Magnetic Resonance Imaging Provides 100% Inspection

Sterility of aseptically packaged food products is obviously extremely important, and the most effective method of making sure that each container is sterile is to test each container.

However, testing each container for presence of bacteria by plating and incubation, measurement of pH change, or ATP bioluminescence testing takes time, effort, and money—and destroys the product. Thus, 100% inspection by these methods represents 100% destruction. Testing of product therefore generally involves statistical sampling, with a much lower percent loss from destructive testing.

Ross Products Div. of Abbott Laboratories, Columbus, Ohio, manufacturer of such liquid nutritional products as Ensure®, Ensure Plus®, Similac®, Introlite®, and Isomil®, wanted to make sure that every container of its aseptically packaged products leaving its plants was sterile. According to Tim Schenz, Research Fellow at Ross Products (phone 614-624-7024), spoilage typically occurs in aseptically packaged products at the rate of about 1 per 10,000 units, compared to less than 1 per 1,000,000 in retorted products. For this reason, most foods sold in aseptic packages in the United States either are low-pH products or are distributed refrigerated to maintain the sterility.

Schenz and his coworkers investigated a variety of techniques for 100% inspection of the company’s products, then decided that magnetic resonance imaging (MRI) might be the best way to go. Their goal was to develop an inspection system that would detect most bacteria that lead to food contamination; minimize package handling by inspecting cases of product instead of individual packages; and inspect the products at line speeds. They noted that as food products spoil, the pH decreases as a result of bacterial proteolysis of proteins and hydrolysis of carbohydrates. They developed a system that would correlate MRI relaxation times with decrease in pH caused by bacterial spoilage and were issued U.S. patent 5,270,650, “Non-Destructive Detection of Spoilage Using Nuclear Magnetic Resonance Spectroscopy,” in 1993.

They then approached IGC Technology Development, a Div. of Intermagnetics General Corp., Latham, N.Y., to test the technique on its products on a production basis. According to Roger Farrell, IGC’s Manager of Marketing and Programs (phone 518-782-1122), MRI is being used in the medical field and is applicable to 100% noncontacting inspection in such industries as pharmaceuticals, pulp and paper, and rubber. IGC subsequently designed and sold two systems to Ross Products, who installed them in two of its plants several years ago. The systems have been in successful operation since then.

One of the advantages of the system, Schenz said, is that it can inspect every container within a packed case at the same time, so that throughput is very high. Ultrasonic testing, another method of 100% inspection, requires a transducer to touch each container and thus is slower; it also doesn’t allow inspection of containers already in sealed cases. Schenz described the system and its application to nutritional products in a paper presented at the Food Science Conference in Norwich, U.K., in September 1998. The paper, “Online MR Imaging for Detection of Spoilage in Finished Packages,” is available on the Internet via IGC’s Web site at www.industrialmr.com.

Farrell described the system as a modified version of IGC’s Clinical Imaging System used for medical purposes. It consists of a low-field (0.15-tesla) nonconductive permanent magnet surrounding a large elliptical tunnel through which a conveyor passes. The system monitors the quality of the individual containers within a shipping case, rejects any case containing a defective container, and identifies which container within the case is defective. Different systems can be designed, he said, with different field strengths and different apertures.

The system installed in Ross Products’ plants works at production-line speeds, checking sealed cases and allowing all the containers in a given batch of a particular product to be tested for spoilage in less than one day.

IGC is currently modifying its software to look at different products, case configurations, and container sizes. The company continues to work with Ross Products and is interested in working with other food manufacturers, as well, particularly those in the dairy industry.

IGC is also participating in studies with the National Center for Food Safety and Technology, Summit-Argo, Ill., on additional products. NCFST is partially funded by the Food and Drug Administration, and one of the center’s objectives is to help FDA understand and be more proactive regarding new technologies such as MRI.

Ken Ghiron, (phone 708-563-1576), Research Assistant Professor at Illinois Institute of Technology, Chicago, has been working at NCFST with other center members for the past year and a half on magnetic thermometry (see the October 1999 issue, p. 86) and MRI. In late September 1999, at IGC’s request, he conducted MRI tests at IGC on aseptically packaged 1-L Tetra Brik containers of soymilk provided by Edensoy, Clinton, Mich.

Soymilk was chosen, he said, because it is one of the largest, fastest-growing markets for aseptically packaged products. Milk is probably larger worldwide, but soymilk is bigger and faster growing in the U.S. His tests compared determination of spoilage of soymilk by MRI, standard plating, pH, and ATP bioluminescence. He used Bacillus stearothermophilus spores and incubated the product for 30 hr. B. stearothermophilus doesn’t produce gas and is therefore difficult to detect by manual inspection, he said, but lowers pH during spoilage. Ghiron is currently analyzing the results of the tests. The next step, he added, is to study leak detection, other spoilage bacteria, additional quality characteristics, and other products, such as aseptic milk.

Magnetic resonance imaging (MRI), a sophisticated form of pulsed nuclear magnetic resonance, produces detailed cross-section images of a sample, whether it is a packaged food or a human body, and can distinguish different chemical and physical properties. In addition to medical purposes, magnetic resonance systems have been used for measuring moisture, fat, and sugar content, ripeness, and other characteristics of foods. This is the first application of MRI in detecting spoilage of foods. Many types of bacteria, mold, and yeast have been detected in inoculated product by MRI, including Aspergillis niger, Bacillus cereus, Bacillus circulans, Bacillus megaterium, Bacillus subtilis, Bacillus stearothermophilus, Candida albicans, Clostridium sporogenes, Escherichia coli, Lactobacillus casei, Pseudomonas aeruginosa, and Staphylococcus aureus.

Here’s how it works: When a product or test sample is placed within a large magnetic field, the direction of the spin of the product’s nuclei aligns with the magnetic field. When a pulse of radiofrequency energy is applied perpendicular to this main, or static, magnetic field, it causes the nuclei to rotate away (nutate) from the main magnetic field. When the RF pulse is stopped, the nuclei tend to return to their equilibrium position in the main field, i.e., “relax” back into alignment, producing an RF signal detected by the instrument’s receiver coil. Advanced computer programs convert that signal into images for medical diagnosis or into measurements for quality control and automation. The time it takes for the atoms to realign with the main magnetic field after the RF pulse can be correlated with characteristics of the product that is being tested, such as changes caused by spoilage, oxygen uptake, and viscosity changes.

The main advantage of MRI, Farrell said, is that it’s noninvasive and noncontacting. It essentially looks inside containers made of plastic, paper (plain or foillined), or glass. It can’t be used with ferromagnetic metal containers, since RF energy doesn’t penetrate them and they distort the uniformity of the static magnetic field.

PATENTS
Apparatus and method for manufacturing twisted pretzels. U.S. patent 5,955,118, filed 10/2/97, issued 9/21/99 to G.J. Powell, assigned to J&J Snack Foods Corp. Describes a device for making twisted pretzels, using a conveyor with a surface for receiving and transporting a rope of dough. An anvil engages the rope of dough so that continued conveyor movement forms a curved dough portion. A twister assembly grabs the two ends of the rope of dough and rotates to form a twisted section of dough. The twister assembly includes a set of plungers that press the two ends against the curved portion to form a twisted pretzel. The exact location and degree to which the ends are pressed against the curved portion are randomly varied so that each pretzel has a different appearance, as occurs with pretzels made by hand.

Method and apparatus for production of multi-flavored and multi-colored chewing gum. U.S. patent 5,955,116, filed 12/17/96, issued 9/21/99 toG. Kehoe et al., assigned to Nabisco Technology Co. Multi-veined chewing gum is produced by injecting different liquid flavors/dyes into a substantially homogeneous gum base. The injection may be performed while the gum composition is being conveyed in one or more extruder head barrels. Ingredients are then partially mixed to displace the colored veins in a direction transverse to the direction of extrusion to create veined, different multi-colored patterns, with different veins of injected liquid additives which provide a swirled or marbleized pattern in the opposing cut ends.

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Products & Literature
LIQUID COATING SYSTEM delivers uniform spray to baking surfaces and food products. Using electrostatic principles, the TotalStat® systems spray release agents onto oven bands, bread pans, cake pans, and muffin pans and specialty oils and postbake oils onto such food products as breads, cakes, crackers, and snacks. The nozzle has no moving parts and is said to provide precise edge-to-edge coverage with the exact amount of coating specified. For a 4-p brochure describing the system, contact TotalStat Div., United Air Specialists, Inc., 4440 Creek Rd., Cincinnati, OH 45242 (phone 888-841-3344 or 513-891-0400)— or circle 366.

BREAD PRODUCTION SYSTEM designed for production of specialty breads such as artisan, French, Italian, and ciabatta. The compact system consists of three components: the RDB-1500 Stress Free Rotary Bread Divider, the RB-40 Bread Rounder

(optional), and the SRM Series Bread Sheeter/Reverse Moulder. The conveyor-fed, rotary knife-cut divider features electronic scaling from 1.5 to 60 oz and produces up to 3,600 pieces/hr. The rounder is designed for sanitation and ease of cleaning. And the sheeter produces such products as baguettes, batards, and pan breads. For more information, contact Baking Machines, 4577-B Las Positas Rd., Livermore, CA 94550 (phone 925-449-3369, fax925-449-2144, www.bakingmachines.com) —or circle 367.

COMPACT X-RAY INSPECTION SYSTEM, the Dylog FB, is designed for inspection of food products in flexible packaging or in bulk. It is designed around a standard flat conveyor and can operate at up to 230 ft/min to keep up with production-line speeds. It can automatically detect and reject any unwanted high-density contaminant, such as glass, metal, stone, bone, and some plastics down to 0.5 mm in size. It can also detect missing or defective products and is not affected by the product, packaging material, temperature, or ingredients. For more information, contact Dylog® USA, 945 Longdale Ave., Longwood, FL 32750 (phone 407-265-9385, fax 407-830-0076)—or circle 368.

PIPELINE MIXER provides in-line processing while meeting the need for rapid and easy cleaning. The mixer features a skewed turbine/stator unit which operates at tight tolerances for continuous, high-speed, high-shear processing of foods and beverages, achieving precise product uniformity in applications involving homogenization, mixing, dispersion, and emulsifying. For more information on the Greerco® Sanitary Pipeline Mixer, contact Chemineer, Inc., 125 Flagship Dr., North Andover, MA 01845 (phone 978-687-0101, fax 978-687-8500)—or circle 369.

SANITATION GUIDE, “101 Questions and Answers on Advanced Oxidation for Food Processing,” discusses use of hydroxyl radicals, ozone, hydrogen peroxide, oxygen, and ultraviolet light for food sanitation. For a copy of the 20-p brochure, contact RGF® Environmental Systems, Inc., 3875 Fiscal Ct., West Palm Beach, FL 33404 (phone 800-842-7771 or 561-848-1826, fax 561-848-9454, www.rgf.com) — or circle 370.

WEIGH BATCHING AND BLENDING SYSTEMS are designed for use with free-flowing and non-free-flowing bulk ingredients. Two types are available: gain-in-weight batching systems and bulk bag loss-of-weight systems. Both systems can handle a variety of materials at high rates. For a copy of a 4-p brochure, “Automated Weigh Batching and Blending Equipment and Integrated Systems,” contact Flexicon Corp., P.O. Box 5269, Phillipsburg, NJ 08865-5269 (phone 888-353-9426, fax 908-859-4820, www.flexicon.com) —or circle 371.

by NEIL H. MERMELSTEIN
Senior Editor