Burgers are still the biggest source of U.S. handheld sales in the food service channel, according to Technomic Inc.’s “Foodservice Prepared Sandwich Category” study.

Sandwiches, burgers and other handheld items accounted for nearly one out of every four dollars spent in food service in 2016, totaling just over $205 billion in sales, the study found. Out of those items, burgers comprised 44% of total U.S. handheld sales, while deli sandwiches weighed in at 13% and submarine sandwiches at 11%.

“Burgers and other traditional sandwiches remain the prominent choice for consumers who eat handhelds away from home today,” said Dave Henkes, senior principal of Technomic’s advisory group. “However, with millennial and Gen-Z consumers growing in spending power and cultural influence, ethnic flavors and non-conventional formats are redefining what Americans typically identify as a standard sandwich. These unique formats and flavors will only grow as these younger populations continue to mature.”

While 53% of all handheld sales occur during the lunch or mid-afternoon daypart, Technomic said, consumers are expanding their sandwich consumption to the morning and other snacking occasions. Food service operators may take advantage of these growing sandwich eating occasions by innovating with breakfast offerings and handheld snacks. Consumer favorites on the rise include formats such as tacos, burritos, breakfast sandwiches and wraps.

The two most important sandwich components ranked by operators and consumers were proteins and carriers.

“It follows that focusing on the quality and variety of meats and breads is critical for operators to stay relevant in the sandwich category,” Technomic said.

While cheeses, toppings and sauces garnered less attention, Technomic said, sauces may be an avenue to explore new flavor profiles, such as spicy, sweet and Asian, to attract millennial and Gen-Z consumers. Trending flavors include sriracha, ghost pepper, ginger, wasabi and chipotle.