Glucosinolates are natural components of many pungent plants that occur as secondary metabolites of most of the Brassicales family, or the cruciferous vegetables. When these vegetables are chewed, a pungent taste arises due to the breakdown products of glucosinolates.
There are a number of vegetables, sprouts and seeds that contain glucosinolates. Table I below is a comprehensive list:
Table 1: Vegetables, sprouts and seeds containing Glucosinolates
Each vegetable, sprout and seed usually contains more than one glucosinolate. However, certain vegetables, sprouts and seeds may contain a predominant amount of one glucosinolate. An example is the following:
- Broccoli and broccoli sprouts contain large amounts of glucoraphanin
- Mustard seeds and Brussel sprouts contain a large amount of Sinigrin
- Garden cress and cabbage contain a large amount of glucotropaeolin
- Watercress contains a large amount of gluconasturtiin
The total number of documented glucosinolates from nature can be estimated to around 132, as of 2011. 1 For purposes of this article, we will focus on the 4 most important glucosinolates and the ones that have been the subject of the majority of medical research. These 4 glucosinolates include:
Gluconasturtiin, also known as phenethylglucosinolate, is a widely distributed glucosinolate in cruciferous vegetables. The name is derived from it occurrence in watercress which has the botanical name Nasturtium officinale.
Glucoraphanin is a glucosinolate distributed in broccoli, Brussel sprouts, cabbage and cauliflower. It is also found in large amounts in young sprouts of cruciferous vegetables, like broccoli sprouts.
Glucotropaeolin is a phytochemical from Tropaeolum majus, which is commonly known as garden nasturtium, Indian cress or monks cress. It is also found in cabbage.
Sinigrin is widely distributed in the plants of the Brassicaceae such as Brussel sprouts, broccoli, horseradish and black mustard seeds.
Table 2 below lists the various foods and the corresponding glucosinolate content.
Each of the vegetables, sprouts and seeds contain the enzyme myrosinase, which is activated when the vegetable, sprout or seeds is damaged (chopped or chewed) in the presence of water. The glucosinolate converts to an isothiocyanate (or thiocyanate) through the enzymatic activity of myrosinase. These isothiocyanates are the defensive substances of the plant.
Thus glucosinolates are the precursors to isothiocyanates through the breakdown of the enzyme myrosinase. Myrosinase activity on the glucosinolate also continues in the gastrointestinal tract through intestinal bacteria which allows for some further formation and absorption of isothiocyanates. 2
Figure 1: Glucosinolates Hydrolysis by Myrosinase (Source: Linus Pauling Institute – Isothiocyanates)
Sulforaphane is obtained from cruciferous vegetables such as broccoli, broccoli sprouts, Brussels sprouts, and cabbages. It is produced when the enzyme myrosinase transforms glucoraphanin into sulforaphane upon damage to the plant (such as from chewing), which allows the two compounds to mix and react.
When cruciferous vegetables are cooked, by either boiling in water, baking, frying or steamed, it prevents the formation of any significant levels of sulforaphane due to the heat inactivating the myrosinase enzyme.
However, the addition of powdered mustard seeds to the heat processed (cooked) cruciferous vegetables significantly increases the formation of sulforaphane. 3
The best way to add mustard seed powder is to grind mustard seeds, in a spice grinder, instead of using pre-powdered mustard seeds. This way you do not use mustard seed powder that has oxidized oils by sitting on the market shelf.
It has also been found that daikon radish added to cruciferous vegetables supports the formation of sulforaphane, even when the daikon radish is heated at 125 °C for 10 min. 4