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Flavour constituents of wasabi
Isothiocyanates (ITCs) are a group of naturally occurring sulphur compounds responsible
for the characteristic flavour of wasabi. The compounds are volatile in nature and are
evolved from plant tissues when they are disrupted eg. in the preparation of food, grating,
cutting, chewing etc. However, plant tissues do not contain ITCs, but contain glucosinolates
which are the precursors of ITCs. Glucosinolates (GSL) are a group of glucosides (ie. they
contain glucose in the structure), stored within the cell vacuoles of all Crucifereae plants.
Glucosinolates are a complex group of _-D thioglucose compounds synthesised from amino
acids. They contain a sulphonic group which is usually bound to sodium or potassium
making then anionic. The sulphate in the sulphonic group is attached through a C=N
bond and different side groups (R) give a wide range of related glucosinolates. Each one has
its own characteristic odour or taste. Due to the presence of the glucose in the molecule
glucosinolates are hydrophilic, non volatile compounds.

When plant tissues are mechanically disrupted or injured (eg. by chewing, crushing or
grating in the preparation of food), the myrosinase is released from the cell wall and in the presence of
adequate moisture myrosinase rapidly hydrolyses the GSLs to yield glucose and an
aglucone. Some of the intermediate steps have not been fully described. The organic
aglucone is unstable and undergoes Lossen Rearrangement to produce sulphate and a
variety of other products.

The nature of the products is dependent on the number of factors, including the structure of the
GSL side chain, the reaction conditions (eg. pH), the presence of cofactors (eg. metal ions,
specific proteins), temperature and duration as well as the age and condition of the plant tissues.
Isothiocyanates (ITCs) are formed from GSLs under neutral and alkaline conditions. However,
GSLs that contain a _-hydroxyl group in their side chain, give rise to ITCs that
spontaneously cyclize to form oxazolidinethiones. Some aromatic and heterocyclic GSLs produce ITCs
which are unstable at pH7 or higher and break down to release the corresponding alcohol and inorganic thiocyanate ions.
However, once formed, ITCs are more stable under acidic conditions. In weakly acidic pH or in
the presence of Fe+2 and/or endogenous nitrile factor, nitriles are produced from aglucone by
autolysis instead of ITC, with the liberation of elemental sulphur. The relative proportion of
ITC to nitriles can vary widely depending upon the conditions of autolysis. Thiocyanate
formation is believed to involve a cofactor, which may also be a protein, since it has been shown to be labile to both heat
and polar organic solvents. Most of the sulphur containing end products formed by the enzymatic and non-enzymatic reactions of GSLs are volatile.
Several ITCs have been reported from previous investigations into wasabi and each ITC has
a specific flavour profile with the complete taste of wasabi being derived from the combined tastes and odours of all the ITCs present.
A summary of different ITCs reported from previous investigations into the ITC content of wasabi tissue.
Allyl ITC has the main effect on the overall taste of wasabi because it is the ITC found in highest concentration in the rhizomes and other plant tissues.
Allyl ITC is also found in the highest concentration in horseradish. While allyl ITC is the main flavour component of
wasabi due to its pungency other ITCs e.g. 6-methylthiohexyl ITC and 7-methylthioheptyl ITC,
by giving their characteristic fresh greenish flavour, may contribute significantly to the total taste profile of wasabi.