CHICAGO – McDonald's said Thursday it will offer fresh beef Quarter Pounders nationwide by the middle of next year, after switching from frozen patties in a number of test markets.

The fresh beef, which McDonald's said will be cooked to order, also will be on the chain's Signature Crafted line, which allows diners to customize their burgers with a choice of bun and toppings. The smaller beef patties at McDonald's restaurants, used on the cheeseburgers and Big Macs, will still come frozen.

The announcement comes just two weeks after the fast-food giant said it had expanded its fresh beef experiment to more than 300 restaurants in the Dallas area.

McDonald's had been testing fresh beef at 55 restaurants across the Dallas-Fort Worth area and 75 restaurants across northeast Oklahoma. It first started testing fresh beef in a handful of restaurants in the spring of last year.

McDonald's Dallas-Fort Worth franchisee Joe Jasper said the tests "received overwhelmingly positive feedback from customers and employees."

"We're proud to have been part of a test that is creating a watershed moment for McDonald's," he said in a statement.

Testing of the burgers, which McDonald's calls "hot off the grill," not only measured what customers think of the burgers, but also the price, kitchen efficiency and other factors.

Switching from frozen to fresh beef signals the importance McDonald's places on moving toward fresher ingredients as consumers demand them.

It already has made moves to clean up other parts of its menu, including removing antibiotics from chicken and preservatives from chicken nuggets. It's also made changes to its burgers to make them taste better, like searing them longer and toasting buns. Its fresh and frozen burgers are both 100 percent beef without fillers, additives or preservatives, McDonald's said.

The burger chain also is hoping fresh beef will improve customers' perception of the brand and bring people back in the door. While sales have improved in the U.S. over the last two years, customer traffic still has not. McDonald's acknowledged recently that it has lost 500 million customer transactions in the U.S. since 2012, when it first moved away from the Dollar Menu.

But adding fresh beef comes with big challenges for McDonald's, which is the largest purchaser of beef in the world. It means changes in everything from shipping to food preparation at the restaurants. It also increases the risk of foodborne illness spreading.

The fast pace of this and other changes at McDonald's has been a cornerstone of CEO Steve Easterbrook's tenure. The move to introduce all-day breakfast, arguably the biggest factor in McDonald's turnaround over the last two years, moved from test to nationwide launch in less than a year.