Burger King has become the latest fast food chain in the United States to add an alternative meat option to its menu, joining an industry-wide push to appeal to growing numbers of health -- and environment-conscious consumers.

In early April, 59 of the fast food giant's restaurants in the St. Louis area started offering the Impossible Whopper, a vegetarian version of its signature burger priced about a dollar higher than the original. Following the successful test run, Burger King announced it would introduce the meatless option nationwide by the end of the year.

"Restaurants are really trying to tap into some of the growing consumer movement around shifting some of their diet or (adopting a) plant-based diet," said Aaron Adalja, assistant professor of food and beverage management at Cornell University.

Amid soaring U.S. demand for meat substitutes -- a market that grew 23 percent in 2018 to exceed $760 million (83.1 billion yen) in sales -- companies like Beyond Meat and Impossible Foods have been leading the charge to develop plant-based products that look and taste like meat.

The offerings of these two California companies target a growing population of so-called "flexitarians," people who for health and environmental reasons have turned to plant-based foods as a replacement for some or most of their regular meat intake.

Founded in 2009, Beyond Meat uses a protein blend derived from mung beans, peas and brown rice in its Beyond Burger, with small amounts of beet and pomegranate for meat-like coloration.

Mitsui & Co., an investor in the company since 2016, said it intends to sell plant-based meat alternatives in Japan, though it has not announced details of its plans yet.

Impossible Foods was established in 2011 and debuted its Impossible Burger at the trendy New York restaurant Momofuku Nishi in 2016. The company uses soy and potato proteins in its burger, as well as an iron-containing molecule called heme that provides both a red color and a distinctive "meaty" flavor.

Both companies have achieved prominence in a crowded market. Beyond Meat products are available in more than 30,000 outlets including health-conscious grocer Whole Foods and the fast food chain Carl's Jr., while Impossible Foods items are used in some 5,000 restaurants in the United States and Asia and will come to supermarkets later this year.

In April, Red Robin became the largest restaurant chain to roll out the Impossible Burger. White Castle also debuted the Impossible Slider that month, offering the plant-based version nationwide for $1.99 as compared to its $1 original slider.

"More customers were telling us they were craving vegetarian options," White Castle Vice President Jamie Richardson told Kyodo News. "Impossible Foods is a great partner, and we are excited to be working with them."

The organic-focused chain Bareburger serves both the Impossible Burger and the Beyond Burger in all of its more than 30 U.S. locations, allowing customers to replace a meat patty with a plant-based one for a price increase of around $3.

Although there are currently six Carl's Jr. restaurants and two Bareburger locations in Japan, neither company plans to offer alternative meat options in the country in the immediate future.

The former said it has not been able to secure a sufficient amount of plant-based meat product for its locations in Japan, while the latter raised the issue of cost as a barrier.

There has also been pushback from critics amid the race to scale up mock-meat production.

"Many of the novel products in the market are highly processed, and they contain components such as soy that has bad press," said Ricardo San Martin, head of the Alternative Meats Lab at the University of California, Berkeley.

"They are also high in salt or contain saturated fats, and the price is still high," he said of the products, many of which use coconut or sunflower oils to mimic the fat content of beef.

The use of new genetic engineering processes prior to data on long-term safety has drawn scrutiny as well, particularly in the case of Impossible Foods. Its signature ingredient -- soy leghemoglobin, a protein that contains heme for a "meaty" color and taste -- is produced through the fermentation of genetically engineered yeast.

In 2017, critics such as the ETC Group raised concerns over the U.S. Food and Drug Administration's inconclusive findings on the safety of consuming the protein. After Impossible Foods conducted further studies, the FDA finally gave its stamp of approval last year.

"I think getting over that hump of the perception that these meat alternatives are heavily processed is going to be a barrier to wider adoption," Cornell's Adalja said. "(But) I think the fact that a couple of major fast food chains have started experimenting with it is certainly promising."