The Why Files The Why Files --

Even better than your nose?
16 AUGUST 2007

Picnic season: Happy food poisoning!
A few years ago, chemist John Lavigne was trying to change the color of light-emitting diodes -- the new high-tech, high-efficiency light bulbs that everybody calls LEDs. When an LED contained chemicals called polythiophenes, he could control the color by adding various chemicals called diamines.

Different diamine, different color.

At which point, the "Eureka!" light bulb lit in Lavigne's head, and he shifted his attention to rotten fish.

Cuts of fish are packaged in plastic and kept in a grocery store freezer
Studying fish spoilage gives a researcher plenty of opportunities to visit the sushi markets -- all in the name of science! Photo: USDA

Lavigne, an assistant professor of chemistry and biochemistry at the University of South Carolina, realized that protein-bearing foods release diamines as it spoils.

He also realized that studying fish would give him plenty of opportunities to visit the sushi markets in search of samples...

"We started out trying to make better light bulbs," Lavigne says. "We thought we had a way of getting better colors; it was really handy but was not useful for light bulbs, but we recognized that we were working with the type of compounds that are found in any protein-containing food that goes bad."

He adds that diamines are "basically what the bacteria spit out" as they devour king salmon or steak tartare. Fortunately, diamines also appear when food carrying even a low level of protein rots for other reasons.

When different diamines bonded with polythiophene, different colored lights appeared in the LED experiments. But could he turn polythiophene-diamine reaction around, and use it to detect specific diamines based on the color of light it absorbed?

"Yes," is the answer. In a study released this week, Lavigne showed that a color change can identify specific diamines. It can also signal the presence of rot hours before it could wrinkle a human nose.

Slab of pink salmon on white paper plate, red card stuck into the meatA lancet could be used to extract a small amount of liquid from a sample for testing. This mockup result shows that the sample is "fair," but the color in this early version is difficult to see. When the sample reaches "poor," your nose needs no help in detecting spoilage! Courtesy John Lavigne, University of South Carolina.

Spoiled rotten!
Spoilage is a burden on the food industry -- and when caused by bacteria, spoilage carries a huge health toll: the Centers for Disease Control estimates that 76 million cases of food-borne illness occur each year in the United States. Even a technology that could detect 10 percent of spoilages cases could help millions of people, Lavigne says.

The detector is akin to the paper strips used to measure pH in chemistry labs, Lavigne says. The heart of the detector -- actually the whole thing -- is a bunch of linked, short polythiophene chains that are trapped in a matrix.

He foresees using the new technology in tiny "dipsticks" that could be rubbed against meat or fish, or as a lancet that stabs the sample. In either case, the tester would give a quick, visual reading on safety.

As the detector touches spoiled food, chemical and physical changes alter how it absorbs light. In other words, the detector changes color. (Absorption governs the color of objects that reflect light: an object that absorbs blue and green, for example, looks red. But to be honest, no way can we Why Filers understand the detailed changes that affect absorption in Lavigne's detector.)

The new study showed that the detector can identify various diamines with 97 percent accuracy, even when they are found in a "highly competitive aqueous media."

That is scientese for "water with a lot of other crud in it," and it suggests that samples might need little or no cleaning before testing. Details like this can help a testing technology move from the lab to the marketplace.

The high-tech "dipstick" changes color according to which specific diamines are present. Courtesy John Lavigne

Caught you!
Another advantage of the new technology is its specificity: each of the 22 diamines tested can be identified by the specific light that bounces off the detector. It may be possible to identify "profiles" of various spoilage bacteria based on the reflections, Lavigne says.

A third advantage is sensitivity: when bacteria decay food, they produce diamines before they make the easier-to-smell amines that our noses immediately recognize as rotten food.

The test is also fast: Results appear within 5 to 10 seconds after the tester touches the sample, Lavigne says. Overall, he hopes the detector will be as simple as a home pregnancy test.

Woman wearing purple leans over a case full of meat, reaching, in the grocery store
Would a simple dipstick or lancet find spoilage at this meat counter? Photo: USDA

Lavigne says his process is an improvement on the current "electronic nose" technology, which uses similar chemistry, but measures electrical, not optical, properties of the polythiophene.

One "electronic nose" is currently being used to sniff explosives to locate deadly land mines left over after wars, Lavigne says. "It's beautiful work... you wave a wand and it starts beeping by looking at the conductivity of these [detector] materials."

But Lavigne is aiming for cheap, and electronics make the electronic nose expensive. The essential polythiophenes used in his detector are already used in specialty paints, and only tiny amounts would be needed for each detector. "A lot of electronic noses tend to be more complicated and expensive ... they don't use them once and throw them away. We were looking for a nice, quick test that is affordable, disposable and cheap. Doing it with color is the most convenient way. When things change color, it's very easy to tell."

-- David Tenenbaum

Related Why Files
• Poison medicine, poison food.
• Plant breeders put food on the table.
• Everything you wanted to know about the science of food.

• A Food Freshness Sensor Using the Multistate Response from Analyte-Induced Aggregation of a Cross-Reactive Poly(thiophene), Organic Letters, (ACS), Mark Maynor et al, Aug. 18, 2007.

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