Food engineer Sundaram Gunasekaran, a professor of biological systems engineering, works with gold. But you won’t find the shiny yellow stuff in his lab; instead, the vials on his bench are mostly purple and red. Gunasekaran works with tiny pieces of gold—nanoparticles—that come in almost every color except gold. And those colors can tell a story.

Gunasekaran’s research focuses on food safety concerns, such as whether a food product was transported and stored properly or whether it has become contaminated. But how can a producer or consumer easily know a product’s history and whether it is safe to eat? That’s where gold nanoparticles come in handy.

“The color of gold nanoparticles will change depending on the size and shape of the particle,” explains Gunasekaran. “At different temperatures, depending on how long you let the particles grow, they acquire different sizes and shapes. And that changes their colors.”

Gunasekaran’s lab is using those color changes for a difficult task—tracking the time and temperature history of a food product as it is packaged, transported and stored. Up to now similar sensors have given consumers some of this information, but they can miss such critical events as, for example, a short temperature spike that could be enough to kick-start the growth of a dangerous microorganism.

The sensors that Gunasekaran and his team are developing provide a more complete and accurate story. The new sensor can differentiate between food stored at high temperatures first and low temperatures second versus a product stored first at low temperatures and then at high temperatures. And that’s thanks to the properties of the gold nanoparticles. The color of the first sample would be different than that of the second because of how and when the particles changed size and shape.

“We’re able to do this because the nanoparticle synthesis is affected by how the particles grow initially versus later,” explains Gunasekaran. “We call this the thermal history indicator, or THI.”

These gold nanoparticle sensors are being patented through the Wisconsin Alumni Research Foundation (WARF), and students in Gunasekaran’s lab won a UW–Madison Discovery to Product award. The student team also won a People’s Choice award in the 2014 Agricultural Innovation Prize competition.

They are now working to further develop and optimize the system. Since different food products travel through different channels, some sensors will be best used to track long-distance travel over the course of a month, while others will track history for only a matter of hours. Some sensors will work best in frozen storage and others will be optimized for various room temperatures.

The goal of optimization is a simple-to-use biosensor customized for any given food product. Gunasekaran envisions the sensors—now being developed as self-adhesive dots or stickers—being used anywhere along the food production channel. Producers, packagers, transporters and even consumers could easily use the biosensors to understand the thermal history of their product, saving time and money and avoiding recalls and health issues.

“There are a number of ways to use this technology, and making a food product’s history easy to see is the key,” says Gunasekaran. “Food is being wasted because people are throwing it out according to an expiration date, or people are getting sick because they eat food that’s gone bad. Those things can be avoided by having a better product safety indicator.”