Application Note

Rheology of : New techniques, capabilities, and instruments

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

heology is the branch of science that deals the type of rheological measuring technique used. with the flow and deformation of materi- An antisettling agent should be tested at low shear R als. Rheological instrumentation and rheo- conditions, simulating settling in a bottle. A sample logical measurements have become essential tools with an antisettling agent will only be stable for a in the analytical laboratories of companies for specific period of time. Therefore, the desired shelf- characterizing ingredients and final products, as life stability must be known or determined. Stabil- well as for predicting product performance and ity is almost exclusively a consumer preference. consumer acceptance. Most food products change their appearance and The materials under investigation can range from texture upon storage. Therefore, an objective, quan- low- fluids to semisolids and gels to hard, titative method for determining their stability is solid-like food products. A knowledge of the rheolog- needed. Rheological measurements can be used to ical and mechanical properties of various food sys- predict shelf-life stability once there is historical tems is important in the design of flow processes for data on a given product. Comparing a sample with quality control, in predicting storage and stability acceptable life versus an unknown will provide a measurements, and in understanding and designing measure of shelf-life at the production stage, with- texture. Quality attributes such as spreadability and out the need to wait the weeks or months that Figure 1 STRESSTECH research rheometer. creaminess are extremely important to the accep- could be necessary for the actual test sample to set- tance of semisolid food products by consumers. tle or phase-separate. In the case of food materials, texture is a key quality factor. Rheological behavior is associated di- A switch in one of the raw ingredients can also rectly with textural qualities such as mouth feel, A knowledge of the rheological have a dramatic effect on the final product. For ex- taste, and shelf-life stability. As an example, rheo- and mechanical properties of ample, if the Florida orange crop is not up to stan- logical measurements are useful in storage stability dards, a producer of fruit juice may have to switch predictions of -based products such as various food systems is important to fruit from California or Spain. Due to the fruit's mayonnaise and salad dressings. geographically different environment, the natural in the design of flow processes for pectin content will differ. A change in formulation will be required to achieve a similar product. Importance of acceptable food quality control, in predicting storage and stability Consumers use subjective tests to determine the Rheological instrumentation p e rceived quality of a food product. For example, measurements, and in the determination of fruit firmness is based on the In principle, every type of consumable food prod- deformation resulting from the physical pressure understanding and designing uct has some rheological characteristic. Most readily applied by the hands and fingers. The toughness or texture. consumable food products contain ingredients that tenderness of meats is based on the effort required have a major impact on the rheology of the final for the teeth to penetrate and chew the flesh tissue. product. Food processors and raw material manufac- Food companies use instruments in an attempt to In food products, small changes in the amount of turers have been aware of the importance of viscos- objectify these subjective customer perceptions and additive can have a dramatic effect on the final prod- ity for many years. To d a y, rheological instrumenta- acceptance criteria. Objective instrument tests for uct. The formulation of popular sports drinks can be tion is considered a required analytical tool utilized texture also rely on deformation of the food mate- used as an example. To produce an acceptable lemon- by food scientists on a daily basis. These instruments rial. Toughness can be defined as the maximum based drink, 1.5% stabilizer/emulsifier is needed in are M i c r o s o f t ® Windows™ (Redmond, WA ) - b a s e d , f o rce required to slice through a sample. Firmness the /oil interface. At 1.5%, the drink is water and measurements are made quickly and easily with can be defined in terms of the force required to de- based. However, an increase to 1.55% changes the the use of straightforward, user-friendly software. form a body of material. product to an oil-based drink and results in a totally The operator simply loads the sample into the in- The quality of a food product depends strongly undrinkable product. In addition, the oil-based prod- strument and selects the appropriate experiment, on its formulation. Certain ingredients called modi- uct will show a drastic increase in viscosity with time and the instrument does the rest. fiers have a larger influence on the finished prod- compared to the water-based drink, which is com- Food products are complex mixtures of different u c t ’s properties than others. Examples of modifiers pletely stable. A simple rheological test could be the ingredients where individual ingredients are mixed are stabilizers, emulsifiers, and structural agents. monitoring of viscosity at low shear rates to establish together to produce a finished product. In many The type of modifier and its desired effect dictate if the correct amount of stabilizer is added. cases, the individual ingredients consist of mixtures

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

16 / JUNE 2000 Figure 4 Steady shear rate sweep on maple syrup. Figure 5 Steady shear rate sweep with first normal stress on honey. of solid as well as fluid components. Most times, above their boiling point; automatic gap setting; re- This phenomenological shear history and loading they are not homogeneous, and the properties vary mote diagnostics capability via modem; and auto- effect require that all structured samples be run un- throughout the sample. Tr a d i t i o n a l l y, single-point matic inertia compensation. In addition, all AT S der user-selectable controlled normal force condi- viscosity tests have been performed using empirical Rh e o S y s t e m s rheometers are designed on a modu- tions. In addition, all samples must be provided a fi- techniques. These simple viscosity experiments ex- lar platform allowing easy upgradability. A wide nite time for their respective internal structure to press the complex rheological response of a sample range of accessories satisfy the most demanding ap- rebuild after loading. into a single parameter, and are not adequate in plications with ease of operation. Considering dynamic oscillation frequency characterizing and/or providing insight into the sweeps, the viscoelastic properties are also depen- quality of food materials. The ingredients used to- User-selectable and quantitative, controlled axial dent on the residual loading history, as shown in day are numerous and expensive, and, as a result, normal force sample loading F i g u re 3. To overcome the sample-dependent load- the cost for controlling these ingredients is high. ing criteria, the rheometers are available with Detailed knowledge and an objective, reproducible, Although transient steady shear and periodic dy- Patented Differential Pressure Normal Force capa- multipoint measurement capable of decomposing namic oscillatory experiments provide information b i l i t y. With this highly sensitive, robust, and accu- the rheological behavior into individual compo- on the rheological properties of food products, they rate axial loading and measurement sensor, con- nents are necessary. The STRESSTECH rheometer do not completely characterize the system. Con- trolled normal force loading and quantitative (ATS RheoSystems, Bordentown, NJ, and R E O - cerning food samples, or any complex, two-phase measurements of normal force, first normal stress LOGICA Instruments AB, Lund, Sweden) used in system, the rheology is dependent on the sample’s difference, and normal stress coefficient can be this study provides all of the required instrument deformation history, loading conditions, and the ax- made as a function of time, temperature, rate, and features and capabilities. ial normal force applied during a measurement. For stress. In addition, the rheology of any material can The rheometer is a research-grade analytical in- example, stress/strain sweeps were performed on a be determined independent of its sensitivity to strument capable of measuring viscous, elastic, and c o m m e rcial mayonnaise product. The results of a loading conditions. viscoelastic properties of liquids, semisolids, and mayonnaise sample run immediately after loading solids (F i g u re 1). The instrument was developed for into the rheometer, and a new sample 300 sec after Sensory evaluation methods for liquid foods use by the serious rheologist, and provides a very loading into the rheometer, are shown in F i g u re 2, broad measurement range, spanning low-viscosity where shear moduli (G′ and G′ ′) are plotted as a Systematic analysis of texture is very important samples such as fruit juice to more viscous products function of shear stress. The sample run without the for food product development. Texture is a key such as creams and salad dressings, semisolids and 300-sec rest period exhibits lower properties since quality factor in food. One of the most important gels through hard cheeses and solid-state samples. the internal structure did not have sufficient time to textural terms obtained is the analysis of thickness. The rheometer incorporates the following fea- rebuild prior to testing. To dissipate this residual Consumers usually associate changes in thickness tures: wide torque, shear stress, temperature, shear loading history, it was determined that a 300-sec de- with changes in viscous behavior of food materials. rate, and frequency range; true Microsoft Windows- lay time was required after a controlled normal force To develop predictive correlation between thickness based operational software; patented Differential loading of 1 Newton. The results indicate that both and rheological properties of foods, it is necessary Pressure Quantitative Normal Force; patented the linear viscoelastic region and the crossover point to understand the deformation process in the Sealed Cell measuring system for testing samples of G′ and G′ ′ are affected by the loading condition. mouth. The flow properties of liquids can be di-

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

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

AMERICAN LABORATORY / 17 APPLICATION NOTE continued

Figure 8 Patented Sealed Cell measuring system.

vided into two main groups: 1) Newtonian, in which 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 re 4 shows these two types of flow properties on pan- cake syrup. The results indicate the regular syrup is predominately Newtonian, while the lite syrup shows non-Newtonian flow properties. Food industries are especially concerned with variations in taste with changes in flow behavior and viscosity. In general, it is known that increases in solution viscosity substantially decrease taste in- Figure 9 Gelation profile of an aqueous system above 100 °C. t e n s i t y. Also, it has been shown that increases in non-Newtonian flow decrease taste intensity. These rheological properties would allow the systemic de- velopment of food products designed for desired texture and taste interactions. Viscosity–taste inter- erties of aqueous gels has been difficult due to the actions are predicted by assuming that the rate of experimental requirement of making small ampli- diffusion of the tasting agent to the surface of the tude dynamic oscillation measurements on low-vis- tongue is the controlling parameter responsible for cosity aqueous solutions above their boiling point. the intensity of a tasting reaction. Although rheological characterization of these sys- tems at elevated temperatures is extremely impor- Quantitative normal force measurements tant to researchers in industry, until now there has not been a viable method available to produce data During mixing or agitation, a viscoelastic fluid on reaction kinetics and rheological properties of will climb the impeller shaft in a phenomenon aqueous solutions during gelatin. known as the Weissenberg effect. This can be ob- A measuring system designed specifically for the s e rved in the home during the mixing of cake or measurement of viscoelastic rheological properties chocolate brownie batter. (G′, G′ ′, tan delta) of solutions above their boiling If a fluid is Newtonian, the viscosity is a constant point is the Sealed Cell shown in Figure 8. and equal to the Newtonian viscosity, and the first The patented Sealed Cell measuring system used and second normal stress differences are zero. How- in conjunction with the STRESSTECH rheometer al- ever, viscoelastic fluids simultaneously exhibit both lows measurements under pressure with full dy- fluid-like, viscous, and solid-like, elastic, behavior namic oscillation and viscometric capability. The and strong normal force response. Manifestation of cell employs a noncontacting, air-bearing seal this behavior, due to a high elastic component, can rather than standard “O” rings. The air-bearing seal create difficult problems in processing and engi- is effectively frictionless, and permits dynamic os- neering design. c i l l a t o ry testing throughout the frequency range of Utilizing the patented Differential Pressure Nor- the instrument. Aqueous samples along with sol- mal Force Sensor, measurements of normal forc e , vent-based systems can be measured above their Figure 10 DYNALYSER complete rheological characteri - first normal stress difference, and normal force co- boiling point. zation system. efficient can be made as a function of shear rate as The gelatin behavior of an aqueous system has shown in F i g u re 5 for a commercial honey product. been studied. First, a dynamic oscillatory tempera- F i g u res 6 and 7 illustrate steady shear viscosity (η) ture scan was performed from 30 to 120 °C at a and first normal stress difference (N1) for lite syrup heating rate of 5 °C/min and a frequency of 0.2 Hz tric cylinders, cone/plate, parallel plate, double con- and chocolate pudding, respectively. Obtaining ac- (F i g u re 9). The results indicate the sample possesses centric cylinders, sealed/pressure cells, and dy- curate data for food materials is complicated by var- a low viscosity of 70 mPa s at room temperature, namic mechanical analysis (DMA) of rods, bars ious factors such as the presence of a yield stress, and gelatin starts at a temperature around 63 °C, as fibers, and films. Special measuring systems for low- time-dependent and shear-dependent behavior, shown by the increase in viscoelastic properties. volume, high-shear rates, and high sensitivity are and chemical reactions occurring during processing The reaction continues as a function of temperature also offered. The measuring geometries can be (e.g., hydration, denaturation, and starc h and time, and the end result is a thick, gel-like con- made in stainless steel, titanium, polycarbonate, or gelatinization). This new measurement capability sistency sample by 120 °C. Of particular interest are any user-defined material. The instrument is sup- will create significant advances in the utility of nor- the results above 100 °C, where the data indicate plied standard with a patented Differential Pressure mal stress data by the . that the properties of the gelled system are stable. Quantitative Normal Force Sensor for reproducible The data integrity is maintained well above the sample loading history, thermal expansion mea- boiling point of water. No other device can obtain surements, and quantitative normal stress measure- Rheological properties of gelling systems above dynamic oscillatory results, especially on low-vis- their boiling point ments. The diffusion air bearing has a low inertia cosity samples, above the sample's boiling point with high axial and radial mechanical stiffness. Food manufacturers and processors relying on due to the mechanical friction limitation of the The rheometer is operated with a separate power the functional properties of aqueous gelling agents cell’s seals and bearings. supply unit that should be left on continuously. are well aware that processing conditions such as This reduces start-up times and makes it possible for time, temperature, and amount of shear during the Rheometer system setup the instrument processor to maintain values as gap heating can alter the final viscoelastic properties and other user-defined settings. and thickening power of the resulting gelled net- STRESSTECH is a modular research rheometer STRESSTECH HR, a high-resolution version of the work, as well as its gelling ability. To date, acquiring with a wide range of measuring systems and acces- instrument, allows measurements at microradian fundamental information on the viscoelastic prop- sories. Measuring systems are available as concen- displacement and extremely low applied torque. continued

18 / JUNE 2000 APPLICATION NOTE continued

Temperature control cells are offered that use cir- tions for unique testing requirements, and can be culating fluid, Joule Thomson effect, and cry o g e n- reset to default values using default buttons. An ex- ics covering the range from –180 to 500 °C. All mea- ample is the Oscillation Frequency Step measuring suring geometries are supported, i.e., cone/plate/ program, where stresses, delay times, integration parallel plate, concentric cylinder, and solids in tor- periods, and sample sizes could be set individually sion and tension. Several high-pressure cells with for all frequencies. Another example is the zooming an upper range of 5800 psig are available. function, 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 be Electronic unit selected. The instrument also performs controlled The instrument electronics are contained within strain and constant shear rate measurements, and the mechanical unit and the instrument is built has automatic gap adjustments and thermal expan- around a dedicated, high-speed 32-bit central proc- sion compensation using the Differential Pressure essing unit (CPU). This consolidation enhances per- Normal Force Sensor. The system enhances mea- formance and versatility due to electrical connec- surement reproducibility since the sample loading tions on the motherboard bus, rather than through history is reproduced identically each time. cables to a separate electronics cabinet. In addition, use of valuable bench space is kept to a minimum. Rheometers for all user levels and applications The motor control is based on digital rather than analog drive technology. The unit comes with a D Y N A LYSER advanced research rheometers (ATS built-in diagnostic system and quick diagnostic ser- Rh e o S y s t e m s / REOLOGICA Instruments AB ) (Fi g - vice port for service engineers. Also included is a ur e 10) are modular research level rheometer systems modem port for remote control operation and fault designed specifically to address the challenging and diagnostics for service. The electronics power sup- diverse testing needs and requirements of the serious ply is designed to operate on a line voltage between rheologist. The instrument’s capability and perfor- 180 and 260 V or 90 and 140 V and an operating mance result from a design and development effort frequency between 47 and 63 Hz. focused exclusively on input and recommendations from rheometer users. The rheometer is designed for testing any rheological significant material, includ- The rheological characterization ing thermoplastics, thermosets, elastomers, semi- of foods provides important 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 A information for engineers and Instruments AB ) is an entry-level research rheome- food scientists to improve and ter system that is fully upgradeable to a STRESS- TECH unit as the user’s needs and requirements dic- optimize their products and tate. DSR QC is a dynamic shear rheometer (DSR) manufacturing processes. designed specifically for routine viscoelastic mea- surements in the QC laboratory. All the rheometers described here are produced Software package according to ISO 9001 and are tested to operate ac- cording to the electromagnetic compatibility rules RheoExplorer 5.0 software (AT S R h e o S y s t e m s ) within the European Community. The instruments is based on the Windows operating platform and are tested to be labeled with the CE-mark. runs under Windows NT 98/2000. The standard software package is a true multitasking interface with selectable user levels, thus providing many ad- Conclusion vantages to the food scientist. It is designed to pro- This article reviewed the important rheological vide flexibility for configuring and using the rheol- characteristics of several different food products, ogy system. The computer is not dedicated to and results generated with a STRESSTECH rheome- simply running the instrument and is available for ter were presented. In addition, a detailed interpre- other use when making measurements. The com- tation of data and correlation of the rheological re- puter can be used for printing previous results, writ- sponse with the physical/chemical properties of ing a report, or performing measurements with an- different semisolid food products were detailed. The other instrument. rheological characterization of foods provides im- The software enables a normal PC to be used as portant information for engineers and food scien- the interface to allow the user to control the instru- tists to improve and optimize their products and ment, and then collect and analyze the resulting manufacturing processes. To d a y, most formulators data. Viscometry, oscillation under stress and strain count on rheological measurements to develop cus- control, creep and recovery, constant rate, yield tomer-favored products with a competitive edge in stress, fast oscillation, process control, and project the marketplace. A reliable research-level rheometer (multiexperiment linking), time-temperature super- and a thorough understanding of food rheology is position, and spectrum transformation packages al- now a necessity for food companies. low the sample to be analyzed via different rheolog- ical procedures. Powerful data analysis capability allows model fitting, graph and table customiza- Mr. Herh is Applications Manager, and Mr. Colo is Presi - dent, ATS RheoSystems, 52 Georgetown Rd., Bord e n - tion, and cut/paste operation to all other Windows- town, NJ 08505, U.S.A.; tel.: 609-298-2522; fax: 609- based software. 298-2795; e-mail: i n f o @ a t s rheosystems.com; home The software includes possibilities to link user- page: www.atsrheosystems.com. Mr. Roye is Western Re - designed methods including instrument setup and gion Manager, ATS RheoSystems, North Hollywood, CA, zero gapping using the Project Software. The dia- U.S.A.; Dr. Hedman is VP, Software Development, R E O - logue windows have many storable, editable func- LOGICA Instruments AB, Lund, Sweden.

20 / JUNE 2000