Useful reference: 97 
 

The name "mustard" is given to various species, the most common being white mustard (B. hirta Moench or Sinapis alba L.), black mustard (B. nigra Koch) and Indian or leaf mustard (B. juncea Coss.). Mustard is cultivated for its seeds which yield oils and are used to make a condiment. It is occasionally grown (like rape or green manure) as a cover crop for fodder. 
 

TOXICITY. Mustard seeds contain a fixed oil (30-35%), which is usually extracted by cold pressing. Though edible, it is used mainly as an industrial oil. A second, very different oil is obtained by grinding the seeds and treating the flour with water to cause a chemical action between an enzyme and a glucoside, thereby producing an oil that is not present as such in the seeds. In black mustard the enzyme myrosinase acts on a glucoside - sinigrin - to produce a volatile, very irritating oil (from which the mustard gas used in the First World war was made). In white mustard the same enzyme acts on a different glucoside - sinalbin - to produce a less irritating oil than that of black mustard. This oil is diluted to make mustard plasters. If the oilcake is used for animal feed, the toxic substances must be evaporated by prolonged steaming (2 hours) or must be extracted. Sinigrin, being water soluble, can be extracted in the following way: the fat-free meal is heated with five parts water to 85 C for one hour and then filtered, after which the residue is washed five times with water. 
 

Mustard meal also contains toxic substances affecting the thyroid.