Journal of the Science of Food and Agriculture J Sci Food Agric 85:2203–2212 (2005)DOI: 10.1002/jsfa.2238

Dough aeration and rheology: Part 3. Effect ofthe presence of gas bubbles in bread dough onmeasured bulk rheology and work input rateNyuk L Chin, Peter J Martin and Grant M Campbell∗Satake Centre for Grain Process Engineering, Department of Chemical Engineering, UMIST, Manchester, M60 1QD, UK

Abstract: The effects of dough aeration on measured bulk rheology of dough were investigated bypreparing doughs with various gas contents and assessing their rheology under large deformation biaxialextension using the SMS Dough Inflation System. Doughs were mixed to various gas contents using alaboratory-scale mixer, the Tweedy 1, at various mixer headspace pressures. In order to consider theeffect of oxidation processes, doughs were mixed in air, in nitrogen and in a controlled oxygen-nitrogenatmosphere in which the partial pressure of oxygen was kept constant. In the latter two cases a flow rateof gas through the mixer was maintained, while the doughs mixed in air had a static headspace. Doughaeration was quantified using density measurements. The dough rheology tests were performed under twodifferent modes: constant volumetric air flow rate; and constant strain rate at two levels, 0.1 and 0.05 s−1.Analysis of the stress–strain data of an inflating dough bubble using an exponential model enabled thestrain hardening index, failure strain and failure stress to be determined. The strain hardening index,failure strain and failure stress decreased with gas content, indicating that gas bubbles in dough disruptthe integrity of dough structure. The faster strain rate tended to give higher values of all parameters andto be more discriminating, while testing at a constant volumetric air flow rate gave lower values. Doughsmixed under a constant partial pressure of oxygen resulted in the strain hardening index being minimallyaffected by the gas content, although failure strain and failure stress decreased with increasing gas contentin the dough. The flow of gas through the mixer appeared to affect dough aeration and rheology, and therate of work input was found to increase with increasing mixer headspace pressures. These suggest thatdough aeration not only affects the rheology of static dough through the physical presence of gas bubblesfollowing mixing, it also affects the development of its rheological properties within the mixer, throughthe physical presence of the bubbles affecting the rate of work input that develops the doughs, and alsothrough the turnover of air that supplies oxygen to facilitate this development. 2005 Society of Chemical Industry

Keywords: bread dough mixing; aeration; rheology; mixer headspace pressure; Dough Inflation System

ABBREVIATIONSCSR constant strain rateCVR constant volumetric air flow rateDIS Dough Inflation System

INTRODUCTIONPrevious papers have highlighted several subtle inter-relationships between dough aeration and rheology,including the effects of mixing speed and pressureon dough development1 and on its aeration andrheological properties.2 This paper underlines the needto quantify the effect of dough aeration on measuredrheology, a subject almost completely neglected in thedough rheology literature.

The effect of aeration on rheology is a complexsubject in a dough system because aeration affects

rheology not only through the physical presence of airbubbles in dough but also through the contributionof oxygen during mixing. Regarding the physicalpresence of bubbles, if the ratio of bubble deformationshear stresses to the bubble surface tension stresses(the capillary number, Ca) is small then the bubblesremain roughly spherical, and Taylor3 showed that inthis case the apparent viscosity of the fluid increaseswith the degree of aeration. If Ca is large and thebubbles deform in the direction of shear the apparentviscosity of the fluid can decrease with the degreeof aeration.4 Llewellin et al5 also demonstrated thatthe viscosity of a Newtonian liquid oscillated atlow frequency (low dynamic Ca) increased with gascontent, and decreased with increasing gas contentwhen oscillated at high frequency (high dynamic Ca).Thus the presence of bubbles in a liquid can affect

∗ Correspondence to: Grant M Campbell, Satake Centre for Grain Process Engineering, Department of Chemical Engineering, UMIST,Manchester, M60 1QD, UKE-mail: grant.campbell@manchester.ac.ukContract/grant sponsor: University Putra Malaysia(Received 4 August 2004; revised version received 8 December 2004; accepted 2 February 2005)Published online 20 June 2005

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parameters describing the bulk rheology in differentdirections depending on the deformation conditions.

The rheology of aerated dough is further compli-cated by the contribution of oxygen, made availableduring mixing through aeration processes, to doughchemistry and hence rheology. Increasing aerationduring mixing increases oxygen availability, whichenhances dough development processes, leading toincreased dough strength. Tsen and Bushuk6 reportedthat mixing dough in oxygen leads to stronger doughswith higher resistance and lower extensibility thandoughs mixed under nitrogen.

The effect of dough aeration on measured rhe-ology has not been investigated directly, althoughBloksma7–9 acknowledged that bubbles in dough con-tribute volume elasticity, but was not concerned thatthis would affect rheological measurements of doughgenerally. Levine10 made allowance for the gas con-tent of the dough in his work on modelling of doughsheeting in terms of dough compressibility but notrheology. The investigation by Newberry et al11 on fer-menting yeasted doughs indirectly demonstrated thatdough aeration affects dough rheology. They foundthat fermenting yeasted doughs demonstrated rheo-logical weakness in a uniaxial elongation test, despiteevidence of increased cross-linking of dough proteinswith fermentation, and suggested this to be attributedto the presence of gas bubbles within the dough. Morerecently, Elmehdi et al12 used ultrasonic probing todemonstrate that the mechanical properties of doughare sensitive to the presence of gas bubbles.

Dobraszczyk and Roberts13 presented approximateequations to calculate the stress, σ , and Hencky strain,ε, in an inflated dough sheet. The approximate natureof these stress and strain calculations was highlightedby Charalambides et al14 who showed through directmeasurement of the strain that both stress and strainwere overestimated, the stress being in error by afactor of four at large strains. However, the standardDough Inflation System (DIS)15 does not allow forthe direct measurement of strain but may still beused to provide comparative measures of stress andstrain. A typical average dough bubble diameter isaround 80 µm (Campbell et al16) and Charalambideset al14 measured the thickness at the top of an inflateddough sheet and found it to decrease to around50 µm. Thus, in the thinner regions of the sheetsignificant deformation of the bubbles appears likelyand the bubbles are likely to affect the local stress andstrain values. Neither Dobraszczyk and Roberts13 norCharalambides et al14 take account of the presenceof bubbles in their calculations. Consideration of theprecise stress and strain fields that occur in the vicinityof the bubbles is beyond the scope of this paper, but itis our intention to illustrate that the presence of thesebubbles is important.

The objective of this paper is to quantify the effectof dough aeration on its rheological properties asmeasured by the DIS.15 The effects of mixer headspacepressure and its gas composition on the aeration

characteristics of the dough both during and aftermixing are also investigated.

MATERIALS AND METHODSDough preparation and analysisStrong flour, President White, and weak flour, SoftPatent, (Nelstrop & Co Ltd, Stockport, UK) wereused to prepare doughs in the Tweedy 1 mixer athigh speed, approximately 72 rad s−1, as described inPart 1 of this series.1 The doughs were mixed in anenclosed chamber at five headspace pressures usingair and nitrogen (above and below atmospheric), andat three pressures using a controlled oxygen:nitrogenmix. In the latter case the oxygen:nitrogen ratio wasvaried when mixing at different pressures in order togive a constant oxygen partial pressure of 0.2 bar. Thepure nitrogen and constant oxygen experiments wereperformed in order to eliminate the effect of differingoxygen availability when mixing in air at differentpressures. Oxygen and nitrogen were supplied fromcompressed gas cylinders and mixed together using aWitt-Gasetechnik gas panel (Witt-Gasetechnik GmbH& Co, Witten, Germany), from which the gas mixturewas supplied to the Tweedy 1 mixer bowl at a flowrate of approximately 5–7 l min−1. When mixing inair the pressure was varied using either a compressedair supply or a vacuum line, without an intentionalor substantial flow-through of gas through the mixerbowl. Mixing under nitrogen was carried out with thenitrogen supplied from a compressed gas cylinder viathe gas mixing panel at a flow rate of 5–7 l min−1. Thedoughs were mixed in the Tweedy 1 mixer equippedwith a computerised work input record system thatmeasures the torque and speed directly for calculationof instantaneous specific power and total specificenergy.1 Doughs mixed for 3 min, correspondingto a work input of approximately 45 kJ kg−1, weresampled for density and rheological measurements.The gas void fraction was calculated based on doughdensity measurements as presented in Part 2 of thisseries.2 Rheological measurements with the DIS wereperformed following the procedures described in Part2 of this series.2 For the doughs mixed in air atdifferent pressures, the DIS was operated under twomodes: constant volumetric air flow rate (CVR) of1602 cm3 min−1, and at constant strain rates (CSR)of 0.1 s−1 and 0.05 s−1. For the doughs mixed underpure nitrogen and under a constant partial pressure ofoxygen, rheological evaluation was carried out just inthe constant strain rate mode at 0.1 s−1.

The dough inflation data were analysed as describedpreviously.2 Stress, σ , and Hencky strain, ε, werecalculated using the approximate equations presentedby Dobraszczyk and Roberts13 and the stress–straindata were interpreted using the exponential equationσ = kεnε to fit the parameters k and n (the strainhardening index), as well as finding the failure strainand failure stress.17 Ten dough samples were tested foreach condition (consisting of five samples from each of

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two replicate runs). The variation of these parameterswith mixing conditions was statistically analysed usingthe generalised linear model of analysis of variance(GLM ANOVA) for the effect of mixer speed andwork input levels. The NCSS 2000 v2.00 statisticalpackage (NCSS Statistical Software, Kaysville, Utah,USA) was used. In the following paragraphs, when thesignificance level P of a group of rheological parameteris reported, the highest value of the group is reported,with all of the other parameters returning significancelevels which were lower than the reported value.

Dough is a complex viscoelastic material; byreducing its rheology to a simple strain hardeningmodel some important aspects of its behaviour can beexpressed relatively easily through the strain hardeningindex, n.13 However, the simplification introducessubstantial uncertainty into the interpretation of thecoefficient k. Since k is a ratio between stress and

strain it should technically be considered as a stiffnessmodulus. However, since in practice the dough isundergoing both viscous flow and elastic stretching,k will also be a function of dough viscosity. Thecurrent work aims to demonstrate that aeration ofthe dough affects its measured rheological parameters,but detailed discussion of the interpretation of theparameter k is beyond the scope of this paper.

RESULTS AND DISCUSSIONDough density and gas void fractionFigure 1 compares the variation in dough densitywith mixing pressure and Fig 2 compares the gasvoid fraction of doughs mixed under the threeheadspace atmosphere conditions: air, pure nitrogenand constant 0.2 bar partial pressure of oxygen. Thelinear relationship between the density of dough mixed

Figure 1. Density of (a) strong and (b) weak flour doughs mixed under a supply of continuous constant oxygen (ž), continuous pure nitrogen (�)and static air (�).

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in air and under constant oxygen with mixing pressureagrees with results from previous work,2,18–20 andlinear regression was used to extrapolate to the gas-freedough density at zero pressure.

The nitrogen data are not linear, in contrast toprevious findings which have generally proved robustlylinear. This finding, which is consistent for both thestrong and the weak flour doughs, may result fromthe continuous flow of nitrogen gas; however, theconstant oxygen work was also carried out under acontinuous flow of gas, yet these results appear linearfor both dough systems. Clearly there is further workto be done to elucidate this; at this stage it alertsus that mixing under a flow of gas is different frommixing in a static headspace, in agreement with thefindings of Marston21 that blowing air over dough

Figure 2. Gas void fraction of (a) strong and (b) weak flour doughsmixed under a supply of continuous constant oxygen (ž), continuouspure nitrogen (�) and static air (�).

during mixing improved bread quality. The constantlyrenewed gas supply also resulted in lower densitiesand higher gas void fractions in the doughs, comparedwith mixing in static air, probably because the oxygenis rapidly removed in the latter case through oxidativereactions within the dough, and is not replaced. Moreremarkable are the apparently very high values ofgas-free dough density resulting from mixing underthe flow of gas at constant oxygen partial pressure.Table 1 lists the gas-free dough densities determinedfrom Fig 1 and the calculated void fractions of gas indoughs mixed at atmospheric pressure. The high gas-free dough densities under constant oxygen, 1.345and 1.322 g cm−3 for the strong and weak flours,respectively, are much higher than previously seen inour laboratories. It may be that the apparently lineartrend would not continue at lower pressures in thiscase; this was not tested as maintaining the oxygenpartial pressure at 0.2 bar at low pressures would haverequired high and unsafe oxygen concentrations. Evenif the relationship is linear, the gas-free densities ofthe doughs mixed under constant oxygen are subjectto greater error than the others since they are basedon a larger extrapolation from fewer data points. Theresult of error in the gas-free dough density wouldbe to alter the absolute magnitude of the calculatedvoidage, but relative comparisons would remain valid.Alternatively, it may be that the much greater oxygenavailability really did affect the development of thedough matrix so as to give these very high densities,perhaps indicating a more closely bound starch–glutennetwork.

The doughs mixed under pure nitrogen werephysically soft and loose. They were also viscous andsticky, and the stickiness was exaggerated when mixingat higher pressures. Doughs mixed under a constantpartial pressure of oxygen displayed tightness and highstiffness and were short. Observations of these physicalcharacteristics of dough agree with previous studiesin terms of shortness but not stickiness;22,23 doughsmixed under pure nitrogen in this study did show signsof over-mixing.

Dough rheology at constant volumetric air flowrateFigure 3 shows the rheological properties of doughsmixed in static air and measured using the DISoperating at CVR. The data are averages of tenmeasurements, and the error bars indicate the standard

Table 1. Gas-free dough density and gas void fraction of doughs

mixed under three atmospheric conditions

Gas-free doughdensity (g cm−3)

Gas voidfraction (%)

Mixing condition Strong Weak Strong Weak

Air (static) 1.261 1.271 5.46 6.06Pure nitrogen (continuous) 1.272 1.290 8.07 10.34Constant oxygen (continuous) 1.345 1.322 7.81 8.85

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Figure 3. (a) Coefficient, (b) index, (c) failure strain and (d) failure stress versus gas void fraction for strong (�) and weak (�) flour doughs inflated atCVR.

deviation of the mean, ie the standard error. Thecoefficient k increased while the strain hardeningindex n, failure strain and failure stress decreasedwith increasing gas void fraction. The strong flourdoughs exhibited higher k, n, failure strain and failurestress than weak flour doughs. The GLM ANOVAindicates that the significance of difference in therheological parameters at different gas void fractionsfor the weak flour doughs (P < 0.01, n = 24) is higherthan the strong flour doughs (P < 0.4, n = 26). Theresults clearly indicate that the measured rheologicalproperties of doughs mixed at different pressuresdepend on the pressure. They do not distinguishwhether these differences arise principally from thephysical presence of the gas bubbles in the dough, orfrom the different availability of oxygen when mixingin air at different pressures.

Dough rheology at constant strain rate of 0.05 s−1

Figure 4 shows the rheological properties of doughmeasured at a CSR of 0.05 s−1. The changes in allthe rheological properties, k, n, failure strain andfailure stress, with gas void fraction displayed asimilar trend to those measured using the CVRmode. The CSR mode gave lower values of kbut higher values of n, failure strain and failurestress. Strong flour dough again displayed highervalues in all the rheological parameters than thecorresponding weak flour doughs. The GLM ANOVAindicated that all the rheological parameters atdifferent gas void fractions are again significantlydifferent (P < 0.000001, n = 36 for strong andP < 0.002, n = 40 for weak). This clearly confirmsthat the gas content in dough significantly affectsits measured bulk rheological properties although,again, whether this results from the presence of

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(a)

(c)

(b)

(d)

Figure 4. (a) Coefficient, (b) index, (c) failure strain and (d) failure stress versus gas void fraction for strong (�) and weak (�) flour doughs inflated atCSR 0.05 s−1.

the bubbles or from oxygen availability is notdistinguishable.

The changes in all the measured rheologicalparameters with gas content showed consistency: kincreased, while n, failure strain and failure stressdecreased with the gas void fraction in dough, for bothflours. As the mass of dough tested was lower fromthe more highly aerated doughs, the observed effectson rheology could have been affected by the mass ofmaterial tested. This was shown not to be the caseby normalising the rheological parameters against thetested dough mass by dividing through by the doughdensity; the magnitude of the gas content effect did notsimply change in proportion to the mass of materialtested. (In any case, the procedure for using DISsamples doughs volumetrically, not by mass, so theobserved effects are relevant to the DIS as it is actually

operated.) The two modes of test, CVR and CSR,yielded the same trend in the changes in rheologicalproperties with gas void fraction, with the latter modeappearing to be slightly more discriminating. In thefollowing section the CSR mode was again applied, butat a strain rate of 0.1 s−1, which gave higher values ofthe rheological parameters and greater discriminationthan at 0.05 s−1.

Dough rheology at constant strain rate of 0.1 s−1

for various mixer headspace conditionsAeration of the dough could have affected the mea-sured bulk dough rheology through the contributionof oxygen made available through aeration, whichchanges dough chemistry, or additionally throughthe physical presence of gas bubbles in the dough.To distinguish whether the effect of gas content on

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the rheological properties of dough was by its phys-ical presence or solely by chemical reaction, doughsmixed under pure nitrogen (to exclude oxygen) and aconstant partial pressure of oxygen were also tested.Comparison of the results from the DIS under thethree different conditions of operation, applied to thedoughs mixed in air, suggested that measurementat a CSR of 0.1 s−1 was most appropriate. The fol-lowing results compare the rheological measurementsthus obtained, for the three different mixer headspaceconditions: static air, a continuous flow of pure nitro-gen, and a continuous flow of oxygen and nitrogenmaintained at a constant oxygen partial pressure of0.2 bar.

Figure 5 shows the rheological properties of doughsmixed under three headspace conditions for the strongand weak flour doughs. Results for doughs mixedunder air and nitrogen are in general agreement,with k slightly increasing and n, failure strain andfailure stress decreasing. However, the results forconstant oxygen are different. Doughs mixed underconstant oxygen showed the opposite trend for k,which decreased with increasing gas content, andwhich showed significantly higher values than thoseobtained under air or nitrogen. In contrast, the strainhardening index n was apparently not greatly affectedby the gas void fraction for doughs mixed underconstant oxygen. These patterns were consistent forboth the strong and the weak flour doughs, givingconfidence in the validity of the findings. The failure

strain and stress tended to decrease with increasingaeration for the constant oxygen condition, in generalagreement with the results from mixing in air andnitrogen.

As noted previously,2 k tends to be inverselycorrelated with n, although this only holds for aparticular dough formulation; both k and n were largerfor the strong flour doughs than for the weak flourdoughs. The parameter n is considered to represent thestrain hardening index and as such can be interpretedrelatively unambiguously. It therefore appears thatin general the presence of bubbles compromises thestrain hardening character of the dough, either throughdilution or weakening of the gluten matrix, anddecreases its failure strain and failure stress. The veryhigh values of k when mixing under constant oxygenindicate a stiffer or more viscous dough. This is thecase despite the oxygen concentration at high pressurebeing much lower than for static air at high pressure(the partial pressure of oxygen in the former case wasmaintained at 0.2 bar, while for the static air at twiceatmospheric pressure, the partial pressure of oxygenwould have been around 0.4 bar). This implies thatblowing even oxygen-depleted air through the mixerheadspace is more effective at enhancing oxidationand dough development than mixing at high pressure,a finding with clear commercial relevance.

The increase in k with increasing mixing pressurefor the doughs mixed in static air suggests an increasein dough strength arising from the increased oxygen

(a) (b)

(d)(c)

Figure 5. (a) Coefficient, (b) index, (c) failure strain and (d) failure stress versus gas void fraction for strong and weak flour doughs mixed under asupply of continuous constant oxygen (ž), continuous pure nitrogen (�) and static air (�).

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availability. This is supported by the observation thatk did not change nearly as much with mixing pressurewhen mixing was carried out under pure nitrogen.The slight increase in k with mixing pressure for thenitrogen-mixed doughs may indicate a stiffening effectof the presence of compressible, elastic bubbles. Whenmixing under a constant oxygen partial pressure, theoxygen concentration was therefore diluted at higherpressures, and this may have decreased its effectivenesssuch that the strength of the dough as indicated byk decreased, despite any stiffening contribution fromthe bubbles.

Strain hardening appeared to remain relativelyconstant when the oxygen partial pressure wasconstant, but decreased with increasing gas contentfor the other two cases. Failure strain and stressdecreased in all cases, indicating that even with astable strain hardening index the presence of bubblesdisrupts the integrity of the matrix. This might suggestthat gas bubbles create local stress concentrationswhich cause bubble failure. The effect on failurestress is more dramatic than on failure strain, ie highlyaerated doughs give somewhat smaller bubbles underdough inflation, but at greatly reduced effort. As notedpreviously,2,17,24 failure strain is correlated with thestrain hardening index. Figure 6 confirms this for thecurrent data under all three modes of operation ofthe DIS, and shows that for a given size of bubble atfailure (ie a given failure strain), the calculated strainhardening index is slightly lower when the DIS isoperated in the CVR mode compared with the CSRmode.

The above comments are tentative: dough chemistryis complex, and the results indicate a lot of subtlety inthe behaviour of the doughs that warrants a much moredetailed study. However, it can be stated unequivocallythat gas content does affect measured rheologicalproperties, at least in part because of the physicalpresence of the bubbles, and that the flow rate ofgas through the mixer headspace also affects bothaeration and rheology. This latter conclusion indicatesa rich field for further studies which may have practicalcommercial relevance. The former conclusion, thataeration affects rheology, implies that many reportedrheological effects of dough formulation or processingcould in fact result from differences in aeration,and that, in order to be meaningful, rheologicalmeasurements of dough should report the dough’sgas content (or at least its density). As this hasseldom been done previously, many studies relying onmeasurements of dough rheology could be consideredin need of reinterpretation.

Work input rate during dough mixingThe work input rate during dough mixing wascalculated by averaging the total work input over a130 s mixing period. Figure 7 shows that increasingmixing pressures resulted in energy being impartedto the dough at a greater rate for all three mixingconditions. The increased work input rate with

Figure 6. Failure strain versus strain hardening index of strong (filledsymbols) and weak flour (empty symbols) doughs mixed at variousconditions: doughs inflated at CVR (+, ); CSR of 0.05 s−1 (�, ♦);CSR of 0.1 s−1 under continuous constant oxygen (ž, °); CSR0.1 s−1 under continuous pure nitrogen (�, �); and CSR 0.1 s−1

under static air (�, �).

Figure 7. The effect of mixing pressure on average work input rate ofstrong (�) and weak (�) flour doughs mixed under static air,continuous nitrogen or continuous constant oxygen.

increasing mixing pressure under pure nitrogen andthe controlled constant partial pressure of oxygen at0.2 bar have ruled out the dough strengthening effectby the chemical oxidation processes in the presenceof oxygen; the effect is entirely due to the presenceof bubbles. (The direct effect of mixing pressure onthe no-load torque measurements was checked byrunning the mixer at various pressures either emptyor filled with water; the mixing pressure did not affectthe work input rate through effects of frictional stresson the mixer shaft such as greater stress imposed onthe shaft as it passes through the PTFE seal duringmixing.)

It may be that the increase in the work input rate isdue to the physical presence of gas bubbles increasingthe viscosity of the dough which thus increases thepower requirement during dough mixing. This issupported by the findings of Cheng and Carreau’s25

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investigation on aerated mixing of shear thinning andelastic fluids, which demonstrated that the powerrequirement in aerated shear-thinning elastic fluidswas almost three times larger than that in ungassedsystems. However, one might expect that the voidfraction and bubble size distribution in the dough whileit is being mixed and is still under pressure (or vacuum)would be independent of the pressure, with differencesin void fraction only arising after the pressure wasreturned to atmospheric. In this case, the effect of thepresence of the bubbles, which should depend only ontheir total surface area and surface tension, should bethe same at all pressures, and the rate of work inputshould be the same. The results in Fig 7 show thatthis is clearly not the case and that mixing pressureaffects dough rheology, as indicated by work input rate,while the dough is still under pressure in the mixer.For this to be the case, the void fraction of bubbleswithin the dough must be dependent on the mixingpressure while it is still being mixed, ie before thepressure has been returned to atmospheric. Campbellet al19 suggested that bubbles in bread dough duringmixing experience an extra compressive stress dueto the viscoelasticity of the dough, such that theirvolume depends primarily on this extra stress andnot on the headspace pressure, thereby explainingthe significantly fewer gas bubbles of the same sizedistribution obtained when mixing under reducedpressures. The finding that the mixer headspacepressure affects the rate of work input supports thismodel of how bubbles behave dynamically within thedough during the mixing process.

CONCLUSIONSAeration of dough significantly affects its measuredbulk rheology, both of the dough during mixing,and of the static dough after mixing. These effectsarise both from the contribution of dough aerationto oxidation and dough development during mixing,and from the physical presence of bubbles whichstiffen and disrupt the dough matrix. The presence ofbubbles decreases the strain hardening character of thedough and its resistance to failure under conditionsof biaxial extension, and increases the rate of workinput to the dough during mixing. Mixing doughsunder a constant flow rate of gas affects aeration andrheology, and appears to be much more effective atsupplying oxygen to the dough than mixing underpressure.

Operating the Dough Inflation System at a fasterstrain rate tended to give higher values of allparameters, while the constant volumetric air flow rategave higher values of k and lower values of n, failurestrain and failure stress than the constant strain ratemode. The constant strain rate approach appeared tobe more discriminating and to give more meaningfuland interpretable results. Bubble failure strain and thestrain hardening index are highly correlated.

For the first time, the effect of aeration or thephysical presence of bubbles in dough on its measuredrheology has been quantified. The indication thatrheological measurements of dough are affected bygas content prompts the recommendation to reportthe gas content (or at least the density) when reportingany dough rheology measurements. It may be thatmany rheological studies of dough reported in theliterature actually signify differences in the aeration ofthe dough.

ACKNOWLEDGEMENTSThe authors gratefully thank Andrew Tassell, RichardWiktorowicz and Air Products plc for the loan ofthe gas mixing panel and supply of gases, the SatakeCorporation of Japan for support in establishing theactivities of the SCGPE and University Putra Malaysiafor financial support to NL Chin’s course at UMIST.

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