Wendy's spent 3 years searching for a single ingredient

Wendy's has spent three years searching for a single ingredient: Blackberries.

The fast-food chain starting hunting down suppliers of the fruit to complement a salad Wendy's plans to offer next summer, the Wall Street Journal reports.

But finding enough blackberries to supply its 6,500 North American restaurants proved to be a bigger challenge than anyone at Wendy's could have ever anticipated.

"It's been a slow, painful journey for us," Dave Kourie, the head of procurement for Wendy's, told the Journal. "We spent 14 months scavenging around the industry, looking at more suppliers than we ever have."

The search for nearly 2 million pounds of blackberries, which involved reviews of more than 30 suppliers, has turned out to be the biggest supply-chain challenge in Wendy's history, according to the Journal.

The challenge highlights the difficulties that fast-food chains face as they evolve to meet growing consumer demand for fresh, unprocessed ingredients.

Chipotle, for example, had to stop selling pork for months at hundreds of its restaurants after discovering that one of its suppliers had violated its animal treatment standards.

The chain also had to start importing beef from Australia after failing to find enough grass-fed, antibiotic-free cattle in the US.

Chipotle is having these difficulties, and yet it's relatively small. The chain has fewer than 1,800 locations, compared to Wendy's which has 6,500 in North America, and McDonald's, which has more than 14,000 in the US.

Despite the challenges, McDonald's is also trying to remove additives from its food. The Big Mac chain is planning to phase out the use of chickens treated with antibiotics over the next two years.

That's going to have a massive impact on how chickens are raised in the US, considering the fact that McDonald's is one of the country's largest buyers of poultry.

As large restaurant chains phase out unprocessed foods, they also have to face new challenges in transporting the food rapidly enough before it spoils.

Wendy's considered adding raspberries to its salads, for example, but later dropped the idea because the berries would become too mushy before being served, according to the Journal's report.