The Truth About Mustard Oil: Behind the "For External Use Only" Label

Mustard oil is a fiery and delicious flavored oil that's used in many cuisines around the world, and it deserves a place in every well-stocked pantry.

During my teens, my dad went through a phase of pickling everything he could. It seemed like every jar he could get his hands on would inevitably be stuffed with spiced vegetables—everything from eggplants and carrots to fruit like green mangoes—and set on our windowsills to allow the pickles to cure in the sunlight.

Indian pickles—or aachars, as they're commonly known—are different than the pickles most non-Indian people are familiar with. While aachars are pickled using salt, acid, and sometimes sugar, they also rely on combinations of spices and oils to flavor the vegetables and create conditions that are inhospitable for harmful bacteria.

In India, the type of oil used in the kitchen changes with geography and has a profound impact on regional cuisines. For example, in the warmer parts of the country, sesame seed oil shows up in many recipes and, as a result, many of the dishes in these parts are characterized by sesame oil's nutty aroma. But in the north, the oil extracted from mustard seeds predominates, lending its unmistakable wasabi-like aroma to many dishes, particularly to aachars like the ones my dad made.

Mustard oil is an extremely popular oil among home cooks in China, Russia, and South Asia, particularly in the northern Indian state of West Bengal as well as in Bangladesh and Pakistan. Unlike oils that are merely infused with mustard seeds, true mustard oil is the fat extracted from the seeds of the mustard plant—in India, the oil is extracted from black mustard seeds from Brassica nigra. While in Russia and China, it's extracted from the brown mustard seeds from Brassica juncea.

Just like most cooking oils, mustard oil has a diverse set of applications. You can use it to sauté or stir-fry vegetables, and its aromatic qualities make it perfect for heating spices and chiles for tadka—the hot, seasoned oil that's used as a finishing touch for many dishes. Also, because it has a particularly high smoke point—about 480°F [249°C], which is higher than canola or even grapeseed oil—it's a great fat to deep-fry stuff in, like battered chunks of fresh fish, and it can be used in place of olive oil when roasting vegetables.

Why Mustard Oil Is Labeled "For External Use Only"

Despite mustard oil's popularity in some parts of the world, the sale of it in the United States for edible purposes is prohibited, and bottles of mustard oil must be sold with labels that indicate "For External Use Only." The obvious question that comes to mind, and one that people often ask me, is this: "Well, if it's okay for folks in India, why can't we cook and consume it here? Indians have been cooking with it for centuries, and they're fine."

To understand why mustard oil's sale is restricted, we need to take a closer look at its composition. All fats and oils are made up of a combination of glycerol and a mixture of molecules called fatty acids, and the composition of the fatty acids in a given oil or fat determines how it behaves. For example, fats that have a high proportion of saturated* fatty acids—animal fats, like lard and tallow, and some plant-derived fats, like coconut oil—will behave like a solid at room temperature. Fats that have a higher proportion of unsaturated fatty acids—olive oil and peanut oil, for example—will behave like liquids at room temperature.

*There are a lot more distinctions between saturated and unsaturated fats.

Mustard oil is rich in unsaturated fatty acids, but it also contains a special type of fatty acid called erucic acid, which lies at the center of the controversy surrounding the oil. Seeds from the brassica family of plants, which includes rapeseed and mustard, in addition to cabbage and kale, all contain varying amounts of erucic acid. Early experimental studies on animals in the 1950s suggested that erucic acid possibly had a role in the development of heart disease.

There were two notable consequences to this research. The first was that, in response to it, Canadian scientists created canola oil (a combination of the words "Canada" and "ola," the old English word for oil—although some say the "ola" is an acronym for "oil, low acid") by carefully breeding rapeseed plants that produced seeds with extremely low levels of erucic acid. (They also refined the oil through a series of industrial processes to strip it of any of its natural mustardy heat to create a more versatile neutral product.)

The other thing that happened was that mustard oil, thanks to its high levels of erucic acid, was restricted for sale for human consumption in America. Mustard oil was only permitted to be sold for use as a therapeutic massage oil, which is why bottles at Indian grocery stores are labeled "For External Use Only." But the safety of consuming erucic acid is still up in the air. Most of the data on erucic acid is obtained from studies on animals, and information on human studies is based on cell culture experiments and epidemiological dietary studies on populations that give conflicting results. To the best of my knowledge, and from what I've read, there are distinct differences in the way different animals respond to erucic acid and metabolize it.

Should You Cook With Mustard Oil?

Honestly, it's up to you whether to use it. But, thankfully, a new option has become available. A few years ago, I came across the first and only FDA-approved edible mustard oil, called Yandilla, which you can purchase online. Produced in Australia, this oil is extracted from seeds from a mustard plant that's been bred specifically to reduce the amount of erucic acid it contains while still maintaining mustard oil's essential flavor— quality neutral-tasting canola oil lacks.

Compared to the other mustard oils available at Indian grocery stores, the color and viscosity of Yandilla are much lighter, but it has the same degree of wasabi flavor. For my part, like other Indians I know, I have no issue with using non-FDA-approved mustard oil whenever I want the kick of its pungent aroma or I want to replicate one of my dad's aachars, but if you're concerned about the potential dangers of ingesting erucic acid, you should give Yandilla a try.

Mustard Oil Flavor Science

The reason Yandilla, with its relatively lower erucic acid content, tastes very similar to other mustard oils is that mustard oil's uniquely pungent character comes from an entirely unrelated molecule: sinigrin.

When black and brown mustard seeds are crushed, sinigrin is converted by an enzyme called myrosinase to allyl isothiocyanate***, the same molecule that gives horseradish and wasabi their fiery nature.

***White (yellow) mustard seeds are rarely used to produce oils, as crushing them produces a far less pungent molecule known as p-hydroxy benzyl isothiocyanate.

Allyl isothiocyanate is soluble in fat and extremely volatile; if you take a big sniff of mustard oil, you will immediately experience the burning sensation caused by allyl isothiocyanate inside your nose. That sensation is caused by a sensitivity of the mucous membranes called chemesthesis, which is triggered by allyl isothiocyanate binding with certain sensory receptors that detect pain and inflammation (the phenomenon is responsible for other, similar burning sensations produced by consuming any of the vegetables from the brassica family, including horseradish and wasabi.)

How to Use Mustard Oil

As I said above, it can be used in any number of ways: frying, sautéing, or included in the mixture for an aachar.

If you want its pungent kick to be particularly powerful, you'll want to heat the oil up, which increases the volatility of its potent aroma molecules, sending more of them into the air and up your nasal passages. For example, to jazz up raita, I'll fry spices in hot mustard oil and drizzle it on top. Just note that while heating the oil gives you a bigger aromatic wallop upfront, it also leads to a slightly less spicy mustard oil on the palate, precisely because you've driven some of those molecules out of the oil and into the air.

In the end, it's best to think of mustard oil just like any other flavorful oil, like olive oil, sesame oil, or walnut oil. I drizzle it over salads. including this spicy cucumber salad, and I use it as a finishing oil for many dishes, whether their focus is vegetables, fish, poultry, or steak—essentially wherever I think its wasabi-like edge will shine.

Read the original article on Serious Eats.