When the chef Asma Khan was growing up in Kolkata, India, she learned that there was very little mustard oil couldn’t do. Dry skin? Weak joints? A common cold? A dab of the oil could cure them all.
But she loves cooking with it most: drizzling it into begun pora, a rich and smoky mashed eggplant, or toasting garam masala in it before adding rice and goat to make tehari.
“You feel it coming through your nose,” said Ms. Khan, the owner of the Indian restaurant Darjeeling Express in London. “There is a part of it which is really pungent. There is a sweeter side. It is all coming from the mustard oil. It is like a living oil.”
Mustard oil, which is derived from the seeds of the mustard plant, is an everyday ingredient in parts of India and the subcontinent — and is particularly essential in Bengali cooking. In the West, though, it doesn’t have the same visibility.
Because undiluted mustard oil has a high quantity of erucic acid, which has been associated with lipid buildup in the heart based on studies of rats, the European Food Safety Authority recommends consuming mustard oil only in small quantities.
In the United States, the Food and Drug Administration banned most brands of mustard oil for consumption, with many labeled “for external use only,” used by home cooks. (Some brands, such as Carrington Farms, are sold specifically for cooking.)
For Ms. Khan and many South Asian cooks, mustard oil is what makes much of their food sing.
“The real taste of the Bengal cuisine comes from the mustard oil,” said Satyen Mazumdar, a former owner of the Indian restaurant Masalawala on the Lower East Side of Manhattan. Mr. Mazumdar’s cooking inspired Masalawala & Sons, an eastern Indian restaurant in Park Slope, Brooklyn, owned by his son, Roni, and the chef Chintan Pandya.
At Masalawala & Sons, mustard oil weaves its way throughout the menu, awakening the bhetki paturi, fish steamed in a banana leaf, and adding a pleasant sting to the kosha mangsho, lush chunks of braised lamb redolent with warm spices.
The mustard plant, which grows across India, yields more than just a fragrant oil. South Asian cooks also use the seeds, which lend earthiness to dishes like lemon rice, and the peppery leaves, a critical ingredient in saag paneer.
Mustard is even part of popular culture: One of the most famous Bollywood scenes, in the movie “Dilwale Dulhania Le Jayenge,” features the actress Kajol running breezily through a bright-yellow mustard field toward her beloved, played by Shah Rukh Khan.
Back in the United States, there is a fledgling movement to change the F.D.A.’s stance on mustard oil. Last year, a group of nutrition specialists published a paper that suggested potential health benefits, like improved insulin resistance, of consuming mustard oil.
Pia Sörensen and Davide Bray, two scientists at Harvard University, are exploring how to reduce the erucic acid content in mustard oil. Dr. Sörensen said that, to prove whether mustard oil is harmful to eat, tests on humans — rather than rats — tracking the health of participants with different levels of mustard oil consumption are necessary. But these studies can take years.
Roni Mazumdar, of Masalawala & Sons, likened mustard oil to monosodium glutamate, or MSG, a flavor-intensifying chemical compound that can be found in a number of dishes, whether ranch dressing or mapo tofu, and faces stigma in the United States. Just like chefs who have educated diners on why MSG is safe to consume, “we are going to create that same cultural context all over with mustard oil,” Mr. Mazumdar said. “We are on that exact same path.”
After all, home cooks and professionals alike prepare dishes with mustard oil all the time.
“Think about it: The largest population in this entire world consumes mustard oil,” said Maneet Chauhan, who runs several restaurants in Nashville, including Chauhan Ale and Masala House. “And the population is not going down, so there has to be something behind that.”