Back to Basics

Use of Computed Radiology in the Food Industry

by Stuart Kleven*

 

Here is a great article on NDT in an area we all know very well but are unfamiliar with in terms of NDT: food. What is great is that the article reminds us of at least three important basics for radiography: that filmless imaging may be faster and cheaper; that image quality indicators should be used to provide knowledge that the necessary sensitivity is achieved; and that the geometry of the discontinuity may require two exposures shot at right angles. Hope you enjoy this article as much as I did. Frank Iddings Tutorial Projects Editor

 

Introduction
The reliability and integrity of food products is of paramount importance and is governed in the United States by the Food and Drug Administration (FDA) and the United States Department of Agriculture (USDA). Strict laws on contamination with organic and inorganic matter are closely monitored by those companies who process food. The inadvertent introduction of metallic objects into food products by processing equipment is an area that is especially closely scrutinized. Metal detectors and other nondestructive testing (NDT) methods are frequently used during inline processing and packaging. The testing equipment is part of the conveyor system. Some food processors have taken steps above and beyond normal requirements and may sample product offline either as part of in process testing or a final test. Typical products tested include: frozen dinners in foil pans with cardboard covers for foreign objects; raw chicken to ensure thorough removal of bones; and hamburger after processing for metallic screening or foreign objects.

 

Testing of Peanut Butter Stock
Peanut butter is a product that is mass produced by processing in a closed pressure vessel unit that chops the peanuts progressively finer and finer until a thick liquid is produced. As the viscous, semisolid peanut butter flows from the processing vessel, an inline, eddy current type metal detector monitors the product as it flows to the jar filling unit. If the metal detector alarm sounds, product is funneled off into 305 by 305 by 356 mm (12 by 12 by 14 in.) boxes in plastic bags. A normal day's production may yield as many as 500 boxes of product that require additional screening. These alarms prompted the quality department of a peanut butter manufacturer to try to find a means of testing the raw stock for broken pieces of the metallic chopping blade and other foreign objects.

---

The use of computed radiology reaches far beyond the testing of castings and welds.

---

After receiving quotations for radiography using film, the manufacturer was going to scrap the peanut butter since the cost of the testing approached the cost of the peanut butter. Upon receiving a request for quotation, we suggested the use of a computed radiologic system for the testing process. The cost associated with this filmless technique was approximately one third the cost of film radiography.

 

Computed Radiology and Standard Film Radiography
A comparison of the computed radiologic process with standard film radiography provides an idea why the process was effective in performing the screening.

In filmless computed radiology, the photostimulable phosphor imaging plate that captures the radiographic image requires no chemicals to process the image. The image is formed when impinging radiation changes the phosphor ions from a +2 to a +3 valence. This extra electron is released when the imaging plate is exposed to a scanning laser. Released electrons recombine in the conduction band where they were originally. This release of electrons emits energy in the form of light that is captured by a photomultiplier tube and then converted to a digital image. The phosphor imaging plate is then ready to use again since the laser reading it erases the previous image. This occurs within 30 to 60 s of processing of the plate.

In comparison, radiographic film, once exposed, cannot be reused. The film technique also requires a separate film processor or manual dip tanks with developer, fixer and water. The exposed film, when processed through the chemicals, releases some silver into the waste stream. This must be treated and/or controlled to prevent contamination of the water supply. In addition, by switching to filmless radiology, the cost of maintaining a film processor and purchase of the chemicals and use of water is eliminated. Time is also saved, since the typical dry to dry time for film processing is approximately 600 to 720 s.

The imaging plates are significantly faster (up to 20x) than radiographic film. This allows for shorter exposure times, reduced energy levels with an improved signal to noise ratio and improved geometry during setup since the source to film distance can be increased to enhance image sharpness without adding exposure time.

The imaging plate has a wide dynamic range and linear exposure response. This allows exposures to vary somewhat without losing sensitivity and compliance with the technique. Readable images have been obtained with exposure variations from 50% under to more than 200% over the normal time. This eliminates the need for costly reshots. The ability of the imaging plate to display a wide range of densities can also eliminate the need for extra shots to cover the full range of thicknesses. Application of preset reading algorithms can simplify the image processing and provide for proper coverage without reshooting.

Image storage is improved as well as the ability to retrieve images. Digital files can be copied and burned to compact disks without loss of information. Archival quality is also improved. Customers can view the exact image in seconds and can conference with the laboratory while viewing the image that has been electronically transferred by e-mail.

 

Computed Radiologic Testing of Peanut Butter Stock
The peanut butter blocks were tested using two shots on 356 by 432 mm (14 by 17 in.) photostimulable phosphor imaging plates. The X-ray exposure time was almost negligible since the peanut butter was easily penetrated by the radiation. To monitor the quality of the image, a wire type penetrameter (image quality indicator) approximating the thickness and density of the blade was placed on the source side of the block. The imaging plates were then processed by the laser plate reader and an image was obtained on the reading station within 60 s. Quick interpretation was easily made on the high resolution monitor and sorting of the boxes with foreign objects was accomplished in less than a day on an entire day's production of rejected peanut butter stock. The resultant images were of extremely high quality, even revealing the folds in the plastic bag used to hold the peanut butter. The wire image quality indicator was easily seen through the 305 to 356 mm (12 to 14 in.) of peanut butter. If there was a question as to the relevancy of an indication on an image, the second exposure made by rotating the box 90 degrees allowed the operator to verify objects that were originally oriented with the blade pieces parallel to the radiation. When placed perpendicular to the X-ray source, the questionable indications were easily identified as to relevancy (Figure 1).

(a)

(b)

Figure 1  - Foreign Objects detected in peanut butter through computed radiology: (a) chopping blade; (b) nail.

 

Conclusion
The use of computed radiology reaches far beyond the testing of castings and welds. The food processing industry has begun to recognize the value in testing speed and reduced cost associated with the process and has used it on food products. The added feature of being able to see the foreign objects provides a level of credibility and certainty to the testing above and beyond the sounding of a simple alarm. The screening of the suspect material has also proved a cost savings since the acceptable material may be placed back into the system, remixed and used. Other industries can also benefit from this technology, such as piping, electronics, composites, plastics and industries that must monitor the presence or absence of an object in an assembly or its relative orientation.

 

*Alloyweld Inspection Company, Inc., 796 Maple Lane, Bensenville, IL 60106; (630) 595-2145; fax (630) 595-2128; e-mail <alloyweldinsp@sbcglobal.net>.

 

Copyright © 2004 by the American Society for Nondestructive Testing, Inc. All rights reserved.