ACS Meeting News: Color-Changing Gels Track Food Quality | March 24, 2014 Issue - Vol. 92 Issue 12 | Chemical & Engineering News
Volume 92 Issue 12 | News of The Week
Issue Date: March 24, 2014 | Web Date: March 21, 2014

ACS Meeting News: Color-Changing Gels Track Food Quality

Nano-based materials indicate age, temperature history of perishables
Department: Science & Technology
News Channels: Analytical SCENE, Materials SCENE, Nano SCENE
Keywords: food safety, indicator, perishables, nanorods, E. coli
Expiration and sell-by dates on foods and beverages don’t always take into account how products are stored during the journey from farm to supermarket. In this video, researchers in China show off gelatin-based materials that might one day be attached to the outside of a food or beverage to report the product’s age and how long it’s been outside of refrigeration. The low-cost indicators go from red to orange to green as a perishable’s quality degrades.
Credit: American Chemical Society

If you’ve ever wondered whether you should drink a carton of milk a week past its sell-by date or whether the six-month-old chicken breast sitting in the freezer is still good, help might be on the way.

Researchers in China have developed an inexpensive gelatinlike material that evolves from red to orange to green as food or beverages are exposed to temperatures that encourage bacteria growth. The gels would be attached to the packaging material. Once the gel takes on a deep-green hue, the scientists say, the product should be thrown away.

Postdoc Chao Zhang of the Chinese University of Hong Kong (CUHK) reported on these food-quality indicators yesterday at the American Chemical Society national meeting in Dallas.

The gels owe their color-changing ability to the gold nanorods and silver chloride embedded within. Just after they’re manufactured, the indicators appear red because their nanorods are uncoated. Over time, though, silver ions deposit onto the rods, turning the gels green. Zhang’s team—which also includes CUHK’s Jianfang Wang and Chun-Hua Yan of Peking University, in Beijing—described the gels’ basic mechanism last year in ACS Nano (2013, DOI: 10.1021/nn401266u).

A nanoparticle-based gel goes from red to orange to green over time as food quality degrades.
Credit: ACS Nano
A clock showing blocks of gel on each hour. The gel at 12 o’clock is red, and as the dial progresses clockwise, the gel turns orange, and then green.
A nanoparticle-based gel goes from red to orange to green over time as food quality degrades.
Credit: ACS Nano

The researchers matched the gels’ rate of color change to the rate at which milk grows Escherichia coli by adding molecules, such as ascorbic acid, that affect how fast silver ions deposit onto the nanorods. Similarly, the team added other molecules, such as lactic acid, so that the gels’ color changes tracked temperature changes to milk. For instance, if a product is exposed to a temperature high enough to encourage rapid E. coli growth, these molecules speed up the gels’ red-to-green transition.

“You have to know the entire temperature history of a food product to know its quality,” Zhang said. Consumers usually don’t know whether a carton of milk has accidently been removed from refrigeration during the journey from farm to supermarket, so estimated expiration dates can sometimes be inaccurate, he explained.

A food quality indicator attached to a carton of Chinese milk.
Credit: Lauren Wolf/C&EN
A carton of Chinese milk with a green food-quality indicator (gel) stuck to its outside.
A food quality indicator attached to a carton of Chinese milk.
Credit: Lauren Wolf/C&EN

“The ability to match time and temperature kinetics to give a clear, actionable indicator is impressive,” said Timothy M. Swager, a chemist at Massachusetts Institute of Technology.

Zhang’s team hopes to tailor the gels for a variety of foods and beverages by tuning the concentrations of the indicators’ ingredients. He reported to the Division of Colloid & Surface Chemistry that he’s also testing semiliquid formulations of the indicators that are easier to mass produce. The team has filed a patent for the technology in China and may eventually commercialize the indicators via Wang’s new company Nanoseedz.

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Bob Barron (March 21, 2014 11:11 AM)
Nice achievement, but too bad it couldn't start green and turn red. Also, I could see where this would perhaps overpredict spoilage since it is on the outside of the container and would equilibrate much more quickly than the relatively large mass of material inside the package which would likely be exposed to a much different mean kinetic temperature. For example, perhaps the sensor would reach 50 F in a few minutes of exposure to that temperature, whereas the milk in the bottle might never get above 40 F in the same time period. I guess in this case, being a little conservative is probably erring in the right direction.

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