Americans love burgers. This is indisputable, experts say. The simple protein patty in a bun is firmly imprinted on the nation’s culinary DNA.

Some data indicates we eat at least one burger a week. During a one-month period a year ago, more than 60 percent of consumers visited a quick-service burger restaurant at least once, while 30 percent sought out a fast-casual one, according to market research firm Mintel.

The ongoing passion between Americans and burgers of all shapes and sizes has given operators an opportunity to experiment and create premium options, Mintel notes, resulting in a wide range of burgers with various types of beef and other protein choices.

“A big part of burgers’ popularity is their familiarity,” says Caleb Bryant, foodservice analyst at Mintel in Chicago. “Most everyone kind of grew up eating burgers, so trying different types of proteins in a burger format takes away some of the mystery.”

Beef remains by far the most popular type of burger, but the descriptors—including Angus, grass-fed, and natural—are expanding. At the same time, more burgers are being made with poultry, seafood, and vegetables.

“Burgers are one of the best platforms for consumers to try new things,” Bryant says. “If you just want a simple cheeseburger, there are plenty of choices, and if you are a bit more adventuresome, burgers are an easy way to try new flavors.”

An operator like Austin, Texas–based Hopdoddy Burger Bar features a range of burger proteins, including three types of beef, plus chicken, turkey, tuna, and vegetarian. It also has had other varieties as specials, including lamb, venison, and antelope.

“You name it, we’ve probably ground it,” says Larry Perdido, cofounder and chef. “For us, burgers go well beyond beef.”

For many burger aficionados, particularly Millennials, a narrative is important.

“They want food with a story, and they want that story to make them feel good about what they are eating,” says Tom Ryan, founder of Denver-based Smashburger, which uses ground Angus beef for its patties that are smashed on the grill with a paddle.

There are three key differentiations in the beef used in burgers, says Meghan Pusey, senior director of integrated communications with the National Cattlemen’s Beef Association: the cattle’s diet, breed, and quality grade.

“The diet can be everything from the type of pasture grass in a particular region to whether the cattle is finished with grass or grain and the kind of grain,” she says. “There’s breed, like Angus or Wagyu, and finally the [U.S. Department of Agriculture (usda)] grading.” Post-harvest actions, like wet or dry aging, can improve tenderness and taste.

All cattle graze in pastures for the majority of their development. While most have a grain diet during the 90–120 days before going to market, a growing number feed on grass entirely, referenced in terms like “grass-fed” and “grass-finished.”

Some marketing terms can be confusing, Pusey says, but others are specific, like “certified organic” for cattle meeting USDA organic program standards, and “naturally raised,” meaning never receiving hormones or antibiotics nor fed animal byproducts.

The difference between frozen and fresh is easily understood. The idea of using fresh ground beef has been a Wendy’s hallmark since the late founder Dave Thomas grilled his first hamburger on a flat-top griddle in Columbus, Ohio, more than 46 years ago.

“His belief was you get a cleaner beef flavor when it’s fresh,” says Lori Estrada, Wendy’s senior vice president of research and development. “The texture is a lot more tender and juicy. Freezing beef changes it chemically.” She says freezing gives it a warmed-over flavor.

At Habit Burger, beef patties are cooked over an open flame with high heat, creating a burger with a charred flavor and texture to go with a powerful image of food being prepared to order.

The Habit burger “is a blend of aged trim for more flavor,” says Adam Baird, vice president of food and beverage for the Irvine, California–based company. “We season quite liberally with a blend of salt, pepper, garlic, and other spices.”

Open-flame cooking creates a unique burger taste like you would have at home on a backyard grill. “The flame underneath really gives a nice caramelization,” Baird says.

Carl’s Jr. and Hardee’s offer three types of beef burgers cooked in flame broilers. In addition to regular ground beef burgers, the sister chains offer Angus beef burgers and natural, grass-fed beef patties that have a slightly earthier flavor, a spokesman says.

The burger at Smashburger is a specific mix of certified Angus.

“Our goal was to develop a juicy, delicious burger that you can taste in every bite,” Ryan says. “We tried everything from meat from a tube to Kobe and everything in between.”

The process began during a blind taste test of more than 300 different mixes that were whittled down to a final four, all of which, it turns out, were certified Angus. “So we started with a flavor and ended up with an adjective: Angus,” he says, pointing out that it wasn’t the other way around.

At Hopdoddy, Angus beef is in most of its burgers, but there’s also a burger using grass-fed beef and another with certified Texas Akaushi beef, a breed of Wagyu.

“Akaushi cattle have a high-grain diet, and there’s much more marbling (intramuscular fat) than conventional beef, so the burger is more like eating a real good Wagyu steak,” Perdido says. “Cooked medium-rare, it is outrageously good.”

The certified Angus at Tampa, Florida–based Burger 21 is a proprietary blend ground by a supplier and pressed into shape in-house.

“We try to handle the beef as little as possible, because working it by hand melts down the fat,” says corporate chef Mike Remes. “We season and cook the burgers medium-rare on the flat top to give them a nice sear.”

Atlanta’s two-unit Yeah! Burger serves beef burgers sourced from grass-fed cattle raised at White Oak Pastures in nearby Bluffton, Georgia.

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Kelly Wallace, marketing and creative director, says the chain is sustainable in all practices, both front and back of the house, and that “the grass-fed beef plays into that. It also affects the taste, which we think is better.”

A distant relative of beef is bison, a meat becoming more popular as both a leaner option and an emotional link to America’s Old West.

Yeah! Burger’s bison burgers—from grass-fed, organically raised animals—are char-grilled rather than cooked on the flat top like the brand’s beef burgers. “People who come for bison choose it because it’s leaner and lower fat,” Wallace says.

Offering leaner bison means more care is required to cook it, says Jesse Gideon, chief operating officer and corporate chef at another Atlanta fast casual, Fresh To Order.

“We use not just the shoulder but the brisket, chuck, and sirloin,” he says. “All have a different reaction to heat, and some are fattier. Bison will dry out without enough fat.”

Your Burger, in Murfreesboro, Tennessee, has burgers made with Angus beef and also with beefalo, a bovine-bison breed. “It’s grass-fed, sustainable, and leaner, but with great taste,” says Kyia Faison, general manager and chef.

The most popular burger meat alternative to beef is ground turkey, which was the original non-beef burger, Mintel’s Bryant says.

The free-range, organic turkey at Yeah! Burger is ground in-house with a blend of herbs. “The entire burger is cooked on the flat-top for a better sear,” Wallace says. “It is very juicy and one of our customers’ favorites.”

Burger 21’s turkey burger uses hormone-free ground breast meat combined with ingredients like breadcrumbs, sour cream, and Dijon mustard. And at Hopdoddy, the ground turkey breast is mixed with sun-dried tomatoes and basil pesto.

Chicken fillets are popular at many limited-service restaurants, but Burger 21’s Remes considers that more a sandwich than a burger. His chicken burger is from a chicken breast, half of which is ground while the other half is cubed and then formed into a patty.

“The reason we both grind and cube is to provide a bite and mouthfeel—the ground chicken has a nice chew, and the chunks will be more of a breast texture,” he says. “We incorporate a little panko as a binder, some seasoning, a little Dijon, and sour cream.”

Hopdoddy’s chicken burger features both light and dark meat, Perdido says, with the latter “providing that extra fat you need in a burger.”

Lamb burgers are also showing up on more menus. One company, Superior Farms of Sacramento, California, offers all-natural lamb patties that appear at several limited-service locations, including AT&T Park, home of baseball’s San Francisco Giants.

“Consumers, especially Millennials, have a much broader palate for protein types, so we’re seeing lamb really appeal to them in the burger space,” says Anders Hemphill, Superior Farms’ vice president of marketing and brand strategy.

Seafood burgers have been on the rise, even though they can provide issues in providing a good burger texture. Many operators have opted for tuna, salmon, and shrimp.

Tuna was Fresh To Order’s first burger offering in 2011, followed by black bean and bison. Sushi-grade tuna is carefully ground, then blended with cilantro, ginger, and other ingredients, and finally formed into patties. They’re then cooked sous vide so the fish proteins lock with one another, and finally finished quickly on the grill.

Fresh To Order’s burgers do well at lunch and “really kill at night time,” says Gideon, who is developing shrimp and salmon burgers.

Burger 21 has ahi tuna and shrimp burgers. In both cases the seafood is chopped into small pieces, mixed with other ingredients, formed into patties, and flash fried. “The vibrant color of the seafood really stands out,” Remes says.

The biggest growth of any burger protein, however, is vegetarian, using ingredients such as mushrooms, quinoa, and black beans. These days, many fast-casual and some quick-service burger restaurants have their own versions with good taste and texture.

“We heard from consumers about having a non-beef burger option,” Wendy’s Estrada says. The company is testing a black bean burger that includes other ingredients like wild and brown rice, farro, quinoa, carrots, corn, and peppers, along with seasonings. “It is healthy, high in protein, and great tasting,” she says.

In most cases, veggie burgers are kept from cross contamination with meat burgers by being cooked on a separate griddle or grill, or in a separate oven.

Vegetables are also being combined with meat in blended burgers. Encouraged by the Mushroom Council, chefs at all types of foodservice outlets are developing mushroom-beef burgers that have fewer calories and less fat and sodium than standard burgers.

Your Burger created a special blended burger with beefalo and mushrooms last year that was so popular it will be back this year. “Beefalo is already a lean type of meat, and we added in the mushrooms for the health and taste factors,” Faison says.

The resulting flavor is something different than that of most burgers that have mushrooms, which are often sautéed and put on top of a beef burger. “People really liked the fact that the mushrooms were cooked right in the burger. It’s a much different taste.”