Keep your fish fresh, thanks to chameleons

October 26, 2021
Original article

Red light, green light. Your first thought may be Squid Game, but these two colors are part of new food technology used for real squid.

Researchers invented a material that changes color to measure how fresh seafood is, inspired by (you guessed it) chameleons. It can save consumers from eating spoiled fish and can keep food waste out of landfills.

Why does your fish smell so fishy?

You probably are familiar with fish that smells… well, too fishy. This unpleasant odor comes from volatile gases in seafood, such as dimethylamine or ammonia. As the temperature of fish rises, its acidity changes, and ammonia is released.

Your salmon or shrimp has definitely spoiled if you can smell this gas — but ammonia can increase to dangerous levels before your nose can detect it. “Seafood easily spoils due to microbial growth that produces volatile amine gases,” said researcher Tao Chen in an interview.

A key part of seafood production is the ability to detect these volatile gases. Current standards take about four hours to find ammonia or dimethylamine in just one sample of fish.

Imagine if the commercial fishing industry had to set aside four hours for every piece of fish in their warehouse. The process would take days, and all the food would be at risk for spoiling. In reality, most fish inspections are done visually and are highly prone to error.

Here is where a team of food scientists and chemists enters the picture. New technology from Chen and his team at the Key Laboratory of Marine Materials detects when seafood has spoiled.

Chameleon skin inspired this material.

The skin of a chameleon can shift its hue to blend into different environments in just a few seconds. In a similar show of colors, the hydrogel can change its fluorescence from red to blue to green in a few minutes. These three colors allow scientists to visualize changes in response to stimuli.

The hydrogel changes color as heat and ammonia levels rise. The technology is easy to use, as the hydrogel can be placed directly into any package to check if fish or shellfish are safe to eat. Though customers should not eat the gel, it will not affect the product’s taste.

The soft material is unique in its ability to change colors. Both chemists and material engineers have struggled to design a synthetic fabric that could change colors. Until this study, scientists have been unable to model the structure of panther chameleon skin in a lab.

“Is it possible to mimic this unique structure into artificial color-changing materials? As described in our paper, the answer is yes,” Chen said.

“Up to now, the responsive color-changing capacity of synthetic materials was still far inferior to that of the natural chameleon skin,” researcher Patrick Théato said in an interview. Théato collaborated with the team in China for this bio-inspired project.

Science: Taste the rainbow.

The team discovered that the secret was in the separation. Instead of placing all fluorescent materials onto one sheet, each color has its own layer.

As seen in the diagram below, at the core of the hydrogel is a red layer that stays true to its hue. A middle blue layer measures the temperature of the seafood, and an outer green layer tests acidity and ammonia levels.

The hydrogel changes color in the presence of ammonia (NH3) or heat. It shifts to a green hue when ammonia is present or becomes more purple as the temperature rises. At either end of the spectrum, the fish in question is unsafe to eat.

The blue hydrogel layer changes color from purplish red to blue when the temperature rises. At 20º Celsius, the material appears to be purple or red, and when the heat rises to 50º Celsius, the hydrogel turns blue. The whole hydrogel turns green when ammonia is present and has no color change if ammonia is not present in the sample.

Not only do the different layers mimic the skin of a chameleon, but they also let scientists test the environmental variables on their own. Existing methods combine heat and ammonia into one reading and are less accurate. For example, current technologies would likely miss a slight change in acidity if the temperature stayed static.

Almost one-third of food in China is wasted.

Chen spoke to his personal motivation to create seafood-focused technology. His team hails from Ningbo, a coastal metropolis in China. “Many people in this city love seafood very much,” Chen said.

However, a significant amount of this catch ends up in landfills. A new study shows that 27% of all food in China is wasted per year. To put that number in perspective, food waste emissions in China are equal to total emissions in the United Kingdom.

How is the country keeping food out of the garbage? Well, President Xi Jinping declared war on food waste last year. As of April 2020, it is illegal to order too much food at a restaurant. ‘Mukbang‘ videos are similarly discouraged and were removed from many social media sites.

Another potential solution? The hydrogel. It can help reduce food waste on an industrial level. Commercial fisheries can use the gel for faster and more accurate readings and take immediate action if some of their seafood is beginning to spoil.

Color in cephalopods.

Théato, Chen, and many of their collaborators are working on a new project inspired by a different type of animal: Cephalopods.

These ocean dwellers – cuttlefish and squids, to name a few – are masters of camouflage. They can change their color faster than a chameleon. The researchers are creating a fluorescent hydrogel that takes notes from octopuses.

The team’s original hydrogel looked to acidity or temperature as the catalyst for color change. The new version has an electric stimulus, which is easier to control and free from any chemicals. It is currently under development for larger-scale applications.

Seems that octopuses are teaching us, after all.

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