According to its "best before" label, that steak you bought hasn't expired yet … but is it really safe to eat? Well, thanks to research being conducted at Canada's McMaster University, the food packaging itself may soon definitively let you know the answer.

What the scientists have created is a patch of specially-treated non-toxic transparent polymer film that could be incorporated into food packaging.

Called Sentinel Wrap, it's coated on one side with a microarray of droplets of DNA molecules known as DNAzymes. When pathogens such as E. coli come into contact with them, those DNAzymes fluoresce. Using an app on a smartphone or other mobile device, users could "read" that fluorescence to determine if the food inside the wrap was spoiled.

"A DNAzyme is a piece of DNA where the sequence has been selected such that it performs a specific activity – in this case it's a cleavage event – in the presence of a specific target," McMaster's Prof. Carlos Filipe explained to us. "Essentially the sequence has been designed so if a bacteria is present, that DNA is going to be broken in a particular location."

"If you can imagine that the DNAzyme is like a rope, and there are scissors that are going to cut that rope in a specific place, those scissors are the presence of E. coli. On one side of the rope, you have a fluorescence molecule, on the other side you have a quencher. If the rope is intact, they cancel each other out. But if you cut it with the scissors, and the two pieces become separate, now the fluorescence can be detected because the quencher is no longer attached."

The scientists believe that mass production of the patch should be relatively cheap and simple, as the DNAzymes are simply inkjet-printed onto the polymer. Additionally, the technology could conceivably be applied to bandages that indicate if wounds are infected, or to wraps for surgical instruments that indicate if they're not sterile.

The research, which was led by Filipe and assistant professor Tohid Didar, is described in a paper recently published in the journal ACS Nano.

For examples of other approaches to food packaging that indicates the freshness of its contents, check out what's been done at the University of Strathclyde and Fraunhofer.