Nondestructive Food Testing

Since nondestructive testing makes it possible to examine the condition or quality of food without damaging it, the use of nondestructive analytical techniques is quickly gaining momentum in the food industry.

Currently, the three most popular, practical, and widely used nondestructive methods are spectroscopy, ultrasound, and nuclear magnetic resonance (NMR), according to Joseph Irudayaraj, Associate Professor in the Dept. of Agricultural and Biological Engineering at Purdue University, West Lafayette, Ind. ([email protected]). Other methods are imaging technology, bar coding, and biosensors.

Spectroscopy
Spectroscopy is an optical technology that has become a key non-contact tool used in the precise analysis and quality control of food components such as protein, sugars, lipids, color, and structure, Irudayaraj relates.

"Using the principle that molecular bonds absorb specific frequencies of light in the electromagnetic continuum depending upon the vibrational, electronic, or rotational states," he says, "a spectroscopic approach utilizes information from these phenomenological events to obtain fundamental food structure and composition information nondestructively."

"Accordingly, the major spectroscopic techniques are ultraviolet (200–400 nm), visible (400–800 nm), near-infrared (800–2400 nm), and mid-infrared (2,500–12,000 nm), based on its absorbance in the electromagnetic spectrum," he elaborates.

To fully understand spectroscopy, it’s necessary to review the characteristics of light. White light is composed of all the colors in the visible spectrum. When white light falls onto an object, certain colors within this spectrum are absorbed by the object, while the rest of the colors are reflected. The light reflected is seen by the eye, and its composite remaining color is interpreted by the eye as the object’s color, as in visible spectroscopy.

"For example," Irudayaraj explains, "when sunlight falls on a clover leaf, both red and blue shades are absorbed to aid in photosynthesis. The other colors are reflected and are interpreted by the eye as green. When the leaf wilts, the absorbed colors change. The change in color could indicate a complex biochemical process, or could be related to composition or a stress condition due to water loss."

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The near-infrared (NIR) region is the region that extends beyond red in the color spectrum and is not visible to humans.

One advantage, Irudayaraj says, is that NIR spectroscopy can typically penetrate much farther into a sample than, say, mid infrared, which is best used for structure determination. "NIR spectroscopy can be very useful in probing bulk material with little or no sample preparation," he points out.

General instrumentation for spectroscopy consists of a source, detector, and dispersive element (such as a prism, or more commonly, a diffraction grating) to allow the intensity at different wavelengths to be recorded. Fourier-transform NIR (FT-NIR) instruments using an interferometer are also common, and have a finer resolution, but cost slightly more. Depending on the sample, the spectrum can be measured by examining the intensity of absorbed, transmitted, or reflected light with respect to the wavelength.

"Infrared technology is relatively inexpensive compared to other nondestructive food testing methods," Irudayaraj says. "For example, a near-infrared instrument could cost $25,000–50,000. Another advantage is portability. A robust visible-infrared spectrometer operable on a production floor for real-time process monitoring will be a significant asset for quality control purposes."

Ultrasound
Ultrasound readings are taken by passing a transducer over the object to be evaluated. Vibrating crystals in the transducer produce high-frequency sound waves.

These sound waves travel through the food material and are detected by a second transducer at the other side or reflected back and are detected by the first transducer, explains John Coupland, Associate Professor in the Dept. of Food Science at Pennsylvania State University, University Park ([email protected]). "The speed of sound and ultrasonic losses can be measured from the difference between the sound waves generated and received."

 Ultrasonic sensors facilitate the automation of food production processes, he relates. "Low-power, high-frequency sound can, in some cases, be the ideal sensor, as it is noninvasive, nondestructive, portable, easy to use, and inexpensive." Basic ultrasound instruments typically cost $10,000–15,000.

"It’s possible to measure size, shape, composition, structure, and movement of food particles with ultrasound," Coupland says. "Ultrasound offers the advantage of being able to measure these properties of opaque foods inside containers and flowing down pipes. Ultrasonic devices are available to measure several simple properties of foods, including depth in a tank, flow rate in a pipe, and composition of simple binary solutions."

Industrial applications include the measurement of the texture, viscosity, and concentration of many solid or fluid foods; composition determination of eggs, meats, fruits and vegetables, dairy, and other products; measurement of a sample’s thickness, flow level, and temperature for the monitoring and control of several processes; and nondestructive inspection of egg shells and food packages.

"More-advanced techniques measure the ultrasonic properties of foods over a range of frequencies and can be used to calculate the size and concentration of particles," Coupland adds. "These technologies are emerging as the best method to characterize emulsions and dispersions. Ultrasound has a great future as a sensing system. New advances in micromanufacturing and electronics allow the development of smaller and cheaper sensors that will allow us to look at foods in new ways at all stages of their manufacture."

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Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon in which the magnetic property of some nuclei is utilized to characterize materials in a noninvasive manner. The instrument consists of a magnet, radiofrequency (RF) transmitter, a coil in which the sample sits that is used to transmit or receive RF signals, and electronics suitable for the experiment.

"When placed inside an NMR instrument, samples that contain atomic nuclei, such as hydrogen, behave like tiny magnets and align themselves with and against the external magnetic field," says Supriyo Ghosh, Applications Scientist for the Minispec Div., Bruker Optics Inc., Woodlands, Tex. ([email protected], www.brukeroptics.com). "Proper application of a radiofrequency pulse from the instrument excites nuclei to a higher energy level opposed to the direction of the magnetic field. While the hydrogen nuclei relax back to their thermodynamic equilibrium state, an electromagnetic signal is generated in the coil."

The hydrogen nuclei in different sample environments relax back at different rates, Ghosh points out. "This electromagnetic signal is processed by the NMR instrument and saved as discrete intensity vs time data. Mathematical analysis of this time-domain (TD) data provides a wealth of information regarding the quantity of constituents, their mobility, and molecular interaction in the sample. Since hydrogen is naturally abundant in food samples, it is a prime choice for analysis by NMR."

TD-NMR instruments can have sensors that are available as benchtop magnets or handheld probes. "Large sample sizes can be measured in a benchtop NMR, without any sample preparation," Ghosh relates. "The handheld instrument poses no restriction on the sample dimension and obtains a signal from a small volumetric region near the surface. The measurement is fast, just a few seconds, and doesn’t require expert technical knowledge to operate.

Since the NMR measurement leaves the sample intact, it can be utilized for other tests. "This truly noninvasive nature of the TD-NMR measurement opens up an excellent opportunity to study dynamic food processing systems," he says.

Because of these conveniences, the versatility of measurement, high reproducibility of the data, and low instrument cost (starting in the $40,000 range), TD-NMR has become a method of choice for routine quality control in the food processing industry.

The first major application of TD-NMR was developed for precise determination of solid fat content in food systems, Ghosh mentions. "Through many years of research in industry and academia, a large array of established applications is available to the food process industry for routine quality control, as well as for research. These applications include the measurement of moisture, oil and protein content, hydration level, droplet size distribution, polar content in deep-frying oil, solid-liquid ratio, and stability of emulsions, along with the study of freezing, drying, and spoilage of food, to name a few."

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