Texture and Viscosity Measurement

The process of evaluating food texture has come a long way since the 1950s, when scientists at the Massachusetts Institute of Technology developed a novel instrument called the strain-gauge denture tenderometer. Designed to simulate the masticatory action of the human mouth, its purpose was to record in an unbiased manner the physical condition of a food sample.

A dentist outfitted the device with a set of plastic dentures, so it’s no surprise the instrument was affectionately known as "Mr. Chewer." Since food slipped out during mastication, the dentures were replaced with a plunger and a plate to simulate their function.

This modified instrument was later used by Alina Szczesniak ([email protected]) for her groundbreaking work to quantify the mechanical parameters of texture. Szczesniak, retired Principal Scientist, General Foods Technical Center, Tarrytown, N.Y., began her career with General Foods, and one of the first projects assigned to her was to "make sense" of texture.

"The long-term plans we developed focused on three questions," Szczesniak relates. "Is texture important? How can it be measured? How can it be created and preserved?"

Szczesniak grouped textural qualities into three main classes: mechanical (considered the most important), geometrical (referring to size and shape), and another related to moisture and fat content. She identified five independent mechanical parameters: hardness, cohesiveness, adhesiveness, viscosity, and elasticity, and two dependent parameters: chewiness and gumminess. Each dependent parameter is composed of two independent parameters; e.g., chewiness equals hardness times cohesiveness. To avoid confusion with its rheological meaning, the term elasticity was later changed to springiness.

Using the modified Mr. Chewer, which she called a texturometer, she then quantified the mechanical parameters of texture, a method later known as texture profile analysis (TPA).

TPA is the sensory analysis of the texture complex of a food in terms of its mechanical, geometrical, fat, and moisture characteristics, as well as the degree that each of these characteristics is present and the order in which they appear from the first bite through complete mastication.

"This method recognizes that the sensory perception of texture is a dynamic process with respect to force, time, temperature, and the influence of saliva," Szczesniak says.

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For her trailblazing work on texture, Szczesniak received the 1985 Nicholas Appert Award, the highest award that the Institute of Food Technologists bestows. She is the only woman thus far to receive this honor.

Subsequent work by others in the area of texture measurement led to such instruments as the TA.XTPlus Texture Analyzer, currently engineered and manufactured by Stable Micro Systems, Ltd., Godalming, Surrey, UK, and distributed in North America by Texture Technologies Corp., Scarsdale, N.Y. (www.texturetechnologies.com).

The TA.XTPlus and its Exponent software package have been specifically designed to address the flexibility customers demand for testing very diverse applications, says Marc Johnson ([email protected]), President, Texture Technologies.

"The hundreds of available probes and fixtures, as well as built-in application studies, allow customers to leverage our test experience and not be forced to create test methods from scratch," Johnson elaborates.

"When applied to TPA," Szczesniak adds, "the instrument offers a big advantage of being computerized and automatically calculating values of the mechanical parameters of texture."

Improving Texture Analysis
Better temperature controls and heightened capacity for data capture are two major improvements in current texture analyzers, according to Mohan Rao ([email protected]), Editor-in-Chief of Journal of Texture Studies.

"With additions of acoustic signal-capturing capabilities, using electronic instruments to capture the crunchiness of food has become prevalent in the industry," Rao points out. "And dynamic measurements of different food strengths and stiffness, instead of just static measurements, are becoming standard in industrial applications."

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While instruments are sufficiently measuring texture, continued improvements in identifying texture notes are still needed, says Malcolm Bourne ([email protected]), Professor Emeritus, Dept. of Food Science and Technology at Cornell University’s New York State Agricultural Experiment Station, Geneva, N.Y. Bourne’s 2002 book, Food Texture and Viscosity, Concept and Measurement (2nd Ed.), is considered the leading reference manual on food texture and viscosity, how these properties are measured, and how they relate to human assessments of textural quality. (Published by Academic Press, ISBN: 0-12-119062-5.)

According to Bourne, a "texture note" is a textural attribute that has been identified by people. For example, firmness, crispness, and creaminess are texture notes. Most foods show 15–30 texture notes from the time of pickup to final swallowing, compared to five taste notes detected on the tongue and hundreds of odor notes detected in the nose.

"Some texture notes, such as hardness or softness, are dominant and easily measured," Bourne elaborates. Others—e.g., tooth packing—are generally not directly identified by measurement.

Some say the interpretation of texture is a sensory attribute, and believe that only people—not instruments—can adequately measure texture, Bourne and Szczesniak point out.

"The universal testing machines that are used to measure textural properties of foods can be configured to use a number of different test principles, for example, puncture, gentle deformation, cutting, extrusion, bending-snapping, crushing, and tensile," Bourne emphasizes. "For the successful evaluation of texture, it’s critical to establish a principle that correlates instrument measurements with what people tell us is texture. Identifying the correct test principle is half the battle. Every test principle will correlate some texture properties and not others. If you use an inappropriate test principle for an instrument, you’ll never get a good correlation with the sensory assessment of texture quality."

Industry trends
Texture measurement is shifting from a theoretical concept to a practical application technology, observes Shirl Lakeway Jr. ([email protected]), President, Food Technology Corp., Sterling, Va. (www.foodtechcorp.com).

"Texture measurement techniques are being transferred from established research through to the factory floor, providing practical benefits to today’s food technologist," he says.

The company offers both simple single-point analysis and comprehensive multi-parameter TPA. Providing texture test solutions to the entire food industry, these systems are often used online or at factory process levels to objectively measure changing process conditions to maintain manufacturing consistency and improve product quality.

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According to Lakeway, manufacturers want to measure texture daily at processing facilities to decrease substandard product, and increase yield and product quality. "The goal is to produce a more consistent and acceptable consumer product, especially in developing countries looking to export. These users require affordable and easy-to-use quality assurance tools to maintain maximum product quality," he says.

There are new ways of measuring certain texture attributes besides firmness, Johnson says. "Now, for example, we can also determine the freshness of bread by evaluating its resilience, stiffness, and springiness. We can even simultaneously capture and analyze a product’s sound profiles."

Johnson adds that sound is a very high-profile textural attribute, particularly for consumers of cereals and snacks. "Food industry researchers are using jagged-line analysis to study the magnitude of force and acoustic signatures that products generate when tested. This type of analysis has been used for a long time to evaluate the effects of moisture and processing differences on a wide variety of foods. It was not a popular technique because it was a cumbersome analysis to conduct. It can now be automated with the click of a button."

When selecting texture analyzing equipment, people demand flexibility more than anything else, Johnson emphasizes. "It’s a tremendous benefit when you can solve a new problem tomorrow with the same instrument that you used to solve a problem yesterday."

Thanks to Szczesniak and Bourne, quantifying texture is now accepted as standard industry practice to solve design challenges and production issues, Johnson adds. "However, textural problems are as varied as customers’ products, manufacturing processes, and supply chains. No single test method can answer all of the textural problems a consumer will experience. Accordingly, texture analysis instruments must be easy to use, must offer methods that differentiate the methods at hand, yet must be flexible enough to handle today’s and tomorrow’s problems."

In the food industry, price drives decisions relative to selection of texture and viscosity testing equipment, according to Robert McGregor ([email protected]), Sales and Marketing Manager, Brookfield Engineering, Middleboro, Mass. (www.brookfieldengineering.com). Brookfield’s VisTex texture and viscosity testing systems include the LFRA Texture Analyzer and the DV-II+ Pro Viscometer.

"Measuring the viscosity of a starch suspension using a pre-determined temperature profile allows food processors to monitor real-world processing conditions in the laboratory," says Sal Iaquez ([email protected]), Vice President of Sales & Marketing, C.W. Brabender Instruments, South Hackensack, N.J. (www.cwbrabender.com).

Brabender offers the Visco-Corder, a rotational viscometer used to measure the viscosity of all foods that have flow characteristics. The company’s Viscograph®-E is used worldwide for measuring the viscosity of starch and of products containing starch. The company’s new Micro ViscoAmyloGraph® is able to evaluate the viscosity of starch or flour samples weighing as little as 5–10 g.

by Linda L. Leake,
Contributing Editor,
Food Safety Consultant,
Wilmington, N.C.
[email protected]