There are three types of mustard consumed in food. Brassica alba (yellow or white mustard), Brassica juncea (brown or Indian mustard), and Brassica nigra (Black Ingra or black mustard).
All species are spicy, with the highest level of spice found in black mustard and the least in white mustard. The zing comes from its isothiocyanate compounds, which develop after grinding the seed in cold water. Grinding releases the enzyme myrosin, which acts on a sinigrin glycoside molecule to produce a sulfur compound. This pungency decreases if the oil is boiled or mixed with other ingredients.
The zest of the oil of B. nigra is from erucic acid and isothiocyanates, which are said to cause accumulation of triglycerides in the blood and are thus considered bad for the heart; however, human trials have not been conducted to confirm these negative health effects.
Mustard oil is not permitted for use as cooking oil in the US and parts of Europe. It is allowed only for external use. Even though mustard oil is used extensively in food in South Asia, the presence of erucic acid and isothiocyanates raises health-related questions. When boiled, oil from black or brown mustard will help reduce the erucic acid and isothiocyanate content.
Additional reading:
Holy Herbs: Modern Connections to Ancient Plants
Asian herbs and their wondrous health-giving properties