The U.S.'s largest retail chain, Walmart, has asked its supplies of fresh, leafy greens to trace their products back to the farm using blockchain technology within one year.

Walmart and Sam's Club today sent a letter to the suppliers informing them of the change.

"Suppliers are expected to have all these systems in place by this time next year," the company said.

The change means that the information gathered by these suppliers will be open and accessible through technology that offers real-time, end-to-end traceability from farm to table, it said.

The announcement comes in the wake of a deadly multistate E. coli outbreak earlier this year linked to Arizona-grown romaine lettuce, which killed five people.

Frank Yiannas, VP of Food Safety at Walmart, explained that while health officials at the Centers for Disease Control told people to avoid eating lettuce that was grown in Yuma, Arizona, it was difficult for consumers to know how to determine where their lettuce was grown.

“None of the bags of salad had ‘Yuma, Arizona’ on them,” he said.

“In the future, using the technology we’re requiring, a customer could potentially scan a bag of salad and know with certainty where it came from.”

Walmart said that with the traditional paper-based method of capturing information that exists at many farms, packing houses and warehouses, tracking down important data from multiple sources is "extremely time-consuming".

With paper-based ledgers, Yiannas mentioned that it may take his team seven days to track down where a product came from. The team has to contact the supplier, get paper records and use those records to contact the company that imported or shipped the product to Walmart’s distribution center.

“The food system is absolutely too large for any single entity to [track],” Yiannas said.

But blockchain changes that.

“We’ve been working with IBM to digitize that, so the information is captured on the farm with a handheld system. It’s [also] captured at the packing house at the supplier,” Yiannas continued.