Measuring Viscosity and Texture
LABORATORY
Rheology is stressful and strainful, M.A. (Andy) Rao told me jokingly when I asked him what challenges lie ahead in the measurement of viscosity and texture of foods. Rao ([email protected]), Professor Emeritus in the Dept. of Food Science & Technology at Cornell University, Geneva, N.Y., and Scientific Editor of the Food Engineering & Physical Properties section of Journal of Food Science, was one of several rheology experts I spoke with for this article.
Rheology, the flow of fluids and deformation of solids under stress and strain, is measured by rheometers, the most versatile types of which are controlled-stress and/or-strain rheometers, Rao said. While glass capillary tubes can be used for measuring the viscosity of low-viscosity liquids, he added, the cone-plate and concentric cylinder geometries are used for measuring more-complex, shear-dependent, viscoelastic behavior of food dispersions and gels.
Other advances include computerization. In the late 1980s, he said, computer chips became very powerful and RAM fairly large. Combined with advances in measuring stress and strain with precision, measurements that used to take half a day now take only a few minutes. New systems can monitor molecular self-assembly (gelation) for even 24 hr, but we need to keep test samples from dehydrating, he said. Now, researchers can focus more on the food sample itself.
One of the challenges ahead is measuring strain resulting from stress applied during thermal processing and high-pressure processing. We need sensors to get signals out and record them, he said, and he hasn’t seen them yet on commercially available rheometers.
It’s an exciting time for food science and food engineering, Rao said. Nanotechnology is pushing people into new structures totally unimagined before. We are now able to look at relatively small particles, such as fibrils, that are 7–10 nanometers in diameter and less than a micrometer long, he said. They form when proteins, such as soy and milk proteins, don’t fold into their usual compact 3-dimensional structure but instead form long, thin, and sometimes hollow shapes. Because of their different length-to-width ratios and stiffness, fibril solutions and gels can provide opportunities for the development of new protein-based functional foods. Rao and colleagues published a concise review of the rheological characteristics of fibril gels in Journal of Food Science in March 2009 (Vol. 74, Issue 3, pp. R47–R55).
Relating Structure and Performance
Next-generation rheological techniques are playing an ever-increasing role in the design of novel functional food products, said Samiul Amin ([email protected]), Rheologist/Senior Scientist at Malvern Instruments, Malvern, UK. Delivering functional benefits as well as delightful sensory experiences requires assembling food ingredients into products that have the correct responses in terms of product stability/breakdown, rheology, texture, and controlled release and delivery of nutraceuticals.
To ensure proper engineering and optimization of these key performance aspects, he said, the links between the food ingredients and formulation, microstructure, and rheology must be firmly established and optimized. This is especially critical for the food industry, as the majority of food systems can be classed as soft condensed matter and/or complex fluids. The characteristic of these systems is that they are usually structured at various length scales, have wide-ranging relaxation mechanisms, and give rise to complex responses when subjected to external stimuli such as shear, temperature, and pH. The resulting microstructure and the corresponding rheological response in such systems is in many cases affected not only by the formulation of the food systems, but also by the processing used in the manufacture of the final food product. The key is understanding how the microstructure and rheology of the food system is affecting the final performance.
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Food systems encompass a wide range of soft-condensed-matter systems, from emulsions and foams to colloidal glasses and gels. Food emulsifiers—surface-active lipids such as monoglycerides—self-assemble in a variety of structures, such as cubic or lamellar, he said, giving rise to a very rich phase behavior, and understanding and establishing the microstructure–rheology–performance linkages in such systems is highly challenging. It requires an understanding of how application- or process-relevant conditions, such as shear rate and temperature, affect the microstructure and how this in turn is linked to the observed rheological response.
Developing this understanding requires expert knowledge in a number of fields, namely rheology, complex fluid microstructure and physics, and colloid science. Since these areas of expertise are generally not present in many food companies, especially small-to-medium-sized companies, he said, Malvern has built an expert system interface into its new Kinexus rotational rheometer, which also incorporates technological innovations in sample preparation and loading, setup and operation, and data analysis and reporting. It has a built-in library of application and performance problems encountered in the food industry and also includes an extensive library of the most commonly encountered soft-condensed-matter systems from the food industry, as well as more-exotic ones from a range of other industries.
The software allows scientists to search for their application problem (stability, ease of spreading, texture, lubrication, etc.) and or microstructure (emulsion, cubic phase, or lamellar gel, etc.) or product form (spreads, mayonnaise, monoglyceride gels, or polysaccharides). Once the topic of interest has been found, both an application note highlighting the problem and the relevant rheometry test sequence, which can be directly run on the rheometer with standard operating procedures, are obtained.
With regard to challenges ahead, in addition to detailed rheological characterization, Amin said, the food industry always has a need for quick quality control tests to ensure that the rheology/viscosity is within specifications. Traditionally, this is done through low-end viscometers, which are generally robust and easy to operate in a QC environment. However, he added, there are several issues with these devices. The main issue is that the data collected on such instruments cannot in most cases be directly compared with data collected on high-end rheometers used in food R&D. This makes it virtually impossible to diagnose QC problems, Amin said.
In addition, measurements are timeconsuming, and cleaning, sample loading, and data management present considerable problems, he said. Rapid development of devices based on micro-electro-mechanical systems is leading toward a new generation of rheology sensors that can ultimately revolutionize QC in the food industry. They offer the potential to carry out rapid determination of advanced rheological parameters, have associated excellent data management systems, and can be integrated into an easy-to-use handheld instrument. Also as a result of their extremely small size, he said, such sensors offer the potential to be implemented as at-line or in-line process sensors, while their anticipated low cost will make disposability a viable option to overcome contamination and cleaning issues.
So, Amin said, a key aspect of next-generation food product design and manufacturing will lie in the ability to link and utilize expert knowledge gained in R&D to optimize product performance and ensure consistency through the manufacturing stages. This will require a new generation of rheology devices with expert systems in the high-end R&D environment and novel robust advanced low-cost rheology sensors in the manufacturing/QC environment. Linking them into an integrated data management system will form the basis for next-generation food design and manufacture, maximizing both product performance and consumer acceptance, as well as optimizing economics of product production.
Analyzing Texture for Quality Control
Robert (Bob) McGregor ([email protected]), Sales and Marketing Manager at Brookfield Engineering Laboratories Inc., Middleboro, Mass., said that a significant evolution taking place in the food industry is the increasing use of texture analysis in QC to perform final acceptance tests on a broad range of products, such as puddings, breads, snack foods, etc. The breakthrough is the price point for texture analyzers, along with increased education of personnel.
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Brookfield, celebrating its 75th anniversary, recently introduced its CT3 Texture Analyzer, which McGregor said provides a low base price for the instrument and low a la carte pricing for test probes—cylinders, cones, punches, balls.
On the technical side, he said, the need to duplicate textural qualities of low-fat and fat-free formulations for many food products necessitates testing that can quantify the similarities and differences in a meaningful and reproducible way. Texture analyzers provide this capability.
Use of software to set up more-elaborate test methods is one of the challenges that lie ahead, according to McGregor. This speaks to the fact, he said, that there are many ways to test a given product and no single approved procedure in most cases. Brookfield is playing a prominent role as methods are refined and, in some cases, standardized, he said. For example, he said, ASTM Committee E18 is investigating the correlation between sensory testing—performed by humans who fulfill the important function of qualifying new formulations—and the ability of instrumentation to mimic/duplicate the type of information that these panels produce when evaluating food products, and Brookfield’s product manager for texture analysis, Len Thibodeau ([email protected]), is chairing the committee.
Reducing Sample Size
Sal Iaquez ([email protected]), Vice President–Sales & Marketing, Food Div., at C.W. Brabender Instruments Inc., S. Hackensack, N.J., said that new developments include viscometers that can measure viscosity of micro samples, in an effort to use less sample to achieve faster results, without sacrificing valuable reproducible data. Along these lines, the company has designed a viscosity measurement instrument specifically for small sample sizes, the Micro-Visco-Amylograph®.
With regard to challenges ahead, he said that bridging laboratory data to “real-world” processes has always been very challenging and it is imperative to link this information from laboratory to production to get a better understanding of processability.
Brabender’s latest instrument in its line of viscosity-measuring equipment for food materials is the Consistometer Model E. This new instrument, Iaquez said, supersedes its legacy model, which has been the standard viscometer for measuring the viscosity and consistency of cream-style corn and baby foods. It utilizes a high-precision torque-cell technology to measure the viscosity of Newtonian and non-Newtonian materials and is well suited to evaluate a wide range of materials in laboratory, processing, and quality control environments. The new instrument, Iaquez said, can be used to measure apparent viscosity and perceived difference in the flow properties of fluids, pastes, slurries, and semi-solids with particulates, characterizing the flow behavior and predicting consistency, processability, and stability of products.
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Measuring Mouthfeel
Prajakta Kamerkar (prajakta. [email protected]), Rheologist and Product Specialist at Anton Paar USA, Ashland, Va., said that with the growing popularity of low-fat foods, the need for their characterization is increasing. To be accepted, low-calorie foods have to feature the same appearance, application, and mouthfeel as their high-calorie counterparts.
For years, rheology has been a valuable tool for food characterization, she said, but recently measurements of tribology—the science and technology related to friction, lubrication, and wear—have been linked to certain aspects of mouthfeel. Anton Paar has developed a measuring chamber that can be used to characterize the tribology of different products in addition to measuring their bulk rheological properties.
The tribology cell for the company’s Physica Modular Compact Rheometer turns the instrument into a fully functional tribometer based on the ball-on-3-plates principle, Kamerkar said. The setup consists of a holder for a half-inch ball and a bottom stage clamping the three friction plates. The bottom stage is movable in all three dimensions, ensuring equal load on all the contact points. The tribometer produces Stribeck curves of friction factor vs sliding speed, showing differences in the lubrication properties that can be related to creaminess and lubricity.
The bulk rheological data collected on the rheometer characterize viscosity and yield behavior, she said, which in turn can be correlated to texture and appearance. Accurate temperature control allows samples to be characterized at various processing and storage conditions. All of this allows food researchers to measure the effects of different ingredients on the mouthfeel of the final product.
The challenge ahead, she said, is to develop a general theory combining the tribological and rheological data for better description of mouthfeel.
Educational Opportunities
Most, if not all, manufacturers of rheology instrumentation offer a range of educational services and materials. They include libraries of application notes available on their Web sites, educational short courses—some on measurement techniques in general and others tied directly to their own instruments—Web-based seminars (“Webinars”), and hands-on instruction at users’ locations. Here are some examples:
Anton Paar offers videos (www.anton-paar.com/US/en/102), “eLearning CDs” on such subjects as “Basics of Viscometry” and “Basics of Rheometry” (www.anton-paar.com/US/en/940), as well as application notes.
Brookfield Engineering Laboratories offers its “Rheology School” (www.brookfieldengineering.com/education/rheology_school.asp), providing papers on rheology; seminars/courses (www.brookfieldengineering.com/education/index.asp), such as “Practical Course on Viscosity Measurement,” “Applied Viscosity Test Methods,” and “Practical Course on Texture Analysis”; and videos showing how to use the company’s instruments.
Malvern Instruments offers numerous Webinars (www.malvern.com/malvern2009webinarsummary), such as “Food Rheology.”
Neil H. Mermelstein, a Fellow of IFT, is Editor Emeritus of Food Technology
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