An Impressive Natural Defense Mechanism

Don Mercer

A number of years ago, I worked on a project to extract a natural mucilage material from yellow mustard seeds.  The mucilage had many interesting properties and potential applications as a thickening agent in various sauces and dressings.  However, there were other aspects of mustard seeds that soon became apparent and quite intriguing.

Any of you who have bitten into a tiny raw mustard seed may have noticed the rather characteristic burning or bitter aftertaste left in your mouth.  What you are experiencing is actually part of mustard’s natural defense mechanism against attack by insects and other threatening predators.

Let’s take a look at the chemistry behind what is occurring at the molecular level to make this happen.  Mustard, and several other related crops, contain an enzyme called “myrosinase” within their cellular structure.  Enzymes are specialised protein molecules that allow them to speed up certain chemical reactions.  When the mustard’s cell walls are damaged, the enzyme is released and comes in contact with compounds in the seed known as “glucosinolates”.  Using small amounts of water present in the seed material, the myrosinase sets about converting the glucosinolates to isothyocyanates.  These bitter isothiocyanates are toxic to the insects and other predators and protect the mustard seeds from attack - a simple yet effective defense mechanism.

The question that probably now arises concerns your safety when consuming mustard seeds and other crops which have the myrosinase enzyme in them.  Fortunately, there is not a major issue here.  Glucosinolates are benign, or harmless to humans.  What we really need to do is prevent them from being converted into isothiocyanates.  We are quite lucky here as well, because it only takes a brief exposure to moderate heat to inactivate the myrosinase.  Since mustard seeds are heated in most food processes and applications, the reaction pathway to the isothiocyanates is shut down, and the problem is resolved.

Natural glucosinolate levels have been reduced in some crops to zero, or near zero.  This means they can be safely used as human or animal feed without the risk of being toxic.  A Canadian success story dates back to the late 1960s and early 1970s when rapeseed was bred to remove its glucosinolates and several other undesirable components.  The new commercially important oilseed crop, with its many health-associated attributes, was named “canola” to distinguish it from its less attractive predecessor.

An interesting off-shoot of the myrosinase-catalysed reaction is its possible use as a natural insecticide.  Glucosinolates can be converted to isothiocyanates which can then be sprayed onto plants to discourage insects and other pests from attacking them.  Using a surface application on the outside of the seeds will have no effect on the consumer. 

Although isothiocyanates are known to have negative effects on human health when ingested, there have been some promising studies to investigate possible medical applications of these compounds under controlled clinical conditions.

From a personal perspective, I have never looked at mustard quite the same as I used to when putting it on a hot dog or hamburger.  There is just so much more here than meets the eye.

Dr Don Mercer is Associate Professor in Food Science, Department of Food Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada; e-mail: dmercer@uoguelph.ca

Permission to reproduce this article is greatly appreciated and acknowledged.

          Tiny mustard seeds are more than just a source of condiments

                                 for hot dogs and hamburgers