13 Fundamentals of Rheology and Spectrometry · F udamentals of Rheology and Spectrometry 119 If we...

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13.1 Rheology

D. Weipert

13.1.1 Introduction

Bread in all its diversity has existed for over6,000 years; rheology as a branch of physics isa great deal younger, though scarcely lessdiverse. What, then, is the relationshipbetween bread and the rheology of dough?Descriptions of the first attempts to measurethe physical properties of food in general andof bread doughs in particular date back to the18th century, when Beccari assessed the qualityand structure of wheat doughs sensorily in1728 and Bolland and Kunis later carried outtests with the Aleurometer in 1836 and 1885respectively. Dough rheology as we know ittoday did not originate until the early 20th

century and was born of necessity. Hungarywas considered the granary of the Austro-Hungarian Empire and exported wheat to therest of Europe. The Hungarian wheat varietieswere popular in North America too – somewere in fact related to the American varieties,having common parents. Hungarian wheatwas much in demand for its quality. So thebreeders made every effort to cultivate andgrow higher-yielding varieties. But the qualityof these new wheat varieties no longer met therequirements of the market. In order to assessthe baking properties of wheat varieties orwheat lots without performing expensive andtime-consuming baking trials, the first recor-ding mixers (Hankoczy) and dough stretchinginstruments (Hankoczy, Rejtö, Gruzl) wereconstructed in the early 20th century; they maybe regarded as the forerunners of theSwanson Working Mixograph in America andalso the Brabender Farinograph andExtensograph and the Chopin Alveograph inEurope. After World War II the situationchanged in that in Germany the BrabenderFarinograph and Extensograph were used todetermine the suitability of the quality wheatlots imported from North America (USA and

Canada) and South America (Argentina). TheAlveograph was used from the start to compareand check the quality of flour batches. Aninteresting description of this development isgiven by Muller and Wassermann (Muller, 1964and 1966; Wassermann, 1993).

The last four measuring instruments – theFarinograph, Mixograph, Extensograph andAlveograph – are used virtually unchanged tothis day in the service of the science and prac-tical task of cereal processing. However, theyuse relatively strong deformation forces as ameasuring principle, and they only describethe properties of the dough in the cold phaseof the bread-making process, during mixingand after fermentation. These two points maybe regarded as disadvantages. The pastingproperties of the cereal starch as a function ofalpha-amylase activity at high temperaturessimilar to those of an oven have been determinedwith the Brabender Amylograph and theHagberg-Perten Falling Number instrumentsince the 1920s and 1960s respectively, andmore recently with the Newport ScientificRapid Visco Analyzer. For technical reasonsthese measurements can only be made inflour/water slurries of various concentrations.

In the early 1930 s, Scott Blair laid thefoundations of fundamental rheometry andrheology of food by measuring and describingthe viscoelastic properties of wheat dough(Schofield and Blair, 1932). During the 1970sand 1980s, fundamental rheology experienceda major upswing with the construction of new,precise and sensitive instruments whosemeasurements permitted an insight into thestructures and behaviour of foods. These newrheometers apply the dynamic, oscillatingmode as a measuring principle, with whichthey can simultaneously record complexviscosity and its components elasticity andplasticity ("pure viscosity"). Moreover, thedeformation forces they apply are extremelysmall; since these do not interfere with the

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13 Fundamentals of Rheology and Spectrometry

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structure of the specimens, they permitcontinuous monitoring of changes in theviscoelastic properties of a dough as a function oftime and temperature. This is really the basis ofthe simulating "recording baking trial" whichmakes it possible to monitor the changes in theviscoelastic properties of the dough in thebaker's oven (Weipert, 1987 and 1992).Bread baking is ultimately a process of theuptake and binding of water by the swellingsubstances (such as proteins and pentosans)in the dough and the rebinding of water by thepasted starch in the heated, baked dough orthe crumb of the bread which results inchanges in the viscosity or consistency of the doughor bread crumb and can therefore be demon-strated well by rheometry. In other words:rheology in general and rheometry in particularare good tools for studying, interpreting,predicting and checking baking properties.

13.1.2 Viscosity and Elasticity of Dough

When mixed with water the flour becomesdough, a cohesive mass in which the glutenforms a three-dimensional network made upof strands and membranes in which the starchgranules are embedded (Amend, 1996 andBloksma, 1990). The viscosity or consistencyof the dough depends on the amount of waterand other ingredients added, but also on theintensity of mixing. The expansion and volumeof the baked products as a quality attribute ofthe flour is the result of the production andretention of gas. In this context the viscosity orconsistency of the dough is initially the maincharacteristic that determines the gas reten-tion necessary for making up a flour into breadand other products. The baker will strive toachieve an optimum consistency which is thickenough to make the dough workable (kneading,moulding) and ensure that it keeps its shapeand on the other hand thin enough to allowthe carbon dioxide generated by the yeast tocause the expansion that results in the desiredleavening of the dough and its baked volume.It was established only recently that a doughcan come about as a "hydrated, unmixed floursystem" through aggregation of the glutenproteins without an input of energy frommixing. Its physical, rheological properties are

somewhat different from those of doughsproduced by mixing; it is firmer and lessextensible, has a high initial viscosity andelasticity but low stability (Unbehend, 2002).Nevertheless, mixing is likely to remainindispensable, for the air bubbles introducedinto the dough by mixing and in which the carbondioxide collects during fermentation are the"starting point" for the pores that result in thebaked volume of the products (Hoseney, 1986).

Doughs without Mixing

Efforts to minimize the energy input necessaryfor making up a dough have led to the devel-opment of the new Rapidojet technique. Onthe basis of observations by Amend (1996),Noll (2002) came to the same conclusion asUnbehend (2002), namely that far less energyis needed for preparing dough than is normallyused in bakeries. With the Rapidojet, Nolldeveloped a fast method of dough preparationthat saves space, time and above all energy.Air and water are introduced at high pressureinto a stream of flour running into a pipe.Within seconds this results in a dough that has

13.1 Rheology

Fig. 53: Baked volume as a function of viscosity and viscoelastic dough properties

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Baked volume = f (gas production + gas retention)Gas production = constant (controllable)Gas retention = f (dough viscosity)Baked volume = f (viscosity + elasticity)Viscosity = constant (controllable)Baked volume = f (elasticity)

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If we view the bread-making process as aninteraction of gas production and gas retentionit may be said that gas production can beadjusted, controlled and kept constant withthe amount of yeast in the formulation, thequantity of the fermentable sugars maltoseand glucose added or present and other technicalmeasures such as fermentation temperatureand time. There remains gas retention as afactor that demands the baker's attention andtechnical skill (Fig. 53). If we assume that gasretention depends directly on the consistencyof the dough, we may expect gas retention toincrease in proportion to consistency. Thisholds true in practice: firm doughs with a highgas retention capacity combined with goodgas production result in a high volume yield.Conversely it is logical that the low gas retentioncapacity of doughs with low viscosity(consistency) results in a low volume yield.Because of their gluten structure the soft andsometimes weak doughs are permeable togas. It is also logical that very firm, short or"bucky" doughs are too strong to be stretchedby the developing carbon dioxide because oftheir firmness and stability. The result is a lowvolume yield. This means that an optimum viscosity orconsistency of the dough is desirable in order

to ensure the highest possible volume yield.Since this factor can be adjusted and controlledby determining water absorption with theFarinograph, it may also be regarded asconstant. The viscoelastic properties of thedough, generally known by bakers and cerealprocessors as "dough characteristics", varyfrom soft and weak to firm and short; it is the"normal" dough characteristics that bring thebest results in each case. The volume of thebaked products is therefore a function ofviscosity and elasticity. If the viscosity can beadjusted (by adding water) and may thereforebe regarded as a constant factor, it is ultimatelyelasticity or the rheological balance betweenextensibility and elasticity that determines thevalue of a wheat flour for baking. Too muchelasticity results in short, bucky doughs; toolittle makes the doughs soft and weak. Doughrheology makes it possible to identify thesevariety-related properties – viscosity andelasticity – quickly and reliably. We also knowways and means by which the viscoelasticproperties can be altered and optimized to acertain extent; here, especially, rheometryhelps with dosing and control. The objective and purpose of rheology is toidentify the basic rheological properties ofsubstances and interpret the changes in theseunder defined measurement conditions. Basic rheological properties are: • strength (solidity),• viscosity,• elasticity, and• plasticity.

13.1 Rheology

Rheology Rheometry

viscoelastic properties similar to those ofdoughs produced by mixing and is just as easy tomake up into bread. The energy introduced by thepressure of the added water is much less than thatnormally required by bakeries (Noll, 2002).

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To establish these properties, rheometry – asub-discipline of rheology – uses a deforma-tion force and measures the effect of this forceon the specimen (in this case the dough) as itsdeformation. The deformation force may begreat or small; the measurement will varyaccordingly. Strength as a further property of a material iseasy to determine. A body, as rheologists callthe substance to be tested, is a viscous massor a fluid if it has no yield point, flows by itsown gravity and is therefore not dimensionallystable. By contrast a solid (solid body) keepsits shape, can only be made to flow by theeffect of a deformation force and has a yieldpoint. A solid can also be a plastic, elastic orviscoelastic body, depending on its structure.Viscosity is an important characteristic of anymaterial; it is made up of the componentselasticity and "pure viscosity" or plasticity. Inthe case of liquids, viscosity may be describedas the internal friction between the moleculesand molecular aggregates; in the case ofsolids it is the cohesion resulting from theirstructure. When determining basic propertiesit is possible to measure viscosity (more

precisely complex viscosity) and its componentelasticity, whereas "plasticity" has to becalculated as an imaginary part of viscosityand the difference between the measuredviscosity and elasticity. A body is termedelastic if it is difficult to deform and regains itsoriginal shape when the deformation force hasceased to act on it. Deformation was thenreversible and the deformation energy appliedwas stored. If, on the other hand, a body iseasy to deform and remains deformed after thedeformation force has ceased to act, the body isirreversibly plastic and the deformation energyhas been lost. The directly measurable and determinablebasic properties "viscosity" and "elasticity"would therefore seem to be the mostimportant characteristics for describing amaterial and for its behaviour as a raw material,in the process itself and finally as an endproduct. That is why dough rheology givesspecial attention to these two properties. Thematerials known to us, including foods, havemainly viscoelastic properties, and thecharacteristics elasticity and plasticity occur indifferent ratios to each other.

13.1 Rheology

Viscosity

Plasticity Viscoelasticity

Elasticity


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13.1.3 Two Kinds of Rheometry

Our modern way of life is unimaginablewithout rheometry. Rheometry is used topredict the avalanche menace in ski-fields, tominimize the risk of a heart attack bymeasuring and influencing the flow propertiesof blood, to estimate the weight-bearingcapacity of shopping bags made frompetropolymers or bio-polymers, and even toincrease the service speed of tennis cracks bydeveloping the right strings for their rackets.Rheology has a multitude of uses and is allaround us. In food production it is used toassess the quality of raw materials and endproducts, and it has therefore become animportant and powerful aid to food technolo-gists. It also aims to determine the texture offoods and thus replace sensory testing in thejudgement of quality. But the latter is hardlylikely to succeed, partly because there are nosuitable measuring techniques so far, andpartly because experienced technologists oflong standing are unlikely to allow themselvesto be ousted or challenged by measuringinstruments.

Rheology has a sub-discipline, rheometry,whose task is to make and explain measure-ments. We speak of empirical (also known asdescriptive or imitative) rheometry and funda-mental (absolute) rheometry, depending onthe measuring principle and the possibilitiesoffered by the instruments used. Empiricalrheometry may also be termed conventionalrheometry. From the point of view of rheolo-gists and practical users, both groups ofinstruments and measuring techniques haveadvantages and disadvantages, most of whichresult from the design of the instruments andthe measuring principle. The users and advo-cates of the two different rheometries regardeach other with suspicion. One rheologist hasdescribed this situation aptly:

• With the empirical methods we don't knowwhat we are measuring, but it works;

• with the fundamental methods we know exactly what we are measuring, but itdoesn't work.

"It works" means that the measurements

reflect the behaviour of a raw material duringprocessing and also the quality of the end pro-duct. How the measurements are interpreteddoubtless depends on the experience of thepeople who carry them out. In practice themeasurements are often performed underconditions different from those of the process.Most of the instruments used in empiricalrheometry are relics of the early days of doughrheology and have scarcely been modified tothis day. They are very common, easy to use,and their results have found a permanent placein the terminology of cereal technologists.

The comment "but it doesn't work", said offundamental rheometry years ago, has nowceased to apply. In the final decades of the lastcentury, rheology in general experienced aconsiderable upswing with the developmentof new, versatile, sensitive, precise andefficient measuring instruments. Theseinstruments were used chiefly in the plasticsindustry to demonstrate the structures of"noble" (and therefore expensive) thermo-plastic polymer melts. Systematic work withmeasuring instruments of this kind yieldedstructural models for wheat and rye doughstoo, and the biopolymer "dough" was foundto conform to the same laws as the organicpolymer melts. Moreover, it also undergoesphase transitions as it passes through thevarious temperature ranges. This discovery isnot only of academic interest; it has also hadpractical value since the technique of freezingdough and heating pre-baked frozen doughportions became common practice at bakeries.But in spite of all the advantages of rheometersand fundamental rheometry, the high price ofsuch measuring instruments and the moreintricate work they require explain why they arenot yet in general use in the laboratories of cerealprocessors. It is a sad fact that bread rolls areless lucrative than plastic polymer melts!

Being a branch of physics, rheology cannotquite do without mathematics; in some areas,in fact, it uses a great deal of mathematics andcalculation. But that should not frighten usand prevent us from using rheometry.Rheology can manage without higher

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mathematics, especially when the resultsare considered in categories of "too much"or "too little" in relation to the desired optimumor used for drawing curves or diagrams.In the meantime simpler measuringinstruments suitable for the routine work ofcereal laboratories have come onto themarket. Some combine the advantages of thetwo rheometries, overcome the old, strictdivisions between empirical and fundamentalrheometry, and are affordable into thebargain. So in future we may expect them to findtheir way into the research and developmentlaboratories more often than in the past.

The use of rheometry in dough rheology hasalready been described in detail in therelevant standard methods (e.g. ICC, AACC,AOAC) and in textbooks on cereal technology.In our discussion of dough rheology anddough rheometry, their application in labora-tories and interpretation of the results we donot intend to repeat this information. Our aimis to give "the practical man" an insight intowhat is going on backstage in nature and inthe measuring instruments and dispel his fearof advanced science in an ivory tower.

13.1.4 Empirical Rheometry

Sensory Testing

There can be no doubt that sensory testing toassess the characteristics of dough and thecrumb and crust of bread is one of the classicand oldest uses of empirical rheometry.The baker uses his hand and his acquiredexperience to judge the viscosity and elasticproperties of his intermediate product (thedough) and his end product (the crumb of thebread). A baker's tester (a specialist trained insensory testing) works almost like an instrument;both the production manager and the customerrely on his "measurements". A tester assessesthe nature of the dough according to itscomponents: "elasticity" (from unsatisfactoryand soft to short) and "surface" (from slimy andwet to dry). He assesses the crumb accordingto elasticity and chewability; his verdict canrange from "soft" to "firm". A baker's languageis more flowery and describes the viscoelasticproperties of the dough more aptly. He woulddistinguish more precisely between "soft"and "weak" on the one hand and "firm" onthe other by describing the dough as "sticky","pliant" or "pliable", "silky" or even "bucky" (ifit "springs back").

13.1 Rheology

These ratings describe the desirable, positivecharacteristics of the dough and its undesirable,negative characteristics. In their flowery termsthey also describe the shred of an RMT 17 roll.The shred of an RMT roll is an importantindicator of the viscoelastic properties of the

dough, and vice versa. The shape of the shredand the appearance of the roll can be predictedfrom the known properties of the dough. Adough portion shaped by flattening and rollingin the RMT standard baking test should openup in the oven to form a "normal" shred.

Semantic description of dough properties

STICKY PLIANT SILKY

17 Rapid Mix Test; chapter 12.9.3 page 115

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In the case of doughs with short, dry propertiesthe shred tends to be wide open; besidesgenetic and environmental factors the causesof this may be unsuitable drying and heatdamage to wheat that has been harvestedwet. A "sewn up" roll with a shred that is stucktogether and unopened is the result of a wetdough surface. An unopened shred in a small,irregularly shaped roll in which the baker'sfingers have left clearly visible marks mayindicate weak flour quality. In conjunction witha large baked volume the unopened shredshows that the flour has quality reserves; suchflour can be used for blending with and impro-ving flours of weak quality. The conclusionsdrawn from the quality characteristic "shred"in the RMT standard baking test can also beapplied to loaves of bread and other bakedproducts.

The ratings acquired by sensory testing areunderpinned by measurements carried out ondoughs with the Farinograph and theExtensograph. These instruments of empiricalrheometry are used in large mills and industrialbakeries. To a certain extent a baker can correct hisdough, made with a small amount of flour, inthe course of preparation. But an industrialbakery must have formulations it can rely on;they must ensure that the products turn outproperly, since no corrections can be made toa large batch of dough. For large-scale baking,objective measurements of the physical andrheological properties of the dough areessential. Results obtained by empiricalrheometry are used effectively for thispurpose too.

Instruments of Empirical Rheology

As we know, the process of bread makingconsists of two phases: a cold phase in whichthe dough is prepared by mixing and left toferment, and a hot phase in which the dough istransformed into bread in the oven.Monitoring the process means showing andchecking the viscosity and viscoelasticproperties in both phases. For technicalreasons empirical rheometry can only carryout the measurements separately.

In the Cold Phase of Bread Making

Bread baking starts with mixing flour andwater to form a dough, followed by fermentation("rising") in a fermentation chamber atcontrolled, slightly elevated temperaturessimilar to those used in practice. Mixing andkneading is simulated with recording mixersunder laboratory conditions; the condition ofthe dough during and after fermentation isshown and described by means of stretchingtests. The experience of the baker (laboratoryworker, production manager, shift supervisor)enables him to read the measurements andcurves thus obtained in order to determine theoptimum flour for a particular product andadjust the recipe accordingly.

Recording Mixers

Generally speaking, modern laboratories usetwo types of recording mixer: the BrabenderFarinograph and the Swanson WorkingMixograph. These two mixers differ funda-mentally in the way they mix and thus themechanical stress to which the dough isexposed, i.e. in the ratio of flour to water andthe amount of water added at the start ofmixing. The sigma-shaped paddles of theFarinograph squeeze and stress the doughrelatively little compared to other types ofmixer (Weipert, 1987b). The amount of waterthat has to be added to achieve a constantconsistency of the dough is determined in apreliminary test before the main test. Theworking parts of the Mixograph are verticalpins that exercise a planetary, rotatingmotion and stretch, squeeze and fold thedough; mixing of this kind subjects thedough to greater mechanical stress than thatof the Farinograh mixer. The doughs are pre-pared with the same amount of added waterirrespective of the water absorption capacityof the flours. This means that evaluation andinterpretation of the curves resulting fromthe measurements differs. For the sake ofcompleteness it should be said at this pointthat the one-arm Alveograph mixer stressesthe dough less than the Farinograph. Withtheir sharp-edged tools, which result in veryintensive mechanical stress, the mixers of theBrabender Do-Corder and the Resistograph

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damage the dough to the point where itcompletely loses its structure ("fatiguepoint"). It is deformed until it becomes liquidand is literally destroyed.

Farinograph

The recording, reading and analysis of aFarinogram, the curve of measurementsobtained with a Farinograph, is described bythe recommendations of the manufacturerBrabender, the Farinograph manual(D'Appolonia and Kunerth, 1984) – a study of theuse of the Farinograph – and finally stipulatedby the standard methods (ICC; AOAC).

In the bowl of a Farinograph the flour ismixed with the water to form a dough; thedough is then developed mechanically andweakened mechanically by over-mixing untilits structure is destroyed. This procedure ismeasured and recorded as kneading resistancein the form of torque by means of a dynamo-meter; the recorded curve is therefore aforce/time diagram from which the work orenergy input can be read off and calculated.The kneading resistance is assumed to bethe viscosity of the dough, although theremaining properties of the dough such asits surface stickiness and adherence to thewalls of the mixer and the paddles contributequite considerably to the measured kneadingresistance. In such tests this was most apparentwith the wheat varieties that produced

doughs with a very sticky surface; the waterabsorption capacity of these flours, whichwas high already, was increased even further,which made the dough softer and more stickystill. In cereal laboratories the viscosity ofdough is often termed consistency. Theviscosity or consistency of the dough isstated in the Farinogram in relative units(FU) specific to the Farinograph, on a scalefrom 0 to 1,000 FU.

In practical baking, determination of viscosityin the Farinograph serves chiefly to establishthe water absorption of a flour. This is the termfor the amount of water that has to be addedto a flour to achieve a viscosity of 500 FU. Thewater absorption of a flour depends on thelatter's water-binding capacity and thusdetermines the yield of the dough andthe amount of water to be added in thepreparation of the dough. Besides theswelling substances in the wheat (proteinsand pentosans), the mechanically damagedstarch granules also contribute to thewater-binding capacity of a flour. The doughconsistency of 500 FU is an empirical value feltto guarantee the best possible processingproperties; it has been adopted in the RMTstandard baking test for determining theamount of water to be added. Different doughconsistencies have proved most suitable forsome other types of baked products thatrequire doughs of a soft or firm consistency.

13.1 Rheology

Fig. 54: Baked volume as a function of dough viscosity and water addition

(the arrows indicate optimum water absorption as determined with the Farinograph).

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The importance and benefit of determiningwater absorption in practical bread baking canbe demonstrated by tests in which the amountof water added is increased or reduced by 5%or 10% as compared to the water absorptiondetermined at 500 FU for four flours withdifferent baking properties (Fig. 54). With allfour flours a reduction of the amount of wateradded caused a noticeable thickening of theconsistency of the dough in the Farinographand resulted in a considerable fall in thevolume yield of the baked product (in this casethe bread rolls in the RMT standard bakingtest). As expected, the addition of more waterresulted in a softer dough consistency, but theeffect was initially a slight rise in the volumeyield at a 5% increase in water absorptionfollowed by a slight fall in the volume yieldat 10% additional water. The flours showeddifferences in water absorption according totheir quality, and the degree of their reactionsto the different amounts of added water variedalso. The fact that good wheat flours respon-ded with increased baked volume to a higherdough yield or a softer dough is a sign thatthey have quality reserves. It also explainswhy some bread formulations require a largeramount of water, which would result in aFarinogram of 450 or even 400 FU. In terms of

volume yield, one and the same amount ofwater led to different results in the productsbaked with the four flours; this again confirmsthe proposition that the viscoelastic propertiesof the dough are more important than itsconsistency.

A method has recently been developed whichalso makes it possible to determine the waterabsorption of rye flours (Brümmer, 1987).Since rye doughs react differently to mixingand rye flours result in a higher dough yieldthan wheat, the water absorption of rye flours ortheir optimum dough yield is read as viscosityafter a mixing time of 10 min.

An analysis of a Farinogram shows thedevelopment time of the dough (up toreaching the 500 FU line), stability (unchangedstructure of the dough without a fall inviscosity) and softening (fall in viscosity) atthe end of the mixing time. Whereas thereadings on the Y axis of the Farinogram,expressed in Farinograph units, denoteviscosity and changes in viscosity during themixing process, the width of the Farinogramcurve is read as the elastic properties of thedough. This empirically based opinion of thecereal processors is correct with the reservation

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Fig. 55: Farinograms of weak and strong flour

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that the width of the curve can be adjustedon the Farinograph itself and thus influenced;it is not an absolute value comparable fromone instrument to another. The viscositycurve of the Farinogram gives information onthe structure of the dough, its tolerance tokneading or the required input of mechanicalenergy and permits conclusions as to theintensity of mixing that is tolerable ornecessary.

Wheat flours described by bakers as "weak"reach the 500 FU mark quickly and show nostability worth the name before undergoing aconsiderable decline in viscosity (Fig. 55).The "strong" flours take longer to developbefore reaching the 500 FU line, where theyremain for some time at good stability andthen show only a minor decline in viscosity.The width of the curve for the two flours dif-fers correspondingly. After reading off thedough development time and stability it ispossible to decide how much mechanical deve-

lopment and energy input is needed. Suchmeasurements support the theory of the spe-cific energy input requirement of flours,which makes it possible to produce good-quality bread from weak flour if the latter'smixing requirements are taken into account(Frazier et al., 1979).

There have always been "strong" flourswhose Farinograms show a second peakafter the dough development time; suchcases have recently become more common,especially with unblended flours from certainnewly bred wheat varieties. The standardmethod recommends reading this secondpeak as dough development time, but doesnot explain the reason for it. A glance at thestructure of wheat gluten shows that it consistsmainly of the fractions gliadin and glutenin(Hoseney, 1986). These two fractions differconsiderably in respect of their molecularstructure and functional properties (Fig. 56and Fig. 57).

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Gluten(Gliadin + Glutenin)

Gliadin Glutenin

Gluten(Gliadin + Glutenin)

Gliadin Glutenin

Fig. 57: Viscoelastic behaviour of rehydrated vital wheat gluten, gliadin and glutenin. (Photographs by Mühlenchemie GmbH & Co. KG. Commercial vital wheat gluten was suspended in 70% v/v ethanol and then centrifuged.The liquid phase was filtered through a filter paper. Solid and liquid phase were then dried at 40°C under vacuum, and rehydrated prior to the rheological demonstration.)

Fig. 56: Schematic representation and demonstration of the structure of gluten and its fractions, gliadin and glutenin

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Whereas the insoluble fraction glutenin isknown to form strand-like shapes called fibrilsthat give the gluten its firmness and elasticity,the gliadin fraction, which is soluble inalcohol, appears as a sticky mass and fillerbetween the fibrils and only contributes to theviscosity of the gluten. Consequently, theviscoelastic behaviour of the gluten and thedough is closely bound up with the ratio ofthese two components. A weak flour (a Cwheat18 variety or a wheat lot with weak gluten)in which the functional properties of thegliadin fraction prevail will bind water quicklybut in smaller amounts; it will form the doughfaster but show a rapid fall in viscosity. Astrong flour (an E or A variety or a wheat lot withstrong gluten) in which the functional propertiesof the glutenin fraction prevail is characterizedby a longer development time and longerstability. In a flour rich in protein or gluten thegliadin fraction present is initially responsiblefor the development of the dough (measuredby achievement of the dough viscosity of 500FU) together with part of the glutenin fraction,whereas a further part of the glutenin fractionrequires more mechanical energy input andproduces a second peak. This behaviour ofdoughs made from strong flour can be demon-strated by applying more mechanical energy(through faster mixing) in the Farinograph (Fig.55). In this case the gliadin component of thedough is "developed" first but is soon weakened,whereas the glutenin component requiresmore energy for development and resiststhe mechanical energy during mixing. Suchbehaviour, known as stiffening, has alreadybeen observed under the standard conditionsof the Farinograph method with some wheatvarieties of American parentage. But similarbehaviour is also found when flours of greatlydiffering quality are blended (as was observedyears ago with the weak Maris Huntsmanvariety and very strong Canadian wheat of theCWRS class). On the basis of their Farinogramssuch blends have been rated poor, althoughsuch a combination of flours with greatly differingproperties in the dough may result in a blendwith very positive effects, as Extensogramsshow. Here too, the reason for such behaviouris the nature of the gluten fractions gliadin and

glutenin, which depends on the variety, andtheir ratio in the gluten. And here too, thegliadin of the weak flour component results inearly dough development and the glutenin ofthe strong flour component leads to stiffening.Although the ratio of the two gluten fractionsis of genetic origin and thus a characteristic ofthe particular variety, it may be influenced bythe environment; besides climatic conditions,such influences are chiefly the result offertilizers. The properties of the gluten and thedough that are characteristic of the variety andmay be influenced by the environment canbe shown even more clearly with theExtensograph.

Besides water absorption, a Farinogram showsother quality characteristics of the dough suchas development time, stability and softening;each of these provides important informationin itself, but together they represent a multitudeof data. To simplify the measurements theValorigraph value was suggested at an earlystage; it integrates these Farinogram charac-teristics in a single number. Read from theFarinogram by means of a special template,this value may lie between the theoreticalfigures 0 (for extremely weak flours) and 100(for extremely strong flours). But these valuescan scarcely be achieved in practice; as a result,the method did not meet with acceptance inspite of some positive aspects. On the otherhand the suggestion of reading a quality number (QN) off the Farinogram as the time taken forthe viscosity (consistency) of the dough to fallby 30 FU after stability met with a positiveresponse and has been introduced into theICC standard method as one of the qualitycharacteristics. This value integrates thedevelopment time and stability of the doughand indicates its softening; determination ofthe QN permits a faster but no less reliableevaluation of the Farinograms. For various reasons the Mixograph has scarcelybeen used in Europe.A new measuring instrument with a number ofuses in the field of food rheology has recentlybeen introduced: the Rheotec Multigraph. Like

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the Farinograph, the instrument works on theprinciple of a recording mixer but with controlledheating of the dough. It records the changes inthe viscosity (consistency) of the dough in thecourse of mixing and heating which reflect theeffect of the proteins, starches and enzymes inthe flour on the binding of water and theviscous properties of the dough. It might besaid that such measurement is a kind of"recording baking test" (Sinaeve et al., 2001).The method is based on the tests for theeffect of additives and baking improvers ondough carried out by Nagao with a modifiedFarinograph (Tanaka et al., 1980).

Stretching Methods – Extensograph versusAlveograph

During fermentation, the dough undergoes aprocess of inflation in which the carbon dioxi-de enlarges the pores and gives the doughgreater volume. The gas retention capacity ofa dough is therefore considered a qualitycharacteristic and shown in the form of exten-sion curves. As a displacement/time functionthe stretching of the fermenting dough may beregarded as a slight deformation, but fortechnical reasons the stretching tests inlaboratories are carried out with greaterdeformation forces. For this reason such testsare rightly classified as empirical methods.

The principle of the stretching tests is that adough made according to the standardmethod and prepared for extension isstretched and an extension curve recordedfrom which characteristics such as viscositycan be read directly and viscoelasticityindirectly. At present two stretching methodsare in common use, carried out with funda-mentally different measuring instruments andprocedures. The methods were developed atthe same time but independently of eachother in regions with different wheat qualitiesand types of bread: the Chopin Alveograph inFrance and the Brabender Extensograph inGermany. Their predecessor was probably theAleurograph and Laboragraph after Muller(1964 and 1966). The extension curves in the Extensographmethod (Extensograms) and in the Alveograph

method (Alveograms) describe the extensionalwork (energy in the case of the Extensogramand W value in the case of the Alveogram)which is understood to be gas retentioncapacity (Faridi and Rasper, 1987, Rasper andPreston, 1991 and Weipert, 1993). In thefurther interpretation the height of the curve(R with the Extensogram and P value with theAlveogram) is understood as resistance toextension and the length of the curve read onthe X axis (E with the Extensogram and L withthe Alveogram) is taken to be extensibility. Ifresistance is now viewed in relation toextensibility, the quotients R/E = ratio and P/Ldescribe the viscoelastic properties of thedough.

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Fig. 58: Extensograms (135 min) of dough with different recipes and mixing processes (with and without salt, sugar, fat, malt flour, ascorbic acid; mixing in the Farinograph or Stephan mixer). ASC - ascorbic acidRMT - Rapid Mix TestFAR - Farinograph

ASC, RMT kneadingRMT recipe, FAR kneadingASC, FAR kneadingRMT without yeast ASCBlank

Malt flourShortening, 3 gSugar, 3 gReferenceSalt, 4.5 g

The question as to the usefulness of Extensogramsand the reliability of the information they yieldas a means of describing the visco-elasticproperties of doughs has been answered bymaking Extensograms of unblended flourswith different dough properties in various

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GRANIT (soft)Protein % WA,% DP VYlow 11.0 54.5 6 542high 16.3 60.0 2 561

W P/L G E R/E97 0.50 21 35 0.658 0.42 19 79 4.3

DIPLOMAT (normal, silky)Protein % WA,% DP VYlow 11.5 51.8 16 598high 18.0 59.6 9 863

W P/L G E R/E245 0.56 25 134 2.2158 0.56 22 130 7.1

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different formulations and using differentmethods of preparation (Bolling and Weipert,1984). The Extensograms reacted verysensitively to the changes in the formulation,the individual ingredients added to the flour inthe RMT standard baking test (salt, ascorbicacid, fat, malt flour, sugar) having a character-istic effect on the curve of the Extensograms(Fig. 58). Even the ascorbic acid alone had avery strong effect. The interaction of all theingredients in the RMT formulation with theflour showed itself in the Extensogram withthe largest area; preparation of the dough inthe Farinograph or in the Stephan mixer duringthe RMT standard baking test made noappreciable difference to the curves of theExtensograms. The most important result wasthat the Extensogram made with salt andascorbic acid according to the standardmethod was found to be practically identicalto the Extensogram of the RMT dough(complete formulation but without yeast). Thisconfirms and justifies the Extensogrammethod as a practical and informativeprocedure.

Extensograms are indeed capable of expressingthe quality of a flour and its suitability formaking different bakery items. Using floursfrom three different wheat varieties withextremely different dough properties (short,normal, soft), each with two different proteinlevels (low and high), it was possible todemonstrate that Extensograms show boththe variety-related quality of the wheat floursand the influence of nitrogen fertilizers (Fig.59). The Extensograms differentiated clearlybetween the flours at both protein levels. Thiscould be seen both from the energy values(area below the curve) and from the ratios(R/E). At a low protein content the Extenso-grams of all three varieties showed higherresistance and lower extensibility, thusindicating flours with shorter dough properties.This was especially evident in the variety withgenetically short dough properties. At highprotein levels, all three varieties producedExtensograms with lower resistance buthigher extensibility, indicating softer doughproperties; again this was especially evident

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Fig. 59: Alveograms and Extensograms of wheat flour with different dough properties and protein content. WA = Water Absorption by Farinograph DP = Dough Property Index (1 = soft, slack, sticky,

25 = short and dry; about 9 is normal and desirable)

VY = Volume Yield in RMT baking trial mL/100 g flour

CARIBO (short)Protein % WA,% DP VYlow 10.3 50.9 25 495high 15.1 56.1 9 703

W P/L G E R/E177 0.55 18 118 3.5106 0.56 20 94 8.3

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in the variety with genetically short doughproperties. In the variety with the "normal"dough properties an increase in the proteincontent of the flour resulted in slightly reducedresistance and increased extensibility, but inboth cases the Extensogram data – includingenergy and the ratio – indicated good qualitywhich was enhanced further by the proteinincrease. The energy values and ratios of allthe Extensograms were in line with the bakedvolumes achieved with these flours. The lowenergy values in conjunction with low ratios(0.6) that indicated soft and weak doughs andthe high ratios (7 and above) in conjunctionwith low energy values that stand for shortdoughs were confirmed by low baked volumes.High energy values and ratios in the optimumrange (about 1.5 to 3.0) in the Extensogramsindicated a flour of good quality and highbaked volumes (Weipert, 1981, 1992 and1993).

The Alveograms recorded at the same timeand with the same flours did not distinguishso clearly between the various flour qualities.Although some differences were found in theW values, the P/L ratio was virtually identicalin all the Alveograms (0.42 - 0.56); this madeit impossible to read off differences in thedough properties. A recommended procedurefor determining the elastic properties of adough directly with the Alveograph is to carryout a second test, a pressure relaxation test,in which the air pressure suddenly stops afterthe formation of the dough bubble and therelaxation of the dough is read off from theresulting curve (Faridi and Rasper, 1987). Thismeasurement procedure was developed onthe lines of the creep recovery or stress relaxationgraphs used in fundamental rheometry andrecorded with a rotating viscometer or rheo-meter (Fig. 63). However, this measurementmethod has not established itself in practicaltesting with the Alveograph.

The reasons why the extension curves of theExtensogram and the Alveogram yield differentinformation lie in the way the tests are carriedout. The most important, most fundamentaland decisive difference between the two ICC

standard methods is already to be found in thepreparation of the dough. The Alveographmethod uses a constant amount of water,which naturally results in doughs of differentconsistency; the Extensograph method assumesthat the doughs are of constant consistencyfollowing determination of optimum waterabsorption in the Farinograph. If the twomethods are assumed to describe the rheologi-cal properties of the dough for processing inthe bakehouse, the Alveograph methodrecords a condition of the dough that is farremoved from its actual rheological conditionat the time of processing into bread or otherproducts because of the addition of a constantamount of water irrespective of the quality ofthe flour; this amount is in any case far toosmall for bakers' doughs. The constantamount of water added to the Alveographdoughs corresponds to a water absorption of50% for all flours irrespective of their quality,whereas today's wheat flours have a waterabsorption capacity between 54% and over60% and are processed into bread at thesewater absorptions, or at the correspondingdough yields. We should not forget that wateris a "plasticizer" that makes the dough softerbut optimizes its consistency if properly dosedand ensures good baked results when combinedwith flour improvers or other ingredients. Withthe addition of 50% or 58% water, for example,depending on its water absorption, one andthe same flour yields dough with greatly differingrheological properties, viscosity and elasticity(Fig. 54).

The other difference in dough preparationbetween the two methods (nature and durationof mixing) is not so fundamentally important.In the Alveograph the measurement itself isperformed by bi-axial stretching, carried outby inflating a piece of dough into a bubblewith an air pump until it bursts. In theExtensograph it is done by uni-axial, linearstretching of a strip of dough with a hook untilit breaks. The speed of deformation is similarfor the two methods; in the Extensograph it is1.5 cm/s. But although the resulting measure-ments, the recorded curves, are supposed toprovide the same information, they have come

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about differently. The Alveogram shows thepressure curve of the air trapped in the doughbubble, whereas the Extensogram is a deform-ation curve from which the resistance toextension (a measure of strength or even theelastic component) and extensibility (as thecompliant, plastic component of the doughproperties) can be read off. The maximumpressure in the Alveogram, the P value, thatdenotes strength, actually shows the yieldpoint of the dough, i.e. the force that has to beexerted in order to start stretching the glutenfibrils in a dough. This P value serves toestimate the dough yield or the amount ofwater to be added. But a pressure curve isvery different from a deformation curve. Adeformation curve can be obtained by recordingthe increase in volume of the expandingdough bubble in a vertical direction (Fig. 62).

A further difference between the two methodswhich is often neglected lies in the time factor,or the duration of the test. An Alveogram isrecorded 28 minutes after the start of mixing;for technical reasons only one measurementcan be performed on each dough specimen(Faridi and Rasper, 1987). An Extensographtest usually consists of three Extensogramsmade at intervals of 45 minutes during thedough resting time. This time factor is importantfor two reasons and must not be ignored.Kneading and moulding for the test cause a"structural activation" of the dough duringwhich the mechanical energy of the mixingand moulding is "stored" in the elasticcomponent and greatly influences the result ofthe measurement (Rasper and Preston, 1991and Weipert, 1981). In this state, resistance toextension is higher and extensibility lower. Thestored energy subsides after about 45 - 60minutes; the taut "springs" of elasticity relaxduring this time and the dough undergoes astructural relaxation or structural recovery sothat its "real, uninfluenced" rheologicalproperties can be measured. The stretchingof a dough resulting from inflation and anincrease in volume during fermentation and inthe early part of the oven phase take place ina relaxed state. Moreover, the effect of theascorbic acid, enzymes and emulsifiers added

as flour improvers or baking ingredients cannaturally be identified better after a longertime of action than after a short one. This effectis therefore only visible to a certain extent inAlveograms (Weipert, 1981 and 1992).

When evaluating the extension curves ofAlveograms and Extensograms it is necessaryto take all these factors into consideration.Only then can the right conclusions be drawnconcerning the properties of the flours andtheir suitability for certain baking purposes.Besides determining the viscosity of a doughit is also extremely important to establish itsviscoelastic properties. An Extensogram revealsboth the viscosity and the viscoelasticity of aflour as a genetic characteristic of the varietyand as the influence of the environment –chiefly the supply of nutrients and the use offertilizers (Fig. 59). It was evident that theExtensograms had clearly recognized andexpressed the dough properties of the wheatvarieties, described as short, normal or soft(Weipert, 1992 and 1993). This was especiallyapparent in flours with a low protein content.Protein levels in the flour that had been raisedby nitrogen fertilizers increased the extensibilityof the dough; the Extensograms of the wheatvariety with genetically short dough propertiestherefore showed normal dough propertieswith balanced viscoelasticity at higher proteinlevels. The variety with normal dough propertiesretained these properties even at a higherprotein level, but its energy value (area belowthe curve) was greater; the soft dough propertiesof the soft variety became softer still. Thesoftening of the dough properties, known bybakers as suppleness or pliancy, is explainedby the increase in the reserve protein componentof the gluten, the gliadin. Nitrogen fertilizationcauses more of this component than of theglutenin component to be formed and stored.But in a dry, warm climate, more glutenin isstored in the wheat grain, and this results inwheat with dry, short dough properties.Unlike glutenin, that determines the strengthand therefore the elastic behaviour of thegluten and the dough by forming strands andmembranes as well as binding large amountsof water, the gliadin component of the gluten

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only contributes to the viscosity (consistency,water binding capacity) of the gluten and thedough. Besides nitrogen fertilization, coolerand wetter environmental conditions favourthe formation of gliadin and result in softer,pliant doughs.

The functional properties and interaction ofthe two components, gliadin and glutenin,have been explained very clearly by Hoseney(1986; Fig. 56). As the photograph shows (Fig.57), the pure gliadin obtained by washing outand isolation is sticky and highly extensible; thepure glutenin is firm, elastic and difficult todeform. It is the ratio and functional propertiesof these two components of the gluten thatdetermine the latter's viscoelastic propertiesand thus the rheological properties of thedough. These properties can be deduced fromthe Farinogram, but they are more apparent inan extension curve like the Extensogram.

Without wishing to question the usefulness ofthe Alveograph method we have to admit, onthe basis of these examples, that the patternand individual characteristic data of theAlveograms do not reveal the dough propertiesof the varieties and the ways in which they arechanged by higher protein levels in the flour –i.e. their current quality. The reasons for thishave already been discussed. For the sake ofcompleteness we should mention that thenecessity of determining optimum waterabsorption has been recognized even by thesupporters of the Alveograph, and that amethod of determining water absorption withthe Alveograph mixer was recently presented(see chapter on Modern Cereal Analysis).Unfortunately it is still not possible to applythe water absorption determined in this wayas the amount of water needed to prepare thedough for the Alveogram recording and thus toindicate the rheological properties of thedough with dough consistencies close tothose used in practice. The biaxial stretchingtest is not fundamentally unsuitable as ameasurement method, as Dobraszcyk hasshown (Dobraszcyk, 2002). At the time of itsdevelopment and use in France theAlveograph method was a suitable means of

characterizing flour: the flours obtained fromwheat varieties with a soft grain structureand with a low protein content and waterabsorption could be described and comparedwell from one lot to the next by means ofAlveograms. But now that even in France thetrend in wheat breeding is towards varietieswith a hard grain structure (which may resultin mechanical damage to the starch grainsduring grinding) and flours with higher proteinlevels and thus greater water absorption, effortsare being made to adjust the Alveographmethod to the new wheat qualities.

The advantages of the Extensograph methodin showing the "rheological" behaviour ofdoughs at a consistency such as is used in theproduction of very different types of bakedgoods have been used to define the term"rheological optimum" (Schäfer, 1972).Schäfer suggested taking this to mean thestate of the dough most suitable for producing abakery item, which would naturally ensure thebest results during baking and an end product ofthe desired quality. The requirement for thisstate is doubtless optimum quality of theflour, but it can be influenced and controlledby flour improvers and ingredients that act onthe properties of the dough. For this purposethere are product ranges offering a choiceof emulsifiers and enzyme preparationsdesigned to achieve the rheological optimumand enhance the final result of baking. Afurther practical application of the rheologicaloptimum lies in the controlled treatment offlours with ascorbic acid at the mill and withenzyme preparations (amylase, proteases,pentosanases, xylanases) and other flour-improving ingredients based on lecithin,cystine, cysteine and emulsifiers, which resultin better inflation of the dough, increasedwater absorption and ultimately better flavourand prolonged shelf life of the baked products.

In practice, a flour can be optimized in respectof its baking properties at a mill by blendingflours with different dough properties. In aflour blend the energy values of theExtensograms of the two flours making up theblend are combined. The energy value of the

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blend lies between the values for the compo-nents in accordance with their ratio in the mix-ture. But the volume yield as a quality charac-teristic of the baked product is higher than thatcalculated from the individual volume yieldsof the blended components (Bolling, 1980).This effect is due to optimization of the vis-coelastic properties of the flour blend and istherefore recognizable from the ratio R/E,which is within the optimum and desiredrange of the Extensogram for the blend. Thisvalue increases with the extent of the differencebetween the dough properties "short" and"soft" of the components of the blend, whichultimately result in "normal" dough propertiesand achievement of a rheological optimum(Schäfer, 1972). But this does not mean thatany arbitrary flour blend with two or morecomponents achieves the desired quality of anormal commercial flour: the componentsmust suit each other and have a high energyvalue as well as a sufficiently high ratio.

To increase the protein content of a flour andimprove its baking properties it is usual to add2 - 3% vital wheat gluten (dried gluten).Rehydrated wheat glutens have differentphysical and rheological properties accordingto the initial quality of the flour, the method ofdrying the gluten and the temperature atwhich it was dried during its production at thestarch factory. When the glutens are added tothe flour, these properties are clearly visiblefrom the viscoelastic properties of the doughand thus ultimately from its baking perform-ance. Even when dried gently, every wheatgluten suffers heat damage which manifestsitself in different degrees of reduction of thewater-binding capacity and extensibility and inan increase in the elasticity of the rehydratedgluten or in its shortness. The properties ofthe rehydrated wheat gluten can be testedsensorily, by hand, or by conducting extensionand shear tests, but an Extensogram of theflour mixture shows most plainly the effect ofthe wheat gluten in conjunction with theproteins of the flour (Weipert andZwingelberg, 1992). A flour with soft, weakdough properties requires a firm wheat glutenthat is not very extensible; a flour with short

dough properties can be improved with a soft,extensible gluten. It is really very surprisingthat the usual addition of about 2% wheatgluten has such a decisive influence on thedough properties of the flour.

All in all it may be said that Extensogramsmake it possible to describe the quality of aflour clearly and with sufficient reliability. Theydescribe the viscosity or consistency of thedough, which can be checked by the waterabsorption determined in the Farinograph. Butwhat is even more important for processingthe flour is that they describe the viscoelasticproperties of the dough and make a consider-able contribution to the quality of the finalbaked product.

The rheological properties of the freshly washedout wet gluten – called "gluten structure" bythe cereal processors – have long been describedby means of stretching by hand in a sensorytest. This sensory rating has been made moreobjective by mechanical, automatic washingand the use of simpler instruments. A measure-ment of this kind carried out with a Glutographor texture analyzer or determined as a glutenindex can doubtless be taken as a guide. But itcannot completely describe the properties ofthe dough (Bloksma, 1990, Weipert, 1998aand Weipert and Zwingelberg, 1992). Thebehaviour of isolated wet gluten and rehydrateddried gluten alone is quite different from theirbehaviour when they are combined withstarch, pentosans, lipids and other ingredientsof dough.

In the case of a wheat flour for bread making,the proteins are expected to form a gluten asquickly as possible; the gluten must bindwater and thus determine the consistency ofthe dough. On the other hand a flour formaking wafers is expected to form gluten lateor preferably not at all, so that the massretains a low viscosity. The suitability of waferflours is determined with the aggregation testand the viscosity test using a flow pipette(Gluzynski et al., 2002). Both are ultimately ameasurement of the viscosity and viscoelasticityof the mass.

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Rheofermentometer and Texture Analyzer

Baked volume and the characteristics of thecrumb are the two most important qualityattributes of baked products. Both aredetermined by the choice of raw materials(wheat, flour, yeast, other ingredients, additivesetc.) and influenced by technical measuresand can be demonstrated for the entire chainof production in the individual phases,starting with the raw material flour, throughthe dough processes and finally in the crumbas the end product. Since we are dealing withphysical properties of the substances it ispossible and appropriate to determine thecharacteristics of the raw materials by rheo-metric methods and observe the effects of thetechnical measures and treatments used.Food rheometry offers a number of measuringinstruments that differ greatly in respect oftheir efficiency, the information they provideand not least their acquisition and runningcosts. The measuring instruments of appliedand fundamental rheology most in demandare those that are simple to use, have a goodprice-to-performance ratio and are suitable formeasuring various different materials.

Rheofermentometer

The Rheofermentometer (Tripette et Renaud/Chopin, Villeneuve la Garenne, France) is aninstrument that measures the interaction ofgas production and gas retention in wheatdoughs from a practical point of view.Maximum CO2 formation and the moment atwhich gas is released from the dough duringfermentation can be read off from a gasformation curve, and the ratio of the amountof gas retained to the overall amount of gascan be calculated. Corresponding to this, acurve for the height of the dough is recorded;it shows the maximum height and the stabilityof the dough (before the CO2 is released), alsoduring fermentation. A simultaneous analysisof the two curves reveals the fermentationproperties of a yeast and a dough under givenconditions and permits conclusions with regardto the characteristics of the raw materials(various flours, yeasts, sugar) and the measuresthat have to be taken to optimize the productionprocess. This viewpoint distinguishes the

Rheofermentometer from the BrabenderFermentograph and the Maturograph. The Rheofermentometer has been usedthroughout the world to investigate the gasretention capacity of different qualities ofthe raw material flour (flour grinds, wheatvarieties, sprout) and their reaction to theaddition of dried gluten and ascorbic acid, andalso to study the effect of maltose and othersugars (sucrose, lactose) and α-amylase on thedevelopment of the gas. Some ingredientssuch as carboxymethyl cellulose have alsobeen known to cause changes in both gasretention and gas formation capacity. Specialattention is given to the effects of the dairyproducts low-fat and full-cream dried milk,whey and caseinate; being surface-activesubstances, these have an enormous effect onbaked volume. These investigations havehelped to optimize the formulations of suchproducts. Tests with the Rheofermentometerhave shown that the fall in baked volumecaused by the storage of frozen dough portionsis caused not by reduced gas retention butsolely by reduced gas formation capacity. Thishas also been taken into account whenoptimizing formulations (ingredients, speed offreezing).

The Rheofermentometer has therefore shownitself to be a useful instrument in practicalbaking. To answer specific questions themeasuring program suggested by themanufacturer of the equipment can and mustbe altered.

Maturograph and Oven-Rise Instrument

For some time the gas formation and gasretention capacity of a dough made with yeasthas been measured with a combination of tworheometric devices, the BrabenderMaturograph and the Oven-Rise Instrument.The Maturograph records the change in volumeof a dough fermenting with yeast by tracingthe shape of the dough specimen with andwithout pressure; in this way it determinesboth the viscoelastic properties of the doughand the time of greatest activity of the yeast orthe end point of fermentation. At this time asample of dough from the same batch is

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"baked" in oil heated to a controlled temperature,and the oven rise of the yeast is describedunder conditions similar to those of an ovenby recording the amount of rise or the loss inweight of the sample. These two methodsform a bridge between the cold phase of thedough in the fermentation chamber and thehot baking phase in the oven in the form of arecording laboratory test without the need fora baker's oven or a direct baking trial.

The two devices can be used successfully andhelpfully for practical and scientific purposesin cereal laboratories. They make it possible toexamine a number of raw materials (wheat,flour, yeast), to optimize flour blends andmethods of flour improvement, to develop,make up and test ready-mixed flours for specialproducts, and ultimately to adjust flour qualitiesto the existing production process or theproduction process to existing flour qualities.Inclusion of the Do-Corder, a recording mixerwith adjustable mixing intensity, in the measu-ring procedure with the Maturograph andOven-Rise Instrument to make up the DMO(Do-Corder-Maturograph-Oven Rise) Systemhas complemented and greatly consolidatedthe information yielded by the test. Thesystem also describes the behaviour of thedough as a reaction to intensive mechanicalstress and recognizes the mixing requirementsand kneading tolerance of the dough. Theintroduction of the DMO System has finallyenabled a more complete description of thesuitability of flours for various different bakingpurposes (Seibel and Cromentoyn, 1964;Brabender 1965, Brabender and Schäfer 1971,Schrader 1984).

Texture Analyzer

The Texture Analyzer (Stable Micro Systems,Goldalming, England) is a universal instrumentthat justifies its popularity in two respects. Itworks on the principle of a compression andtension measuring device with a range ofdifferent tools. Universal instruments of thiskind, with the same measuring principle, haveexisted for a long time. But because of its sizeand ease of operation, this device is alsosuitable for cereal laboratories. On the one

hand its versatility enables the user to makesimple, quick and objective measurements ofviscosity with materials of different consistenciesand structures such as whipped cream,mustard, ketchup, starch gel and also wet gluten,dough, and the crumb of baked products. It iseven possible to measure the fracturestrength of crispbread and biscuits. On theother hand, a feature of the universal nature ofthe instrument is that the free choice of loadsmakes it possible to describe the flow propertiesof a substance in the sense of fundamentalrheometry.

Being a tensile instrument it is similar to aMini-Extensograph that can record the structures(viscoelastic properties) of the wet gluten andthe dough strands by measuring their extension(using a small sample and the Kieffer rig). Thismakes it possible to describe both the qualityof the wheat or flour as raw materials and theeffect of the additives and ingredients on theproperties of the dough. Used in the compression mode the instrumentsimulates the baker's finger, and by penetratingthe specimen (depth of penetration as a functionof the force applied and time) it ascertains theviscosity of the dough (and thus its waterabsorption) (Tscheuschner and Auermann,1964) and also the crumb characteristics of thefinished bread. Compression of a sample ofbread crumb defined in terms of geometricdimensions makes it possible to record anddescribe the texture of the sample even moreprecisely and reliably – useful for describingthe chewiness and staling of the crumb.Similarly, a suitable measuring technique canbe used to describe the cooking properties ofpasta. Further measuring cells have recentlybeen developed that make it possible tomeasure the "stickiness" of a surface(dough, bread crumb, pasta, rice) and stateit in terms of numbers, or to describe thecharacteristics of dough by means of biaxialstretching, as for the Alveograph(Dobraszcyk, 2002).

Hot Phase of the Bread-Making Process

Whereas the focus of attention in the coldphase of bread making is on the swelling

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substances of the flour, particularly the proteins,it is starch and its pasting behaviour thatdominate tests to show the behaviour ofdoughs in the hot phase, i.e. the actualbaking. Starch only starts to swell intensivelyat elevated temperatures; it binds water andgelatinizes, losing its crystalline structure. Butthe gelatinizing and already gelatinized starchis exposed to enzymatic breakdown throughthe activity of α-amylase. In the quick break-down process of the enzymes the starch losesits ability to bind and hold water; this resultsin bread with an inelastic, soft, wet and veryoften unchewable crumb which makes theproduct inedible. On the other hand, optimumenzyme activity is necessary for optimumresults in the baked product. So it is essentialto determine the activity of the α-amylase in aflour or any other ground product in order toachieve the desired result. If the enzymeactivity is too high, activity-inhibiting agents are added or suitable measures taken; if it istoo low it can be optimized by adding enzymepreparations.

For practical and technical reasons, the activityof the α-amylase is ascertained indirectly bymeasuring the viscosity in a flour-and-waterslurry. When interpreting the measurementsobtained by various methods we have to beaware of the fact that:

• the observed changes in viscosity are notdue solely to the interaction betweenthe starch and the enzyme; they alsoreflect the water-binding capacity of theswelling substances in the flour, and

• although the gelatinization of the starchis a function of the elevated temperatureor the heat energy introduced, theconcentration of the slurry and thetemperature gradient with which the risein temperature in the slurry is controlledhave an enormous effect on the gelatini-zation process of the starch. So themeasurements made with the Amylo-graph, the Rapid Visco Analyzer and theFalling Number apparatus are not directlycomparable.

Amylograph and Falling Number

The Amylograph is a rotational viscometerwith a measuring system consisting of a roundvessel, in which the flour-and-water slurry isheated under controlled conditions, and asensor to record the changes in viscosityduring the measuring time. The pins of themeasuring device cause turbulences in theslurry; these are necessary to preventsedimentation of the starch, but they makeit impossible to calculate the viscosityprecisely in absolute physical units. Theviscosity of the slurry is therefore stated astorque in Brabender or Amylogram units.The measurements that can be read offinclude the temperature and the viscosity atmaximum gelatinization; these provide moredifferentiated information than a viscosityvalue alone.

The Amylogram, a viscosity curve showingthe gelatinization of the starch in the flour-and-water slurry, reflects the changes inwater binding capacity of the swelling andpasting starch and the enzymatically andmechanically decomposed starch gel. Inthe way the standard Amylograph methodis used it offers a suitable means of describingthe pasting properties of rye flour slurries.Rye starch gelatinizes at lower temperaturesthan wheat starch, especially if it is sprout-damaged as a result of poor environmentalconditions. Then the task is to find outwhether the rye is suitable for baking bydetermining the temperature and viscosityat the pasting peak of the Amylogram. Thisapplies to wheat too, but only if it is assumedto be sprout-damaged. The starch of theflour slurry from a wheat lot that is notsprout-damaged normally gelatinizes laterand at higher temperatures, towards theend of the temperature range of anAmylogram or the measurements from anAmylograph. However, a very high viscosityat the pasting peak yields little informationin relation to the amount of time that hasto be invested. For such wheat flours thequick and simple determination of the FallingNumber is sufficient.

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Falling Number determination is a simple andquick method in which the viscosity of a flour-and-water slurry is stated as the number ofseconds a pestle takes to penetrate the starchgel. Measurement of the viscosity in a FallingNumber tube does not start until 60 secondsafter stirring, when the viscous properties ofthe gelatinized starch slurry have already beenchanged by the α-amylase present in the flourand the mechanical force of stirring. TheFalling Number is therefore a one-point measure-ment of the residual viscosity of the starch gel,not a continuous measurement like that of theAmylograph. The Falling Number method canbe used for both wheat and rye, although thelimits of the measurements differ. This resultsnot least from the different water bindingcapacities of the swelling substances of wheatand rye. In rye flours too, the Falling Numbercan be used with sufficient accuracy for indirectdetermination of α-amylase activity and thesuitability of a flour for baking.

There is not a close enough relationshipbetween the Amylograph and Falling Numbermethods to permit a direct comparison of themeasurements. Firstly, the ratio of flour towater (concentration of the slurry) and thetime/temperature gradient of the heatingdiffer; secondly, the Amylograph method is acontinuous measurement over a period of upto 45 minutes, whereas in the case of theFalling Number the measurement of residualviscosity does not start until after 60 secondsof stirring. For these reasons it is not possibleto allocate an Amylogram value to a corre-sponding Falling Number. Nevertheless, anumerical relationship between the twomethods can be achieved by comparing a largenumber of measurements and calculating amathematical-statistical regression. But thisrelationship only applies to the harvest of asingle year and has to be reviewed or re-calculated for each new harvest.

By using nomograms in a double-logarithmicsystem it is possible to make up optimizedmixtures from rye flours with different FallingNumber and Amylogram data (Weipert,1987c). However, the percentages of the

enzyme-active components with low datadepend on the height of the data of the low-enzyme components; as a rule these percen-tages tend to be low.

Rapid Visco Analyzer

Whatever the advantages for which theBrabender Amylograph (and the Viscograph,intended chiefly for the starch industry) isappreciated, it has disadvantages too. Firstlyit requires a very large sample for testing, andsecondly the recording of a pasting curve istime-consuming. Several attempts haverecently been made to develop and market a"micro-Amylograph". The development of theRapid Visco Analyzer (Newport Scientific,Sydney, Australia) was and is still the biggestand most successful step towards simplifyingand broadening the investigation and descrip-tion of the pasting properties of starch andproducts containing starch (Weipert, 1998a).Because of its versatility it is in general use inthe field of food analysis (milk, soups, sauces,salad dressings etc.).

The Rapid Visco Analyzer is also a rotationalviscometer that unites the advantage ofrequiring only a small sample (2-4 g) with thepossibility of setting to any desired temperaturegradient. The temperature profile of a test canbe adjusted in such a way that the test startsat any chosen temperature, which rises slowlyor rapidly and remains constant for a timebefore cooling down in the desired steps.In practice this means that the test can beperformed at a constantly high temperature inthe manner of the Falling Number test, withslow or faster heating in the manner of anAmylograph or with controlled heating andcooling as in a Viscograph. A close correlationhas been found between the measurementsfrom these two methods and the results of theFalling Number and Amylograph tests; thiscorrelation enables the Rapid Visco Analyzerwith the already standardized methods ICC"stirring number" (similar to the FallingNumber) and "rapid pasting" (similar to theAmylograms and Viscograms) to be adoptedby cereal laboratories and used "seamlessly"(Weipert, 1998a).

13.1 Rheology

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Interpretation of Brabender viscograms andthe RVA rapid pasting curves:

Fig. 60: Temperature ramps (upper set of curves) and pasting curves (lower set of curves) of a wheat flour recorded in the Rapid Visco Analyzer at different temperature gradients: SN: constant 95 °C; ST1: 12 °C/min; ST2: 6 °C/min; ST3: 3 °C/min; ST4: 1.5 °C/min

The possibility of programming and determiningthe duration of a test for the pasting behaviourof starch in a starch/water or flour/waterslurry oneself, according to needs, and thusmonitoring the behaviour of the starch in therelevant process is very much appreciated byusers of the Rapid Visco Analyzer. A quickmethod can doubtless yield a reliable result asa guide, but the process of making and bakingbread takes rather longer. In order to describethe pasting behaviour of wheat and ryestarches in flours for baking and to assess it inthe manner of an Amylogram, the measuringtime in which the starch swells and gelatinizesmust be taken into account. The starch grainshave time to absorb and bind the water, toswell, to be "annealed", and finally to gelatinizecompletely or incompletely, depending onthe amount of free water available. One andthe same flour/water slurry shows differentviscosities and temperatures at the pastingpeak according to the length of the measuringtime. The shorter the measuring time, thehigher the viscosity; it is therefore highest in the

"stirring number" method, similar to the FallingNumber (Fig. 60). To save time by shorteningthe measuring period may mean a loss ofinformation (Weipert, 1998a), especially if thequality data from the time-consumingAmylograph method are still used on thegrounds of experience. Nevertheless, a "quicktest" of this kind may serve as an initialguide. Farther-reaching decisions require theintroduction of new critical values for each ofthe suggested temperature profiles in thecourse of measurement. The Rapid ViscoAnalyzer can measure both fast and slowly.

13.1.5 Fundamental Rheometry

Fundamental rheometry came into being withthe pioneering work of G. W. Scott Blair – andit is characteristic that the material he used forhis trials was a wheat dough (Schofield andBlair, 1932). This substance, that was initially aproblem to the rheologists because of its"memory" (meaning the stored energy of itselastic behaviour), subsequently took rheo-metry and rheology a great step forward.

There is a relationship between conventionaland fundamental rheometry. They use asimilar deformation force, but in fundamentalrheometry this is variable and therefore capableof describing the flow properties of a materialunder different loads or stresses in a test witha universal viscometer. The result is a flowcurve, or stress strain curve, in which changesof stress are recorded over changes in strain.The stress in the curve is either the chosendeformation force, by which the change instrain is measured, or it is a measure of theresistance to deformation if the strain is variedunder controlled conditions during the test. Inboth cases the viscosity is calculated from thetwo physical values stress and shear rate, andsince the magnitude of the deformation force(measured area and force) and of the strain isdefined, it is expressed in absolute physicalunits. These physical units permit a directcomparison of results from viscometers madeby different manufacturers. Moreover, bycalculating the viscosity, a flow curve can be"redrawn" as a viscosity curve (Fig. 61); the twotypes of curve yield the same information, and

13.1 Rheology


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it is up to the person interpreting the results tochoose the type of curve he prefers. The twotypes of curve show the flow properties of thesubstance tested; in particular they indicateany shear-dependent or time-dependentanomalies of flow behaviour that may occur inthe test. The viscosity curve of a dough yieldsvery important information on the rheologicalproperties of the dough under different deform-ation forces. It shows that the dough has ayield point, and that its viscosity (consistency)falls as the load increases. This property isknown as structural viscosity or shear-thinningand is caused and explained by the orientationof the molecules and aggregates in the flowfield. It means that when exposed to only alow deformation force, such as stretching bythe fermentation processes during the restingtime, a dough has a higher viscosity thanduring transportation through the pipes ortesting with the Farinograph or Extensograph(Fig. 61). This justifiably raises the question ofhow to describe the state of a dough at the lowdeformation forces in the process with amethod that uses high deformation forces(Tanaka, et al., 1980). In other words: withhigh deformation forces it is only possible todetermine the mechanical properties of thedough, not its rheological properties.

A comprehensive work by several authors hasbeen published on the subject of food rheologyin general and the advantages of fundamentalrheometry, including dough rheology, in particular.It deals with the importance of rheology forexplaining and improving the quality of foods(Weipert and Tscheuschner, 1993).

The deformation curve of an Extensogram andthe stress strain curve are similar in appearanceand take a similar course (Fig. 62). It is hopedthat this fact will lead to greater acceptance offundamental rheometry in cereal laboratories.There is no similarity to the Alveogram, since itis a pressure curve and not a deformationcurve. If a ruler is placed behind the expandingdough bubble so that the increase in the sizeof the dough bubble can be measured, theresulting deformation curve made up of themeasured points also shows similarity to theExtensogram.

Viscometer

The instruments used in rheometry differ inrespect of their measuring principle, theirmode of use and thus the presentation of theresults. In rheometry the flow behaviour of amaterial is monitored between two parallelflat plates, in the circular gap between two

13.1 Rheology

Fig. 61: Stress strain curves of wheat flour doughs with different dough properties recorded with a rotational viscometer (left) and a viscosity curve showing "yield point" and "shear thinning" (right)

60

40

20

2 3 4 5 D

Shearing velocity (s -1)

char.D

Shea

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A

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C CT-

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Deformation

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coaxial cylinders, between two round parallelplates, between a cone and a plate or finally ina capillary tube. The rotational viscometershave shown themselves to have variousadvantages and are therefore the most widelyused. A rotational viscometer determines theviscosity of a material in a simple test; in themeasuring gap of the instrument the conditionsare "stationary" and the flow is laminar. Arelatively simple and therefore inexpensiveviscometer makes it possible to record stressstrain curves that reveal certain informationabout the dough. The yield point and theviscous properties of a dough can be read offfrom the shape and pattern of a stress straincurve and the (automatically) re-calculatedviscosity curve. From the viscosity it ispossible to determine the water absorption orthe volume yield of the dough. The shape ofthe stress strain curve reveals the propertiesof the dough: a dough with short propertieshas a steep stress strain curve, whereas thecurve of a dough with soft, weak properties isflatter (Fig. 61). And finally the stress straincurve makes it possible to assign a numericalvalue to the surface stickiness of a dough(Weipert, 1987a, 1992, 1998 and 1998b). So itis no wonder that such a method, whichrequires very little specimen material (10 gflour), is used successfully in the breeding ofwheat (Fig. 61).

Rheometer

A rheometer can measure viscosity in thesame way, but it can also show the elasticproperties of a material in a second test inwhich the specimen is briefly stressed bysudden shearing at a controlled rate, and thestress then suddenly ceases. As a rule the testonly takes a few seconds and producesmeasurements in the form of a curve. Thecurve rises sharply in relation to the stress andfalls again more or less steeply when theshearing suddenly stops. The falling end of thecurve shows the elastic properties of thematerial; it reveals a reversible elasticdeformation and an irreversible plasticdeformation (Fig. 63). The measurement is"unsteady" because of the sudden changes inthe flow field of the specimen.

A special class of rheometers consists ofinstruments which enable the rheologicalproperties of a material to be demonstrated ina single test. To do so they operate in thedynamic oscillating or vibratory mode: insteadof shearing simply by rotation in one directionthey perform an oscillating measuringdeformation in which the amplitude of theoscillations (excursion) and their frequency(movements within a unit of time) can becontrolled (Fig. 64). Viscosity is measured astorque according to the familiar method and

13.1 Rheology

Fig. 62: Comparison of recorded curves: Extensogram, Alveogram and stress-strain curve

Extensogram

empirical fundamental

Time / Extension Shear velocity

Alveogram Stress-straincurve

Dough Rheology Dough Rheology

Resi

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Pres

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shown as complex viscosity, but the "stiffness"of the material is also recorded as storedelastic energy. The result of the measurementis a sinusoid ("wavy") curve. A comparison ofthe curve thus produced with the controlleddeformation curve reveals a phase shiftmeasured in angular units between 0° and90°. The smaller the phase shift, the "stiffer"and more elastic is the tested material. The"plastic" component of viscosity, the energyloss, cannot be measured; it is calculated asan imaginary component, the difference betweencomplex (total) viscosity and the stored (elastic)viscosity. "Plastic" viscosity divided by elasticviscosity denotes the viscoelastic behaviour ofthe material tested. For reasons of simplicitythe results are usually stated in the form ofmeasured moduli that have to be convertedinto viscosity values by calculation. Theconversion factors for a measurement areconstant, so that the moduli G* (complexshear modulus), G I (storage modulus) and G II

(loss modulus) stand for the relevant viscosities(complex or total viscosity, elastic viscosityand plastic viscosity). Viscoelasticity is calcu-lated as tan delta (tangent delta or loss angle),the quotient from G II divided by G I. If this issmaller than 1, since G I is greater than G II, itdescribes an elastic material; if it is greater,the material is plastic. The greater the deviationof tan delta from this quotient 1, the moredistinctly does the viscoelastic behaviour of thematerial tend in one direction of viscoelasticityor the other, i.e. elasticity or plasticity.

This highly efficient, sensitive and elegantmethod of recording and displaying the "true"rheological properties of foods has made agreat contribution to understanding the specificcharacteristics and behaviour of raw materialsand foods during processing and ultimately toexplaining why consumers like one productand dislike another. The definite advantagesof the dynamic oscillating method for studyingand identifying structural changes in foodsduring processing can be documented byDMTA (Dynamic Mechanical ThermalAnalysis). If a starch/water slurry is heated asin an Amylograph test and the changes inviscosity, elasticity, plasticity and tan delta aremeasured, several curves similar to anAmylogram are obtained (Weipert, 1995).

13.1 Rheology

Fig. 63: Creep recovery / stress relaxation curves of a gluten or a dough

800

600

0 Shearing time

immediate retarded

BE

Burgers Model

KV

N2

H2

Stress Relaxation

elastic(reversible)

viscous(irreversible)

H2

N2

H2

N2

F

H1

Bu=KV-M

=( )-(H2-N2)

N1KV (N1 H1)

H1N1

HNKV=

Fig. 64: DMTA of a starch slurry: storage (G ) and loss (G ) moduli and tan delta over the time in the course of heating (Weipert, 1995)

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At the beginning of such a test the loss modulusG II was greater than the storage modulus G I,showing that the starch slurry had the proper-ties of a liquid at low temperatures (Fig. 64). Athigher temperatures, following increasedwater absorption and gelatinization of thestarch, the situation was reversed: the storagemodulus G I was greater than the loss modulusG II, indicating that the properties of the starchgel were becoming more solid. These changeswere expressed even more clearly by the courseof tan delta, which was well above 1 at thebeginning of the test and well below 1 aftergelatinization of the starch. This means thatstarch gel has predominately the elasticproperties of a "solid". These observationsconcerning the changes in the viscoelasticproperties of the starch slurry were accom-panied by measurements of the temperatureof the heating medium and of the slurry itself.It was found that the temperature curve of theslurry (Ts) followed the temperature curve ofthe heating medium (Tw) with some delay, butthat a slight rise in the temperature curve ofthe slurry occurred at the beginning of gelati-nization. This additional delay was caused bythe fact that the starch took the heat energyout of the slurry in order to gelatinize. In adough the transformation from a soft,"plastic" mass into a firm crumb in which the"elastic" properties predominate is evenmore evident. This shows that such a test isuseful for identifying and demonstrating thechanges in the properties of a flour in thecourse of processing.

Measurements with such instruments offundamental rheology have opened up newways and means of analyzing the structureand properties of doughs. By carrying out afrequency sweep (in which the amplituderemains constant and only the frequency ischanged as required) or an amplitude sweep(in which the frequency remains constant andthe amplitude varies) it is possible to recordthe flow properties of a dough at differentdeformation forces. Both the viscosity and theelasticity or viscoelasticity of the dough arerecorded synchronously and simultaneouslyin a single measurement. This is a simple,quick and elegant way of differentiatingbetween doughs with a firm elastic or soft andplastic structure (Fig. 65). It has also been

13.1 Rheology

Dynamic oscillating mode

Fig. 65: Deformation (strain) test in the dynamic oscillating mode on wheat flour doughs with extremely different dough properties

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10-4

G G

Et

aI

II

10-1 10-4

% strain

firm

weak

GI

GII

eta*

eta*

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observed that at an extremely low deforma-tion load wheat dough shows a plateau ofelastic behaviour, since its structure is notdamaged during this part of the measure-ment; the dough does not show the expected

structural viscosity as its viscosity decreasesunder increasing deformation forces. Thisobservation has been used to develop a "non-destructive" testing method, in fact one whichscarcely touches the dough, in the form of a"recording baking test" in which the dough ismonitored over the desired length of time atrising baking temperatures and falling coolingtemperatures under conditions simulating theprocess in the baker's oven (Weipert, 1987aand 1992). The viscosity and elasticity curvesare related to the curve of an Amylogram,since they show the gelatinization propertiesof the starch in interaction with other flourconstituents and additives. But in this case wehave a dough of the consistency usual inbread making, and so they show the propertiesand interaction of these two most importantcomponents of a flour and a dough in thebaking process. They demonstrate the doughproperties resulting from the gluten at thebeginning of the process, in the oven stageand as a final result after baking. The measure-ments after cooling show the properties of thebaked dough, which only differs from thecrumb of the bread in that the inflation ismissing (Fig. 66).

Although still comparatively new, the "recor-ding baking test" method has already shownits value and potential in a few publications.Baking trials using flours from wheat varietieswith different dough properties have shownthat the viscoelastic properties of the doughsare preserved into the baked crumb. Thebaked crumb is doubtless firmer and moreelastic than the dough, but the crumb of awheat flour with soft dough properties issofter than that of a wheat flour with firmdough properties. Furthermore, the methodshowed the effect of the different doughyields, of ascorbic acid, various enzyme pre-parations, emulsifiers and other ingredientson the viscosity and viscoelastic properties ofthe dough and the crumb, in respect of extentand also time and temperature (Fig. 67). Theinfluence of the oven temperature was shownwith an enzyme-active rye flour by carrying outrecording baking tests using slowly and rapidlyrising temperature gradients (3.5 °C/min and

13.1 Rheology

Fig. 66: "Recording baking test" - changes in viscosity (G*), elasticity (storage modulus G I), plasticity (loss modulus G II), sample height (delta L) and viscoelasticity (tangent delta) in the course of heating and cooling

G

G

(

Pa)

tan

Delt

aTe

mpe

ratu

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C)

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1Rad/S 0.2 % STRAN RALLE • 0.5% YEAST 150min RESTING TIME

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

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Fermenting Oven stage Cooling Time (min)

Time (min)

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7 °C or 17.5 °C/min respectively). It was alsopossible to simulate the process of producingbread rolls from frozen dough portions. In themeasuring device of the rheometer a doughwas frozen to -18 °C, heated to +100 °C andcooled down to +30 °C in one cycle duringwhich the changes in viscosity and the visco-elastic properties were recorded continuously.So far the recording baking test is the onlymethod by which doughs can be tested rheo-logically in their full formulation, includingyeast (Weipert, 1987a, 1992, 1995 and 1998b).

there are limits to its uses. A rotational rheo-meter working on the principle of shearing iswell able to show the rheological properties offluids (in coaxial cylinders) and pasty substan-ces (by the plate/cone or plate/plate system),but it fails with solids (Weipert, 1987a, 1992,1997 and 1998b). On the other hand, a rheo-meter working in the compression mode mightbe unable to show the rheological propertiesof fluids, but its measurement range coverspasty substances (such as dough) and solidsof different consistencies (bread crumb, cerealgrains) (Weipert, 1997). In the compressionmode the measured moduli are termed E* forthe complex modulus, E I for the stored modu-lus and E II for the loss modulus. Both dynamicoscillating measuring principles, the shearingmode and the compression mode, are equallysuitable for expressing the rheological proper-ties of materials, complex viscosity and elasti-city or viscoelasticity. But since they measurethe rheological properties of the specimens ina highly sensitive and precise manner andrepresent them simultaneously and synchro-nously, they require friction-free suspension oftheir working parts in air bearings and com-plex computer software for control and evalu-ation. This makes them expensive to buy,maintain and operate (Weipert, 1993). But thenew information acquired through the measu-rements justifies their use.

But despite the versatility of the rheometer,

13.1 Rheology

Fig. 67: "Recording baking test" – viscosity (G*) and viscoelasticity (tangent delta) of doughs from one wheat flour treated with ascorbic acid, α-amylase and protease

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100

10-1

120

0

G* (

Pa)

Tan

Del

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Tem

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C

0 50Time (min)

Tan Delta Temperature

G0

BlankProteaseα - AmylaseAscorbic acidTemperature

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• Amend D, 1996. Grundlagen der Teigbildung beiWeizen- und Roggenteigen. Handbuch Backwaren,Chapter 3.3.1. Behr's Verlag, Hamburg.

• Bloksma AH, 1990. Dough structure, doughrheology and baking quality. Cereal Foods World35(2):237-294.

• Bolling H, and Weipert D, 1984. Zur Beurteilungder Eigenschaften von Weizenteigen mit Hilfe desExtensogramms. Getreide Mehl Brot 38(5):131-136.

• Bolling H, 1980. Zur Optimierung derBackeigenschaften von Weizenmischungen unterbesonderer Berücksichtigung spezifischerRohstoffeigenschaften. Getreide Mehl Brot 34(12):310-314.

• Brabender CW and Schäfer W, 1971. NeueErgebnisse teigrheologischer Untersuchungsver-fahren (New achievements in testing the dough forrheological properties). Mühle Mischfutter-technik108(6):69-71; (8):101-103.

• Brabender CW, 1965. Physical dough testing –past, present and future. Cereal Science Today 10,291-304.

• Brabender CW and Schäfer W, 1971. NeueErgebnisse teigrheologischer Untersuchungsver-fahren. Mühle Mischfuttertechnik 108(5):69-71, 101-103.

• Brabender CW, 1965. Physical dough testing –past, present, and future. Cereal Science Today(10):291-304.

• Brümmer JM, 1987. Ermittlung der Wasserauf-nahme von Roggenmehltypen für den Sauerteig-Standard-Backversuch. Mühle Mischfuttertechnik124(3):306, 309-310.

• D'Appolonia BL and Kunerth WH, 1984. TheFarinograph handbook. AACC, St.Paul, MN, USA.

• Dobraszczyk B, 2002. Blowing bubbles. EuropeanBaker 32(1):34-38.

• Faridi H and Rasper VF, 1987. The Alveographhandbook. AACC, St. Paul, MN, USA.

• Frazier PJ, Brimblecombe FA, Daniels NRW andRussel Eggitt PW, 1979. Besseres Brot aus schwäche-rem Weizen – rheologische Überlegungen. GetreideMehl Brot 33(10):268-271.

• Gluszynski M, Brümmer JM and Lindhauer MG,2002. Glutenaggregationstest. Merkblatt derArbeitsgemeinschaft Getreideforschung e.V.Detmold Nr. 153.

• Hoseney CR, 1986. Principles of cereal scienceand technology. AACC, St. Paul, MN, USA.

13.1 Rheology

13.1.6 Outlook for the Future

Whether conventional or fundamental,rheometry will remain an established andimportant feature of the production of qualitybread and other baked products. The choice ofmeasuring instruments and methods willdepend on the level and purpose for whichthey are to be used. Both rheometries, theconventional and the fundamental, haveadvantages and disadvantages; an idealrheometry would combine the advantages ofboth. But ultimately it is the task of man – thecereal expert and the rheologist – to use theinstruments and interpret the results. He hasto ensure that Finagle's Law does not apply,namely that:

The viscosity and viscoelastic behaviour ofdoughs and the end products is and willalways be the information we have and theinformation we need. And since it is availableit offers a guarantee of reliable productionprocesses and good quality in the endproducts.

13.1.8 References

The information we have is not the information we want.

The information we want is not the information we need.

And the information we need is not available.

13.1.7 Acknowledgments

The author of these lines wishes to thank Ms.A. Stüwe and Mr. V.G. Rao for the drawingsthat have made the opinions expressed muchmore readable and easier to understand.


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• Muller HG, 1966. Teigrheologische Studien II.Empirische Konsistenzmessungen. Brot Gebäck20(3):51-54.

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