A study of some physical properties of flour doughs in relation to their breadmaking qualities

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Halton, P. and Blair, G. W. S. 1936. A study of some physical properties

of flour doughs in relation to their breadmaking qualities. Journal of

Physical Chemistry (1952). 40 (5), pp. 561-580.

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A STUDY OF SOME PHYSICAL PROPERTIES OF FLOUR DOUGHS IN RELATION TO THEIR

BREAD-MAKING

QUALITIES

P. HALTON1

ResearchAssociation ofBritish Flour Millers,St. _{Albans, England}

AND

G. W. SCOTT BLAIR

Departmentof Physics, RothamstedExperimental Station, Harpenden, England

Received_AprilS, 1935

INTRODUCTION

Attempts tomeasure the physical propertiesofflour_doughswere made by Kosutány (4) as _early as 1907. This author pulled out a _cylinder

of flour dough at a constant rate, and measured _{tjhe stress build-up.2}

Although not clearly distinguishingbetween viscous and elasticproperties, the author’sinterpretation ofhis _{data points clearly to} a number of the conclusions reached inthe _presentwork.

It iscurious that this valuable work,in _{which Kosutány}came so near

to _{separating and measuring specificphysicalproperties, should have}lain fallow for so _{many years.} From 1907 to 1932 work on _{the physical}

properties of flour doughswas confined almost _{entirely to the production}

ofinstruments_measuringa_complexmixtureof properties,which,although in some cases ofrealvalue in _{the bread-making industry, threw}little or

no _light on _{the physical nature of the problem.} In 1932-33 threepapers were published by Schofield and Scott Blair _(8), in which certain of the physical properties ofdoughswere _{separated andindependently}measured. Forthesakeofconvenience these paperswillbereferredtoas

_I,

II,

andIII.

In paper I it was _emphasized that flourdough belongs to a _{group of}

materialsinwhicha _highdegreeofplasticityiscombinedwithconsiderable elasticity. When under stress the relative amounts of _plastic

(non-1

By mutual agreement the authors’ names are in alphabetical order and no

seniorityisimplied.

8

Terzaghi(9)developsvery similarideas. The constant whichhecalls “degree of elasticity” (p. 79) is closely connected with relaxation time (vide infra). Terzaghi’swork_{refers to soils and clays,}but their behavior isin some ways

strik-ingly similartothatofflourdoughs. Seealsoarecentpaper by . P.Wolarowitsch andK. I. Samarina (10) dealingwith some physical properties of flour doughs.

561

THB JOURNALOFPHYSICAL CHEMISTRY, VOL.40.NO,5

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recoverable) and elastic (recoverable) deformation depend on the time

of_durationof thestress. An_{extended significance}was _givento Maxwell’s

relaxation time,sothatthe_equation

tr —

ii/n

in which tr = relaxation time, = _viscosity, and n = _{shear (rigidity)}

modulus,3 could be applied to a material such as flour _dough, in which

neither nor n isaconstant.

It was foundthat and tr decreasewith _increasing stress and increase with increasing deformation. Fall in _viscosity with _{increasing stress} is a _phenomenonwell knowninmany colloidalsystems, andhasbeencalled “structuralviscosity” (6). Increaseinviscositywithincreasingstrainis

a common characteristic of _{metals, and}is called“work-hardening.” In paper

II,

these_{two properties}were more

_fully

studied_byobserving

the rate of shear of cylinders of dough hung vertically, and allowed to elongateunder theactionofgravity (themethodof rheograms).

The_{Maxwell equation}couldonlybetested_{satisfactorily after the most} suitable valueofmodulusto use in calculatingviscositiesfrom relaxation times was known. _{Experiments to} decide _{this point} were described in paper

III,

and agreement was found to be as _satisfactory as could be expected whenthe best value for the modulus was used. These experi-ments also made it clear that doughshows _{two other properties} charac-teristicof metals, namely, elastic hysteresis and elastic after-effect.

The formercauses the

_rigidity

modulusto fallslowlyas stress israised, andalsoas stressis_lowered,butat_{the point at which the} sign of dS/d< changes,4themodulus increases abruptly. The latter means that elastic deformations are not recoveredinstantly, so thatunlesstimeis_givenfor slow recovery to take place, certain deformations will be regarded as

permanent which are in reality recoverable. This would lead to

con-siderable errors in _{determining viscosity} and modulus (see experimental section). In viewofthe partialunderstanding which this treatment had already given, it seemed advisableto investigate furtherthe _relationship between thesefundamental physicalproperties, and thosequalities of the dough which are of importancein the bread-making industry. For this purposethePhysicsDepartmentof the_{Rothamsted Experimental Station} and the ResearchAssociationofBritishFlour Millers decidedto_cooperate in thefurtherstudyof_{the problem.}

8_{Note that forflourdoughs, Poisson’s}ratio_being

0.5,we can assume therigidity modulus tobeequal toone-thirdof Young’s modulus. The shearingstresslikewise

isone-thirdof the loadingstress. Inthe present paper, the term“rigiditymodulus”

isoftenabbreviated to “modulus,”sinceno other modulus isdiscussed.

4_Where£is

The _general principles havenow been_{elucidated, and} although much detailrequires yet to be _{filled in, thepresent paper gives} a_descriptionof the conclusions to date.

EXPERIMENTAL

By far the most serious difficulties that have been encountered were

those associated with the _{reproducibility} of measurements on different testpiecesfromthesame _dough. In_papers

_I,

II,

andIII accurate replica-tion on different test _pieces was not attempted, and _{although the}

phe-nomena described could be _{repeatedly observed,} it was realized that no

fully

satisfactory technique existed for such _replication. In the course

ofthe_{presentinvestigation much time} has_{been spent}indeveloping such

a _{technique, and}the methodfinallyadoptedhasprovedon thewholetobe satisfactory.

TABLE 1

Theeffectofmixingon theviscosityandmodulusofthe dough

TIME OF MIXING TIME OFRESTING

AFTER MIXING VISCOSITY* MODULUS*

minutes minutes

3 0 6.38 X 10s 4.29 X 10*

30 4.92 X 106 3.94 X 10* 60 4.04 X 10« 3.73 X 10*

12 0 2.92 X 106 3.02 X 10*

30 3.55 X 10= 3.41 X 10*

60 3.51 X 10« 3.50 X 10*

*_{All dataforviscosities (ij) and moduli}

(re)in thispaperare _givenino.o.s. units, but it mustbebornein mindthat theyrefer only to arbitraryfixed conditions of

stressandstrain.

It isnecessarytomakeas_homogeneous a _doughas _{possible, and}it has beenfoundthatmachinemixing oftheflourandwatergivesthemost satis-factory results. The _longerthe time of _{mixing, the} more _homogeneous

is the _{finished dough,} but excessive _mixinghas a _{very marked} effect on

the_{dough’s physicalproperties.}

Fromthe dataintable1itcan beseen thatexcessive_{mixing considerably} lowers _{the viscosity} and modulus of the dough, but that theseincrease againon _resting. Such_{treatment,however,permanently}lowers the tensile strength ofthe _dough.

It willbeshownlaterthat the_{generaltendency}on _aginga _doughisfor the viscosityandmodulusboth to fall. It isonly afterprolongedmixing that the _{opposite effect} more than compensatesfor this fall, producing a

min-HALTON AND G. W. SCOTT BLAIR

utes _{mixing, the} effect ofstandingis_{the opposite of}that producedby12 minutes mixing.

The sample of doughistransferredfrom the mixerto a _“gun,” consist-ing of a hollow metal cylinder, 5 cm. _{long and}2.5 cm. in _diameter, fitted with a _plunger. To the bottom of the _cylinderis fitted a solidpiece of metaldrilledwitha hole3.5cm. _{long and}0.5cm. indiameter. Thisfirst gun,into whichthe doughcan be_placedbymeans ofa_spatula,thus

avoid-ing handling it, is_{too big} to fit conveniently on to the _{apparatus, and}

therefore the _{extended dough cylinderis squeezed straight into} a second smaller gun 15 cm. _long and 1 cm. in _diameter, "which is fitted with an

end piece similarto that ofthe first_gun. The dough isforcedfrom this secondgunstraighton to thesurfaceofa bathof_mercury.

Statisticalanalysis5showedthattherewas no _greatererror in comparing

testpiecesfrom differentdoughsthanincomparing piecesfromthesame

dough. It thusappearedthatthe chiefsource oferror _{lay in the method}

of _preparationofthe testpieces. It was foundthat theforceapplied to the guns very largely affected the physical properties of the prepared doughcylinder. A system ofpulleys and weightswas thereforeused for manipulation ofthe_{guns, and, provided} that the_weightswere _small, we

found very little alteration of_{the properties} of the doughs. The impor-tanceofa _carefullystandardizeduse of_{these guns cannot}be_{too strongly} emphasized.

Duringextrusion_{the doughcylinder}swells,thisswellingbeingingeneral greaterforgoodthanforpoor qualityflours. _{The exact connection,} how-ever,isnot clearlyunderstood.

The dough cylinders have been examined by the twogeneral methods described in papers I,

II,

and

III,

namely (a) rheograms, and (b) the mercury troughextensimeter, but the technique ofthelatter methodhas beenextended and developed.

(a) The rheogram method. This method has been further developed and has now reached a _stage at which theresults are _excellently

repro-ducible, and, although not free from _errors, it provides the most satis-factoryway at present availableforseparatingtheeffectsof work-harden-ing (rise in viscosity with rising strain) and structural viscosity (fall in viscositywithrisingstress).

In mostdoughs thesetwo propertiesappear,undertheconditionsofthe rheogram experiments, approximately to cancel out. Without commit-ting ourselvesto _any assessment of the _{degreeof accuracy of} individual samples, an examination ofthe data obtainedfrom a _study of some _sixty

doughs made from a series of twenty different flours, leads to the

con-clusion that certain flours tend to show higher or lowerdegrees of

work-6_{Our best thanksare due to Mr. F.Yates of theRothamsted Experimental}

hardening than others, and that this property generally persists when differentamounts ofwaterand timesof_agingare used. Suchdifferences do _{not, however, correlate directly} with flour quality, and we conclude that weakness in a flourdue to an unsuitable degree of work-hardening istheexceptionrather than therule.

Certain practical difficulties are encountered in _using the rheogram techniquein thecase of_{stickydoughs, anddoughs}of_{poor tensilestrength.}

Moreover, both viscosity and _rigidity modulus can be calculated from

a _single test on the extensimeter (vide infra); therefore the _rheogram methodwas not _employed further in this investigation.

(b) The extensimeter. This instrument, which is an _improved model of the extensimeter described in papers I and

_III,

is shown diagram-matically infigure 1.

A_{doughcylinderA,} about10cm. _longby0.7cm. in_{diameter, made} as

described above,isfloatedon a _{mercury bath.} Theendsof this cylinder

are connectedbymeans ofcork“chairs”and cottonthreadsto two small scales B, which are observed through low-power microscopes, C. The

IT

-<5> r

Fig. 1. The mercury bathextensimeter

smallest divisions on the scales are 0.013 cm. in length, _{and readings} to

one-tenthofthiscan beestimatedwithafair_{degreeof accuracy.} Toone scaleis fastenedasteelspringD, the otherendofwhichissecurelyattached tothe_{framework of the apparatus;}theotherscale isconnectedby cotton to a small_{winch (E), which} can bewound either byhand or _bya small motor.

During experiments the dough is protected by a _cover, the felt lining

ofwhichis_{damped toprovide}ahumid_{atmosphereto prevent drying out}

of the dough surface.

When thewinch (E)iswound_up,the_{dough and}scalesare moved to the

left, and this extends the spring D. The dough is therefore subjected to a_stress,thevalueof whichis_{proportional to}the extensionofthe spring, and inversely proportional to the cross section of _{the dough} cylinder. The spring iscalibrated by noting theextensioncaused_{byhangingweights} ofvarioussizesfrom it when placedina vertical position. Thediameter

W.

of shearingstressto_{velocity gradient}(i.e., rate ofchangeof non-recoverable strain),we can either fix therateof change.ofstrainandmeasure thestress, or fix the stress and measure the rate of change of strain. The latter has _{in practice} been found to be _byfar the simpler method to use, and has been the basis for most of the experiments described in this paper. Before doing an _experiment, the necessary deflection ofthe spring to givethe desiredstress isfirstcalculatedfroma _{knowledge of}the constant for the spring and the cross _{section of the dough cylinder.} The winch

is then rapidly wound up until the shift in the scale attached to the spring corresponds to this required stress. For ordinary purposes we

worked with an _arbitrary tensile stress of 1500 _{dynes per square} centi-meter (shearing stress 500 _{dynes per square} centimeter).

Understress_{the doughcylinderextends, and}forfiveminutesthis exten-sionistaken_{upby slowly winding the winch}so that thedeflection of the

right-handscaleis_keptconstant. _Bythismeans thestressis_keptalmost constant, the riseinstresswithslightthinning ofthe_{dough cylinder not} being generally great enough to introduce any serious error, especially when _{relative, rather than} absolute, viscosities of doughs are _required. Bymeasuring the extensionatminute intervals it wouldappearthatthe

mean _{viscosity for} each minute could be _determined, and from this an

indication of the amount of work-hardening (i.e., change in viscosity withstrain) assessed. Thisis_{not,however,practicable,}sinceelastic after-effect is takingplace during thewhole process (seeintroduction).

The stress is released at the end of the five minutes and the _dough allowed to relaxuntil no further_change in length_{takes place (this} takes about three to _{five minutes).} The difference inthe length of the dough cylinder before any stress is applied to it and at the end of relaxation givesa measure of the non-recoverablestrain caused_bythe stressacting for five minutes.6 From this a mean _viscosity can be determined by dividing the non-recoverable strain per unit time into the stress. This is free from errors due to elasticafter-effect.

The amountof recoverableor elasticdeformationis _{obtained by noting} the changein length of the dough cylinderbetweenthe time _{of releasing} the stress and the end of relaxation. The value of the shear modulus whichisgivenbythe ratio

change in shearingstress changeinrecoverablestrain

8_{Strictlyspeaking, the strains shouldbe}calculated from log,i/i0,asin the rheo-gram calculation, but for the small strains used the method described here is

is a mean value fora _changeinstress from 500 to 0 dynes,7the modulus fallingprogressivelyduring the lowering ofthestressduetoelastic hystere-sis (see introduction).

Effectof temperature

Experiments have shown that the _viscosity of a _{typical dough} falls by about 10per cent perdegree Centigrade rise in temperature and the modulus by about5_{per cent.} Thismakesit desirableto exercisecareful control over _{the temperature. The} extensimeter is not _easily thermo-stated, and, moreover, the processes of dough preparation should also becarried out ata_{constant temperature.} It would thusbebestto carry outallmeasurementsina constant temperatureroom.8

Although a suitable constant temperature room has now been built, thefactthat itwas not availablefor the earlierwork meantthatwe could

onlymakedirectcomparison between resultsobtainedover _{periodsduring}

which laboratory temperature didnot _{fluctuate very widely,}andin

conse-quencewe havehadto_foregomakingas fulluse ofour dataas we should otherwise havebeenable to do. _Further, since the viscosity falls about twice as fast (with rise in temperature) as does themodulus, it is clear thatthe all-important9 viscosity modulus ratio (relaxation time)is_higher, the lower the temperature. Thissupportsthe viewheld bysome bakers thatdough shouldbefermented andput intotheoven ataslowa

tempera-ture as isconsistent withthe satisfactory working of theyeast.

The temperaturecoefficients also_vary withthe age ofthe dough, and it isthusclearthatthetemperature at whichit is agedplaysan _important

part in defining its physical properties at any given time, and hence in determining thequality of theresulting bread. Further experiments are

needed to explorethis field.

WATER ABSORPTION

When determining the valueof a flour inthe bakehouse the first step isto turn it intodoughby mixingwithwaterandother ingredients. The bakerdoesnotuse aconstantratioofflourto waterfor allsamples, other-wisesome _{of his doughs}wouldbetoo softandsticky, whileotherswould betoo tough, extremesofcondition whichnot onlycause serious difficul-tiesin _{the handling}ofthedoughs,butwhichdonot result inbread repre-sentativeof the valueofthe flour. In _{view of this, the baker}varies the

7_{The stress isnever allowed to fall quite to}

zero, owing to the necessity for keeping the cotton taut, but thefinalstressis very small, and isthesame in all

experiments.

8_{Caremust beexercisedto ensure}

properventilationand soprevent the danger of mercury poisoning(see Stock (9)).

amount of waterheadds toeachfloursothat_{his doughs}are _easytohandle and in _{general bake} into as _{good quality} bread as his various flours are

capable of making.

Weare therefore facedwith theproblemof determiningthe significance ofthis optimum amount ofwater, or “water absorption” as it iscalledin the bakehouse, and alsowith the _{necessity of} determining some method

wherebyit couldbefixed. In assessingwater absorptionthebakerrelies

on hissense of touch,and one of the impressionswhich_helpsinhis judg-ment of correct absorption is the extent to which _{the dough sticks} to hishands.10

It appeared to us that thebakermade up his doughsso thatthe sticki-ness was _justshortof beinga _trouble,andon this_assumptionwe basedour

TABLE 2

Effectofwater contenton viscosityandmodulus_offlourdoughs

WATER CONTENT

— 1_gal.

-1gal. Normal +igal. + 1_gal.

No.1Manitoba:

10.0 7.5 5.8 4.8 4.1

Modulus (X 104)... 4.1 3.6 3.1 2.6 2.1 BarussoPlate:

15.0 8.3 5.7 4.4 3.5

Modulus (X104) ...",... 4.6 4.0 3.4 2.8 2.2

Australian:

7.5 4.8 3.5 2.8 2.3

5.5 4.6 3.6 2.6 1.6

Normal absorptions: No. 1 _Manitoba, 15.6_gals, persack; Barusso _Plate, 15.3

gals, persack; Australian, 14.0_{gals, per sack.}

first attempt to connect water absorption with a _{physical property,} so

that by themeasurementof thelatter we couldfix the former.

Measurements of stickinesswere madebymeasuringtheforcerequired

to overcome the adhesion of a metal weight to the surface of a _dough.

Themethodusedwas a modification of those proposed_{by Kachinski} (3) and others (1, 2, 5, 7) formeasuringthe stickinessofsoils. It has, how-ever,notbeen_{possible to make the measurements}reproducibleenough to

use as a means of_{assessingwater absorption.}

The connection between water content andtwo other _physical proper-ties, namely, viscosity and rigidity modulus,was next investigated.

For this a series of flours was _obtained, and from each flour several doughs were made _{containing different} amounts of water. The data

10_{For experiments on the psychological}

obtained by measuring the viscosities and moduli of the doughs made from three of these flours, a No. 1 _Manitoba, a _{Barusso Plate, and} an

Australianare _givenin table 2.

Each flour was examined _{at the absorption} chosen in the bakehouse whichwe havecalled “normal,”andatfour other watercontents

_±§

gal-lonand ±1 gallon per 280-lb. sackofflour.11 _{Each dough}was fermented forfourhours, atthe end ofwhichtimesampleswere _{taken for viscosity}

and modulus measurements. In figure 2 curves nre drawn _{showing the} relationship betweenthese two properties at each of the five water

con-tentsforeachof thethree flours.

An examination ofthesecurves showsthat witheachflourboth viscosity and modulus fell with increasing water content and vice versa. The

relative effects on these _{two properties} were not the _same, and differed from flour to flour. Theeffectsofa change of2 _gallonsin water content

on _viscosityandmodulusare _givenin table_3,the_{figuresrepresenting}the difference in values for the driest and wettest _doughs expressed as a

percentage of theaverage.

These figures show that the effect bothon _viscosityand modulus was

least in thecase ofthe Manitoba, whichisin_keepingwiththe_experience

of the bakerthatthis typeofflourhasa_{much greater tolerance to changing}

water contentthan anyother. ThePlateflourdifferedfromtheManitoba chiefly on account of _{the much greater} effect of changing water content

on _{viscosity, while in the}case of theAustralianboth viscosityand modulus, and particularlythe latter, were more _markedlyaffectedthan in thecase

of the Manitoba. It is _probable that a _big upward change in viscosity with decreasing water content is ofmuch less account in the bakehouse thana_bigincreasein_modulus,asthelatter wouldmakethe_doughs“dead” and lifeless (seelater).

Thethree doughs ofparticular interestare the “normal”_{doughs, and} it

will benoticedthat these _{doughs differed veryconsiderably in viscosity,} but_only to a small extent in modulus. It is _probablethat the baker is unconsciously more influenced in choosing his water absorption by the moduli _{of his doughs than by their viscosities, and} in consequence we

may be able to use measurements of the former property as a means of determining correct water absorption. This point,however, needsmuch furtherinvestigationbefore it can besettled.

GENERAL CONSIDERATIONS

Although it is_{quitepossible to make}a _{doughfrom any flour}whatsoever

so thatit shall haveanydesired_viscosityor modulus _(withinwidelimits), byusing the appropriate amount of water, it iswell known that a poor flour cannot_{by any}suchmeans bemade to _give a _{dough ofsatisfactory}

plastic and elastic properties. The reason for this is clearly seen from

table 3. It ishere apparentthat if a _“strong” flour _{dough (Manitoba)} is_comparedwitha“weak” _{(Australian) at the}same viscosity,the modulus of the strongflourismuchlowerthanthatof the weak. If the comparison ismadeatthesame _{modulus value,}the_{strong flour}hasby farthe greater viscosity. The intermediate Plate flour fallsbetweenthe two extremes. This suggests that the relaxation time

( / ,

see _paper

_I)

is of primary importance. This suggestion has been _{amply verified.} It is, of course, clearthatwe _{cannot regard}relaxationtimeas a constant ofa _dough,since it varieswithboth stressanddeformation,but ithas becomeincreasingly evident during the course ofthisw'orkthatif doughs are _comparedunder

similar conditionsofstressand_strain, andifthese_{conditions approximate} as _closelyas _possibleto those obtained in the commercial dough, a

com-parison ofrelaxation times (“viscosity^-modulusratios”) gives a _primary measure of the differences in flour quality (although as is shown later, many other factors have to be taken into consideration as _well). In

of a_plastic film_{by which the}elastic elements are connected. It is_quite possible that the elements are _{capable of} complete elastic recovery, but there is at _present no criterion for testing this. The time of relaxation isa characteristicoftheconnectedstructure as awhole andits valueisas

much determined by the elasticity of the elements as _{by the viscosity}

associated with their plastic

junctions....

In _relating these deductions to theknown structure ofthe dough, one _{may safelyidentify}the elastic elementswiththe protein partoftheflour.”

When a _doughrises under the action of yeast, it is _advantageous for

as_higha_percentageas_possibleof the deformation to beelastic (recover-able). Non-recoverabledeformationsimplyflow ofthecell-walls, leading to theirrupture, and collapse ofthe dough dueto inabilityforit to hold itsshape,resulting inaloaf having_{large andbadly shaped holes,}a _poor

volume, and badover-allshape. Bigelastic extension_{resulting from low} modulus tendsto producea _bigrisewhenthe dough isfirstplacedin the oven, and hencebig loafvolume.

TABLE 3

Effectofchangein watercontent

MANITOBA PLATE AUSTRALIAN

83 124 106

64 70 105

The property by which an _{extended dough} on release recovers a _high

percentage ofitsextension, is calledby bakers_{“spring.”} It isclearthat the extent to which thedough fails to return to its _{original length after} suchan extensionwill_dependon howfartheelastic elements have slipped pastone another. Thisdependson theamount offrictionbetween them, which, as we have seen, corresponds to the viscosity of the dough (as normallymeasured). The higher the viscosity, theless_{the slippage.} But

the amountof slippage dependsnot onlyon _{the viscosity,}butalsoon the internal stress set up in the elastic elements. If we think of these as

coiled springs, it is easy to see that the

_{“lighter”}

the _{springs (i.e.,} the

lowertheir_{moduli), the}lessstresswillbebuiltupforany givenextension, andhencethelesswillbe_{the slippage}fora_{givenviscosity.} Thusa_dough

showing good“spring” willhavea_{relatively high viscosity}andlow modu-lus, whereasa _doughhavingbadspringwillhavea_{relatively low viscosity}

and _high modulus.

It isnow clear_{why the baker}attachesso _{much importance to “spring”}

in his doughs. Good spring means a _{high viscosity modulus} ratio _(big

It is_{pertinent toenquire}as tothe significance of the extent of the varia-tion of viscosity and modulus with stress, strain, and stress _{history for} differentflours. Experiment has shownthat even _{widely different flours}

show a _{very similar degree of elastic hysteresis, and} no marked differ-ences in their elastic after-effect behavior. _{As explained} in paper III elasticafter-effect is _{important in} that unlessattention is _{paid to} elimi-nate its effect, it is liable to interfere seriously with the correct deter-mination ofviscosity.

TENSILE STRENGTH

In_{the bakehouse occasional doughs}are encounteredwhich“tear” _badly during thebaker’s_manipulationand_duringtherisingofthe_{doughs under} the pressure of the _{gas generated inside them. Such doughs} are said to be “short” and bakeinto unsatisfactorybread. Owingto the_tearing, excessive gas leakage occurs which results in _poor loafvolume, and the actual tearinggives the outside of theloafa _{ragged appearance.} In addi-tion,the insides ofsuch_{loaves easily crumble whenpressed}withthe fingers. It thus appears that for such flours tensile strength is also a factor of

major importance. This propertyis now _beinginvestigated.

AGING AND FERMENTATION OF DOUGHS

In the process of bread making the doughs are _{kept for} some hours before bakinginto bread. Not _only do _{their handlingproperties depend} on thelengthofthisperiod,butthe_typeofbreadalsoshowsconsiderable variation.

In order to determine the _changeswith _age in the physical properties of such doughs, and to findwhatconnections exist betweenthese changes and bakehouse behavior, a series of flourswas obtained whichhadbeen previously examined by thebaker. The doughswere _{made up}with the same _ingredients as had been used in the bakehouse and were _kept at 27°C., samplesbeingtaken at hourly intervals for viscosity andmodulus measurements,whichwere madeat room _temperature.

The data obtained on four ofthese _flours, a No. 1 _Manitoba, a No. 3

Manitoba, a _{Barusso Plate, and}a SouthAustralian, are _givenin table _4,

andcurves drawnfromthesedataare _givenin_{figures3, 4,} and5.

It will be noticed that no data are given for freshly made doughs. This is because of the _difficulty of _{obtaining reproducible}measurements

on them. _{Rapid changes} take place _{during this initial period and} are

probably connected with the rate of absorption of water by the flour. Afterone-half toone hourtheseeffects disappear, and thephysical proper-tiesthenchangeina normalandregularfashion.

In the case ofthe above four flours,the changesin physical properties

TABLE4

Effect of fermentationon viscosity,modulus, andtheij/raratio

FLOUR TIMEIN HOURS viscosity X10e MODULUSX10* _/ RATIO

No. I Manitoba 1.23 3.97 3.16 126

2.12 3.82 3.02 126

3.05 3.79 3.21 118

4.93 3.32 2.99 111

6.00 3.27 3.21 102

6.93 3.25 3.09 105

No.3Manitoba 1.10 3.25 2.61 124

2.03 2.84 2.37 120

3.08 2.68 2.53 105

5.02 1.88 2.18 86

6.07 2.04 2.18 93

7.03 1.83 2.03 90

BarussoPlate 1.18 3.33 3.53 94

2.03 3.46 3.47 100

3.13 2.83 3.23 87

5.02 2.56 3.08 83

6.00 2.45 2.76 89

7.07 2.07 2.39 89

Australian 1.07 2.52 3.21 78

2.05 2.18 2.92 74

3.00 1.97 2.82 70

5.00 1.39 2.32 60

6.13 1.00 2.06 48

7.00 0.76 1.58 48

A

3

2

/

Fig.3. The effectofageofthe doughon theviscosity

-BLAIR

The data given in table 5 have been obtained from the curves _(see

of each _{flour the viscosity} fellmore _rapidly than the modulus, resulting

in theratioofthese_{two properties}also_fallingwithincreasingtime. In _{attempting to} correlate thesechanges with the changes which took placeinthé_{handling properties of the} doughsin thebakehouse, we were

at once confrontedwith thefact that the baker reportedthat the No. 1

Manitoba_{doughimproved in body}and springas fermentation_progressed, that the Plate remained unchanged, and that the No. 3 Manitoba and Australian _{doughs became softer}withincreasingtime.

TABLE 5

Changein physical properties with time

FLOUR

VISCOSITYAT

ACTUAL

PERCENT-1sthr. 7thhr. DECREASE

4.05 X 10*

3.01 X 10*

3.17 X 10*

1.83 X 10*

0.88 X 10*

1.18 X 10* 22 39 3.37 X 10® 2.12 X 106 1.25 X 106 37

2.84 X 10* 0.77 X 10® 2.07 X 10* 70

FLOUR

MODULUS AT

ACTUAL

PERCENT-let hr. 7thhr. DECREASE

3.19 X 10*

2.66 X 101

3.05 X 104

2.04 X 104

0.14 X 104

0.62 X 104

4 23 3.62 X 10*

3.23X 104

2.55 X 104

1.87X 104

1.07 X 104

1.36 X 104

30

Australian... 42

FLOUR

VISCOSITY-MODULUS RATIOAT

ACTUAL DECREASE

PERCENT-DECREASE

1sthr. 7thhr.

No.1Manitoba... 127 104 23 18

No.3Manitoba... 123 89 31 28

Plate... 94 87 7 7

Australian... 79 48 31 39

This improvement or _toughening is associated by bakers with good quality. Severalflours which have shown this response to fermentation in the bakehouse have been examined _{in the laboratory,} and in every

case both viscosityandmodulushave beenfound to decreasewith aging; in fact, no flour has _yet been examined which showed a rise in either property during fermentation. It hasbeen_{noticed, however,} that those flours which have been _reported as toughening in the bakehouse, were

those which showed the smallest fall-off _{in physical properties} when examinedinthe_laboratory.

G. SCOTT BLAIR

baker, dough does toughen,12 but this _happens with all _{flours, and} in addition _{the effect disappears} on _resting. The toughening the baker speaksofinconnectionwithgoodqualityfloursonly,isconsidered_{by him} to be dueto theaction of fermentation.

Careful tests have now been carried out in the bakehouse in which freshly made doughswere _comparedwith fermented _{doughs made} from

the same _{Manitoba flour, which} the baker considered had toughened. When the doughswere mouldedside_{by side,} one ineach _{hand, and}then allowed a few minutes to rest, it was _{reported by} the baker that when tested _by “feel” _{the older dough} was _{very slightly} the softer. It thus appearsthat bakers havebeenmistakenin theirimpressions thatcertain doughstoughenduring fermentation.

When a series of replicate doughs is made from the same flour and allowed to ferment_{for varyingtimes,}it isfoundthat withincreasingtime the volume and crumb _quality of the bread atfirstimproves, and then fallsoff. This improvementis consideredbythebakerto bedueto what is calledthe_{“ripening”} ofthe_{dough, the}actualtimeto obtain maximum improvementvaryingwiththe amount of yeast, andwiththe_typeofflour used. “Strong” flours like Manitoba require much longer fermentation for optimum results than “weak” flours _{like English} or Australian. In-creasing the amountof yeastin thedoughdecreasesthetimeofripening. Since viscosity, modulus, and relaxation time all fall consistently as

the dough ages,it isdifficult to explain the initial improvement inbread quality withtime offermentation in terms ofthe changestakingplace in thesephysical properties.

Thedecreasein_rigiditymoduluswithtimeisin itself_desirable,butsince it is the ratio of _{viscosity to} modulus which is of _{primary importance} in determining flour quality, the general result ivould be _{expected to} be a _fall-off, and not an improvement in bread _quality. Also, if a fall in

both of these _{physical properties} were _{desirable, then} a _similar

improve-ment inbread qualityto thatwhichtakes place withfermentationcould be _{brought about by using} more water in the dough. This is not so, andeven whenthebaker adds too much water to theflour,hestillgetsan

improvement in breadquality with increasing fermentation time. Although this improvement or _{“dough ripening” may} be _partly due to changestakingplacein some _{physicalproperty other than viscosity}or

modulus,itis_{possible to account}forit on _purelymechanical lines. Before a_goodloaf can be_{made, the necessary cell structure} hasto bebuilt _upin the dough, andthiscellstructure mustbedeterminedbythe number and distribution of the _yeast cells. Now normal bakehouse _mixing is

com-parativelycrude, andinconsequencethisdistributionis_{probably anything}

12_See

but uniform. Owing to the _activity of the yeast, however, the dough swells,andthis probably helpsto spread theyeast cells. In_{addition, at} variousstagesoffermentation, the bakerknocksthegasout of thedough andmoulds it up, thusagain helping towardsmore uniformdistribution.

During fermentation theyeast cells multiply, and thus as time _goes on

the gas-producing centers increase innumber.

If the above_pictureis_{correct, and}it is_{the building up}ofthenecessary cellstructure whichdetermines how muchfermentationis_{required for any} flour to give itsbest bread, then it shouldbe _{possible to} cut down this time by theuse ofmore _{yeast and/or thorough mixing.} That thisisso

is _{well known,} and it has moreover been _{verified experimentally by} us.

During fermentationwe therefore havetwo processes goingon side_by side, an _improvement due to the _{multiplication} and better distribution ofthe_{yeast, and}a_{falling-off in bread-makingquality}dueto thedecrease in value of_viscosityand relaxation time. The improvement dueto the yeast appearsto be_{comparativelyindependent ofthe physicalproperties} of the _dough. With a _{good quality} flour suchas _Manitoba, thefall-off in physicalpropertiesisso_slightthat_{good bread isproduced}over a_large

range oftime, whileon theotherhandthefall-off_{in physical properties}is so _greatwitha _poorqualityflour like English, thatthe best breadis pro-duced early, and is_{followed by}a_rapidfall-offin quality.

Returning to the data in table5,the No. 1 Manitobahadthe_highest

initial viscosity and showed the smallest decrease _{in this property} with time. Thisflouralsohadthehighestviscosity-modulus ratio either after

one or after seven hours. Thisisall inkeeping withthe _{generalquality} of theflourasshowninthe bakehouse, whereit behavedas_{by far the}best ofthe four.

TheNo.3_{Manitoba,however, had}amuchlower

_initial

_viscosityanda

much greater fall _{in this property} with time. _{This low viscosity} was

accompaniedbyalow_modulus,sothattheinitialvalueof theratioofthese two was _{high,although}it fellconsiderablywithtime. This flourwas not

up to standard forits_{grade, andalthough}it produced bread of excellent volume (low modulus) the best loafwas “thrown” _{relatively early, and}

thelaterloaveshad_{much poorer“crumbs,”owing}to thefall_{in viscosity.} The Plate flourhad a _{much higher viscosity than the No.}3 Manitoba throughout theseven _hours, butthiswas _{accompaniedby} a rather high

modulus. The ratio was lowerthanthat for the No. 3 Manitoba at the beginning offermentation, but decreasedvery little, so that at the later times the two flours approached one anotherin this _respect. ThePlate

the best bread _{would probably} have had greater volume, it would have beenmade earlier, and mighthavehada _{poorer crumb} structure.

TheAustralian flourwithitslowinitial viscosity, lowviscosity-modulus ratio, and considerable fall in both with time, would be _{expected to} be much the _poorest flour of the four, and this was found to be so in the

bakehouse.

So far we have _only dealt with_doughs containing yeast. This ingre-dient is necessary to produce the gas which builds up dough structure, buthas_{it any other function inbread-making?}

Testshavebeen _{carried out in which the physicalproperties of doughs} withandwithout_{yeast have}been_compared. The general results ofthese tests showthat in small amounts such as are usedincommercial bread-making, the effect of the yeaston the_viscosityandmodulus of the dough is_{probably notgreat enough to} beof_{importance in}the bakehouse. _Very large amounts of yeast do affect viscosity, forexample, in one case 8_per

cent yeast loweredthe viscosity from3.6 X 106to 2.9 X 106. The effect

on themodulus and on the rate of change of either propertywith age of the doughwas, however, insignificant.

CONCLUSIONS

1. Viscosity and rigidity modulus appear to be of _{major importance.} The viscosity must be _{high enough to prevent} undesirable _flowing-out ofthe _dough, but on the otherhand themodulus must below, to allow bigelastic expansion under_{the relatively low} pressure ofgasinside a fer-menting dough. The relaxation time (which is the ratio of these two properties) is_perhaps themostimportantsingle criterion ofquality.

2. The water content ofa _{dough determines}the magnitude ofits

vis-cosity and modulus, and it is desirable thatvariations in water content should haveassmallan effecton these_propertiesas_possible,thus helping

towards making the flour more _{fool-proof in} the bakehouse. Whether water absorption is _important in other ways, apart from financial

con-siderationsto thebaker, isnot known.

3. The_degreeto which viscosityand moduluschange duringthe aging of the dough is of _{the utmost importance.} The fall _{in viscosity} with time is _probably a _{major factor in determining the fermentation} toler-ance of a _flour, and not only is the absolute rate offall of this property important,butequallysoisits relativerate comparedwiththerateoffall of modulus,sincethisdeterminesthefallin relaxation time.

4. Tensile_strengthisa_{major factor in determining}the_{extensibility}and

5. Stickinessis_{important in affecting}the _{dough’s handling properties;} the dough mustnot be_{too sticky to work}over that_rangeof moisturebest suited to its other properties, nor should excessive stickiness develop during fermentation.

6. It is realized that such properties as _{work-hardening, structural}

viscosity, elastic hysteresis, and elastic after-effect must playtheirparts in determining the behavior of dough in _{the bakehouse,} but although their significance is _{not yet fully understood,} it iscertainthat they are

only of secondary importance in determining the relative values of dif-ferent flour samplesexcept perhaps in certain abnormal cases.

7. It is considered that measurements of the physical properties of doughs,as faras_possibleinabsoluteunitsandunder standardreproducible conditions, should lead to a far better appreciation of flour _qualitythan

any number of empirical tests, and should afford a sound basis for the control ofqualityin flour.

SUMMARY

Methods described in earlier papers for measuring the viscosity and rigiditymodulus of flourdoughs have been extended anddeveloped.

The physical properties of dough are _markedly affected _by excessive handling, either during thepreparation of the dough itselfor _{during the}

preparation of the test piece. The methods used have therefore to be carefully controlled.

Viscosity and modulus measured under standard conditions of stress andstrainbothdecreasewithincreasingwater content or with_increasing age ofthe dough.

Goodbread-makingqualityisassociatedwitha _{relatively high viscosity}

andlow modulus; the relaxation time,i.e., viscosity-modulusratio, there-fore appearstobethechief singlecriterionof_quality.

Yeast in small amounts has little effect on _viscosity or _{modulus, and}

its importance in bread-making appears to be _entirely due to its gas-producing activities.

Tensile _strength is a _{major factor in determining}the extensibility and gas-holdingproperties ofa_dough,butworkon this propertyisstillattoo earlya _stage to bediscussed.

Stickiness is an _{independent property which}can be _roughlymeasured. Its principal importance lies in its effect on the _{handling properties} of the _dough.

Our best thanksare due to Dr. _{E. A. Fisher,} Directorof the Research Association of British _{Flour Millers,} and to Dr. R. K. Schofield of the PhysicsDepartment, Rothamsted Experimental Station, for helpful sug-gestionsduringthe progress ofthis work.

REFERENCES

(1) Ballu,T.: Ann. agron.4, 373 (1934).

(2) Bouyoucos, G. J.: SoilSci.34, 393 (1932).

(3) Kachinski,N.A.: Studieson the Physical PropertiesofSoilandon the

Root-systems of Plants (in Russian), published by Selkolkhozghiz, Moscow (1931); also Proc. 2ndIntern. Congr. SoilSci. Comm._I,p.153,Leningrad

(1930).

(4) KosutAny, T.: Der ungarischeWeizenunddasungarische Mehl. Verlag Mol-narok Lapja, Budapesth (1907).

(5) Okhotin,V. V., and _Smirnof, O. F.: Pedology U. S. S.R. 2,237 (1934).

(6) Ostwald, W.:Kolloid-Z. 36, 99 (1925), andmany other papers. (7) Pankof,A. M.: Pedology U. S. S.R. 1, 80 (1934).

(8) Schofield,R.K., and Scott Blair, G.W.: PaperI, Proc._Roy. Soc.London

138A,707 _(1932);PaperII, ibid. 139A,557 _(1933);Paper

III,

ibid. 141A,

72(1933). SeealsoSchofield and Scott Blair: Mühlenlab.4,41_(1934).

(9) Stock: Z. angew. Chem. 39, 461 (1926), and others.