It is not legal to sell Mustard Oil in the US for food use because it contains 20 to 40% Erucic Acid, and the Food and Drug Administration (FDA) limit is about 3% (Erucic Acid has been shown to cause cancer in rats); The FDA isn’t happy either about its levels of allyl isothiocyanate, derivatives of which can be used for chemical weapons. It has to be labelled in the US as being “for external use only.”
Rapeseed oil had the same problem until the Erucic Acid was bred out of the crop to become Canola seed.
Mustard Oil is very popular in Northern India. All Indians heat it till it smokes to improve the flavour. This, though, also neutralizes the risk from the allyl isothiocyanate in it, which is destroyed during cooking. Every housewife knows this procedure: heat it until it starts to smoke, then lower heat down to the temperature you actually want to cook at. For this reason, it shouldn’t be used raw in salads. Nevertheless, some Bengalis say they will use small amounts of it raw for flavouring.
Mustard Flavoured Oils
Mustard Flavoured Oils are usually soy oil or canola oil flavoured with allyl isothiocyanate, or flavoured with real mustard oil, but in proportions such that the other oil dilutes the Erucic Acid down to permissible levels.
Use only a little for flavour, and always heat it first until it smokes, then lower to the temperature you want.
Per two tablespoons of mustard oil, 2 tablespoons of any plain-tasting oil plus 1 teaspoon mustard powder.
Store in fridge for up to 6 months.