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Aeration During Bread Dough Mixing
0960–3085/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part C, December 2004 Food and Bioproducts Processing, 82(C4): 261–267
AERATION DURING BREAD DOUGH MIXING I. Effect of Direction and Size of a Pressure Step-change During Mixing on the Turnover of Gas N. L. CHIN, P. J. MARTIN and G. M. CAMPBELL* Satake Centre for Grain Process Engineering, Department of Chemical Engineering, UMIST, Manchester UK
T
he aeration of bread dough during mixing has previously been modelled as a balance between gas entrainment and disentrainment into and out of the dough, from which the gas turnover time was determined. This paper tests this model by mixing dough in a high-speed Tweedy-type mixer which undergoes a pressure step-change midway through mixing. The volume of gas entrained in the dough following the pressure change was calculated from its density. The volume of gas entrained was found to reach a steady state much more quickly following a pressure step-decrease than an increase, appearing to signify gas turnover rates over twice as large. Thus, gas disentrainment was found to be enhanced following a pressure step-decrease in a manner not incorporated into the model. The size of the pressure step-decrease did not appear to affect the gas turnover time, but increasing the pressure step-increase size decreased the gas turnover times. The weak flour doughs exhibited higher gas turnover rates during mixing than the strong flour doughs, in both pressure stepchange directions and at all step-sizes. Keywords: dough; aeration; mixing; pressure; entrainment; disentrainment.
INTRODUCTION
Marston, 1986), while mixing at vacuum pressure towards the end reduces the gas content in dough to produce bread with finer crumb texture (Cauvain, 2000). The first model describing bread dough aeration during mixing was presented by Campbell and Shah (1999). They modelled bread dough aeration during mixing in terms of the dynamic balance between the gas entrainment and disentrainment processes. They applied a mass balance on the gas in the dough, from which the rate constants describing entrainment and disentrainment were proposed. The mass balance of gas in dough based on a unit volume of gas-free dough is:
Mixing is an important stage in a breadmaking process because it is where the gas bubbles in the dough are first created. These subsequently evolve into the gas cells of the final loaf crumb which give bread its aerated structure. The mixing stage is critical in modern breadmaking processes, such as the Chorleywood bread process (CBP), because it is the stage where bakers have greatest control over product characteristics. The gas bubble structure created in bread dough during mixing has a direct effect on the gas cell structure in the baked loaf (Baker and Mize, 1941; Cauvain, et al., 1999; Cauvain, 2000; Campbell, 2003). Bread dough aeration during mixing is not well understood. For example, the occurrence of excessive aeration which is experienced using industrial scale CBP mixers is still not explained, although this is an important problem resulting in poor loaf structure. Pressure-vacuum mixing, with approximate pressures 2.5– 0.35 bar absolute (Spooner, 1999), is often used to rectify this problem. Mixing above atmospheric pressure in the beginning provides oxygen which facilitates dough development and produces whiter loaves (Hay, 1950; Todd and Hawthorn, 1957;
dm _i m _o ¼m dt
(1)
_ i is the where m is the mass of gas entrained in a dough, m _ o is mass flow rate of gas being entrained into a dough, m the mass flow rate of gas being disentrained from a dough and t is time. All entrained gas bubbles are believed to be at the same pressure, and this is assumed to be equal to the mixer headspace pressure, P. Using the ideal gas law, the mass of gas entrained is written as:
*Correspondence to: Dr G.M. Campbell, Satake Centre for Grain Process Engineering, Department of Chemical Engineering, UMIST, Manchester, M60 1QD, UK. E-mail: [email protected]
m¼ 261
PVMw RT
(2)
262
CHIN et al.
where V is the volume of gas entrained in the dough, Mw is the molecular weight of gas, R is the universal gas constant and T is the absolute temperature. Entrainment of gas into the dough occurs when dough surfaces come into contact, entrapping a volume of gas during mixing. Entrainment is assumed to occur at a constant volumetric rate, v such that the mass flow rate of _ i , may be written as: gas being entrained into the dough, m PMw _i ¼ v m RT
(3)
Disentrainment of gas must also occur, but a mechanism was not proposed by Campbell and Shah. They simply assumed disentrainment to be proportional by a disentrainment coefficient, k, to the mass of entrained gas in the dough: _ o ¼ km ¼ k m
PVMw RT
(4)
Substituting equations (2) –(4) into equation (1) gives equation (5) and, when expressed on a volume basis, equation (6): dm PMw ¼ (v kV) dt RT dV ¼ (v kV) dt
(5) (6)
Equation (6) is a first-order differential equation that has the solution: v v V(t) ¼ þ V0 ekt k k
(7)
where V0 is the initial volume of gas entrained. Equation (7) indicates that, following a step-change in headspace pressure during dough mixing, the volume of entrained gas in the dough changes exponentially from the initial to the steady state value, V1 ¼ v/k. At steady state the mean gas residence time, or turnover time, is given by V1/v ¼ 1/k. Conversely, the disentrainment coefficient k could be considered as the turnover rate, the mean number of times an element of gas is replenished in a given time at steady state. This model can be investigated experimentally by measuring the change in dough density following a step-change of mixer headspace pressure midway through mixing; from such data the disentrainment coefficient may be determined. Campbell and Shah (1999) applied the above approach to measure the rate of gas turnover in dough following a step-increase in pressure. Their results suggested turnover times of the order of 1–2 min in Tweedy 1 and 35 mixers. However, industrial practice is to use a pressure stepdecrease towards the end of mixing, resulting in a decrease in gas content which occurs much faster than Campbell and Shah’s results suggest. This suggests that the disentrainment of gas during mixing is faster, and the turnover time is shorter, following a step-decrease in the mixer headspace pressure, than following a step-increase. This paper investigates experimentally the gas turnover rates in dough by using both step-increases and decreases in
headspace pressure during dough mixing. The rate of gas turnover during mixing was also evaluated by using different pressure step sizes in both pressure change directions. METHODS AND MATERIALS Flour Preparation Two varieties of English wheat, Hereward (hard) and Consort (soft) were laboratory milled to provide strong and weak flours for this study. The wheat was conditioned using the Satake Laboratory Conditioner (Satake UK Ltd, Stockport, UK) to a moisture content of 16% (w.b.) and tempered in airtight containers for at least 18 h before being milled using the Bu¨hler Laboratory Mill Type MLU-202 (Bu¨hler Ltd, Switzerland) to an extraction rate of 80%. Flour was combined from the three break rolls, the three reduction rolls and from bran processed through the Bu¨hler Laboratory Impact Finisher MLU-302 (Bu¨hler Ltd, Switzerland). The flour was stored for a month prior to use. Dough Mixing and Experimental Design Doughs were mixed using a high-speed Tweedy 1 mixer at a nominally constant speed of 720 rpm and with a dough loading of around 630– 650 g based on the formulation given in Table 1. Two types of experiment were performed: single pressure mixes and pressure step-change mixes. The single pressure experiments gave values of the gas-free dough density. The pressure step-change experiments allowed disentrainment coefficients to be determined. The single pressure mixes were conducted at various pressures ranging from 0.07 –1 bar (absolute). Doughs were mixed for 120 s, at which point a coherent (although not fully or optimally developed) dough was formed. For the pressure step-change experiments, two investigations were performed. In the first investigation, four sets of experiments, which included both the pressure stepchanges, increase and decrease, to and from 1 bar, were conducted. In the second investigation, pressure stepincreases from 1 bar and pressure step-decreases to 1 bar with five step-sizes ranging from 0.2 to 1.3 bar were conducted. For each pressure step-change experiment, a dough was mixed at the first pressure, P1, for 120 s before a rapid pressure step-change, within a few seconds, into the second pressure, P2, where mixing continued for a different period of time ranging from 0 to 140 s in 12 – 17 time intervals. Following the second mixing period any mixer headspace pressure was rapidly released such that the mixer reached atmospheric pressure, again within a
Table 1. Dough formulation based on 400 g flour loading. Ingredient Flour Water Strong Weak Improver (Diamond)a Salt
Percentage on flour weight 100 60 55 1.100 1.875
a
Full-fat soya flour, emulsifier E472e, dextrose, vegetable oil, flour treatment agent E300 and ascorbic acid.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C4): 261–267
AERATION DURING BREAD DOUGH MIXING I few seconds. A separate dough was produced for each time interval. In all experiments, the water temperature was controlled such that the dough temperature after 120 s of mixing was 30 + 18C.
Dough Density and Turnover Time Dough density was measured at atmospheric pressure using the double cup system placed on a Precisa Electronic Balance 125A as described by Campbell et al. (2001). Dough samples of about 10 g were weighed in air and then immersed in xylene (Fisher Scientific, Loughborough, UK, rxylene ¼ 0.86059 g cm23), maintained at 308C with a water-filled jacket. The dough density at atmospheric pressure, ratm, was calculated from the sample weight in air, wair and, while immersed in xylene, wxylene:
ratm ¼
wair r wair wxylene xylene
(8)
Six doughs samples were removed from the mixer immediately after each mixing trial, and their average density calculated. The gas-free dough density, rgf, was obtained by plotting the graph of dough density vs mixing pressure, P, and extrapolating the regression line back to zero absolute pressure (Campbell et al., 1993, 1998):
ratm ¼ rgf sP
(9)
where s is the slope of graph. The standard deviation of rgf was calculated following Campbell et al. (1993). The volume of gas entrained at time t, Vatm(t), is related to its density, ratm(t), by: Vatm (t) ¼
rgf 1 ratm (t)
(10)
Dough densities were measured at atmospheric pressure, thus it is the volume of gas entrained when the dough has been removed to atmosphere, Vatm, which is calculated, rather than the volume of gas entrained whilst the dough is in the mixer under the headspace pressure, V. It is again assumed that all of the bubbles have the same pressure, and that this is equal to the external pressure. Thus, by the ideal gas law the two values are related by the pressure ratio: V¼
Patm Vatm P
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Table 2. Characteristics of the flour used. Analysis
Strong
Weak
Moisture content (w.b.) (%) Ash (d.b.) (%) Protein (%) Farinograph peak (min)a Farinograph absorption (%)a Falling number (s)
12.5 0.58 10.5 4.5 57.5 353
12.0 0.51 7.3 3.0 48.5 216
a
Mixing using a Farinograph was carried out at Farinograph absorption of 500 BU consistency.
RESULTS AND DISCUSSION Flour Characteristics Table 2 shows the characteristics of flours used based on the standard analytical chemical tests by the Approved Methods of the American Association of Cereal Chemists (AACC, 1995). The moisture content was determined using the oven drying method (method 44-15A), the ash content was determined using the Leco Ash, Moisture and Volatiles Analyser MAC-400 (Leco Corporation, USA), the protein content (N 5.75) was determined following the Total Kjeldahl Nitrogen method by Nessler (Berigari, 1975); the Farinograph peak and flour water absorption were measured in the Brabender Farinograph at consistency of 500 BU following method 54-21 and the falling number following method 56-81B.
Gas-free Dough Densities Figure 1 shows the density of doughs mixed from strong and weak flours at various pressures. The weak flour produced doughs with higher gas-free dough density, 1.2837 + 0.0007 g cm23 (1 SD) than the strong flour, 1.2674 + 0.0005 g cm23. These gas-free dough densities were used to calculate the volume of gas entrained in the dough.
(11)
Since they are proportional, the form of equation (7) remains the same, and may be written as: Vatm (t) ¼ V1,atm þ (V0,atm V1,atm )ekt
(12)
For simplicity, all of the results are expressed in their atmospheric pressure equivalents, unless stated otherwise. The disentrainment coefficient, the initial and the steady state volume of gas entrained following a pressure change during dough mixing were determined by fitting the model in equation (12) to the experimental data for each separate step-change. The parameters were fitted by minimizing the sum of the normalized errors.
Figure 1. Dough density against mixing pressures for strong and weak flour doughs.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C4): 261–267
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CHIN et al. Effect of Pressure Step-change Direction
The volume of entrained gas following the pressure stepchange is presented in Figure 2 (note that the x-axis in these and similar graphs refers to the time elapsed after applying the pressure change). The volume changes exponentially, apparently approaching a steady-state value, in accordance with equation (12). The rate of change of entrained volume appears very similar when the step-change is in the same direction. However, the curves appear steeper following a step-down than a step-up. The weak flour doughs appear to have higher gas content and approached steady state more rapidly than the strong flour doughs. The fitted model can be seen to match closely the experimental data. Figure 3 shows the fitted disentrainment coefficients over the range of mixing pressure ratios. The disentrainment
Figure 3. Disentrainment coefficient following a step-change in pressure against pressure ratio: step-direction experiments.
coefficients obtained are significantly greater following a pressure step-decrease than an increase, by up to a factor of 3. The weak flour doughs tend to give a higher k as displayed by the more rapid changes in attaining steady state gas content following the pressure step-changes. The average ratios of k values from step-decrease to step-increase are about 2.3 and 1.4 for the strong and the weak flour doughs, respectively. This finding suggests that gas disentrainment coefficients are not equivalent for a step-decrease and a step-increase in pressure during dough mixing, and the pressure step-change direction has an effect on the gas disentrainment process in dough during mixing. It is possible that when the pressure is reduced midway through mixing, the mechanism of disentrainment is enhanced, or an additional disentrainment mechanism may occur. It was assumed that the bubbles are all at the headspace pressure, so it would be expected that the initial volume of gas per unit volume of gas-free dough entrained following the pressure step-change is proportional to the ratio of mixing pressures, as is apparent in Figure 4. Thus the variation in disentrainment coefficient against the ratio of mixing pressures, seen in Figure 3, can also be represented against the initial gas entrainment, calculated at the mixer pressure P2, as shown in Figure 5.
Figure 2. Entrained gas volume response following a step-increase and decrease in pressure at t ¼ 0. (a) Strong flour and (b) weak flour. Symbols represent experimental data, solid lines represent the fitted model described by equation (12).
Figure 4. Initial entrained gas volume following a step-change in pressure against pressure ratio: step-direction experiments.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C4): 261–267
AERATION DURING BREAD DOUGH MIXING I
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Figure 5. Disentrainment coefficient against initial entrained gas volume following a step-change in pressure: step-direction experiments.
Effect of Pressure Step-change Size The response of entrained gas volume following a range of pressure step-increases is presented in Figure 6, and the response following a range of step-decreases is presented in Figure 7. As shown above, the weak flour doughs again reached higher gas contents than the strong flour doughs, and also appeared to achieve steady state more rapidly. Figure 8 illustrates the effect of the mixing pressure ratio on the disentrainment coefficients. The ratios of k from step-decrease to step-increase range from 4 to 8, and k tends to increase with the investigated pressure step-sizes of 0.2 –1.3 bar. These ratios are around twice as large for the strong flour than weak flour doughs. The k values obtained following the pressure step-decrease display more scatter than in the pressure step-increase. In general, it appears that the step-decrease size does not affect the k values greatly, whereas increasing pressure step-increase size increases k. The first and second investigations on the pressure step-change experiments were from nominally identical experiments. However, the disentrainment coefficients obtained, as illustrated by Figures 3 and 8, vary by up to a factor of 2. The nature of these experiments involves day-to-day variation which can obscure effects considerably. For this reason, comparisons are only made between experiments conducted in the same set. As shown above, the observed variation in disentrainment coefficient against mixing pressure ratio may also be displayed against the initial volume of entrained gas, calculated at the mixer pressure P2, presented in Figure 9.
CONCLUSIONS Campbell and Shah’s (1999) model of bread dough aeration during mixing was investigated by conducting pressure step-change experiments in both directions, pressure increases and pressure decreases. The investigations have identified effects of pressure step-change direction and size on the fitted disentrainment coefficients. The corresponding turnover times are about 1 min in a pressure step-increase and less than 30 s for the pressure step-
Figure 6. Entrained gas volume response following a pressure stepincrease of five sizes at t ¼ 0. (a) Strong flour and (b) weak flour. Symbols represent experimental data, solid lines represent the fitted model described by equation (12).
decrease. This is in line with industrial experience, where turnover times found from pressure step-up experiments would be too slow to achieve the observed reduction in entrained gas over the final 25 s of mixing. The air turnover times in weak flour doughs are consistently higher than in the strong flour doughs. The variation of disentrainment coefficient with mixing pressure ratio indicates a shortcoming within Campbell and Shah’s model. Once nominally steady-state mixing has been reached, the disentrainment coefficient should be a constant value dependent only on the flour and mixer characteristics, thus the true turnover time is can be conceptualised unambiguously. However, the current model yields steady-state turnover times which vary depending on the size and direction of the applied step-change in pressure;
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C4): 261–267
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Figure 8. Disentrainment coefficient following a step-change in pressure against pressure ratio: step-size experiments.
Figure 9. Disentrainment coefficient against initial entrained gas volume following a step-change in pressure: step-size experiments.
builds on this observation to develop a population balance model of aeration during dough mixing. Figure 7. Entrained gas volume response following a pressure stepdecrease of five sizes at t ¼ 0. (a) Strong flour and (b) weak flour. Symbols represent experimental data, solid lines represent the fitted model described by equation (12).
thus the true steady-state turnover time is not able to be defined and measured unambiguously. At best, it could be thought to lie in the middle of the range bounded by the step-increase and step-decrease experiments. Since the gas entrainment mechanism has been relatively clearly conceptualized, it is thought that the problem lies with the modelling of disentrainment. The increased disentrainment rates following a step-down in pressure may be due to additional disentrainment mechanisms, or just an enhancement of a single mechanism. It has been noted that the variation in disentrainment coefficient appears to be a function of the initial volume of entrained gas following the step-change, and this insight may provide the best route to resolving the problem. Paper 2 of this series
NOMENCLATURE k m _ m Mw P R s t T v V w
disentrainment coefficient mass of gas entrained in a dough per unit volume of gas-free dough mass flow rate of gas per unit volume of gas-free dough molecular weight of gas mixer headspace absolute pressure universal gas constant gradient of dough density versus mixing pressure time absolute temperature volumetric gas entrainment rate volume of gas entrained per unit of gas free dough weight of dough sample
Greek symbols r dough density Subscripts 1 infinite time after pressure step-change 0 zero time after pressure step-change 1 pressure 1
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C4): 261–267
AERATION DURING BREAD DOUGH MIXING I 2 atm gf i o
pressure 2 atmospheric conditions gas free state flow into the dough flow out of the dough
REFERENCES AACC, 1995, Approved Methods of the American Association of Cereal Chemists (AACC Inc., Minnesota, USA). Baker, J.C. and Mize, M.D., 1941, The origin of the gas cell in bread dough, Cereal Chem, 18(1): 19–34. Berigari, M.S., 1975, Determination of Total Protein in Plant Tissues from Nitrogen Analysis by a Modified Kjeldahl Digestion and Nesslerization Method, Argonne National Laboratory Argonne, (IL), pp 62–68. Campbell, G.M., 2003, Bread aeration, in Breadmaking: Improving Quality, Cauvain S.P. (ed.), (Woodhead, Cambridge, UK), pp 352 –374. Campbell, G.M. and Shah, P., 1999, Entrainment and disentrainment of air during bread dough mixing and their scale-up of dough mixers, in Bubbles in Food, Campbell, G.M., Webb, C., Pandiella, S.S. and Niranjan, K. (eds) (Eagan Press, Minnesota, USA), pp 11–20. Campbell, G.M., Rielly, C.D., Fryer, P.J. and Sadd, P.A., 1993, Measurement and interpretation of dough densities, Cereal Chem, 70(5): 517– 521. Campbell, G.M., Rielly, C.D., Fryer, P.J. and Sadd, P.A., 1998, Aeration of bread dough during mixing: Effect of mixing dough at reduced pressure, Cereal Foods World, 43(3): 163 –167. Campbell, G.M., Herrero-Sanchez, R., Payo-Rodriguez, R. and Merchan, M.L., 2001, Measurement of dynamic dough density and the
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effect of surfactants and flour type on aeration during mixing and gas retention during proofing, Cereal Chem, 78(3): 272–277. Cauvain, S.P., 2000, Principles of dough formation: it’s got form, Eur Baker, 38: 24– 28. Cauvain, S.P., Whitworth, M.B. and Alava, J.M., 1999, The evolution of bubble structure in bread doughs and its effect on bread structure, in Bubbles in Food, Campbell, G.M., Webb, C., Pandiella, S.S. and Niranjan, K. (eds) (Eagan Press, Minnesota, USA), pp 85 –88. Hay, J.G., 1950, Improved method of making bread with untreated and unbleached flour, UK patent 646311. Marston, P.E., 1986, Dough development for breadmaking under controlled atmosphere, J Cereal Sci, 4(4): 335– 344. Spooner, T F., 1999, Controlled atmosphere, Baking Snack, February: 96–102. Todd, J.P. and Hawthorn, J., 1957, Improvement in manufacture of bread, UK Patent 771361.
ACKNOWLEDGEMENTS The authors gratefully acknowledge support from the Biotechnology and Biological Sciences Research Council (UK) and the Satake Corporation of Japan in establishing the activities of the SCGPE. The authors would like to thank Arkady Craigmillar Ltd., Wirral, UK, for providing dough improvers. N.L.C. acknowledges financial support from Universiti Putra Malaysia for her course at UMIST. The manuscript was received 3 September 2003 and accepted for publication after revision 28 July 2004.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C4): 261–267
AERATION DURING BREAD DOUGH MIXING I. Effect of Direction and Size of a Pressure Step-change During Mixing on the Turnover of Gas N. L. CHIN, P. J. MARTIN and G. M. CAMPBELL* Satake Centre for Grain Process Engineering, Department of Chemical Engineering, UMIST, Manchester UK
T
he aeration of bread dough during mixing has previously been modelled as a balance between gas entrainment and disentrainment into and out of the dough, from which the gas turnover time was determined. This paper tests this model by mixing dough in a high-speed Tweedy-type mixer which undergoes a pressure step-change midway through mixing. The volume of gas entrained in the dough following the pressure change was calculated from its density. The volume of gas entrained was found to reach a steady state much more quickly following a pressure step-decrease than an increase, appearing to signify gas turnover rates over twice as large. Thus, gas disentrainment was found to be enhanced following a pressure step-decrease in a manner not incorporated into the model. The size of the pressure step-decrease did not appear to affect the gas turnover time, but increasing the pressure step-increase size decreased the gas turnover times. The weak flour doughs exhibited higher gas turnover rates during mixing than the strong flour doughs, in both pressure stepchange directions and at all step-sizes. Keywords: dough; aeration; mixing; pressure; entrainment; disentrainment.
INTRODUCTION
Marston, 1986), while mixing at vacuum pressure towards the end reduces the gas content in dough to produce bread with finer crumb texture (Cauvain, 2000). The first model describing bread dough aeration during mixing was presented by Campbell and Shah (1999). They modelled bread dough aeration during mixing in terms of the dynamic balance between the gas entrainment and disentrainment processes. They applied a mass balance on the gas in the dough, from which the rate constants describing entrainment and disentrainment were proposed. The mass balance of gas in dough based on a unit volume of gas-free dough is:
Mixing is an important stage in a breadmaking process because it is where the gas bubbles in the dough are first created. These subsequently evolve into the gas cells of the final loaf crumb which give bread its aerated structure. The mixing stage is critical in modern breadmaking processes, such as the Chorleywood bread process (CBP), because it is the stage where bakers have greatest control over product characteristics. The gas bubble structure created in bread dough during mixing has a direct effect on the gas cell structure in the baked loaf (Baker and Mize, 1941; Cauvain, et al., 1999; Cauvain, 2000; Campbell, 2003). Bread dough aeration during mixing is not well understood. For example, the occurrence of excessive aeration which is experienced using industrial scale CBP mixers is still not explained, although this is an important problem resulting in poor loaf structure. Pressure-vacuum mixing, with approximate pressures 2.5– 0.35 bar absolute (Spooner, 1999), is often used to rectify this problem. Mixing above atmospheric pressure in the beginning provides oxygen which facilitates dough development and produces whiter loaves (Hay, 1950; Todd and Hawthorn, 1957;
dm _i m _o ¼m dt
(1)
_ i is the where m is the mass of gas entrained in a dough, m _ o is mass flow rate of gas being entrained into a dough, m the mass flow rate of gas being disentrained from a dough and t is time. All entrained gas bubbles are believed to be at the same pressure, and this is assumed to be equal to the mixer headspace pressure, P. Using the ideal gas law, the mass of gas entrained is written as:
*Correspondence to: Dr G.M. Campbell, Satake Centre for Grain Process Engineering, Department of Chemical Engineering, UMIST, Manchester, M60 1QD, UK. E-mail: [email protected]
m¼ 261
PVMw RT
(2)
262
CHIN et al.
where V is the volume of gas entrained in the dough, Mw is the molecular weight of gas, R is the universal gas constant and T is the absolute temperature. Entrainment of gas into the dough occurs when dough surfaces come into contact, entrapping a volume of gas during mixing. Entrainment is assumed to occur at a constant volumetric rate, v such that the mass flow rate of _ i , may be written as: gas being entrained into the dough, m PMw _i ¼ v m RT
(3)
Disentrainment of gas must also occur, but a mechanism was not proposed by Campbell and Shah. They simply assumed disentrainment to be proportional by a disentrainment coefficient, k, to the mass of entrained gas in the dough: _ o ¼ km ¼ k m
PVMw RT
(4)
Substituting equations (2) –(4) into equation (1) gives equation (5) and, when expressed on a volume basis, equation (6): dm PMw ¼ (v kV) dt RT dV ¼ (v kV) dt
(5) (6)
Equation (6) is a first-order differential equation that has the solution: v v V(t) ¼ þ V0 ekt k k
(7)
where V0 is the initial volume of gas entrained. Equation (7) indicates that, following a step-change in headspace pressure during dough mixing, the volume of entrained gas in the dough changes exponentially from the initial to the steady state value, V1 ¼ v/k. At steady state the mean gas residence time, or turnover time, is given by V1/v ¼ 1/k. Conversely, the disentrainment coefficient k could be considered as the turnover rate, the mean number of times an element of gas is replenished in a given time at steady state. This model can be investigated experimentally by measuring the change in dough density following a step-change of mixer headspace pressure midway through mixing; from such data the disentrainment coefficient may be determined. Campbell and Shah (1999) applied the above approach to measure the rate of gas turnover in dough following a step-increase in pressure. Their results suggested turnover times of the order of 1–2 min in Tweedy 1 and 35 mixers. However, industrial practice is to use a pressure stepdecrease towards the end of mixing, resulting in a decrease in gas content which occurs much faster than Campbell and Shah’s results suggest. This suggests that the disentrainment of gas during mixing is faster, and the turnover time is shorter, following a step-decrease in the mixer headspace pressure, than following a step-increase. This paper investigates experimentally the gas turnover rates in dough by using both step-increases and decreases in
headspace pressure during dough mixing. The rate of gas turnover during mixing was also evaluated by using different pressure step sizes in both pressure change directions. METHODS AND MATERIALS Flour Preparation Two varieties of English wheat, Hereward (hard) and Consort (soft) were laboratory milled to provide strong and weak flours for this study. The wheat was conditioned using the Satake Laboratory Conditioner (Satake UK Ltd, Stockport, UK) to a moisture content of 16% (w.b.) and tempered in airtight containers for at least 18 h before being milled using the Bu¨hler Laboratory Mill Type MLU-202 (Bu¨hler Ltd, Switzerland) to an extraction rate of 80%. Flour was combined from the three break rolls, the three reduction rolls and from bran processed through the Bu¨hler Laboratory Impact Finisher MLU-302 (Bu¨hler Ltd, Switzerland). The flour was stored for a month prior to use. Dough Mixing and Experimental Design Doughs were mixed using a high-speed Tweedy 1 mixer at a nominally constant speed of 720 rpm and with a dough loading of around 630– 650 g based on the formulation given in Table 1. Two types of experiment were performed: single pressure mixes and pressure step-change mixes. The single pressure experiments gave values of the gas-free dough density. The pressure step-change experiments allowed disentrainment coefficients to be determined. The single pressure mixes were conducted at various pressures ranging from 0.07 –1 bar (absolute). Doughs were mixed for 120 s, at which point a coherent (although not fully or optimally developed) dough was formed. For the pressure step-change experiments, two investigations were performed. In the first investigation, four sets of experiments, which included both the pressure stepchanges, increase and decrease, to and from 1 bar, were conducted. In the second investigation, pressure stepincreases from 1 bar and pressure step-decreases to 1 bar with five step-sizes ranging from 0.2 to 1.3 bar were conducted. For each pressure step-change experiment, a dough was mixed at the first pressure, P1, for 120 s before a rapid pressure step-change, within a few seconds, into the second pressure, P2, where mixing continued for a different period of time ranging from 0 to 140 s in 12 – 17 time intervals. Following the second mixing period any mixer headspace pressure was rapidly released such that the mixer reached atmospheric pressure, again within a
Table 1. Dough formulation based on 400 g flour loading. Ingredient Flour Water Strong Weak Improver (Diamond)a Salt
Percentage on flour weight 100 60 55 1.100 1.875
a
Full-fat soya flour, emulsifier E472e, dextrose, vegetable oil, flour treatment agent E300 and ascorbic acid.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C4): 261–267
AERATION DURING BREAD DOUGH MIXING I few seconds. A separate dough was produced for each time interval. In all experiments, the water temperature was controlled such that the dough temperature after 120 s of mixing was 30 + 18C.
Dough Density and Turnover Time Dough density was measured at atmospheric pressure using the double cup system placed on a Precisa Electronic Balance 125A as described by Campbell et al. (2001). Dough samples of about 10 g were weighed in air and then immersed in xylene (Fisher Scientific, Loughborough, UK, rxylene ¼ 0.86059 g cm23), maintained at 308C with a water-filled jacket. The dough density at atmospheric pressure, ratm, was calculated from the sample weight in air, wair and, while immersed in xylene, wxylene:
ratm ¼
wair r wair wxylene xylene
(8)
Six doughs samples were removed from the mixer immediately after each mixing trial, and their average density calculated. The gas-free dough density, rgf, was obtained by plotting the graph of dough density vs mixing pressure, P, and extrapolating the regression line back to zero absolute pressure (Campbell et al., 1993, 1998):
ratm ¼ rgf sP
(9)
where s is the slope of graph. The standard deviation of rgf was calculated following Campbell et al. (1993). The volume of gas entrained at time t, Vatm(t), is related to its density, ratm(t), by: Vatm (t) ¼
rgf 1 ratm (t)
(10)
Dough densities were measured at atmospheric pressure, thus it is the volume of gas entrained when the dough has been removed to atmosphere, Vatm, which is calculated, rather than the volume of gas entrained whilst the dough is in the mixer under the headspace pressure, V. It is again assumed that all of the bubbles have the same pressure, and that this is equal to the external pressure. Thus, by the ideal gas law the two values are related by the pressure ratio: V¼
Patm Vatm P
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Table 2. Characteristics of the flour used. Analysis
Strong
Weak
Moisture content (w.b.) (%) Ash (d.b.) (%) Protein (%) Farinograph peak (min)a Farinograph absorption (%)a Falling number (s)
12.5 0.58 10.5 4.5 57.5 353
12.0 0.51 7.3 3.0 48.5 216
a
Mixing using a Farinograph was carried out at Farinograph absorption of 500 BU consistency.
RESULTS AND DISCUSSION Flour Characteristics Table 2 shows the characteristics of flours used based on the standard analytical chemical tests by the Approved Methods of the American Association of Cereal Chemists (AACC, 1995). The moisture content was determined using the oven drying method (method 44-15A), the ash content was determined using the Leco Ash, Moisture and Volatiles Analyser MAC-400 (Leco Corporation, USA), the protein content (N 5.75) was determined following the Total Kjeldahl Nitrogen method by Nessler (Berigari, 1975); the Farinograph peak and flour water absorption were measured in the Brabender Farinograph at consistency of 500 BU following method 54-21 and the falling number following method 56-81B.
Gas-free Dough Densities Figure 1 shows the density of doughs mixed from strong and weak flours at various pressures. The weak flour produced doughs with higher gas-free dough density, 1.2837 + 0.0007 g cm23 (1 SD) than the strong flour, 1.2674 + 0.0005 g cm23. These gas-free dough densities were used to calculate the volume of gas entrained in the dough.
(11)
Since they are proportional, the form of equation (7) remains the same, and may be written as: Vatm (t) ¼ V1,atm þ (V0,atm V1,atm )ekt
(12)
For simplicity, all of the results are expressed in their atmospheric pressure equivalents, unless stated otherwise. The disentrainment coefficient, the initial and the steady state volume of gas entrained following a pressure change during dough mixing were determined by fitting the model in equation (12) to the experimental data for each separate step-change. The parameters were fitted by minimizing the sum of the normalized errors.
Figure 1. Dough density against mixing pressures for strong and weak flour doughs.
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CHIN et al. Effect of Pressure Step-change Direction
The volume of entrained gas following the pressure stepchange is presented in Figure 2 (note that the x-axis in these and similar graphs refers to the time elapsed after applying the pressure change). The volume changes exponentially, apparently approaching a steady-state value, in accordance with equation (12). The rate of change of entrained volume appears very similar when the step-change is in the same direction. However, the curves appear steeper following a step-down than a step-up. The weak flour doughs appear to have higher gas content and approached steady state more rapidly than the strong flour doughs. The fitted model can be seen to match closely the experimental data. Figure 3 shows the fitted disentrainment coefficients over the range of mixing pressure ratios. The disentrainment
Figure 3. Disentrainment coefficient following a step-change in pressure against pressure ratio: step-direction experiments.
coefficients obtained are significantly greater following a pressure step-decrease than an increase, by up to a factor of 3. The weak flour doughs tend to give a higher k as displayed by the more rapid changes in attaining steady state gas content following the pressure step-changes. The average ratios of k values from step-decrease to step-increase are about 2.3 and 1.4 for the strong and the weak flour doughs, respectively. This finding suggests that gas disentrainment coefficients are not equivalent for a step-decrease and a step-increase in pressure during dough mixing, and the pressure step-change direction has an effect on the gas disentrainment process in dough during mixing. It is possible that when the pressure is reduced midway through mixing, the mechanism of disentrainment is enhanced, or an additional disentrainment mechanism may occur. It was assumed that the bubbles are all at the headspace pressure, so it would be expected that the initial volume of gas per unit volume of gas-free dough entrained following the pressure step-change is proportional to the ratio of mixing pressures, as is apparent in Figure 4. Thus the variation in disentrainment coefficient against the ratio of mixing pressures, seen in Figure 3, can also be represented against the initial gas entrainment, calculated at the mixer pressure P2, as shown in Figure 5.
Figure 2. Entrained gas volume response following a step-increase and decrease in pressure at t ¼ 0. (a) Strong flour and (b) weak flour. Symbols represent experimental data, solid lines represent the fitted model described by equation (12).
Figure 4. Initial entrained gas volume following a step-change in pressure against pressure ratio: step-direction experiments.
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Figure 5. Disentrainment coefficient against initial entrained gas volume following a step-change in pressure: step-direction experiments.
Effect of Pressure Step-change Size The response of entrained gas volume following a range of pressure step-increases is presented in Figure 6, and the response following a range of step-decreases is presented in Figure 7. As shown above, the weak flour doughs again reached higher gas contents than the strong flour doughs, and also appeared to achieve steady state more rapidly. Figure 8 illustrates the effect of the mixing pressure ratio on the disentrainment coefficients. The ratios of k from step-decrease to step-increase range from 4 to 8, and k tends to increase with the investigated pressure step-sizes of 0.2 –1.3 bar. These ratios are around twice as large for the strong flour than weak flour doughs. The k values obtained following the pressure step-decrease display more scatter than in the pressure step-increase. In general, it appears that the step-decrease size does not affect the k values greatly, whereas increasing pressure step-increase size increases k. The first and second investigations on the pressure step-change experiments were from nominally identical experiments. However, the disentrainment coefficients obtained, as illustrated by Figures 3 and 8, vary by up to a factor of 2. The nature of these experiments involves day-to-day variation which can obscure effects considerably. For this reason, comparisons are only made between experiments conducted in the same set. As shown above, the observed variation in disentrainment coefficient against mixing pressure ratio may also be displayed against the initial volume of entrained gas, calculated at the mixer pressure P2, presented in Figure 9.
CONCLUSIONS Campbell and Shah’s (1999) model of bread dough aeration during mixing was investigated by conducting pressure step-change experiments in both directions, pressure increases and pressure decreases. The investigations have identified effects of pressure step-change direction and size on the fitted disentrainment coefficients. The corresponding turnover times are about 1 min in a pressure step-increase and less than 30 s for the pressure step-
Figure 6. Entrained gas volume response following a pressure stepincrease of five sizes at t ¼ 0. (a) Strong flour and (b) weak flour. Symbols represent experimental data, solid lines represent the fitted model described by equation (12).
decrease. This is in line with industrial experience, where turnover times found from pressure step-up experiments would be too slow to achieve the observed reduction in entrained gas over the final 25 s of mixing. The air turnover times in weak flour doughs are consistently higher than in the strong flour doughs. The variation of disentrainment coefficient with mixing pressure ratio indicates a shortcoming within Campbell and Shah’s model. Once nominally steady-state mixing has been reached, the disentrainment coefficient should be a constant value dependent only on the flour and mixer characteristics, thus the true turnover time is can be conceptualised unambiguously. However, the current model yields steady-state turnover times which vary depending on the size and direction of the applied step-change in pressure;
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Figure 8. Disentrainment coefficient following a step-change in pressure against pressure ratio: step-size experiments.
Figure 9. Disentrainment coefficient against initial entrained gas volume following a step-change in pressure: step-size experiments.
builds on this observation to develop a population balance model of aeration during dough mixing. Figure 7. Entrained gas volume response following a pressure stepdecrease of five sizes at t ¼ 0. (a) Strong flour and (b) weak flour. Symbols represent experimental data, solid lines represent the fitted model described by equation (12).
thus the true steady-state turnover time is not able to be defined and measured unambiguously. At best, it could be thought to lie in the middle of the range bounded by the step-increase and step-decrease experiments. Since the gas entrainment mechanism has been relatively clearly conceptualized, it is thought that the problem lies with the modelling of disentrainment. The increased disentrainment rates following a step-down in pressure may be due to additional disentrainment mechanisms, or just an enhancement of a single mechanism. It has been noted that the variation in disentrainment coefficient appears to be a function of the initial volume of entrained gas following the step-change, and this insight may provide the best route to resolving the problem. Paper 2 of this series
NOMENCLATURE k m _ m Mw P R s t T v V w
disentrainment coefficient mass of gas entrained in a dough per unit volume of gas-free dough mass flow rate of gas per unit volume of gas-free dough molecular weight of gas mixer headspace absolute pressure universal gas constant gradient of dough density versus mixing pressure time absolute temperature volumetric gas entrainment rate volume of gas entrained per unit of gas free dough weight of dough sample
Greek symbols r dough density Subscripts 1 infinite time after pressure step-change 0 zero time after pressure step-change 1 pressure 1
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pressure 2 atmospheric conditions gas free state flow into the dough flow out of the dough
REFERENCES AACC, 1995, Approved Methods of the American Association of Cereal Chemists (AACC Inc., Minnesota, USA). Baker, J.C. and Mize, M.D., 1941, The origin of the gas cell in bread dough, Cereal Chem, 18(1): 19–34. Berigari, M.S., 1975, Determination of Total Protein in Plant Tissues from Nitrogen Analysis by a Modified Kjeldahl Digestion and Nesslerization Method, Argonne National Laboratory Argonne, (IL), pp 62–68. Campbell, G.M., 2003, Bread aeration, in Breadmaking: Improving Quality, Cauvain S.P. (ed.), (Woodhead, Cambridge, UK), pp 352 –374. Campbell, G.M. and Shah, P., 1999, Entrainment and disentrainment of air during bread dough mixing and their scale-up of dough mixers, in Bubbles in Food, Campbell, G.M., Webb, C., Pandiella, S.S. and Niranjan, K. (eds) (Eagan Press, Minnesota, USA), pp 11–20. Campbell, G.M., Rielly, C.D., Fryer, P.J. and Sadd, P.A., 1993, Measurement and interpretation of dough densities, Cereal Chem, 70(5): 517– 521. Campbell, G.M., Rielly, C.D., Fryer, P.J. and Sadd, P.A., 1998, Aeration of bread dough during mixing: Effect of mixing dough at reduced pressure, Cereal Foods World, 43(3): 163 –167. Campbell, G.M., Herrero-Sanchez, R., Payo-Rodriguez, R. and Merchan, M.L., 2001, Measurement of dynamic dough density and the
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effect of surfactants and flour type on aeration during mixing and gas retention during proofing, Cereal Chem, 78(3): 272–277. Cauvain, S.P., 2000, Principles of dough formation: it’s got form, Eur Baker, 38: 24– 28. Cauvain, S.P., Whitworth, M.B. and Alava, J.M., 1999, The evolution of bubble structure in bread doughs and its effect on bread structure, in Bubbles in Food, Campbell, G.M., Webb, C., Pandiella, S.S. and Niranjan, K. (eds) (Eagan Press, Minnesota, USA), pp 85 –88. Hay, J.G., 1950, Improved method of making bread with untreated and unbleached flour, UK patent 646311. Marston, P.E., 1986, Dough development for breadmaking under controlled atmosphere, J Cereal Sci, 4(4): 335– 344. Spooner, T F., 1999, Controlled atmosphere, Baking Snack, February: 96–102. Todd, J.P. and Hawthorn, J., 1957, Improvement in manufacture of bread, UK Patent 771361.
ACKNOWLEDGEMENTS The authors gratefully acknowledge support from the Biotechnology and Biological Sciences Research Council (UK) and the Satake Corporation of Japan in establishing the activities of the SCGPE. The authors would like to thank Arkady Craigmillar Ltd., Wirral, UK, for providing dough improvers. N.L.C. acknowledges financial support from Universiti Putra Malaysia for her course at UMIST. The manuscript was received 3 September 2003 and accepted for publication after revision 28 July 2004.
Trans IChemE, Part C, Food and Bioproducts Processing, 2004, 82(C4): 261–267