Each year, some 50 million people suffer food poisoning in the U.S. alone, including more than a million cases of potentially lethal salmonella poisoning.
So finding ways to prevent the spread of this and other kinds of bacteria is an important goal. But it is hard to detect bacteria in food products. The most common detection methods involve techniques such as microbiological culturing, polymerase chain reactions, high-performance liquid chromatography, and mass spectrometry, to name just a few.
These methods are complex, expensive, and time-consuming. And they require highly trained technicians to perform them. Consequently, few food companies and outlets have access to this kind of technology, and consumers have to take the hygiene of most foods they buy on trust.
Now that looks set to change thanks to the work of Jonghee Yoon and pals at the Korea Advanced Institutes of Science and Technology in South Korea. These guys have found a quick and cheap way to spot bacteria on the surface of foods in just a few seconds. They say their technique could be easily used in food processing lines and even fitted to standard home fridges.
The new technique is simple in principle. Bacteria such as salmonella have hair-like flagella that they use to propel themselves across surfaces. This movement turns the surface of contaminated food into an ocean of writhing microörganisms. It is this movement that Yoon and co have worked out how to spot.
Their method is straightforward. When a red, coherent laser beam hits biological tissue, it is scattered through the material. This scattering causes the light to interfere, creating a random pattern called laser speckle.
Since bacteria on the surface of food also scatter light, this influences the speckle. And as the bacteria move, the speckle pattern changes. “By detecting the decorrelation in the laser speckle intensity patterns from tissues, the living activities of microörganisms can be detected,” say Yoon and co.
All that is needed to monitor this change is a camera that can record the change over a few seconds. Yoon and co use one that takes images at a rate of 30 times a second and then process the images by subtracting one from another to reveal any difference.
They’ve put their gear through its paces with a set of experiments on chicken breast. They began by contaminating samples of chicken breast with the common bacteria Escherichia coli and Bacillus cereus, which are common causes of foodborne illness. They then zapped each of the samples, and a control, with a laser while recording the speckle with a camera.
The results clearly show the utility of the technique. The image subtraction technique quickly reveals which samples are contaminated and to what degree. The technique picks up both types of bacterial contamination, although it cannot distinguish between them. It also demonstrates that uncontaminated meat shows little or no change in the laser speckle pattern over time.
That’s an interesting result. Monitoring laser speckle is quick and easy to do with cheap equipment that can be retrospectively fitted to food processing lines. And it requires little specialized expertise.
Crucially, the technique does not require contact with the meat and so can be done at a distance. It can also see through transparent plastic packaging, which does not influence the speckle pattern.
That could have an important impact in many parts of the world, particularly in developing countries that do not have easy access to microbiology laboratories. And the equipment is so cheap and simple that it could easily be fitted to ordinary refrigerators designed for the home.
There are limitations, of course. Although the technique detects different types of bacteria, it cannot distinguish between them. And of course, it cannot spot contaminants that do not change the laser speckle over time. So it wouldn’t pick up viral contaminants, such as norovirus, which is responsible for five million causes of foodborne illness a year in the U.S. Neither does it detect the toxins produced by bacteria, which can cause illness even when the bacteria have been killed off.
Nevertheless, the new technique has the potential to significantly improve food hygiene and thereby reduce the number of cases of food poisoning each year. And that can’t be bad.