An isothiocyanate is the chemical group contained in a number of phytochemicals found in cruciferous vegetables, such as broccoli, cauliflower, cabbage, and Brussels sprouts. These compounds give the flavoring to foods such as wasabi, horseradish, and mustard. While toxic in large quantities, there is quite a bit of evidence that isothiocyanates have anti-cancer properties at the concentrations found in foods. Some isothiocyanates are also used industrially and in agriculture to control pests and diseases.
There are many different kinds of isothiocyanates. All have a basic NCS structure, but have different organic groups attached to the nitrogen group. Different plants often make dissimilar isothiocyanates, and sometimes a plant makes multiple types. All are toxic to humans at high concentrations, and can cause severe irritation of the eyes.
Isothiocyanates are toxic to the plants that produce them. Due to this, they are stored as part of a larger molecule called a glucosinolate. In this form, they are inactive. The attack of the plant by something that chews its tissues, such as a caterpillar, releases an enzyme called a myrosinase. This enzyme breaks down the glucosinolate, releasing an isothiocyanate to drive the attacker away.
The compound responsible for the particular taste of horseradish, wasabi, and mustard is allyl isothiocyanate (AITC). This compound is released from the breakdown of mustard seed glucosinolates. It can be distilled to a high concentration and is then known as volatile oil of mustard. Used in small amounts as a flavoring agent in the food industry, at higher concentrations it is used in crop protection.
Other isothiocyanates of note from foods include sulforaphane, which is found in broccoli, cabbage, and Brussels sprouts, and phenethyl-isothiocyanate (PEITC). The latter compound is found in watercress. Also, benzyl isothiocyanate (BITC) is found in cabbage, Indian cress, and garden cress. Broccoli sprouts are a particularly good source of sulforaphane.
Aside from their contribution to the flavor of these foods, these compounds are of interest because of their effects on human health. Some studies have found that eating five or more servings of cruciferous vegetables a week significantly reduces cancer risk. There are a number of studies examining the role of specific isothiocyanates in inhibiting the development of various types of cancer.
The method of preparation of the cruciferous vegetables can have a significant effect on their amount of isothiocyanates, and their ability to prevent cancer. For the plant to produce the most isothiocyanate, it needs to have the myrosinase enzyme active. High heating inactivates this enzyme, and fewer isothiocyanates will be available if the vegetables are overly cooked. Bacteria in the intestine do have some myrosinase activity and will still release some of these compounds. Steaming and mild microwaving are recommended, but raw cruciferous vegetables have the highest concentrations of isothiocyanates.
Industrial uses of isothiocyanates include the use of methyl isothiocyanate as a precursor to a number of industrial compounds. It is also used as a soil fumigant in agriculture. In scientific research, phenylisothiocyanate is used for a traditional method of sequencing amino acids.