CHAMPAIGN, Ill. -- Ultrasound's ability to see where human eyes cannot is being fine tuned to improve food inspection and make plastic-sealed food containers safer and potentially longer lasting.
Quality control in food packaging now depends on human visual inspection. Not only does the inspection slow production lines, people doing the work can only spot defects in seals down to about 50 microns in size -- the thickness of a human hair -- said Scott A. Morris, a professor of food science and of agricultural engineering at the University of Illinois.
"If you get a void or a piece of food across the area that is to be sealed, then you will have a conduit for bacteria to grow in and move through," Morris said.
So far, U. of I. tests with a newly developed pulse-echo acoustic technique have spotted defects of less than 10 microns, which is one-fifth the diameter of a human hair. A new target is 6.5 microns.
Findings of some of the research -- funded by the U.S. Food and Drug Administration and by the Value-Added Research Opportunities Program of the U. of I. Agricultural Experiment Station -- were published in the March issue of the Journal of Food Protection.
Just how much can the ultrasound see? "We can distinguish what's in the channel of a defect -- if it's air, if it's water or if it's tiny strands of protein," Morris said. "We can tell if a hole is empty or full. We can look at other contaminants, such as grease or dust, that may be in the seal and affect its strength. We're looking at a number of different failures of the joining system. Potentially, we could reduce the error inherent with visual inspection."
The Food Protection article looked at artificially constructed defects ranging from 10 to 37 microns in diameter and formed between two layers of polyethylene films used in plastic food bags or microwavable packages. The authors were Morris, William D. O'Brien Jr., a professor of electrical and computer engineering and researcher in the U. of I. Bioacoustics Research Laboratory, former graduate students Amjad A. Safvi and Harold J. Meerbaum, and FDA researcher Carol L. Harper.
Follow-up research is examining defects taken from actual production samples.
"The reality of food processing is that it must go very quickly to remain competitive. The implications of this research are that an acoustical method may prove to be the basis for the development of an accurate on-line sensor capable of 100 percent inspection of hot seals at production-line speeds," the authors concluded. "This, in turn, should eliminate the economic bottleneck that has restrained the widespread use of these types of packages."
In a study submitted for publication, Morris and O'Brien document how their ultrasound process outperforms other non-destructive imaging techniques such as machine-vision, X-ray, infrared and magnetic resonance, as well as ultrasound's advantages over commonly used destructive methods.