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Effect of the drying rate on the complex viscosity of wheat flour dough transforming into crust and crumb during baking
Journal of Cereal Science 58 (2013) 290e297
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Effect of the drying rate on the complex viscosity of wheat flour dough transforming into crust and crumb during baking F.M. Vanin a, b,1, C. Michon c, T. Lucas a, b, * a
IRSTEA, UR TERE, 17 Avenue de Cucillé, CS 64427, F-35044 Rennes, France Université européenne de Bretagne, France c AgroParisTech, JRU n 1145 Engineering Processes Food, AgroParisTech-Inra-Cnam, 1 avenue des Olympiades F-91300 Massy Cedex, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2013 Received in revised form 3 May 2013 Accepted 12 June 2013
This study aimed at characterizing the effect of hydrothermal dynamics on the dough rheology, in order to develop a complete dough viscosity model valid at different locations during baking. The dough rheology was characterised using dynamic mechanical thermal analysis (DMTA). Temperature and water content (WC) were monitored during DMTA. At high heating rates (15e30 C/min), relevant to the top crust, viscosity behaved as if WC was kept constant, in spite of dehydration (37%); such similarity was valid up to 80 C (stage A). Beyond, the viscosity decrease observed in the samples at constant WC was replaced by a long-lasting plateau (stage B, 3e4 106 Pa.s), attributed to WC reduction below w37%. Above the boiling water temperature, the logarithm of viscosity increased linearly with decreasing WC (stage C). At lower heating rates (5 C/min), relevant to the bottom crust, viscosity was two-fold higher than that at higher heating rates, suggesting lower oven-rise. The viscosity decrease, observed at high temperatures (>80 C) for samples at constant WC, was not observed if drying occurred late (case of crumb beneath the crust); instead, viscosity increased up to levels close to that of the top crust (2 e3 107 Pa.s at WCw20%). Despite these deviations, viscosity as a WC function was modelled with a unique equation set. Ó 2013 Published by Elsevier Ltd.
Keywords: Water content Heating rate Rheological properties Starch gelatinisation
1. Introduction The rheological properties of bread dough have been widely characterised experimentally (Dobraszczyk and Morgenstern, 2003), but generally not in conditions that represent the baking process, especially in relation to the crust formation. First, viscosity has been thoroughly measured at ambient and/or constant temperatures, and less often during dynamic heating, as encountered during bread baking. When heating rates were applied, they were appropriate to the bread core, 3e5 C/min (Collar et al., 2007; Rouillé et al., 2010). The highest comparable values were 9e11 C/min (Bloksma, 1980; Gupta et al., 2012; Singh and Bhattacharya, 2005), while in real conditions instantaneous heating rates for the crust are between 14 and 34 C/min (Dogan, 2002; Vanin et al., 2009), at the beginning of the baking process. Moreover, rheological measurements applicable to the crust should
* Corresponding author. IRSTEA, UR TERE, 17 Avenue de Cucillé, CS 64427, F35044 Rennes, France. Tel.: þ33 (0)2 23 48 21 77; fax: þ33 (0)2 23 48 21 15. E-mail address: [email protected] (T. Lucas). 1 Present address: Food Engineering Department, ZEA-FZEA, University of São Paulo, USP, 13635-900 Pirassununga, SP, Brazil. 0733-5210/$ e see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jcs.2013.06.003
ideally be conducted under dynamic drying; and most of the data available were obtained at a constant water content, or at water content variations more relevant to the dough (Collar et al., 2007; Gupta et al., 2012; Hibberd, 1970; Phan-Thien and Safari-Ardi, 1998; Smith et al., 1970; Webb et al., 1970). Second, relevant rheological conditions around an expanding gas cell during the baking are biaxial, large strain and low strain rates (Dobraszczyk and Morgenstern, 2003). However with the exception of biaxial extension (Dobraszczyk and Morgenstern, 2003; Rouillé et al., 2005), other tests used in combination with dynamic heating reproduced only approximately the expected deformation of dough films over a gas bubble during baking. On the other hand, bread dough has been characterised at very low uniaxial shear rates, between 102 and 103 s1 for uniaxial testing mode (Bloksma, 1990) and at rates of 5 102 s1 (Rouillé et al., 2005) and 101 s1 (Dobraszczyk and Salmanowicz, 2008) under biaxial extension. Most of these strain rates failed to reproduce those encountered during baking, which range from 1 to 2 103 s1 (Bloksma, 1990). Despite of these deficiencies, a comparison between data reported from some studies (Bloksma, 1980; Dreese et al., 1988; Lassoued-Oualdi, 2005; Vanin et al., 2010), at different heating
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rates, constant water content, unleavened dough, and under biaxial (lubricated squeezing mode) or uniaxial (tension mode) deformation, showed that variations of viscosity follow the same pattern, and three stages were identified at around the following transition temperatures: 55, 75, and 100 C (Fig. 1). First, Log h* (where h* stands for complex viscosity) showed a slight decrease until dough temperature reached w55e60 C (stage 1, Fig. 1). Stage 1 was not observed in all studies, especially when the values measured were below the sensitivity threshold of the apparatus (Lassoued-Oualdi, 2005; Vanin et al., 2010; Rolee and LeMeste, 1999; Rouillé et al., 2010). Between 55 and 75 C, an almost linear increase in Log h* was observed (stage 2). This stage was attributed to starch gelatinisation and/or to protein coagulation; however, no consensus has been reached in the literature on this issue (Dreese et al., 1988; Lassoued-Oualdi, 2005). Finally, between w75 and 100 C, Log h* decreased almost linearly (stage 3), an observation that was not common to all the studies, because the range of temperature was limited in upper values (Bloksma, 1980), or because water content started to decrease above 95 C (Vanin et al., 2010). The mechanisms related to the third stage received little interpretation in the literature (Dreese et al., 1988; Rolee and LeMeste, 1999). However, some hypotheses could be proposed, as underlined in some studies (Dreese et al., 1988; Lassoued-Oualdi, 2005; Rolee and LeMeste, 1999; Vanin et al., 2010): the softening of swollen starch granules (Rolee and LeMeste, 1999), the growth of gas bubbles (LassouedOualdi, 2005) and likely the degradation of the gluten network. The former hypothesis is likely to reduce the modulus. The second hypotheses take into account the increase of dough porosity and then its compressibility. As there is a higher quantity of gas in the sample, gas bubble walls become thinner; it is easier to bend them (Lassoued-Oualdi, 2005). Concerning the latter hypothesis, different studies characterised the effect of wheat gluten on dough rheology (Dreese et al., 1988; Rouillé et al., 2010; Stathopoulos et al., 2008; Toufeili et al., 2002). All those studies, with the exception to Rouillé et al. (2010), did not observe any significant increase of modulus (G0 or G00 ), or yet a little small increase (Toufeili et al., 2002), for gluten dough in the temperature range of starch gelatinisation. In this way and once that the present study also evaluated the effect of water content reduction during the “baking”, the changes in viscosity in this range of temperature should rather be discussed in terms of starch gelatinisation (and water content reduction if observed). Finally, it could be underlined that at a constant water content, the uniaxial tensile mode yielded results consistent with lubricated squeezing measurements, leading to
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biaxial extension of the dough (Fig. 1), although at low extension rates when compared to the ones observed during baking (Vanin et al., 2010). Previous modelling of transport and deformation during bread baking did not attempt to respect these experimental trends of mechanical behaviour of dough as entry parameters. Dough viscosity was considered to increase with the temperature in a monotonic manner, starting at about 60 C, which is generally accepted as the onset temperature of starch gelatinisation (De Cindio and Correra, 1995; Zhang and Datta, 2006). The dependency on water content is particularly crucial for a proper description of the crust setting in two-dimensional geometry, as attempted by some authors (De Cindio and Correra, 1995; Zanoni and Peri, 1993; Zhang and Datta, 2006), but this has not been implemented in baking models so far. Thus, the general objective of this study was to bring a proper description of the crust setting under conditions as close as possible to the ones which control baking. An average hydrothermal pathway for the top bread crust during baking was recently reproduced inside a dynamic mechanical thermal analysis (DMTA) rheometer, providing information on the effect of water content on rheological properties at the realistic conditions of the crust during baking (Vanin et al., 2010). For temperatures higher than 90 C, complex viscosities in the crustlike samples were 17 times higher than in samples kept at constant water content. The water content in two samples heated at the same rate under immediate or delayed drying conditions differed by about 7% (wet basis), and their complex viscosity by about a factor of 10, for temperatures higher than 100 C. A first contribution of this new study was supported by an experimental study to investigate the variations of complex viscosity in the water content-temperature domain, choosing and designing hydrothermal pathways relevant to the different locations in the dough during baking and/or induced by variations in the process parameters (oven air temperature and velocity, steam injection, and radiation). Examples of hydrothermal pathways observed in top and bottom crusts of genuine bread are presented in Fig. 2a. Different hydro-thermal pathways imply different extents of starch gelatinisation, as illustrated in the hydrothermal domain of gelatinisation. Indeed, Fig. 2a also presents the three major endothermic peaks observed by calorimetric analysis performed on dough. The first peak corresponds to cooperative, watermediated melting of starch crystallites, accompanied by swelling, the second peak to the melting of the remaining crystallites, and the third to the amyloseelipid complex melting transition. A second contribution was to propose, from these experimental data, a complete model of viscosity, ideally depending on local temperature and water content, for further use in the mathematical models of baking, including transport and volume changes. 2. Materials and methods 2.1. Preparation of dough samples and experimental procedure
Fig. 1. Compilation of literature data about temperature changes in complex viscosity (h) at constant water content (expressed in g of water per 100 g of wet dough e wb) for different recipes, heating rates, and testing modes. Refer to the text for the details on the mechanisms involved in stages 1, 2 and 3. Offsets between data sets can be attributed to changes in the recipe.
Dough was prepared with 2000g wheat flour (T55, Moulins Soufflet Pantin, France), 40g salt (La Baleine, France), 20g baking improver, and 1140g water. The ingredients were mixed in a spiral vertical mixer (KitchenAid) with a fixed bowl and a whip-shaped leaves turning following a planetary movement for 17 min. The final temperature of the dough was 25 1 C. At the end of the mixing procedure, the dough was covered with a plastic film for 15 min; then it was divided into two parts, and covered again with a plastic film for 10 min, enabling dough relaxation. Thirty grams of dough, previously floured with microcrystalline cellulose (MCC) (FMC BioPolymer, France) to prevent dough surface stickiness
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2.2. Rheological measurements Dynamic mechanical thermal analysis (DMTA) MKIIIE (TA Instruments, USA) with tensile testing was used to analyse the rheological properties of dough under drying. Dough strips were fixed in the immobile and mobile parts of the tensile system of the DMTA using clamps. Preliminary tests were performed in order to set the dynamic frequency (1 Hz), the static tensile stress (self-adjusted to 5% higher than the amplitude of dynamic stress) and the strain (0.04%) (Vanin et al., 2010). It must be emphasised that, in relation to the strain observed in the superficial layers during baking, the strain selected for DMTA tests was negligible in comparison with that in the crust under real conditions because the effect of two factors were followed (temperature and water content). It would have been too difficult to interpret if a third parameter (the effect of large deformations on the rheological behaviour of dough) was added. Moreover, the chosen frequency was rather large in comparison to the real rates to which the dough is submitted (order of magnitude 103 s1). As it is necessary to have no changes in the rheological properties during one measurement, a frequency of 1 Hz allowed carrying out one measurement every approximately 2e3 s, a time during which the dough did not evolve too much even when drying conditions were very quick. The sample was heated up by only circulating hot air around it during the DMTA tests. DMTA and temperature measurements were performed simultaneously. Means and standard deviations were determined from 4 repetitions for each different heating rate. In order to determine the water content in the dough samples, another series of experiments was also carried out, and measurements were stopped at different times (three separate samples for each different time). Water content was determined after oven-drying at 104 C for 24 h, and means were determined based on three samples. Dough thickness was not measured during drying. In order to reproduce different hydro-thermal pathways as encountered in the crust during bread baking (see Fig. 2a), several temperature ramp protocols were previously tested by adjusting the parameter values. Four different heating rates were fixed and evaluated. Complex uniaxial viscosity (h*) was calculated as follows (equations (1) and (2)):
h* ¼ h0 þ j$h00 ¼ Fig. 2. Water content and viscosity in the crust as a function of crust temperature; (a) experimental water content in bread crust obtained in the literature with different baking protocols (oven temperature, dough size, oven type, .), with reference to the state diagram of starch-water mixtures (gelatinisation and glass transition curves adapted from Vanin et al., 2010); measurements from this study were performed on bread dough using the same recipe as for dough strips used for DMTA measurements; temperature was measured at 0.5e1 mm from the top surface and water content was determined on a crust sample encompassing dried regions apparently dry regions; (b) experimental water content in DMTA samples (average of 4 runs) obtained with different heating protocols; (c) complex viscosity (average of 4 runs) in DMTA samples obtained from the different heating protocols presented in b.
without developing any affinity for water, were laminated with the manual lamination system provided with the Chopin Alveograph. Ten round trips produced a dough strip of 10 10 2.5 mm3. The initial water content was determined from three dough samples after oven-drying at 104 C for 24 h. In a set of samples, paraffin oil was spread over the surfaces of dough strips to delay dehydration during DMTA measurements; these samples were referred to as “under delayed drying”. Samples submitted to immediate drying while heating were referred to as “under drying”.
* h ¼
00 E
u
0 E þ j$
u
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E002 þ E02
u
(1)
(2)
where u is the pulsation in rad s1 (¼2p$frequency) and E0 , E00 , the storage and the loss modulus, respectively and finally h0 is termed the dynamic viscosity, and is equivalent to the ratio of the stress in phase with the rate of strain to the amplitude of the rate of strain. The term h00 is referred to as the out-of-phase viscosity, and is equivalent to the ratio of the stress 90 out of phase with the rate of strain to the amplitude of the rate of strain in the forced oscillation. 2.3. Temperature measurements Previously calibrated thermocouples (type T, Ø ¼ 0.2 mm, TCSA, France) were connected to an HP 34970A data logger. Temperatures were recorded every second. Means and standard deviations were determined from 4 repetitions for each different heating rate (Vanin et al., 2010).
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3. Results 3.1. Hydrothermal pathways simulated in the DMTA oven compared to those in a bread baked in a commercial oven In order to clearly identify each hydrothermal pathway, the average heating rate was calculated from experimental data of temperature in dough strips comprised between 30 and 80 C, and assuming a linear increase of temperature over this range, which was mainly the case here. Values were equal to 1 C/min, 5 C/min, 15 C/min, and 30 C/min. The protocol at 15 C/min was similar to that of a former study carried out by Vanin et al. (2010). Fig. 2b presents the average experimental temperature-water content trajectory in DMTA samples, accompanied by their respective standard deviations determined from the repetitions (runs). The temperature increase in the DMTA sample was highly repeatable, with an average standard deviation of 1.5 C. The maximum standard deviation of 4.5 C observed for the samples heated at 30 C/min may be attributed to variations in the positioning of the thermocouple in thickness of the dough sample, and to the high temperature increase observed in this heating protocol. Similarly, water content measurements from DMTA samples were also highly repeatable, with an average standard deviation of 0.66%. The maximum standard deviation of 3.42% was also observed for the samples heated at 30 C/min. Hydrothermal pathways achieved in the DMTA oven were compared with the ones obtained within the crust thickness, as reported in the literature or in loaves of bread purchased for the purpose of this study (Fig. 2a and b). Pathways presenting heating rates at 15 and 30 C/min (Fig. 2b) were close to most hydrothermal pathways encountered in top crusts of genuine bread (Fig. 2a), i.e. they pass through or tangentially approach the second endotherm in the hydrothermal domain of gelatinisation, which corresponds to a melting transition due to crystallite disruption. In pathways with heating rates at 1 and 5 C/ min, dehydration predominated upon heating, and the first and second endotherms in the hydrothermal domain of gelatinisation, respectively, were not crossed. Such a case is encountered in practice in the bottom crust during pan baking (Fig. 2a), or possibly at the top crust during part-baking. While extreme, the pathway at 1 C/min is a key condition in the experimental plan, enabling the study of the effect of combined water content and temperature, independent of the major modifications in macromolecular structure (starch gelatinisation, protein coagulation). Finally, dough at constant water content heated at a rate close to 5 C/min is relevant to crumb at the centre of the dough/bread loaf, and was considered as the reference condition. In practice, water content could be kept constant only below 100 C (Vanin et al., 2010), and the whole pathway (pathway at 5 C/min under delayed drying) was closer to the crumb located between the top crust, and the crumb at the core, that is dehydrated above 100 C (Fig. 2a). While water content measurements are destructive and timeconsuming, measurements performed during the baking are very few and therefore the hydrothermal pathways in genuine crusts are therefore low-resolved. Hence, hydrothermal kinetics were also plotted in Fig. 3 for DMTA samples under drying, and for crusts formed in a genuine bread loaf (Dogan, 2002; Hallstrom, 1998; Marston and Wannan, 1976; Thorvaldsson et al., 1999; Wagner et al., 2007; Zanoni and Peri, 1993). Time-course changes in temperature and water content obtained in the present study widely covered the domain defined in the literature for the top crust during bread baking for a variety of operating conditions (Fig. 3). Crossing of the first endotherm in the hydrothermal domain of gelatinisation (Fig. 2b) was confirmed by a significant decrease of
Fig. 3. Evolution of temperature (a) and water content (b) in samples under drying for different hydrothermal pathways performed inside the DMTA oven and at the surface of bread loaves, as reported in the literature.
birefringence in microscopy images for most pathways, either simulated in the DMTA oven or observed in genuine crusts (Table 1). This was consistent with a previous report made by Burt and Russell (1983), who showed that birefringence in images mainly disappeared, whether the second endotherm was crossed or not. Birefringence was still observed in samples heated at 1 C/min for temperatures as high as 85 C, although to a lesser intensity than in crude dough (Table 1). In fact, despite the successive adjustments of the protocol to decrease the drying rate, it could not be stated whether the pathway at a heating rate of 1 C/min crossed the hydrothermal interval defined by the first endotherm or not. 3.2. Changes in complex viscosity of dough upon immediate drying of variable intensity The pathways that presented dehydration simultaneous to heating and starch structural changes (Fig. 2b) are analysed in this section. Given the simultaneous changes in water content and temperature, mechanical properties should ideally be analysed in a three-dimensional graph with both factors. However, such a mode of representation is hard to deal with and discuss (Vanin et al., 2010). Successive two-dimensional representations of mechanical properties as a function of temperature and as a function of water content were thus presented. Fig. 2c presents changes in complex viscosity as a function of dough temperature. Changes in complex viscosity obtained at 15 C/ min under drying and delayed drying in the present study were identical to the ones obtained in a previous study, despite changes in flour batch and mixing protocols between the two studies (Vanin et al., 2010).
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Table 1 Microscopic images of crusts obtained in a convective oven set at two extreme values of temperature (T ¼ 100 C representative of partial baking, T ¼ 250 C), and DMTA ribbon dough samples heated at different heating rates (1, 5 and 30 C/min), took at different temperature e water content in order to evidence the loss of birefringence at passing the first endotherm of gelatinisation. Genuine bread crust
Toven ¼ 100 C
DMTA ribbon dough samples
1 C/min
5 C/min
Genuine bread crust 30 C/min
Toven ¼ 250 C
Raw dough
At passing the 1st endotherm (55e60 C)
Between 1st and 2nd endotherms (85e90 C)
>100 C
The higher the heating rate (30 C/min versus 1 C/min), the lower the extent of drying at a given temperature (respectively, 39% against 29% of water content at 80 C, for instance, see Fig. 2b, as well). The initial period where viscosity remained constant was a consistent finding at high heating rates (15e30 C/min). This period was in the same range of temperature, as observed in samples at constant water content (see Figs. 1 and 2c). This period was followed by a steep increase related to starch granule swelling and gluten aggregation (or strengthening of gluten network due to thermal coagulation of proteins), followed by a plateau. Once more, this was similar to what was observed in samples at constant water content (Fig. 2c). It should be remembered that pathways of DMTA samples at high heating rates and at constant water content were not very different (Fig. 2b), both passing the first two endotherms in the hydrothermal domain of starch gelatinisation, and initially favouring heat transfer upon dehydration. A long lasting plateau at high heating rates replaced the decrease in viscosity observed at constant water content and attributed to the disruption of swollen starch granules (Rolee and LeMeste, 1999) and likely the degradation of gluten network (80e 110 C approximately). The onset of the plateau at high heating rates could be associated either to the crossing of the second endotherm in the hydrothermal domain of gelatinisation, or to dough water content decreasing below 37%, approximately (Fig. 2b and c). This point will be further discussed based on the results obtained at lower heating rates. The onset temperature of the plateau was higher for samples at high heating rates than at constant water content, thus explaining the higher viscosity values reached in the plateau. Finally, complex viscosity increased steeply as the temperature exceeded 110 C; such behaviour was very different from that observed in samples at constant water content. All of these trends were formerly evidenced at 15 C/min (Vanin et al., 2010), and confirmed at higher heating rates (30 C/min) in
the present study. Increasing the heating rate above 15 C/min did not affect the changes in complex viscosity too much. Although differences were observed for temperature and water content kinetics, between the heating rates of 15 and 30 C/min (Fig. 3), they became small when combined in the state diagram (Fig. 2b), which explained the similarity in the viscosity behaviour between these two heating rates. Different from high heating rates, the initial increase in complex viscosity at 1 and 5 C/min was related to a decrease in water content. At 1 C/min for instance, complex viscosity increased almost 40 times, while temperature increased only up to 50 C, but water content decreased to 35% (Fig. 2b and c). In fact, the reduction in water content took more time at low heating rates, more probably affecting the distribution of water by diffusion at microscopic and molecular levels, and explaining the difference in viscosity behaviour; this point would require further investigation. Likewise, the crossing of the first endotherm in the hydrothermal domain of gelatinisation, at about 60 C (Fig. 2b), occurred much later in the drying process and was not accompanied by a steep increase in complex viscosity, different from what was observed at high heating rates (Fig. 2c). For example, considering the interval of temperature between 60 and 80 C, viscosity rose by about 2e5 times for samples heated at 1 and 5 C/min, respectively, and about 7e9 times for samples heated at 15 and 30 C/min, respectively. Level-off in viscosity was observed at 1 and 5 C/min (Fig. 2c), while water content decreased below 37%, and temperatures were still low (50 and 70 C, respectively). All these results showed the predominant effect of water content at low heating rates; such effect will be further discussed below (Fig. 4). As for high heating rates, complex viscosity increased steeply again in the vicinity and above the temperature of boiling water (Fig. 2c), while water vaporisation was enhanced. Finally, regardless of the heating rate, the strong reduction in water content below 5e10% was accompanied by a
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ones observed in the present study at the same range of water content (by a factor from 0.5 to 3 per % of water content reduction). Stage B (Fig. 4a) marked the transition between stage A, characterised by a steep increase in the logarithm of viscosity, and stage C, which presented a slower and linear increase of the logarithm of viscosity. The range in water content over which stage B took place shortened with decreasing heating rates (see dashed vertical lines in Fig. 4a). This was consistent with the interpretation of results presented above, in Fig. 2b and c, and may be explained by the different degree of starch gelatinisation. Stage C (Fig. 4a) exhibited a linear increase in logarithmic viscosity, with a slope of 0.3 per % in water content reduction (calculated over the range of 16e26%) no matter the heating rate. Similar linear trends in logarithmic viscosity were reported for semolina dough, noodles, or biscuit dough (Maache-Rezzoug et al., 1998; Manohar and Rao, 1999; Yu and Ngadi, 2006). However, slopes were slightly higher than those observed in the present study, 0.5 to 1 per % of water content reduction for noodle dough, and 1.3 for semolina extruded pasta. The offset observed in stage C between linear curves at low and high heating rates mainly resulted from the different length of stage B. Considering stages B and C globally, high heating rates acted as reducers of dough viscosity at a given water content, possibly limiting the extent of starch gelatinisation when compared with low heating rates. Finally, it should be noted that the effect of the heating rate was less dominant when viscosity was plotted against water content. This drove us to propose an empirical model of viscosity dependent on water content to implement the present data in future baking models. Equations are presented in Table 2 according to stages A to C presented above; their numerical applications were also drawn in Fig. 4a to enable the reader to evaluate the quality of the adjustment. Fig. 4. Normalised complex viscosity h*/h0*, average of 4 runs versus dough water content for samples under drying, heated at 1, 5, 15, and 30 C/min. Time-course changes in water content were (linearly) interpolated, and each interpolated water content was associated with a complex viscosity; these supplementary data are represented with an open symbol. The different sets of data were adjusted by the models (plain lines) reported in Table 2. Refer to the text for the description of stages A, B and C; the lowest and highest water contents of stage B are marked by dashed and plain vertical lines, respectively; (b) stage A in comparison with literature data.
glass transition and complex viscosity values higher than 108 Pa. s (Fig. 2b and c). Fig. 4 presents changes in complex viscosity as a function of water content in the dough. Complex viscosity was normalised to its initial value (at 44.5% water content) to eliminate this source of variation between the tested conditions. Globally, three stages could be distinguished in relation to water content (stages A, B, and C, Fig. 4a). In stage A (Fig. 4b), where water content was maintained between 45 and 37%, an important rise in complex viscosity was observed. It should be emphasised that at 5 and 15 C/min, a period exhibiting no change in viscosity was noticed, with a steep increase in viscosity occurring at lower water content than the initial one. This was consistent with the interpretations above that are based on results presented in Fig. 2b and c. The duration of this period was related to the predominance of heating upon dehydration. It should also be noted that data at 1 C/min and 30 C/min did not have enough resolution at high water contents to fully support the discussion of such a period. Data from literature at different water contents (41e45% wb) obtained at constant temperature (Berland and Launay, 1995; Hibberd, 1970; Smith et al., 1970; Webb et al., 1970) presented a rise in viscosity (by a factor of 1.1 per % of water content reduction) in accordance with the
3.3. Changes in the complex viscosity of dough upon delayed drying This section focuses on the sample heated at 5 C/min and covered with oil to delay dehydration. Again, such a pathway is relevant to the crumb located beneath the top crust (Fig. 2aeb). Complex viscosity in Fig. 4 was considered constant, as long as water content did not decrease, erasing the variations observed in Fig. 2c, below 100 C. Such simplification appears reasonable in modelling the viscosity for its implementation into a baking model. Similar to other pathways discussed in Section 3.2, stages B and C applied to complex viscosity in the samples under delayed drying (Table 2). The low level of complex viscosity at stage B was explained by the fact that, when water content started decreasing in the samples under delayed drying, complex viscosity was already decreasing on its way back to its initial value (considering the value of 1 in Fig. 2c). Unlike other pathways, stage B started with the initial water content of the dough before drying (instead of the unique threshold, at 37%). Consistent with this interpretation of stage B length in terms to degree of starch gelatinisation, the lower limit in water content of the plateau for 5 C/min under delayed drying should have been even lower than that observed for 15e 30 C/min. However, this was not observed. Instead, the lower limit of this plateau seemed to correspond to that observed at the same heating rate (5 C/min) under drying, suggesting an effect of the heating rate on the length of stage B. However, to be further discussed, this issue would deserve additional data, between 12.2 and 32.9% of water content. Finally, complex viscosity during stage C presented the highest increase among all pathways. No explanation for this finding could be proposed at this stage of the study.
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Table 2 Models of the normalised complex viscosity h* =h*0 as a function of water content Xw and heating rate (R2 > 0.95). h*0 refers to Xw ¼ 44.5%. Stages A, B, and C refer to Fig. 4. * expressed in g of water per 100g of wet dough, **in g of wet dough per g of water, ***in g1/2 of wet dough per g1/2 of water. Heating rate ( C/min) Under immediate drying (crust-like) 1 Stage A
5
* Log10 hh* 0
Stage C
15e30
¼ aXw þ b
with a ¼ 0.205* b ¼ 9.151
Stage B
Under delayed drying (crumb-like)
* Log10 hh* 0
ðXw bÞ c2
¼ ae
þ de
5
ðXw bÞ f
with a ¼ 0.827 b ¼ 37.0* c ¼ 1.596*** d ¼ 0.707 f ¼ 2.532**
with a ¼ 0.678 b ¼ 37.0* c ¼ 1.148*** d ¼ 0.560 f ¼ 1.359**
with a ¼ 37.0 * Log10 hh* ¼ aXw þ b
with a ¼ 31.0
with a ¼ 17.0
with a ¼ 3.2
with a ¼ 5.829 102* b ¼ 3.58
with a ¼ 5.824 102* b ¼ 3.40
with a ¼ 5.828 102* b ¼ 2.86
with a ¼ 8.534 102* b ¼ 3.30
h* h*0 ¼ a
0
4. Discussion and conclusion Different hydrothermal pathways in dough strips mimicking those reported in the literature at different dough locations during baking were successfully reproduced in the oven of a DMTA rheometer. At high heating rates relevant to a typical top crust (15e 30 C/min) despite of decreasing water content, dough viscosity behaved as if water content was kept constant, as long as temperature remained below 80 C (stage A); this behavior comprised an initial period with no change in viscosity followed by an increase at about 60 C, related to the onset of starch gelatinisation. Conversely to high heating rates, complex viscosity at low heating rates, relevant to a bottom crust during pan baking for instance, increased with decreasing water content; this was attributed to the enhanced distribution of water at the microscopic scale. The decrease in viscosity observed in samples at constant water content above 80 C was not observed in these samples under drying, and was replaced by a long-lasting plateau (stage B); this was related to the reduction in water content below approximately 37%, regardless of the heating rate. Viscosity values at the plateau as well as the length of this plateau were respectively lower (by a factor of 2) and longer at higher heating rate (15e30 C/min compared to 1e5 C/min), possibly affecting the extent of starch gelatinisation. It may be assumed that high heating rate would extend dough extensibility and oven-rise. Above the boiling temperature of water, the logarithm of viscosity increased linearly with decreasing water content, at a rate only dependent on water content (stage C). Stages B and C were shown to be independent of the heating rate, for 15 and 30 C/min, which covers a wide range of bread-baking conditions for the top crust. It should be remembered that past baking models that included transport and volume changes were based on a temperaturedependent model of viscosity. Results from the present study showed that such an approach is invalid for the bottom crust and remains valid up to 80 C for the top crust, which corresponds to the first couples of minutes in most baking conditions. Beyond 80 C, the viscosity in dough surface layers at the top is expected to be much higher than in the crumb at the core because of a reduction in water content. These high levels of viscosity encountered in the crust, compared with the crumb at the core, account not only for the cessation of the oven rise, but also for the resistance to squeezing resulting from the expansion of inner dough layers (Wagner et al., 2008).
Finally, the crumb located beneath the top crust was studied, a case in which drying is subsequent to starch gelatinisation. Different from the crumb at the core, which maintains its initial water content, this progressive dehydration of 3e5 points would maintain its viscosity (plateau during stage B); this may be enough to provide higher resistance to squeezing forces exerted by the expansion of inner dough layers (Wagner et al., 2008). Stages B and C both applied to this latter case. Particularly, the slope during stage C was greater than in other pathways. Viscosity at high levels of dehydration (20% for instance, which seems reasonable for the crumb very close to the top crust) did not differ much from that of samples heated at 15e30 C/min under drying (mimicking the top crust); this suggested a relative continuity of viscosity along a profile in bread from the surface to the core. In spite of deviations between hydrothermal dynamics, complex viscosity as a function of water content followed similar trends, leading to a unique set of equations for modeling stages A to C, with obviously different values of the parameters for each heating rate. Acknowledgements This study was carried out with financial support from the Council of Brittany (France). The authors also wish to thank Juliette Bodet for her technical assistance in the experimental work. References Berland, S., Launay, B., 1995. Shear softening and thixotropic properties of wheat flour doughs in dynamic testing at high shear strain. Rheologica Acta 34, 622e628. Bloksma, A.H., 1980. Effect of heating rate on viscosity of wheat flour doughs. Journal of Texture Studies 10, 261e269. Bloksma, A.H., 1990. Rheology of the breadmaking process. Cereal Foods World 35, 228e236. Burt, D.J., Russell, P.L., 1983. Gelatinization of low water content wheat starch water mixtures (A combined study by differential scanning calorimetry and light microscopy). Starch/Starke 35, 354e360. Collar, C., Bollain, C., Rosell, C.M., 2007. Rheological behaviour of formulated bread doughs during mixing and heating. Food Science and Technology International 13, 99e107. De Cindio, B., Correra, S., 1995. Mathematical modeling of leavened cereal goods. Journal of Food Engineering 24, 379e403. Dobraszczyk, B.J., Morgenstern, M., 2003. Rheology and the breadmaking process. Journal of Cereal Science 38, 229e245. Dobraszczyk, B.J., Salmanowicz, B.P., 2008. Comparison of predictions of baking volume using large deformation rheological properties. Journal of Cereal Science 47, 292e301. Dogan, I.S., 2002. Dynamic rheological properties of dough as affected by amylases from various sources. Nahrung-Food 46, 399e403.
F.M. Vanin et al. / Journal of Cereal Science 58 (2013) 290e297 Dreese, P.C., Faubion, J.M., Hoseney, R.C., 1988. Dynamic rheological properties of flour, gluten and gluten-starch doughs. Cereal Chemistry 65, 348. Gupta, S., Shimray, C.A., Rao, G.V., 2012. Influence of organic acids on rheological and bread-makingcharacteristics of fortified wheat flour. International Journal of Food Sciences and Nutrition 63, 411e420. Hallstrom, B., 1998. Heat and mass transfer in baking ovens. In: Asp, N.-G. (Ed.), Cereal Science and Technology in Sweden. Lund University, Lund, p. 174. Hibberd, G.E., 1970. Dynamic viscoelastic behaviour of wheat flour doughs. Part III: the influence of the starch granules. Rheologica Acta 9, 501e505. Lassoued-Oualdi, N., 2005. Structure alvéolaire des produits céréaliers de cuisson en lien avec les propriétés rhéologiques et thermiques de la pâte: Effet de la composition Sciences Alimentaires. Ecole Nationale Supérieure des Industries Agricoles et Alimentaires, Massy, Fr, p. 163. Maache-Rezzoug, Z., Bouvier, J.M., Allaf, K., Patras, C., 1998. Effect of principal ingredients on rheological behaviour of biscuit dough and on quality of biscuits. Journal of Food Engineering 35, 23e42. Manohar, R.S., Rao, P.H., 1999. Effects of water on the rheological characteristics of biscuit dough and quality of biscuits. European Food Research and Technology 209, 281e285. Marston, P.E., Wannan, T.L., 1976. Bread baking: the transformation from dough to bread. The Bakers Digest, 24e28. Phan-Thien, N., Safari-Ardi, M., 1998. Linear viscoelastic properties of flour-water doughs at different water concentrations. Journal of Non-Newtonian Fluid Mechanics 74, 137e150. Rolee, A., LeMeste, M., 1999. Effect of moisture content on thermomechanical behavior of concentrated wheat starch-water preparations. Cereal Chemistry 76, 452e458. Rouillé, J., Chiron, H., Colonna, P., Della Valle, G., Lourdin, D., 2010. Dough/crumb transition during French bread baking. Journal of Cereal Science 52, 161e169. Rouillé, J., Della Valle, G., Lefebvre, J., Sliwinski, E., vanVliet, T., 2005. Shear and extensional properties of bread doughs affected by their minor components. Journal of Cereal Science 42, 45e57. Singh, A.P., Bhattacharya, M., 2005. Development of dynamic modulus and cell opening of dough during baking. Journal of Texture Studies 36, 44e67.
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Smith, J.R., Smith, T.L., Tschoegl, N.M., 1970. Rheological properties of wheat flour doughs. III. Dynamic shear modulus and its dependence on amplitude, frequency, and dough composition. Rheologica Acta 9, 239e252. Stathopoulos, C.E., Tsiami, A.A., Schofield, J.D., Dobraszczyk, B.J., 2008. Effect of heat on rheology, surface hydrophobicity and molecular weight distribution of glutens extracted from flours with different bread-making quality. Journal of Cereal Science 47, 134e143. Thorvaldsson, K., Stading, M., Nilsson, K., Kidman, S., Langton, M., 1999. Rheology and structure of heat-treated pasta dough: influence of water content and heating rate. Food Science and Technology-Lebensmittel-Wissenschaft & Technologie 32, 154e161. Toufeili, I., Lambert, I.A., Kokini, J.L., 2002. Effect of glass transition and cross-linking on rheological properties of gluten: development of a preliminary state diagram. Cereal Chemistry 79, 138e142. Vanin, F.M., Lucas, T., Trystram, G., 2009. Crust formation and its role during bread baking. Trends in Food Science & Technology 20, 333e343. Vanin, F.M., Michon, C., Trystram, G., Lucas, T., 2010. Simulating the formation of bread crust in a DMTA rheometer. Journal of Cereal Science 51, 277e283. Wagner, M., Lucas, T., Le Ray, D., Trystram, G., 2007. Water transport in bread during baking. Journal of Food Engineering 78, 1167e1173. Wagner, M., Zhang, L., Quellec, S., Dioursat, C., Flick, D., Trystram, G., Lucas, T., 2008. Role of the crust formation on local expansion during bread bakling. In: Campbell, G.M., Scanlon, M.G., Pyle, D.L. (Eds.), 2008. Bubble in Foods, vol. II. AACC International Inc., pp. 241e250. Webb, T., Heaps, P.W., Russell Eggitt, P.W., Coppock, J.B.M., 1970. A rheological investigation of the role of water in wheat flour doughs. International Journal of Food Science & Technology 5, 65e76. Yu, L.J., Ngadi, M.O., 2006. Rheological properties of instant fried noodle dough as affected by some ingredients. Journal of the Science of Food and Agriculture 86, 544e548. Zanoni, B., Peri, C., 1993. A study of the bread-baking process. I: a phenomenological model. Journal of Food Engineering 19, 389e398. Zhang, J., Datta, A.K., 2006. Mathematical modeling of bread baking process. Journal of Food Engineering 75, 78e89.
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Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Effect of the drying rate on the complex viscosity of wheat flour dough transforming into crust and crumb during baking F.M. Vanin a, b,1, C. Michon c, T. Lucas a, b, * a
IRSTEA, UR TERE, 17 Avenue de Cucillé, CS 64427, F-35044 Rennes, France Université européenne de Bretagne, France c AgroParisTech, JRU n 1145 Engineering Processes Food, AgroParisTech-Inra-Cnam, 1 avenue des Olympiades F-91300 Massy Cedex, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2013 Received in revised form 3 May 2013 Accepted 12 June 2013
This study aimed at characterizing the effect of hydrothermal dynamics on the dough rheology, in order to develop a complete dough viscosity model valid at different locations during baking. The dough rheology was characterised using dynamic mechanical thermal analysis (DMTA). Temperature and water content (WC) were monitored during DMTA. At high heating rates (15e30 C/min), relevant to the top crust, viscosity behaved as if WC was kept constant, in spite of dehydration (37%); such similarity was valid up to 80 C (stage A). Beyond, the viscosity decrease observed in the samples at constant WC was replaced by a long-lasting plateau (stage B, 3e4 106 Pa.s), attributed to WC reduction below w37%. Above the boiling water temperature, the logarithm of viscosity increased linearly with decreasing WC (stage C). At lower heating rates (5 C/min), relevant to the bottom crust, viscosity was two-fold higher than that at higher heating rates, suggesting lower oven-rise. The viscosity decrease, observed at high temperatures (>80 C) for samples at constant WC, was not observed if drying occurred late (case of crumb beneath the crust); instead, viscosity increased up to levels close to that of the top crust (2 e3 107 Pa.s at WCw20%). Despite these deviations, viscosity as a WC function was modelled with a unique equation set. Ó 2013 Published by Elsevier Ltd.
Keywords: Water content Heating rate Rheological properties Starch gelatinisation
1. Introduction The rheological properties of bread dough have been widely characterised experimentally (Dobraszczyk and Morgenstern, 2003), but generally not in conditions that represent the baking process, especially in relation to the crust formation. First, viscosity has been thoroughly measured at ambient and/or constant temperatures, and less often during dynamic heating, as encountered during bread baking. When heating rates were applied, they were appropriate to the bread core, 3e5 C/min (Collar et al., 2007; Rouillé et al., 2010). The highest comparable values were 9e11 C/min (Bloksma, 1980; Gupta et al., 2012; Singh and Bhattacharya, 2005), while in real conditions instantaneous heating rates for the crust are between 14 and 34 C/min (Dogan, 2002; Vanin et al., 2009), at the beginning of the baking process. Moreover, rheological measurements applicable to the crust should
* Corresponding author. IRSTEA, UR TERE, 17 Avenue de Cucillé, CS 64427, F35044 Rennes, France. Tel.: þ33 (0)2 23 48 21 77; fax: þ33 (0)2 23 48 21 15. E-mail address: [email protected] (T. Lucas). 1 Present address: Food Engineering Department, ZEA-FZEA, University of São Paulo, USP, 13635-900 Pirassununga, SP, Brazil. 0733-5210/$ e see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jcs.2013.06.003
ideally be conducted under dynamic drying; and most of the data available were obtained at a constant water content, or at water content variations more relevant to the dough (Collar et al., 2007; Gupta et al., 2012; Hibberd, 1970; Phan-Thien and Safari-Ardi, 1998; Smith et al., 1970; Webb et al., 1970). Second, relevant rheological conditions around an expanding gas cell during the baking are biaxial, large strain and low strain rates (Dobraszczyk and Morgenstern, 2003). However with the exception of biaxial extension (Dobraszczyk and Morgenstern, 2003; Rouillé et al., 2005), other tests used in combination with dynamic heating reproduced only approximately the expected deformation of dough films over a gas bubble during baking. On the other hand, bread dough has been characterised at very low uniaxial shear rates, between 102 and 103 s1 for uniaxial testing mode (Bloksma, 1990) and at rates of 5 102 s1 (Rouillé et al., 2005) and 101 s1 (Dobraszczyk and Salmanowicz, 2008) under biaxial extension. Most of these strain rates failed to reproduce those encountered during baking, which range from 1 to 2 103 s1 (Bloksma, 1990). Despite of these deficiencies, a comparison between data reported from some studies (Bloksma, 1980; Dreese et al., 1988; Lassoued-Oualdi, 2005; Vanin et al., 2010), at different heating
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rates, constant water content, unleavened dough, and under biaxial (lubricated squeezing mode) or uniaxial (tension mode) deformation, showed that variations of viscosity follow the same pattern, and three stages were identified at around the following transition temperatures: 55, 75, and 100 C (Fig. 1). First, Log h* (where h* stands for complex viscosity) showed a slight decrease until dough temperature reached w55e60 C (stage 1, Fig. 1). Stage 1 was not observed in all studies, especially when the values measured were below the sensitivity threshold of the apparatus (Lassoued-Oualdi, 2005; Vanin et al., 2010; Rolee and LeMeste, 1999; Rouillé et al., 2010). Between 55 and 75 C, an almost linear increase in Log h* was observed (stage 2). This stage was attributed to starch gelatinisation and/or to protein coagulation; however, no consensus has been reached in the literature on this issue (Dreese et al., 1988; Lassoued-Oualdi, 2005). Finally, between w75 and 100 C, Log h* decreased almost linearly (stage 3), an observation that was not common to all the studies, because the range of temperature was limited in upper values (Bloksma, 1980), or because water content started to decrease above 95 C (Vanin et al., 2010). The mechanisms related to the third stage received little interpretation in the literature (Dreese et al., 1988; Rolee and LeMeste, 1999). However, some hypotheses could be proposed, as underlined in some studies (Dreese et al., 1988; Lassoued-Oualdi, 2005; Rolee and LeMeste, 1999; Vanin et al., 2010): the softening of swollen starch granules (Rolee and LeMeste, 1999), the growth of gas bubbles (LassouedOualdi, 2005) and likely the degradation of the gluten network. The former hypothesis is likely to reduce the modulus. The second hypotheses take into account the increase of dough porosity and then its compressibility. As there is a higher quantity of gas in the sample, gas bubble walls become thinner; it is easier to bend them (Lassoued-Oualdi, 2005). Concerning the latter hypothesis, different studies characterised the effect of wheat gluten on dough rheology (Dreese et al., 1988; Rouillé et al., 2010; Stathopoulos et al., 2008; Toufeili et al., 2002). All those studies, with the exception to Rouillé et al. (2010), did not observe any significant increase of modulus (G0 or G00 ), or yet a little small increase (Toufeili et al., 2002), for gluten dough in the temperature range of starch gelatinisation. In this way and once that the present study also evaluated the effect of water content reduction during the “baking”, the changes in viscosity in this range of temperature should rather be discussed in terms of starch gelatinisation (and water content reduction if observed). Finally, it could be underlined that at a constant water content, the uniaxial tensile mode yielded results consistent with lubricated squeezing measurements, leading to
291
biaxial extension of the dough (Fig. 1), although at low extension rates when compared to the ones observed during baking (Vanin et al., 2010). Previous modelling of transport and deformation during bread baking did not attempt to respect these experimental trends of mechanical behaviour of dough as entry parameters. Dough viscosity was considered to increase with the temperature in a monotonic manner, starting at about 60 C, which is generally accepted as the onset temperature of starch gelatinisation (De Cindio and Correra, 1995; Zhang and Datta, 2006). The dependency on water content is particularly crucial for a proper description of the crust setting in two-dimensional geometry, as attempted by some authors (De Cindio and Correra, 1995; Zanoni and Peri, 1993; Zhang and Datta, 2006), but this has not been implemented in baking models so far. Thus, the general objective of this study was to bring a proper description of the crust setting under conditions as close as possible to the ones which control baking. An average hydrothermal pathway for the top bread crust during baking was recently reproduced inside a dynamic mechanical thermal analysis (DMTA) rheometer, providing information on the effect of water content on rheological properties at the realistic conditions of the crust during baking (Vanin et al., 2010). For temperatures higher than 90 C, complex viscosities in the crustlike samples were 17 times higher than in samples kept at constant water content. The water content in two samples heated at the same rate under immediate or delayed drying conditions differed by about 7% (wet basis), and their complex viscosity by about a factor of 10, for temperatures higher than 100 C. A first contribution of this new study was supported by an experimental study to investigate the variations of complex viscosity in the water content-temperature domain, choosing and designing hydrothermal pathways relevant to the different locations in the dough during baking and/or induced by variations in the process parameters (oven air temperature and velocity, steam injection, and radiation). Examples of hydrothermal pathways observed in top and bottom crusts of genuine bread are presented in Fig. 2a. Different hydro-thermal pathways imply different extents of starch gelatinisation, as illustrated in the hydrothermal domain of gelatinisation. Indeed, Fig. 2a also presents the three major endothermic peaks observed by calorimetric analysis performed on dough. The first peak corresponds to cooperative, watermediated melting of starch crystallites, accompanied by swelling, the second peak to the melting of the remaining crystallites, and the third to the amyloseelipid complex melting transition. A second contribution was to propose, from these experimental data, a complete model of viscosity, ideally depending on local temperature and water content, for further use in the mathematical models of baking, including transport and volume changes. 2. Materials and methods 2.1. Preparation of dough samples and experimental procedure
Fig. 1. Compilation of literature data about temperature changes in complex viscosity (h) at constant water content (expressed in g of water per 100 g of wet dough e wb) for different recipes, heating rates, and testing modes. Refer to the text for the details on the mechanisms involved in stages 1, 2 and 3. Offsets between data sets can be attributed to changes in the recipe.
Dough was prepared with 2000g wheat flour (T55, Moulins Soufflet Pantin, France), 40g salt (La Baleine, France), 20g baking improver, and 1140g water. The ingredients were mixed in a spiral vertical mixer (KitchenAid) with a fixed bowl and a whip-shaped leaves turning following a planetary movement for 17 min. The final temperature of the dough was 25 1 C. At the end of the mixing procedure, the dough was covered with a plastic film for 15 min; then it was divided into two parts, and covered again with a plastic film for 10 min, enabling dough relaxation. Thirty grams of dough, previously floured with microcrystalline cellulose (MCC) (FMC BioPolymer, France) to prevent dough surface stickiness
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2.2. Rheological measurements Dynamic mechanical thermal analysis (DMTA) MKIIIE (TA Instruments, USA) with tensile testing was used to analyse the rheological properties of dough under drying. Dough strips were fixed in the immobile and mobile parts of the tensile system of the DMTA using clamps. Preliminary tests were performed in order to set the dynamic frequency (1 Hz), the static tensile stress (self-adjusted to 5% higher than the amplitude of dynamic stress) and the strain (0.04%) (Vanin et al., 2010). It must be emphasised that, in relation to the strain observed in the superficial layers during baking, the strain selected for DMTA tests was negligible in comparison with that in the crust under real conditions because the effect of two factors were followed (temperature and water content). It would have been too difficult to interpret if a third parameter (the effect of large deformations on the rheological behaviour of dough) was added. Moreover, the chosen frequency was rather large in comparison to the real rates to which the dough is submitted (order of magnitude 103 s1). As it is necessary to have no changes in the rheological properties during one measurement, a frequency of 1 Hz allowed carrying out one measurement every approximately 2e3 s, a time during which the dough did not evolve too much even when drying conditions were very quick. The sample was heated up by only circulating hot air around it during the DMTA tests. DMTA and temperature measurements were performed simultaneously. Means and standard deviations were determined from 4 repetitions for each different heating rate. In order to determine the water content in the dough samples, another series of experiments was also carried out, and measurements were stopped at different times (three separate samples for each different time). Water content was determined after oven-drying at 104 C for 24 h, and means were determined based on three samples. Dough thickness was not measured during drying. In order to reproduce different hydro-thermal pathways as encountered in the crust during bread baking (see Fig. 2a), several temperature ramp protocols were previously tested by adjusting the parameter values. Four different heating rates were fixed and evaluated. Complex uniaxial viscosity (h*) was calculated as follows (equations (1) and (2)):
h* ¼ h0 þ j$h00 ¼ Fig. 2. Water content and viscosity in the crust as a function of crust temperature; (a) experimental water content in bread crust obtained in the literature with different baking protocols (oven temperature, dough size, oven type, .), with reference to the state diagram of starch-water mixtures (gelatinisation and glass transition curves adapted from Vanin et al., 2010); measurements from this study were performed on bread dough using the same recipe as for dough strips used for DMTA measurements; temperature was measured at 0.5e1 mm from the top surface and water content was determined on a crust sample encompassing dried regions apparently dry regions; (b) experimental water content in DMTA samples (average of 4 runs) obtained with different heating protocols; (c) complex viscosity (average of 4 runs) in DMTA samples obtained from the different heating protocols presented in b.
without developing any affinity for water, were laminated with the manual lamination system provided with the Chopin Alveograph. Ten round trips produced a dough strip of 10 10 2.5 mm3. The initial water content was determined from three dough samples after oven-drying at 104 C for 24 h. In a set of samples, paraffin oil was spread over the surfaces of dough strips to delay dehydration during DMTA measurements; these samples were referred to as “under delayed drying”. Samples submitted to immediate drying while heating were referred to as “under drying”.
* h ¼
00 E
u
0 E þ j$
u
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E002 þ E02
u
(1)
(2)
where u is the pulsation in rad s1 (¼2p$frequency) and E0 , E00 , the storage and the loss modulus, respectively and finally h0 is termed the dynamic viscosity, and is equivalent to the ratio of the stress in phase with the rate of strain to the amplitude of the rate of strain. The term h00 is referred to as the out-of-phase viscosity, and is equivalent to the ratio of the stress 90 out of phase with the rate of strain to the amplitude of the rate of strain in the forced oscillation. 2.3. Temperature measurements Previously calibrated thermocouples (type T, Ø ¼ 0.2 mm, TCSA, France) were connected to an HP 34970A data logger. Temperatures were recorded every second. Means and standard deviations were determined from 4 repetitions for each different heating rate (Vanin et al., 2010).
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3. Results 3.1. Hydrothermal pathways simulated in the DMTA oven compared to those in a bread baked in a commercial oven In order to clearly identify each hydrothermal pathway, the average heating rate was calculated from experimental data of temperature in dough strips comprised between 30 and 80 C, and assuming a linear increase of temperature over this range, which was mainly the case here. Values were equal to 1 C/min, 5 C/min, 15 C/min, and 30 C/min. The protocol at 15 C/min was similar to that of a former study carried out by Vanin et al. (2010). Fig. 2b presents the average experimental temperature-water content trajectory in DMTA samples, accompanied by their respective standard deviations determined from the repetitions (runs). The temperature increase in the DMTA sample was highly repeatable, with an average standard deviation of 1.5 C. The maximum standard deviation of 4.5 C observed for the samples heated at 30 C/min may be attributed to variations in the positioning of the thermocouple in thickness of the dough sample, and to the high temperature increase observed in this heating protocol. Similarly, water content measurements from DMTA samples were also highly repeatable, with an average standard deviation of 0.66%. The maximum standard deviation of 3.42% was also observed for the samples heated at 30 C/min. Hydrothermal pathways achieved in the DMTA oven were compared with the ones obtained within the crust thickness, as reported in the literature or in loaves of bread purchased for the purpose of this study (Fig. 2a and b). Pathways presenting heating rates at 15 and 30 C/min (Fig. 2b) were close to most hydrothermal pathways encountered in top crusts of genuine bread (Fig. 2a), i.e. they pass through or tangentially approach the second endotherm in the hydrothermal domain of gelatinisation, which corresponds to a melting transition due to crystallite disruption. In pathways with heating rates at 1 and 5 C/ min, dehydration predominated upon heating, and the first and second endotherms in the hydrothermal domain of gelatinisation, respectively, were not crossed. Such a case is encountered in practice in the bottom crust during pan baking (Fig. 2a), or possibly at the top crust during part-baking. While extreme, the pathway at 1 C/min is a key condition in the experimental plan, enabling the study of the effect of combined water content and temperature, independent of the major modifications in macromolecular structure (starch gelatinisation, protein coagulation). Finally, dough at constant water content heated at a rate close to 5 C/min is relevant to crumb at the centre of the dough/bread loaf, and was considered as the reference condition. In practice, water content could be kept constant only below 100 C (Vanin et al., 2010), and the whole pathway (pathway at 5 C/min under delayed drying) was closer to the crumb located between the top crust, and the crumb at the core, that is dehydrated above 100 C (Fig. 2a). While water content measurements are destructive and timeconsuming, measurements performed during the baking are very few and therefore the hydrothermal pathways in genuine crusts are therefore low-resolved. Hence, hydrothermal kinetics were also plotted in Fig. 3 for DMTA samples under drying, and for crusts formed in a genuine bread loaf (Dogan, 2002; Hallstrom, 1998; Marston and Wannan, 1976; Thorvaldsson et al., 1999; Wagner et al., 2007; Zanoni and Peri, 1993). Time-course changes in temperature and water content obtained in the present study widely covered the domain defined in the literature for the top crust during bread baking for a variety of operating conditions (Fig. 3). Crossing of the first endotherm in the hydrothermal domain of gelatinisation (Fig. 2b) was confirmed by a significant decrease of
Fig. 3. Evolution of temperature (a) and water content (b) in samples under drying for different hydrothermal pathways performed inside the DMTA oven and at the surface of bread loaves, as reported in the literature.
birefringence in microscopy images for most pathways, either simulated in the DMTA oven or observed in genuine crusts (Table 1). This was consistent with a previous report made by Burt and Russell (1983), who showed that birefringence in images mainly disappeared, whether the second endotherm was crossed or not. Birefringence was still observed in samples heated at 1 C/min for temperatures as high as 85 C, although to a lesser intensity than in crude dough (Table 1). In fact, despite the successive adjustments of the protocol to decrease the drying rate, it could not be stated whether the pathway at a heating rate of 1 C/min crossed the hydrothermal interval defined by the first endotherm or not. 3.2. Changes in complex viscosity of dough upon immediate drying of variable intensity The pathways that presented dehydration simultaneous to heating and starch structural changes (Fig. 2b) are analysed in this section. Given the simultaneous changes in water content and temperature, mechanical properties should ideally be analysed in a three-dimensional graph with both factors. However, such a mode of representation is hard to deal with and discuss (Vanin et al., 2010). Successive two-dimensional representations of mechanical properties as a function of temperature and as a function of water content were thus presented. Fig. 2c presents changes in complex viscosity as a function of dough temperature. Changes in complex viscosity obtained at 15 C/ min under drying and delayed drying in the present study were identical to the ones obtained in a previous study, despite changes in flour batch and mixing protocols between the two studies (Vanin et al., 2010).
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Table 1 Microscopic images of crusts obtained in a convective oven set at two extreme values of temperature (T ¼ 100 C representative of partial baking, T ¼ 250 C), and DMTA ribbon dough samples heated at different heating rates (1, 5 and 30 C/min), took at different temperature e water content in order to evidence the loss of birefringence at passing the first endotherm of gelatinisation. Genuine bread crust
Toven ¼ 100 C
DMTA ribbon dough samples
1 C/min
5 C/min
Genuine bread crust 30 C/min
Toven ¼ 250 C
Raw dough
At passing the 1st endotherm (55e60 C)
Between 1st and 2nd endotherms (85e90 C)
>100 C
The higher the heating rate (30 C/min versus 1 C/min), the lower the extent of drying at a given temperature (respectively, 39% against 29% of water content at 80 C, for instance, see Fig. 2b, as well). The initial period where viscosity remained constant was a consistent finding at high heating rates (15e30 C/min). This period was in the same range of temperature, as observed in samples at constant water content (see Figs. 1 and 2c). This period was followed by a steep increase related to starch granule swelling and gluten aggregation (or strengthening of gluten network due to thermal coagulation of proteins), followed by a plateau. Once more, this was similar to what was observed in samples at constant water content (Fig. 2c). It should be remembered that pathways of DMTA samples at high heating rates and at constant water content were not very different (Fig. 2b), both passing the first two endotherms in the hydrothermal domain of starch gelatinisation, and initially favouring heat transfer upon dehydration. A long lasting plateau at high heating rates replaced the decrease in viscosity observed at constant water content and attributed to the disruption of swollen starch granules (Rolee and LeMeste, 1999) and likely the degradation of gluten network (80e 110 C approximately). The onset of the plateau at high heating rates could be associated either to the crossing of the second endotherm in the hydrothermal domain of gelatinisation, or to dough water content decreasing below 37%, approximately (Fig. 2b and c). This point will be further discussed based on the results obtained at lower heating rates. The onset temperature of the plateau was higher for samples at high heating rates than at constant water content, thus explaining the higher viscosity values reached in the plateau. Finally, complex viscosity increased steeply as the temperature exceeded 110 C; such behaviour was very different from that observed in samples at constant water content. All of these trends were formerly evidenced at 15 C/min (Vanin et al., 2010), and confirmed at higher heating rates (30 C/min) in
the present study. Increasing the heating rate above 15 C/min did not affect the changes in complex viscosity too much. Although differences were observed for temperature and water content kinetics, between the heating rates of 15 and 30 C/min (Fig. 3), they became small when combined in the state diagram (Fig. 2b), which explained the similarity in the viscosity behaviour between these two heating rates. Different from high heating rates, the initial increase in complex viscosity at 1 and 5 C/min was related to a decrease in water content. At 1 C/min for instance, complex viscosity increased almost 40 times, while temperature increased only up to 50 C, but water content decreased to 35% (Fig. 2b and c). In fact, the reduction in water content took more time at low heating rates, more probably affecting the distribution of water by diffusion at microscopic and molecular levels, and explaining the difference in viscosity behaviour; this point would require further investigation. Likewise, the crossing of the first endotherm in the hydrothermal domain of gelatinisation, at about 60 C (Fig. 2b), occurred much later in the drying process and was not accompanied by a steep increase in complex viscosity, different from what was observed at high heating rates (Fig. 2c). For example, considering the interval of temperature between 60 and 80 C, viscosity rose by about 2e5 times for samples heated at 1 and 5 C/min, respectively, and about 7e9 times for samples heated at 15 and 30 C/min, respectively. Level-off in viscosity was observed at 1 and 5 C/min (Fig. 2c), while water content decreased below 37%, and temperatures were still low (50 and 70 C, respectively). All these results showed the predominant effect of water content at low heating rates; such effect will be further discussed below (Fig. 4). As for high heating rates, complex viscosity increased steeply again in the vicinity and above the temperature of boiling water (Fig. 2c), while water vaporisation was enhanced. Finally, regardless of the heating rate, the strong reduction in water content below 5e10% was accompanied by a
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ones observed in the present study at the same range of water content (by a factor from 0.5 to 3 per % of water content reduction). Stage B (Fig. 4a) marked the transition between stage A, characterised by a steep increase in the logarithm of viscosity, and stage C, which presented a slower and linear increase of the logarithm of viscosity. The range in water content over which stage B took place shortened with decreasing heating rates (see dashed vertical lines in Fig. 4a). This was consistent with the interpretation of results presented above, in Fig. 2b and c, and may be explained by the different degree of starch gelatinisation. Stage C (Fig. 4a) exhibited a linear increase in logarithmic viscosity, with a slope of 0.3 per % in water content reduction (calculated over the range of 16e26%) no matter the heating rate. Similar linear trends in logarithmic viscosity were reported for semolina dough, noodles, or biscuit dough (Maache-Rezzoug et al., 1998; Manohar and Rao, 1999; Yu and Ngadi, 2006). However, slopes were slightly higher than those observed in the present study, 0.5 to 1 per % of water content reduction for noodle dough, and 1.3 for semolina extruded pasta. The offset observed in stage C between linear curves at low and high heating rates mainly resulted from the different length of stage B. Considering stages B and C globally, high heating rates acted as reducers of dough viscosity at a given water content, possibly limiting the extent of starch gelatinisation when compared with low heating rates. Finally, it should be noted that the effect of the heating rate was less dominant when viscosity was plotted against water content. This drove us to propose an empirical model of viscosity dependent on water content to implement the present data in future baking models. Equations are presented in Table 2 according to stages A to C presented above; their numerical applications were also drawn in Fig. 4a to enable the reader to evaluate the quality of the adjustment. Fig. 4. Normalised complex viscosity h*/h0*, average of 4 runs versus dough water content for samples under drying, heated at 1, 5, 15, and 30 C/min. Time-course changes in water content were (linearly) interpolated, and each interpolated water content was associated with a complex viscosity; these supplementary data are represented with an open symbol. The different sets of data were adjusted by the models (plain lines) reported in Table 2. Refer to the text for the description of stages A, B and C; the lowest and highest water contents of stage B are marked by dashed and plain vertical lines, respectively; (b) stage A in comparison with literature data.
glass transition and complex viscosity values higher than 108 Pa. s (Fig. 2b and c). Fig. 4 presents changes in complex viscosity as a function of water content in the dough. Complex viscosity was normalised to its initial value (at 44.5% water content) to eliminate this source of variation between the tested conditions. Globally, three stages could be distinguished in relation to water content (stages A, B, and C, Fig. 4a). In stage A (Fig. 4b), where water content was maintained between 45 and 37%, an important rise in complex viscosity was observed. It should be emphasised that at 5 and 15 C/min, a period exhibiting no change in viscosity was noticed, with a steep increase in viscosity occurring at lower water content than the initial one. This was consistent with the interpretations above that are based on results presented in Fig. 2b and c. The duration of this period was related to the predominance of heating upon dehydration. It should also be noted that data at 1 C/min and 30 C/min did not have enough resolution at high water contents to fully support the discussion of such a period. Data from literature at different water contents (41e45% wb) obtained at constant temperature (Berland and Launay, 1995; Hibberd, 1970; Smith et al., 1970; Webb et al., 1970) presented a rise in viscosity (by a factor of 1.1 per % of water content reduction) in accordance with the
3.3. Changes in the complex viscosity of dough upon delayed drying This section focuses on the sample heated at 5 C/min and covered with oil to delay dehydration. Again, such a pathway is relevant to the crumb located beneath the top crust (Fig. 2aeb). Complex viscosity in Fig. 4 was considered constant, as long as water content did not decrease, erasing the variations observed in Fig. 2c, below 100 C. Such simplification appears reasonable in modelling the viscosity for its implementation into a baking model. Similar to other pathways discussed in Section 3.2, stages B and C applied to complex viscosity in the samples under delayed drying (Table 2). The low level of complex viscosity at stage B was explained by the fact that, when water content started decreasing in the samples under delayed drying, complex viscosity was already decreasing on its way back to its initial value (considering the value of 1 in Fig. 2c). Unlike other pathways, stage B started with the initial water content of the dough before drying (instead of the unique threshold, at 37%). Consistent with this interpretation of stage B length in terms to degree of starch gelatinisation, the lower limit in water content of the plateau for 5 C/min under delayed drying should have been even lower than that observed for 15e 30 C/min. However, this was not observed. Instead, the lower limit of this plateau seemed to correspond to that observed at the same heating rate (5 C/min) under drying, suggesting an effect of the heating rate on the length of stage B. However, to be further discussed, this issue would deserve additional data, between 12.2 and 32.9% of water content. Finally, complex viscosity during stage C presented the highest increase among all pathways. No explanation for this finding could be proposed at this stage of the study.
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Table 2 Models of the normalised complex viscosity h* =h*0 as a function of water content Xw and heating rate (R2 > 0.95). h*0 refers to Xw ¼ 44.5%. Stages A, B, and C refer to Fig. 4. * expressed in g of water per 100g of wet dough, **in g of wet dough per g of water, ***in g1/2 of wet dough per g1/2 of water. Heating rate ( C/min) Under immediate drying (crust-like) 1 Stage A
5
* Log10 hh* 0
Stage C
15e30
¼ aXw þ b
with a ¼ 0.205* b ¼ 9.151
Stage B
Under delayed drying (crumb-like)
* Log10 hh* 0
ðXw bÞ c2
¼ ae
þ de
5
ðXw bÞ f
with a ¼ 0.827 b ¼ 37.0* c ¼ 1.596*** d ¼ 0.707 f ¼ 2.532**
with a ¼ 0.678 b ¼ 37.0* c ¼ 1.148*** d ¼ 0.560 f ¼ 1.359**
with a ¼ 37.0 * Log10 hh* ¼ aXw þ b
with a ¼ 31.0
with a ¼ 17.0
with a ¼ 3.2
with a ¼ 5.829 102* b ¼ 3.58
with a ¼ 5.824 102* b ¼ 3.40
with a ¼ 5.828 102* b ¼ 2.86
with a ¼ 8.534 102* b ¼ 3.30
h* h*0 ¼ a
0
4. Discussion and conclusion Different hydrothermal pathways in dough strips mimicking those reported in the literature at different dough locations during baking were successfully reproduced in the oven of a DMTA rheometer. At high heating rates relevant to a typical top crust (15e 30 C/min) despite of decreasing water content, dough viscosity behaved as if water content was kept constant, as long as temperature remained below 80 C (stage A); this behavior comprised an initial period with no change in viscosity followed by an increase at about 60 C, related to the onset of starch gelatinisation. Conversely to high heating rates, complex viscosity at low heating rates, relevant to a bottom crust during pan baking for instance, increased with decreasing water content; this was attributed to the enhanced distribution of water at the microscopic scale. The decrease in viscosity observed in samples at constant water content above 80 C was not observed in these samples under drying, and was replaced by a long-lasting plateau (stage B); this was related to the reduction in water content below approximately 37%, regardless of the heating rate. Viscosity values at the plateau as well as the length of this plateau were respectively lower (by a factor of 2) and longer at higher heating rate (15e30 C/min compared to 1e5 C/min), possibly affecting the extent of starch gelatinisation. It may be assumed that high heating rate would extend dough extensibility and oven-rise. Above the boiling temperature of water, the logarithm of viscosity increased linearly with decreasing water content, at a rate only dependent on water content (stage C). Stages B and C were shown to be independent of the heating rate, for 15 and 30 C/min, which covers a wide range of bread-baking conditions for the top crust. It should be remembered that past baking models that included transport and volume changes were based on a temperaturedependent model of viscosity. Results from the present study showed that such an approach is invalid for the bottom crust and remains valid up to 80 C for the top crust, which corresponds to the first couples of minutes in most baking conditions. Beyond 80 C, the viscosity in dough surface layers at the top is expected to be much higher than in the crumb at the core because of a reduction in water content. These high levels of viscosity encountered in the crust, compared with the crumb at the core, account not only for the cessation of the oven rise, but also for the resistance to squeezing resulting from the expansion of inner dough layers (Wagner et al., 2008).
Finally, the crumb located beneath the top crust was studied, a case in which drying is subsequent to starch gelatinisation. Different from the crumb at the core, which maintains its initial water content, this progressive dehydration of 3e5 points would maintain its viscosity (plateau during stage B); this may be enough to provide higher resistance to squeezing forces exerted by the expansion of inner dough layers (Wagner et al., 2008). Stages B and C both applied to this latter case. Particularly, the slope during stage C was greater than in other pathways. Viscosity at high levels of dehydration (20% for instance, which seems reasonable for the crumb very close to the top crust) did not differ much from that of samples heated at 15e30 C/min under drying (mimicking the top crust); this suggested a relative continuity of viscosity along a profile in bread from the surface to the core. In spite of deviations between hydrothermal dynamics, complex viscosity as a function of water content followed similar trends, leading to a unique set of equations for modeling stages A to C, with obviously different values of the parameters for each heating rate. Acknowledgements This study was carried out with financial support from the Council of Brittany (France). The authors also wish to thank Juliette Bodet for her technical assistance in the experimental work. References Berland, S., Launay, B., 1995. Shear softening and thixotropic properties of wheat flour doughs in dynamic testing at high shear strain. Rheologica Acta 34, 622e628. Bloksma, A.H., 1980. Effect of heating rate on viscosity of wheat flour doughs. Journal of Texture Studies 10, 261e269. Bloksma, A.H., 1990. Rheology of the breadmaking process. Cereal Foods World 35, 228e236. Burt, D.J., Russell, P.L., 1983. Gelatinization of low water content wheat starch water mixtures (A combined study by differential scanning calorimetry and light microscopy). Starch/Starke 35, 354e360. Collar, C., Bollain, C., Rosell, C.M., 2007. Rheological behaviour of formulated bread doughs during mixing and heating. Food Science and Technology International 13, 99e107. De Cindio, B., Correra, S., 1995. Mathematical modeling of leavened cereal goods. Journal of Food Engineering 24, 379e403. Dobraszczyk, B.J., Morgenstern, M., 2003. Rheology and the breadmaking process. Journal of Cereal Science 38, 229e245. Dobraszczyk, B.J., Salmanowicz, B.P., 2008. Comparison of predictions of baking volume using large deformation rheological properties. Journal of Cereal Science 47, 292e301. Dogan, I.S., 2002. Dynamic rheological properties of dough as affected by amylases from various sources. Nahrung-Food 46, 399e403.
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