The role of tyrosinase in food science
Tyrosinase (EC 1.14.18.1) is a copper-containing enzyme, also known as polyphenol oxidase, that is widely distributed across many different organisms. It serves various biological functions, including melanin biosynthesis in animals and pigment formation in plants and fungi. In the food industry, its primary significance lies in its role in enzymatic browning, which occurs when plant tissues are damaged and exposed to oxygen. This reaction can lead to undesirable changes in color, flavor, and nutritional value, shortening a product's shelf life.
The chemical process of enzymatic browning
The enzymatic browning process begins when plant cells are ruptured, releasing tyrosinase and phenolic compounds that are normally kept separate. The enzyme catalyzes the oxidation of these phenolic substrates to produce o-quinones. These o-quinones are highly reactive and can undergo further polymerization reactions, either spontaneously or with other cellular components like amino acids and proteins, eventually forming dark-colored pigments known as melanins. This entire cascade of reactions is what causes the visible browning in foods like sliced apples or bananas.
Foods naturally containing tyrosinase
Tyrosinase activity is present in a wide variety of foods. Here are some of the most common examples:
- Mushrooms: Mushrooms, particularly the common button mushroom (Agaricus bisporus), are one of the richest and most frequently studied sources of tyrosinase. The rapid darkening of a cut mushroom's surface is a clear demonstration of this enzyme at work.
- Fruits: A number of fruits contain tyrosinase, with varying levels of activity. The enzyme is responsible for the browning seen in cut bananas, apples, and avocados, with avocado pulp being a particularly rich source.
- Vegetables: Besides potatoes, which are well-known to contain tyrosinase, other vegetables also show enzymatic browning. This includes eggplant and lettuce, especially when damaged or cut.
- Crustaceans: In addition to plant-based sources, tyrosinase is also found in many invertebrates, including crustaceans like shrimp and crabs. The black spots that can appear on shrimp after being caught and stored are a result of melanin synthesis mediated by tyrosinase.
Controlling tyrosinase activity in food
Controlling the activity of tyrosinase is crucial for extending the shelf life and maintaining the quality of fresh produce. Several methods can be used to inhibit the enzyme and prevent browning:
- Acidulation: Applying a mild acid, such as lemon juice or vinegar, can lower the pH below the optimal range for tyrosinase activity, effectively slowing down or stopping the browning reaction.
- Blanching: Heat treatment, like blanching, can denature the tyrosinase enzyme, destroying its function entirely. This is a common practice in the food industry before freezing fruits and vegetables.
- Oxygen Exclusion: Since oxygen is a necessary component for the oxidation process, limiting its access to the food can prevent browning. Vacuum-sealing and immersing sliced produce in water are examples of this method.
- Antioxidants: Ascorbic acid (Vitamin C) acts as a reducing agent that converts the initial o-quinones back to phenolic compounds, delaying the formation of melanin.
Comparison of foods containing tyrosinase
The following table compares some common foods in terms of tyrosinase presence and browning characteristics.
| Food Item | Primary Type of Tyrosinase | Speed of Browning | Effective Anti-Browning Method |
|---|---|---|---|
| Mushrooms | Fungal Tyrosinase | Very Fast | Acidulation (e.g., citric acid), cooking |
| Apples | Plant Tyrosinase (PPO) | Medium | Lemon juice, vacuum sealing |
| Avocados | Plant Tyrosinase (PPO) | Very Fast | Lemon juice, oxygen exclusion |
| Potatoes | Plant Tyrosinase (PPO) | Medium-Slow | Water immersion, boiling (blanching) |
| Shrimp | Crustacean Tyrosinase | Fast (post-mortem) | Cold storage, antioxidants (e.g., sulfite) |
Sources of tyrosinase beyond food
While this article focuses on dietary sources, it's important to recognize that tyrosinase is not exclusive to food items. The enzyme is found throughout the biological world and has significant applications in various industries. Microorganisms, such as certain bacteria (Bacillus aryabhattai) and fungi (Neurospora crassa), are also major producers of tyrosinase and are utilized for industrial purposes, including bioremediation and the production of L-DOPA for pharmaceuticals. Furthermore, cosmetic research actively seeks tyrosinase inhibitors from natural sources to reduce skin hyperpigmentation. These broader applications demonstrate the enzyme's multifaceted role beyond the kitchen.
Conclusion
Tyrosinase is the natural enzyme behind the brown discoloration seen in many foods. It is most notably found in fruits like avocados and apples, vegetables such as potatoes, and fungi like mushrooms. By causing the oxidation of phenolic compounds upon exposure to air, it leads to enzymatic browning, which diminishes a food's visual appeal and quality. However, simple and effective methods like using acids, applying heat, or limiting oxygen can control this reaction. While a common frustration for home cooks, the wide presence of tyrosinase in nature makes it a subject of extensive research, with applications reaching from food science to medicine and environmental technology. The continued development of natural and synthetic tyrosinase inhibitors holds promise for improving food preservation techniques and addressing dermatological issues related to hyperpigmentation.
Keypoints
- Mushrooms are a potent source: Button mushrooms are particularly rich in tyrosinase, which is why they brown quickly when sliced.
- Browning is an oxidation reaction: When cut, fruits and vegetables release the tyrosinase enzyme, which reacts with oxygen to turn phenolic compounds into dark pigments.
- Control with acids: The activity of tyrosinase can be effectively inhibited by lowering the pH with acidic substances like lemon juice or vinegar.
- Apples and avocados brown readily: These fruits are well-known for their rapid browning due to high levels of tyrosinase activity.
- Tyrosinase is widespread in nature: The enzyme is not limited to produce; it is also found in bacteria, fungi, and crustaceans, serving different biological roles.
- Inactivation prevents browning: Heating foods (blanching) can destroy tyrosinase, while methods that limit oxygen exposure also help preserve color.
- Industrial applications exist: Beyond food science, tyrosinase is studied for its use in pharmaceuticals, cosmetics, and environmental cleanup.
Faqs
Question: Is it safe to eat a food that has browned due to tyrosinase? Answer: Yes, it is generally safe to eat. The browning is a non-toxic chemical reaction, though it may result in a less appealing flavor or texture.
Question: How can I stop a cut apple or avocado from turning brown? Answer: To prevent browning, you can coat the cut surface with an acidic solution, such as lemon juice, or immerse it in cold water. You can also use commercial anti-browning products that often contain antioxidants.
Question: Is tyrosinase the same as the amino acid tyrosine? Answer: No, they are not the same. Tyrosinase is an enzyme that acts upon the amino acid tyrosine, converting it and other phenolic compounds into melanin pigments during the browning process.
Question: Do all mushrooms contain tyrosinase? Answer: Most mushrooms contain tyrosinase, but the amount and activity can vary by species. This is why some mushrooms, like the common button mushroom, brown much faster than others.
Question: Does cooking destroy tyrosinase? Answer: Yes, heating foods to a high temperature, such as through blanching, will denature the tyrosinase enzyme and permanently prevent the browning reaction.
Question: Does freezing food prevent tyrosinase activity? Answer: Freezing alone does not inactivate the enzyme and browning can occur upon thawing. For best results, foods should be blanched before freezing to denature the enzyme.
Question: Are some people more sensitive to the effects of tyrosinase? Answer: While tyrosinase is generally harmless in food, it's worth noting that some individuals with conditions like phenylketonuria (PKU) must monitor their intake of the amino acid phenylalanine, which is used to make tyrosine, due to enzyme deficiencies in their body.
Question: What other foods besides fruits and vegetables contain tyrosinase? Answer: Tyrosinase is found in a wide variety of biological sources. Beyond common produce, it is present in fungi like mushrooms, various bacteria, and in crustaceans such as shrimp and crabs.
Question: Why is tyrosinase activity a major concern for the food industry? Answer: Tyrosinase causes enzymatic browning that can lead to significant economic losses for the food industry by reducing the shelf life, commercial value, and quality of fresh-cut produce.
Question: How is tyrosinase used in other industries? Answer: Tyrosinase is used in other industries for various purposes, including the development of L-DOPA for pharmaceuticals, bioremediation, and in cosmetics for developing skin-whitening agents.
