Unfortunately, foods that are safe to eat can look exactly the same to the naked eye as food that's teeming with bacteria. But a prototype of a new test using "two-faced" droplets could let food companies check if their products are safe to eat before shipping them out, either by using a smartphone or simply by looking at them.

Currently, the most basic food safety tests involve a lot of waiting. Since bacteria are easier to find in bigger batches, food samples are placed in a dish and left for a few days to let any colonies grow to more easily-detectable sizes. Other methods can amplify the amount of bacterial DNA in a sample for quicker results, but these come at a higher cost and need more specialized equipment.

There are more simplified testing systems currently in the pipeline, including one built using a virus that latches onto E. coli and fluoresces, and another that detects gases associated with decay. A new test, developed at MIT, makes use of "two-faced" droplets called Janus emulsions, after the Roman god with two faces.

These droplets are made up of a fluorocarbon half and a hydrocarbon half, and normally, the heavier fluorocarbon side will sink to the bottom. Seen from above in that state, the droplets will look clear, but if viewed from another angle they become a bit murkier, as light bounces around inside. The MIT researchers took advantage of this mechanism to design their test.

The team attached a molecule to the surface of the hydrocarbon half, on the top of the droplet. Since this molecule binds to a protein commonly found in some types of E. coli, the droplets will roll over and clump together in the presence of the bacteria, making the droplets look hazy from above.

Janus droplets without E coli, left, and with E. coli, on the right (Credit: Qifan Zhang)

To translate this system into a visible food safety test, the researchers put a Petri dish full of the droplets on top of a QR code, those square barcode patterns that can be scanned with a smartphone app. If the system detects E. coli, the liquid turns opaque, rendering the code unreadable by the phone.

"The great advantage of our device is you don't need specialized instruments and technical training in order to do this," says Qifan Zhang, lead author of the study. "That can enable people from the factory, before shipping the food, to scan and test it to make sure it's safe."

The new test isn't quite ready for the factory floor though, as researchers still need to find a way to incorporate food samples that need to be tested into the droplets. The next steps are to make the test more sensitive, and extend it to seek out other food contaminants.

"You could imagine making really selective droplets to catch different bacteria, based on the sugar we put on them," says Suchol Savagatrup, another author of the study.

The research was published in the journal ACS Chemical Science.

Source: MIT

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