The Glucosinolate-Myrosinase System
In short, the answer to the question, "Is there myrosinase in mustard?" is an unequivocal yes. The enzyme myrosinase is an integral component of mustard seeds and other plants within the Brassicaceae family, such as broccoli, kale, and cabbage. The story behind mustard’s signature flavor is a perfect example of plant biochemistry in action. Myrosinase exists within the plant's cells, stored separately from its substrate compounds, known as glucosinolates. This clever compartmentalization prevents any reaction from occurring while the plant is intact. However, once the plant tissue is damaged—for example, by grinding the seeds or chewing the leaves—the myrosinase and glucosinolates mix, triggering a powerful chemical defense mechanism.
The most important aspect of this process is the hydrolysis reaction. Myrosinase catalyzes the cleavage of the β-glucoside bond in glucosinolates, releasing a glucose molecule and an unstable intermediate. This intermediate then rearranges itself to form various products, most notably isothiocyanates. These volatile compounds are what give mustard its characteristic bite and pungent aroma. The specific type of isothiocyanate produced depends on the variety of mustard seed. For instance, brown and black mustard seeds contain the glucosinolate sinigrin, which produces the volatile and highly pungent allyl isothiocyanate. In contrast, white or yellow mustard seeds contain sinalbin, which produces the less pungent but still spicy para-hydroxybenzyl isothiocyanate.
This reaction is highly sensitive to environmental factors. Temperature, pH, and the presence of water are all crucial for the myrosinase enzyme to function optimally. Hot water and acids, such as vinegar, can denature the myrosinase, deactivating the enzyme and resulting in a milder flavor. This is why prepared mustards often have a milder taste than freshly ground mustard, as the vinegar and heat used in processing inhibit the enzyme. Conversely, using cold water when preparing mustard powder creates the most intense heat, as it allows the enzyme to operate at its peak efficiency. The complex biochemistry of mustard has intrigued scientists for centuries and remains an area of study due to the potential health benefits of isothiocyanates, which are known for their anti-inflammatory, antibacterial, and antioxidant properties.
The Function of Myrosinase in Mustard
Plant Defense Mechanism
Myrosinase functions as a key component of the plant's defense system. When an herbivore chews on a mustard plant, the myrosinase and glucosinolates are mixed, producing toxic isothiocyanates that deter pests and pathogens.
Flavor and Pungency Production
The distinctive heat and flavor of mustard and other cruciferous vegetables are a direct result of the myrosinase-catalyzed hydrolysis of glucosinolates. This is the mechanism by which the familiar flavor is created during preparation or consumption.
Nutrient Activation
The same chemical process that produces pungency also creates bioactive compounds with potential health benefits. The formation of isothiocyanates and other breakdown products is a critical step in making these phytonutrients available. For example, the myrosinase from mustard can be added to broccoli powder to increase the bioavailability of sulforaphane, a potent isothiocyanate known for its health benefits.
Chemical Diversity in Defense
Different varieties of mustard contain different glucosinolates, leading to distinct isothiocyanates upon hydrolysis. This chemical diversity allows the plant to have a broad spectrum of defense against various threats.
Food Processing Control
The stability of the myrosinase enzyme is a critical factor in food processing. Heating mustard during processing can intentionally inactivate the enzyme to produce a milder product, while cold preparation preserves its activity for maximum pungency.
Myrosinase Activity Across Mustard Types
The myrosinase enzyme's activity and the specific glucosinolates it acts on vary depending on the type of mustard seed. This variation accounts for the different flavor profiles and pungency levels found in black, brown, and yellow mustards.
| Feature | Yellow/White Mustard (Sinapis alba) | Brown Mustard (Brassica juncea) | Black Mustard (Brassica nigra) |
|---|---|---|---|
| Primary Glucosinolate | Sinalbin | Sinigrin | Sinigrin |
| Primary Isothiocyanate | p-hydroxybenzyl isothiocyanate | Allyl isothiocyanate | Allyl isothiocyanate |
| Flavor Profile | Milder, less pungent, but still spicy | Strong, pungent, and hot | Most pungent and sharp flavor |
| Myrosinase Activity | Can be highly effective, producing a strong initial sensation | Robust activity when mixed with water | Similar robust activity to brown mustard |
| Pungency Source | Primarily from the isothiocyanate produced from sinalbin | Volatile allyl isothiocyanate causes the intense heat | Volatile allyl isothiocyanate is the main source of heat |
How the Reaction is Triggered
The glucosinolate-myrosinase reaction is a prime example of a plant's sophisticated defense mechanism. Within the mustard seed, the myrosinase and glucosinolates are kept separate by compartmentalization. The myrosinase is typically stored in specialized vacuoles, while the glucosinolates are located in neighboring cells. When the cell walls are broken—for example, by grinding the seeds and adding water—the enzyme and substrate come into contact. The subsequent hydrolysis reaction generates the pungent mustard oil, providing the plant with protection from insects and other predators. This is why whole mustard seeds have little to no flavor until they are crushed and moistened. The process is not just a culinary trick but a fundamental part of the plant's survival strategy. Interestingly, myrosinase activity can be influenced by other factors, including the presence of ascorbic acid (vitamin C). Some studies have shown that adding ascorbic acid can enhance myrosinase activity, potentially increasing the yield of health-promoting isothiocyanates.
Conclusion
Yes, myrosinase is a crucial enzyme present in mustard seeds, playing a vital role in both the plant's defense mechanism and the flavor we associate with the condiment. By catalyzing the breakdown of glucosinolates into potent isothiocyanates when the seeds are crushed and mixed with water, myrosinase is directly responsible for mustard's characteristic pungency. The interaction of myrosinase with different glucosinolates across mustard varieties, and its sensitivity to factors like temperature and acid, explains the diverse range of flavors available in prepared mustards. This biochemical partnership not only provides flavor but also produces compounds with significant biological activity, including antibacterial and anticancer properties, demonstrating a fascinating intersection between botany, food science, and health.
Further Reading
For more in-depth scientific information on this topic, a useful resource is the article from Frontiers in Plant Science titled, "Myrosinase-dependent and –independent formation and control of isothiocyanate products of glucosinolate hydrolysis", which provides detailed information on the chemical pathways and factors influencing the myrosinase reaction.
