heology is the branch of science that dealswith the flow and deformation of materi-als. Rheological instrumentation and rheo-

logical measurements have become essential toolsin the analytical laboratories of food companies forcharacterizing ingredients and final products, aswell as for predicting product performance andconsumer acceptance.

The materials under investigation can range fromlow-viscosity fluids to semisolids and gels to hard,solid-like food products. A knowledge of the rheolog-ical and mechanical properties of various food sys-tems is important in the design of flow processes forquality control, in predicting storage and stabilitymeasurements, and in understanding and designingtexture. Quality attributes such as spreadability andcreaminess are extremely important to the accep-tance of semisolid food products by consumers.

In the case of food materials, texture is a keyquality factor. Rheological behavior is associated di-rectly with textural qualities such as mouth feel,taste, and shelf-life stability. As an example, rheo-logical measurements are useful in storage stabilitypredictions of emulsion-based products such asmayonnaise and salad dressings.

Importance of acceptable food rheology

Consumers use subjective tests to determine thep e rceived quality of a food product. For example,the determination of fruit firmness is based on thedeformation resulting from the physical pressureapplied by the hands and fingers. The toughness ortenderness of meats is based on the effort requiredfor the teeth to penetrate and chew the flesh tissue.Food companies use instruments in an attempt toobjectify these subjective customer perceptions andacceptance criteria. Objective instrument tests fortexture also rely on deformation of the food mate-rial. Toughness can be defined as the maximumf o rce required to slice through a sample. Firmnesscan be defined in terms of the force required to de-form a body of material.

The quality of a food product depends stronglyon its formulation. Certain ingredients called modi-fiers have a larger influence on the finished prod-u c t ’s properties than others. Examples of modifiersare stabilizers, emulsifiers, and structural agents.The type of modifier and its desired effect dictate

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Rheology of foods: New techniques, capabilities, and instruments

Peter K.W. Herh, Steven M. Colo, Nick Roye, and Kaj Hedman

the type of rheological measuring technique used.An antisettling agent should be tested at low shearconditions, simulating settling in a bottle. A samplewith an antisettling agent will only be stable for aspecific period of time. Therefore, the desired shelf-life stability must be known or determined. Stabil-ity is almost exclusively a consumer preference.Most food products change their appearance andtexture upon storage. Therefore, an objective, quan-titative method for determining their stability isneeded. Rheological measurements can be used topredict shelf-life stability once there is historicaldata on a given product. Comparing a sample withacceptable life versus an unknown will provide ameasure of shelf-life at the production stage, with-out the need to wait the weeks or months thatcould be necessary for the actual test sample to set-tle or phase-separate.

In food products, small changes in the amount ofadditive can have a dramatic effect on the final prod-uct. The formulation of popular sports drinks can beused as an example. To produce an acceptable lemon-based drink, 1.5% stabilizer/emulsifier is needed inthe water/oil interface. At 1.5%, the drink is waterbased. However, an increase to 1.55% changes theproduct to an oil-based drink and results in a totallyundrinkable product. In addition, the oil-based prod-uct will show a drastic increase in viscosity with timecompared to the water-based drink, which is com-pletely stable. A simple rheological test could be themonitoring of viscosity at low shear rates to establishif the correct amount of stabilizer is added.

A switch in one of the raw ingredients can alsohave a dramatic effect on the final product. For ex-ample, if the Florida orange crop is not up to stan-dards, a producer of fruit juice may have to switchto fruit from California or Spain. Due to the fruit'sgeographically different environment, the naturalpectin content will differ. A change in formulationwill be required to achieve a similar product.

Rheological instrumentation

In principle, every type of consumable food prod-uct has some rheological characteristic. Most readilyconsumable food products contain ingredients thathave a major impact on the rheology of the finalproduct. Food processors and raw material manufac-turers have been aware of the importance of viscos-ity for many years. To d a y, rheological instrumenta-tion is considered a required analytical tool utilizedby food scientists on a daily basis. These instrumentsare M i c r o s o f t ® Windows™ (Redmond, WA ) - b a s e d ,and measurements are made quickly and easily withthe use of straightforward, user-friendly software.The operator simply loads the sample into the in-strument and selects the appropriate experiment,and the instrument does the rest.

Food products are complex mixtures of differentingredients where individual ingredients are mixedtogether to produce a finished product. In manycases, the individual ingredients consist of mixtures

R

A knowledge of the rheologicaland mechanical properties ofvarious food systems is importantin the design of flow processes forquality control, in predictingstorage and stabilitymeasurements, and inunderstanding and designingtexture.

Figure 1 STRESSTECH research rheometer.

Figure 2 Strain sweep on mayonnaise. Figure 3 Frequency sweep on mayonnaise.

AMERICAN LABORATORY / 17

of solid as well as fluid components. Most times,they are not homogeneous, and the properties varythroughout the sample. Tr a d i t i o n a l l y, single-pointviscosity tests have been performed using empiricaltechniques. These simple viscosity experiments ex-press the complex rheological response of a sampleinto a single parameter, and are not adequate incharacterizing and/or providing insight into thequality of food materials. The ingredients used to-day are numerous and expensive, and, as a result,the cost for controlling these ingredients is high.Detailed knowledge and an objective, reproducible,multipoint measurement capable of decomposingthe rheological behavior into individual compo-nents are necessary. The STRESSTECH rheometer(ATS RheoSystems, Bordentown, NJ, and R E O -LOGICA Instruments AB, Lund, Sweden) used inthis study provides all of the required instrumentfeatures and capabilit ies.

The rheometer is a research-grade analytical in-strument capable of measuring viscous, elastic, andviscoelastic properties of liquids, semisolids, andsolids (F i g u re 1). The instrument was developed foruse by the serious rheologist, and provides a verybroad measurement range, spanning low-viscositysamples such as fruit juice to more viscous productssuch as creams and salad dressings, semisolids andgels through hard cheeses and solid-state samples.

The rheometer incorporates the following fea-tures: wide torque, shear stress, temperature, shearrate, and frequency range; true Microsoft Windows-based operational software; patented DifferentialPressure Quantitative Normal Force; patentedSealed Cell measuring system for testing samples

above their boiling point; automatic gap setting; re-mote diagnostics capability via modem; and auto-matic inertia compensation. In addition, all AT SR h e o S y s t e m s rheometers are designed on a modu-lar platform allowing easy upgradability. A widerange of accessories satisfy the most demanding ap-plications with ease of operation.

User-selectable and quantitative, controlled axialnormal force sample loading

Although transient steady shear and periodic dy-namic oscillatory experiments provide informationon the rheological properties of food products, theydo not completely characterize the system. Con-cerning food samples, or any complex, two-phasesystem, the rheology is dependent on the sample’sdeformation history, loading conditions, and the ax-ial normal force applied during a measurement. Forexample, stress/strain sweeps were performed on ac o m m e rcial mayonnaise product. The results of amayonnaise sample run immediately after loadinginto the rheometer, and a new sample 300 sec afterloading into the rheometer, are shown in F i g u re 2,where shear moduli (G′ and G′ ′) are plotted as afunction of shear stress. The sample run without the300-sec rest period exhibits lower properties sincethe internal structure did not have sufficient time torebuild prior to testing. To dissipate this residualloading history, it was determined that a 300-sec de-lay time was required after a controlled normal forc eloading of 1 Newton. The results indicate that boththe linear viscoelastic region and the crossover pointof G′ and G′ ′ are affected by the loading condition.

This phenomenological shear history and loadingeffect require that all structured samples be run un-der user-selectable controlled normal force condi-tions. In addition, all samples must be provided a fi-nite time for their respective internal structure torebuild after loading.

Considering dynamic oscillation frequencysweeps, the viscoelastic properties are also depen-dent on the residual loading history, as shown inF i g u re 3. To overcome the sample-dependent load-ing criteria, the rheometers are available withPatented Differential Pressure Normal Force capa-b i l i t y. With this highly sensitive, robust, and accu-rate axial loading and measurement sensor, con-trolled normal force loading and quantitativemeasurements of normal force, first normal stressdifference, and normal stress coefficient can bemade as a function of time, temperature, rate, andstress. In addition, the rheology of any material canbe determined independent of its sensitivity toloading conditions.

Sensory evaluation methods for liquid foods

Systematic analysis of texture is very importantfor food product development. Texture is a keyquality factor in food. One of the most importanttextural terms obtained is the analysis of thickness.Consumers usually associate changes in thicknesswith changes in viscous behavior of food materials.To develop predictive correlation between thicknessand rheological properties of foods, it is necessaryto understand the deformation process in themouth. The flow properties of liquids can be di-

Figure 4 Steady shear rate sweep on maple syrup.Figure 5 Steady shear rate sweep with first normal stress on honey.

Figure 6 Steady shear rate sweep and first normal stress on syrup.

Figure 7 Steady shear rate sweep and first normal stress on chocolate pudding.

vided into two main groups: 1) Newtonian, inwhich a sample's viscosity is independent of ap-plied shear, and 2) non-Newtonian, in which a sam-ple's viscosity is dependent on applied shear. F i g u re4 shows these two types of flow properties on pan-cake syrup. The results indicate the regular syrup ispredominately Newtonian, while the lite syrupshows non-Newtonian flow properties.

Food industries are especially concerned withvariations in taste with changes in flow behaviorand viscosity. In general, it is known that increasesin solution viscosity substantially decrease taste in-t e n s i t y. Also, it has been shown that increases innon-Newtonian flow decrease taste intensity. Theserheological properties would allow the systemic de-velopment of food products designed for desiredtexture and taste interactions. Viscosity–taste inter-actions are predicted by assuming that the rate ofdiffusion of the tasting agent to the surface of thetongue is the controlling parameter responsible forthe intensity of a tasting reaction.

Quantitative normal force measurements

During mixing or agitation, a viscoelastic fluidwill climb the impeller shaft in a phenomenonknown as the Weissenberg effect. This can be ob-s e rved in the home during the mixing of cake orchocolate brownie batter.

If a fluid is Newtonian, the viscosity is a constantand equal to the Newtonian viscosity, and the firstand second normal stress differences are zero. How-ever, viscoelastic fluids simultaneously exhibit bothfluid-like, viscous, and solid-like, elastic, behaviorand strong normal force response. Manifestation ofthis behavior, due to a high elastic component, cancreate difficult problems in processing and engi-neering design.

Utilizing the patented Differential Pressure Nor-mal Force Sensor, measurements of normal forc e ,first normal stress difference, and normal force co-efficient can be made as a function of shear rate asshown in F i g u re 5 for a commercial honey product.F i g u res 6 and 7 illustrate steady shear viscosity (η)and first normal stress difference (N1) for lite syrupand chocolate pudding, respectively. Obtaining ac-curate data for food materials is complicated by var-ious factors such as the presence of a yield stress,time-dependent and shear-dependent behavior,and chemical reactions occurring during processing(e.g., hydration, protein denaturation, and starc hgelatinization). This new measurement capabilitywill create significant advances in the utility of nor-mal stress data by the food industry.

Rheological properties of gelling systems abovetheir boiling point

Food manufacturers and processors relying onthe functional properties of aqueous gelling agentsare well aware that processing conditions such astime, temperature, and amount of shear during theheating can alter the final viscoelastic propertiesand thickening power of the resulting gelled net-work, as well as its gelling ability. To date, acquiringfundamental information on the viscoelastic prop-

18 / JUNE 2000

erties of aqueous gels has been difficult due to theexperimental requirement of making small ampli-tude dynamic oscillation measurements on low-vis-cosity aqueous solutions above their boiling point.Although rheological characterization of these sys-tems at elevated temperatures is extremely impor-tant to researchers in industry, until now there hasnot been a viable method available to produce dataon reaction kinetics and rheological properties ofaqueous solutions during gelatin.

A measuring system designed specifically for themeasurement of viscoelastic rheological properties(G′, G′ ′, tan delta) of solutions above their boilingpoint is the Sealed Cell shown in Figure 8.

The patented Sealed Cell measuring system usedin conjunction with the STRESSTECH rheometer al-lows measurements under pressure with full dy-namic oscillation and viscometric capability. Thecell employs a noncontacting, air-bearing sealrather than standard “O” rings. The air-bearing sealis effectively frictionless, and permits dynamic os-c i l l a t o ry testing throughout the frequency range ofthe instrument. Aqueous samples along with sol-vent-based systems can be measured above theirboiling point.

The gelatin behavior of an aqueous system hasbeen studied. First, a dynamic oscillatory tempera-ture scan was performed from 30 to 120 °C at aheating rate of 5 °C/min and a frequency of 0.2 Hz(F i g u re 9). The results indicate the sample possessesa low viscosity of 70 mPa s at room temperature,and gelatin starts at a temperature around 63 °C, asshown by the increase in viscoelastic properties.The reaction continues as a function of temperatureand time, and the end result is a thick, gel-like con-sistency sample by 120 °C. Of particular interest arethe results above 100 °C, where the data indicatethat the properties of the gelled system are stable.The data integrity is maintained well above theboiling point of water. No other device can obtaindynamic oscillatory results, especially on low-vis-cosity samples, above the sample's boiling pointdue to the mechanical friction limitation of thecell’s seals and bearings.

Rheometer system setup

STRESSTECH is a modular research rheometerwith a wide range of measuring systems and acces-sories. Measuring systems are available as concen-

tric cylinders, cone/plate, parallel plate, double con-centric cylinders, sealed/pressure cells, and dy-namic mechanical analysis (DMA) of rods, barsfibers, and films. Special measuring systems for low-volume, high-shear rates, and high sensitivity arealso offered. The measuring geometries can bemade in stainless steel, titanium, polycarbonate, orany user-defined material. The instrument is sup-plied standard with a patented Differential PressureQuantitative Normal Force Sensor for reproduciblesample loading history, thermal expansion mea-surements, and quantitative normal stress measure-ments. The diffusion air bearing has a low inertiawith high axial and radial mechanical stiffness.

The rheometer is operated with a separate powersupply unit that should be left on continuously.This reduces start-up times and makes it possible forthe instrument processor to maintain values as gapand other user-defined settings.

STRESSTECH HR, a high-resolution version of theinstrument, allows measurements at microradiandisplacement and extremely low applied torque.

Figure 10 DYNALYSER complete rheological characteri -zation system.

Figure 9 Gelation profile of an aqueous system above 100 °C.

Figure 8 Patented Sealed Cell measuring system.

APPLICATION NOTE continued

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tions for unique testing requirements, and can bereset to default values using default buttons. An ex-ample is the Oscillation Frequency Step measuringprogram, where stresses, delay times, integrationperiods, and sample sizes could be set individuallyfor all frequencies. Another example is the zoomingfunction, which is presented both in the Vi s c o m e-t ry Stress Step and the Oscillation Frequency Step,allowing any number of steps and increments to beselected. The instrument also performs controlledstrain and constant shear rate measurements, andhas automatic gap adjustments and thermal expan-sion compensation using the Differential PressureNormal Force Sensor. The system enhances mea-surement reproducibility since the sample loadinghistory is reproduced identically each time.

Rheometers for all user levels and applications

D Y N A LYSER advanced research rheometers (AT SR h e o S y s t e m s / REOLOGICA Instruments AB ) (F i g -u re 10) are modular research level rheometer systemsdesigned specifically to address the challenging anddiverse testing needs and requirements of the seriousrheologist. The instrument’s capability and perfor-mance result from a design and development effortfocused exclusively on input and recommendationsfrom rheometer users. The rheometer is designed fortesting any rheological significant material, includ-ing thermoplastics, thermosets, elastomers, semi-solids, and/or fluids systems.

VISCOTECH (AT S R h e o S y s t e m s / R E O L O G I C AInstruments AB ) is an entry-level research rheome-ter system that is fully upgradeable to a STRESS-TECH unit as the user’s needs and requirements dic-tate. DSR QC is a dynamic shear rheometer (DSR)designed specifically for routine viscoelastic mea-surements in the QC laboratory.

All the rheometers described here are producedaccording to ISO 9001 and are tested to operate ac-cording to the electromagnetic compatibility ruleswithin the European Community. The instrumentsare tested to be labeled with the CE-mark.

Conclusion

This article reviewed the important rheologicalcharacteristics of several different food products,and results generated with a STRESSTECH rheome-ter were presented. In addition, a detailed interpre-tation of data and correlation of the rheological re-sponse with the physical/chemical properties ofdifferent semisolid food products were detailed. Therheological characterization of foods provides im-portant information for engineers and food scien-tists to improve and optimize their products andmanufacturing processes. To d a y, most formulatorscount on rheological measurements to develop cus-tomer-favored products with a competitive edge inthe marketplace. A reliable research-level rheometerand a thorough understanding of food rheology isnow a necessity for food companies.

Temperature control cells are offered that use cir-culating fluid, Joule Thomson effect, and cry o g e n-ics covering the range from –180 to 500 °C. All mea-suring geometries are supported, i.e., cone/plate/parallel plate, concentric cylinder, and solids in tor-sion and tension. Several high-pressure cells withan upper range of 5800 psig are available.

Electronic unit

The instrument electronics are contained withinthe mechanical unit and the instrument is builtaround a dedicated, high-speed 32-bit central proc-essing unit (CPU). This consolidation enhances per-formance and versatility due to electrical connec-tions on the motherboard bus, rather than throughcables to a separate electronics cabinet. In addition,use of valuable bench space is kept to a minimum.The motor control is based on digital rather thananalog drive technology. The unit comes with abuilt-in diagnostic system and quick diagnostic ser-vice port for service engineers. Also included is amodem port for remote control operation and faultdiagnostics for service. The electronics power sup-ply is designed to operate on a line voltage between180 and 260 V or 90 and 140 V and an operatingfrequency between 47 and 63 Hz.

Software package

RheoExplorer 5.0 software (AT S R h e o S y s t e m s )is based on the Windows operating platform andruns under Windows NT 98/2000. The standardsoftware package is a true multitasking interfacewith selectable user levels, thus providing many ad-vantages to the food scientist. It is designed to pro-vide flexibility for configuring and using the rheol-ogy system. The computer is not dedicated tosimply running the instrument and is available forother use when making measurements. The com-puter can be used for printing previous results, writ-ing a report, or performing measurements with an-other instrument.

The software enables a normal PC to be used asthe interface to allow the user to control the instru-ment, and then collect and analyze the resultingdata. Viscometry, oscillation under stress and straincontrol, creep and recovery, constant rate, yieldstress, fast oscillation, process control, and project(multiexperiment linking), time-temperature super-position, and spectrum transformation packages al-low the sample to be analyzed via different rheolog-ical procedures. Powerful data analysis capabilityallows model fitting, graph and table customiza-tion, and cut/paste operation to all other Windows-based software.

The software includes possibilities to link user-designed methods including instrument setup andzero gapping using the Project Software. The dia-logue windows have many storable, editable func-

The rheological characterizationof foods provides importantinformation for engineers andfood scientists to improve andoptimize their products andmanufacturing processes.

Mr. Herh is Applications Manager, and Mr. Colo is Presi -dent, ATS RheoSystems, 52 Georgetown Rd., Bord e n -town, NJ 08505, U.S.A.; tel.: 609-298-2522; fax: 609-298-2795; e-mail: i n f o @ a t s rheosystems.com; homepage: www.atsrheosystems.com. Mr. Roye is Western Re -gion Manager, ATS RheoSystems, North Hollywood, CA,U.S.A.; Dr. Hedman is VP, Software Development, R E O -LOGICA Instruments AB, Lund, Sweden.

APPLICATION NOTE continued