significantly different from the ones obtained for CNS dough. These curves looked like those measured for yeasted and unyeasted doughs by Hallström (1988), who explained this difference by the effect of dough density on internal transport of water (Rask, 1989). F1 dough heated up more slowly than the other ones, which was attributed to the lower volume and its negative effect on the evaporation/ condensation mechanism of heat transport through the dough. Loaf expansion during baking was reflected by the changes in loaf sections (Fig. 2). For every composition but F1, the loaf section area increased during the first 5 min to a maximum at a value of 20e60% larger than the initial loaf (dough). Comparison with temperature measurements showed that maximum expansion for all loaves, i.e. 10 min in F1 dough and 5 min for others, was obtained when core temperature (3 cm) was 60  C, in agreement with former results reported on oven spring and dough expansion (Bloksma, 1990; Wang and Sun, 1999). F1 loaf displayed the same trends, but for larger time values. Because of the suppression of the soluble fraction, F1 dough has larger values of elongational viscosity, 5e10 times those of other doughs, which could explain its lower initial loaf volume (Rouille et al., 2005c). Its larger (elongational) viscosity values precluded bubble growth and its lower sugar content limited CO2 production during proofing. After 7e10 min, shrinkage occurred and a final plateau value, slightly lower than the maximum value, was reached after 10e15 min. These results were likely due to the constraints imposed by the formation of crust which led to inner densification (Zhang et al., 2007). Observation of CNS bread sliced after 3, 5 and 10 min of baking made it possible to distinguish, visually, the bright and white homogenous mass at the centre and the crumb, a little greyer and dull cellular material (Fig. 3a). According to these observations, dough to crumb transition (noted D/C) was nearly complete after 5 min, the time for which expansion is a maximum. Dough section decreased by progressive conversion into crumb, schematically according to a reduction of an elliptic area. This area was materialised by a dotted line in the photograph of slice cut after 3 min (Fig. 3a). The internal ellipse was considered as the D/C transition zone and its location related to thermocouple positions. The values of temperature at different locations in dough (or crumb, see Fig. 3a, 5 min), and for different baking times (3, 5 and 10 min), from Fig. 1a, were compared to the state (dough or crumb) observed at these locations, after slicing and drawing the ellipse on the photo. The larger temperature value at which dough still existed was 56  C and the lower one at which crumb appeared was 69  C, which suggested that the dough/crumb transition (D/C) occurred in the interval between 55 and 70  C. This result was confirmed for other dough recipes (Fig. 3b), including F1, using the same rationale. Furthermore, temperature at the centre of the loaf was in the interval [55, 70  C], when loaves reached a maximum expansion, underlining the link between the concomitance of D/C transition and maximum expansion. These results also suggest that the D/C transition set the maximum expansion and determined the loaf volume, whatever be the dough composition. These macroscopic structural changes have already been observed indirectly by Thorvaldsson and Skjöldebrand (1998), when measuring local water contents in dough and crumb during baking. They found that a slight increase of water content occurred at the dough centre at 70  5  C, which was supposed to be related to the opening of the pore structure. Wagner et al. (2007) performed accurate measurements of water content by removing samples during baking of breads in glass moulds, heated in a convective ovens. They found that the D/C transition was in the interval [55, 65  C]. Despite the difference of experimental conditions, our results were in good agreement on the thermal interval of the D/C transition. For the conditions of bread moisture content during baking (40e50% on a total wet basis), various phase transitions and molecular reactions occurred in this interval, starch gelatinization starting, gluten thermosetting. The contributions of both phenomena to D/C transition were analysed in the following. Table 1 Temperatures, enthalpies and percentage of gelatinization of doughs (and crumb) taken at different times and locations during baking, measured by DSC. Dough Time (min) Location Tonset ( C) Tmax ( C) Enthalpy (J/g db) %Gelatinization CNS 0 3 Dough Dough Crumb Crumb 54.5 57.0 59.5 No peak 63 64 65 No peak 12.2 7.1 4.3 0 0 42 65 100 Dough Dough Crumb Crumb 53.5 55.5 60 No peak 61 63.5 66 No peak 9.6 8.2 3.4 0.0 0 14 64 100 Dough Dough Crumb Crumb 56 56 57 58 64 63.5 63 63.5 13.5 11.0 8.1 6.5 0 19 40 52 Dough Dough Crumb Crumb 56.5 58 60 nd 64 65.5 65.5 nd 8.1 7.0 4.9 nd 0 14 39 nd Dough Dough Crumb Crumb 55.5 56 61.5 63.5 64 64 66.5 69 12.3 8.3 3.8 1.5 0 33 69 88 5 Defatted CNS 0 3 5 F1 (insoluble) 0 3 5 F1 þ 2 F2 (soluble) 0 3 CNS þ 0.1%Pin 0 3 5 5 nd: non-determined. J. Rouillé et al. / Journal of Cereal Science 52 (2010) 161e169 167 Fig. 5. Dynamic thermomechanical analysis of wheat flour doughs: (a) variations of storage modulus E0 for CNS (B), F1 (,), F1 þ F2 (*), F1 þ 2F2 (6) and defatted CNS (A) doughs with sample temperature (b) storage (empty symbols) and dissipative (filled symbols) moduli of gluten (,, -) (MC ¼ 55%) and starch suspension (B, C) (45%) and (c) comparison of variations of relative storage moduli (values normalized for E0 (25 C)) of doughs from CNS wheat flour (-), gluten (,) and starch (B). 168 J. Rouillé et al. / Journal of Cereal Science 52 (2010) 161e169 3.2. Starch gelatinization during the baking process DSC thermograms were measured in excess of water for CNS bread samples taken in dough and crumb parts at different baking times, before baking (t0), dough and crumb for 3 min, and 5 min in crumb (Fig. 4, from bottom to top). Accurate sampling of dough in the loaf baked after 5 min was thought too uncertain, as illustrated by the tiny ellipse in Fig. 3a, to evaluate starch gelatinization by DSC. At t0, the thermogram showed a gelatinization endotherm with a peak at 63  C, noted by mark D (Fig. 4) with enthalpy value of 12.2 J/g, when integrated between A and B. Those values were very close to those found in the literature for wheat flour (Chevallier et al., 2000). The area of the endothermic peak continuously decreased with increasing baking time and finally disappeared for crumb after 5 min of baking, but it was still significant (4.3 J/g) in the crumb at 3 min, meaning that not all starch gelatinized in the crumb. These trends were similar for all loaf compositions, but with varying values of percentage of starch gelatinization (%G) as reported in Table 1. For F1 dough, the lower values of internal temperature during baking may explain the incomplete starch gelatinization (52% in crumb at 5 min), whereas in the case of (F1 þ 2F2) dough, it could be attributed to the lower availability of water due to pentosans and low molecular weight sugars. Finally the most striking feature was that, according to Table 1, crumb could contain 35e60% of non-gelatinized starch. This result might be explained by considering starch thermal transition under limited water content. The thermogram of initial dough (MC ¼ 45%) classically exhibited two successive peaks, the first endotherm corresponding to starch gelatinization and the second one to the melting endotherm characterized by a wider peak at Tm ¼ 83  C (Fig. 4b), a value close to the one encountered for wheat starch at the same hydration (Champenois et al., 1995). Looking into temperature kinetics (Fig. 1), it appeared that, for instance at 1 cm after 3 min, temperature was lower than 80  C, even for thermocouples located in the crumb. So, the significant values of non-gelatinized starch amount found in crumb samples (% G  69% after 3min, Table 1), were explained by the temperature they experienced, lower than Tm, whatever be the specific influence of composition. Although starch melting was involved in D/C transition, the significant amount of non-gelatinized starch in crumb and the spread of the thermal interval of its state change suggested that it was not its major cause. So, the thermomechanical behaviour of starch and gluten therefore deserve more attention in order to better ascertain the changes involved in this transition. 3.3. Dynamic and mechanical thermal analysis of flourewater doughs Variations of elastic modulus E0 , obtained by DMA for various dough systems, were presented as a function of sample temperature (Fig. 5a). They all exhibited similar trends. E0 values decreased slightly as temperature increased from 25 to 50  C, likely due to the influence of temperature on gluten mobility, in line with the decrease of dough viscosity observed in this temperature range (Bloksma, 1990). A strong increase was clearly observed in the interval (55e75  C), over about one decade (106e107Pa), E0 reaching a peak at temperatures varying from 68 to 75  C, according to dough composition. This result was in agreement with measurements made by Angioloni and Dalla Rossa (2005), Schiraldi et al. (1996), and Singh and Battacharya (2005), who found this increase in the temperature interval [60, 80  C] for wheat dough. When examining the influence of dough composition, only differences of E0 values between CNS and F1 dough were found to be significant. The larger values encountered for F1 (Fig. 5a), two to three times, might be explained by the lack of plasticizing action of molecules contained in the soluble fraction (F2, mainly small molecules), as discussed before, on their viscous properties at ambient temperature (Rouille et al., 2005c). The absence of soluble components in F1 might also be inferred for the earlier transition of F1, because of the increased amount of available water content. Whatever the role of composition, the increase of dough rigidity might be related to the D/C transition as it occurred in the same interval of temperature. Singh and Battacharya (2005) associated this modulus increase to the gas cell opening in the dough, which was another manifestation of dough/crumb transition. Working on flour and glutenestarch doughs, Dreese et al. (1988) attributed the observed dynamic rheological changes to the starch fraction and more precisely, the increase of rigidity to the progressive swelling of starch granules. The variations of modulus found by Rolee and LeMeste (1999) on wheat starchewater preparations presented trends similar to the ones shown here. They were characterized by a dramatic increase of E0 , over four decades in the range of temperature 50e60  C, for MC > 50%. The authors explained this result by granule swelling, mainly those of smaller size, on a theoretical basis for the increase of particle volume fraction of a suspension. Although the order of magnitude might not agree completely with the theory, since swollen granules can hardly be considered as rigid spheres, this finding suggested that starch gelatinization would be involved in dough stiffening by an increase of granule volume. However, for a starch sample at 45% MC (wb), the increase of modulus was much less significant, less than one decade (Fig. 5b). This result challenged the interpretation of starch granule swelling as the main cause for D/C transition, since this event was reflected in DMA by an increase of modulus of more than one decade. Furthermore, our preceding DSC results (progressive decrease of %G) showed that starch granules continuously swelled and disrupted in dough and then in crumb. Indeed, the agreement on critical temperatures for this event suggested that starch gelatinization would rather be involved in the final setting of the crumb, related to cell opening cessation (Singh and Battacharya, 2005). Conversely, an increase of storage modulus close to one decade was found for hydrated gluten in the same temperature interval [55, 70  C] (Fig. 5b). It appeared contradictory with results from Dreese et al. (1988) who did not observe any significant increase of modulus for commercial gluten dough. Conversely, Toufeili et al. (2002) found the same trend for larger temperature (80  C) but lower moisture content (40%). The presence of the remaining starch granules when extracting gluten (10%) has been inferred for this result (Stathopoulos et al., 2008). However, 10% of remaining granules would only have induced a modulus increase by a factor of about 2, when considering results obtained by Dreese et al. (1988), i.e. much lower than the factor 8 observed in our case (Fig. 5b). So, these results would rather suggest that gluten aggregation was also a possible explanation for the stiffening occurring during dough/ crumb transition. Finally, to underline the influence of temperature of the studied fractions, we represented the thermal variations of the ratio of storage modulus to its initial value at 25  C, E0 /E0 (25 C), in order to compare the behaviour of starch, gluten and wheat dough (Fig. 5c). This figure showed that the three samples displayed a peak at a temperature close to 70  C. Defining RE0 ¼ E0 ((70  C)/E0 (25  C)), we found that RE0 values ranked in the following order: RE0 (starch) < RE0 (gluten) < RE0 (wheat dough). Looking at peak magnitude, and taking into consideration the preceding results from the literature, it was finally suggested that the stiffening of the dough during the dough/crumb transition could be assigned for two-thirds to gluten aggregation and for one-third to starch granule swelling. J. Rouillé et al. / Journal of Cereal Science 52 (2010) 161e169 4. Conclusions Using an instrumented pilot-scale baking oven, it was possible to define the thermal conditions under which the dough/crumb transition occurred in French bread, i.e. between 55 and 70  C. It was barely completed after 5 min of baking, for most dough compositions. It was completely achieved at the same time as the maximum oven rise, as shown by the continuous increase of bread section during baking. Flour minor components did not influence much of these changes since final density was mostly controlled by its value after proofing. Using DSC and DMTA, D/C transition was shown to take place after the beginning of starch gelatinization, but ended before complete melting of starch. It was associated with the increase of modulus in the same temperature interval. Hence, dough stiffening might be more due to gluten aggregation rather than to the progressive swelling of starch granules. Finally, to better understand the role of soluble fractions and the lipid/puroindoline ratio in this transition, it will be important to achieve their characterisation in state diagrams, a purpose for which these results provide a significant contribution. Indeed, these structural changes occurring at microscopic and molecular levels in breads have a real impact on loaf expansion and texture setting during baking. 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