Foods that have tyrosinase and its role in browning and biology

The well-known browning of a cut apple or avocado is caused by the enzyme tyrosinase. This copper-containing enzyme is widely distributed in nature and found in various foods, most notably mushrooms, fruits, and vegetables. Understanding which foods have tyrosinase provides insight into both food spoilage and important biological processes.

Quick Summary

Tyrosinase, an enzyme found in many mushrooms, fruits, and vegetables, is primarily responsible for enzymatic browning and pigment formation. It catalyzes the oxidation of phenolic compounds, leading to the discoloration of fresh produce. The enzyme is also found in a variety of other organisms, including insects and mammals, where it plays roles in defense and pigmentation.

Key Points

• Mushrooms are a potent source: Fungi, especially the common button mushroom (Agaricus bisporus), are cheap and readily available sources of tyrosinase.

• Enzymatic browning culprit: Tyrosinase is the key enzyme responsible for the browning of cut fruits and vegetables, including apples, avocados, and potatoes.

• Dual biological roles: Beyond food browning, tyrosinase is essential for melanin production in animals and serves as a defense mechanism in plants.

• Inhibition is key to preservation: Methods to inhibit tyrosinase include using antioxidants like vitamin C, adjusting pH with citric acid, and heat treatment.

• Tyrosinase is an enzyme, tyrosine is an amino acid: It is important to distinguish tyrosinase, the enzyme, from tyrosine, the amino acid substrate that is converted during the browning process.

What is tyrosinase?

Tyrosinase, also known as polyphenol oxidase (PPO), is a metalloenzyme containing copper at its active site. It catalyzes the hydroxylation of monophenols (compounds with one hydroxyl group) and the oxidation of o-diphenols (compounds with two hydroxyl groups) to produce o-quinones. These o-quinones are highly reactive and undergo further reactions to form melanin and other dark pigments. This reaction is responsible for the undesirable browning of fresh produce, but it also serves important biological functions in various organisms.

Foods containing tyrosinase

Tyrosinase is prevalent across many plant-based and fungal foods. Its presence explains why certain produce changes color so quickly after being cut or bruised. For consumers, understanding these sources can help in preventing food spoilage.

Mushrooms: The most common food source

Mushrooms are one of the most widely studied and commercially used sources of tyrosinase. The Agaricus bisporus, or common button mushroom, contains high levels of this enzyme. The brown discoloration you see on mushrooms as they age or are bruised is a direct result of tyrosinase activity. Researchers often use mushroom tyrosinase as a model for studying the enzyme due to its accessibility and high activity.

Fruits and vegetables with high tyrosinase activity

Many common fruits and vegetables contain active tyrosinase, contributing to their post-harvest browning. These include:

• Apples: Sliced apples turning brown is a classic example of tyrosinase at work. The enzyme oxidizes phenolic compounds released from damaged cells, forming brown pigments.
• Avocados: After being cut open, the green flesh of an avocado quickly turns brown due to tyrosinase activity. The reaction occurs when the enzyme, normally compartmentalized within the plant's cells, is exposed to air.
• Bananas: The browning of a banana peel and pulp as it ripens is another prominent example. Research has even investigated using banana peels as a source for tyrosinase isolation.
• Potatoes: Peeled or cut potatoes can turn brown or black when exposed to air, another consequence of tyrosinase action.
• Eggplants: The dark discoloration on a sliced eggplant is a result of this same enzymatic browning process.

The functional role of tyrosinase beyond browning

While often associated with food spoilage, tyrosinase plays several crucial roles in living organisms. In plants, the enzyme's involvement in browning is thought to be a defense mechanism against pests and injury. The quinones produced can be toxic to invading microorganisms. In other organisms, tyrosinase is vital for broader biological processes.

• Melanin biosynthesis in mammals and other organisms: In mammals, tyrosinase is the key enzyme responsible for melanogenesis, the process of producing melanin pigments that determine the color of skin, hair, and eyes. Similarly, in insects, tyrosinase is essential for wound healing and the hardening and pigmentation of their outer cuticle, a process known as sclerotization.
• Immune response: In invertebrates, tyrosinase plays a critical role in the innate immune system. It helps encapsulate and destroy invading pathogens.
• Environmental detoxification: Microbial tyrosinases have been explored for industrial applications, including the degradation of phenolic pollutants in contaminated water.

How to inhibit tyrosinase in foods

For the food industry and home cooks, controlling tyrosinase activity is essential for preserving the quality and aesthetic appeal of fruits and vegetables. Several inhibitors can be used to prevent enzymatic browning.

• Ascorbic acid (Vitamin C): A powerful antioxidant found in citrus fruits like lemons and limes. The acid effectively reduces the o-quinones back to colorless diphenols, stopping the browning reaction.
• Citric acid: Found in many fruits, this acid lowers the pH of the food, which inhibits tyrosinase activity, as the enzyme works best at a neutral pH.
• Cooking: Heat denatures the enzyme, permanently disabling its function. This is why cooked potatoes don't turn brown like raw ones.
• Exclusion of oxygen: Since tyrosinase uses molecular oxygen to catalyze its reaction, limiting oxygen exposure can prevent browning. Vacuum sealing or covering cut fruit in water are simple methods.

Tyrosinase vs. Tyrosine: A quick comparison

Feature	Tyrosinase	Tyrosine
Classification	An enzyme (protein).	An amino acid (the substrate).
Function in Food	Catalyzes the browning of foods.	A component of proteins found in many foods.
Biological Role	Key in melanin synthesis and defense.	Precursor to neurotransmitters like dopamine and hormones.
Examples in Foods	Found in mushrooms, apples, and bananas.	Found in cheese, meat, fish, and soy.
Inhibition/Control	Inhibited by acids, antioxidants, or heat.	Not inhibited, but consumed as a nutrient from protein-rich foods.

Conclusion

Tyrosinase is an interesting enzyme found in a wide variety of foods, with mushrooms, fruits, and vegetables being the most notable sources. While its activity is often viewed as a negative trait leading to the enzymatic browning of produce, this discoloration is a result of its natural function. This function extends far beyond simple food spoilage, encompassing crucial biological roles in defense and pigmentation in various organisms. For culinary purposes, controlling tyrosinase is a matter of understanding its reaction, but its presence highlights a fundamental aspect of natural biochemistry. Knowing what foods have tyrosinase empowers both food producers and consumers to better preserve food quality and appreciate the science behind everyday observations.

A list of common foods with tyrosinase activity

• Mushrooms: All types, especially button and portobello.
• Apples: Most varieties.
• Avocados: All varieties.
• Bananas: Ripening fruit.
• Potatoes: All varieties.
• Eggplants: All varieties.
• Pears: Most varieties.
• Lettuce: Particularly when bruised or cut.
• Peaches: Exposed flesh.

A note on tyrosinase in mushrooms

Not all mushrooms contain the same level of active tyrosinase. Some strains are cultivated to have lower enzymatic activity to reduce post-harvest browning. However, mushrooms generally contain some form of tyrosinase, whether in its active or latent form, which is why they are a key example in food science.

Citations

ACS Publications, "Mushroom Tyrosinase: Recent Prospects," 2003. NIH, "Microbial Tyrosinases: Promising Enzymes for Pharmaceutical ...," 2014. ResearchGate, "isolation and purification of tyrosinase from different plant sources," 2017. BioMed Central, "Microorganisms as a source of tyrosinase inhibitors: a review," 2017. NIH, "A comprehensive review on tyrosinase inhibitors," 2019. Wiley Online Library, "Effect of ascorbic acid on tyrosinase and its anti-browning...," 2021. Forest Essentials India, "How to Reduce Melanin in Skin: 8 Natural Tips," 2025. Wiley Online Library, "Microbial Tyrosinases: Promising Enzymes for Pharmaceutical...," 2014. ScienceDirect, "Banana peel wastes as a source of tyrosinase useful...," 2022. Medical News Today, "Tyrosine foods: Health benefits, what to eat, and more," 2021. WebMD, "6 Foods High in Tyrosine and Why You Need It," 2024.

In-depth explanation of the browning process

The browning process, scientifically known as enzymatic browning, begins when plant tissues are damaged—for example, by cutting, bruising, or pests. This damage breaks down cellular compartments. Specifically, tyrosinase, which is normally isolated in a different cellular location (like chloroplasts), comes into contact with its phenolic substrates, which are stored in the plant's vacuoles. In the presence of oxygen from the air, the enzyme catalyzes the oxidation of these colorless phenolic compounds into colored o-quinones. These quinones then polymerize, either with themselves or with other plant compounds like amino acids and proteins, to form larger, darker pigments called melanins. The rate and intensity of this browning can vary significantly based on factors like pH, temperature, and the specific plant's genetics. For instance, some apple varieties are more prone to browning than others due to differences in their tyrosinase and phenolic content.

Applications of tyrosinase in the food industry

Interestingly, while typically seen as a negative, controlled tyrosinase activity is intentionally leveraged in some food production processes. For example, in the production of black tea, tyrosinase facilitates the oxidation of catechins in the leaves, which contributes to the characteristic color and flavor. The enzyme's role in the fermentation of cocoa beans also helps develop the rich color and complex flavor profile of chocolate.

Conclusion

Tyrosinase is an integral part of food biochemistry, responsible for the browning seen in everyday fruits and vegetables like apples and mushrooms. This process is a result of the enzyme's reaction with phenolic compounds upon exposure to oxygen, and it serves as a defense mechanism in the plant. From preventing spoilage with inhibitors like citric and ascorbic acid to appreciating its role in creating the color of tea and chocolate, understanding where tyrosinase exists offers valuable insight into the food we consume.

Frequently Asked Questions

Which common foods have high tyrosinase activity?

Foods with high tyrosinase activity include mushrooms, apples, bananas, avocados, potatoes, and eggplant. The enzyme is responsible for the discoloration seen in these items after they are cut or bruised and exposed to air.

Why does a sliced apple turn brown?

A sliced apple turns brown due to the action of tyrosinase, or polyphenol oxidase. When the apple's cells are damaged, the enzyme is exposed to oxygen and phenolic compounds. Tyrosinase catalyzes the oxidation of these compounds into dark-colored pigments known as melanins.

Can I prevent foods from browning naturally?

Yes, natural methods to prevent browning include soaking cut produce in lemon or other acidic juice (containing citric acid) to lower the pH, submerging them in water to limit oxygen exposure, or blanching them with heat to denature the enzyme.

Does cooking destroy tyrosinase?

Yes, cooking or heating foods effectively denatures the tyrosinase enzyme, destroying its function. This is why cooked fruits and vegetables do not turn brown in the same way as their raw counterparts.

What is the difference between tyrosinase and tyrosine?

Tyrosinase is a copper-containing enzyme, while tyrosine is an amino acid that serves as a substrate for tyrosinase. Tyrosinase converts tyrosine into melanin and other pigments, but they are not the same substance.

Is tyrosinase harmful to consume?

No, consuming foods containing tyrosinase is not harmful. The browning it causes is a natural reaction. However, some individuals with certain health conditions may need to monitor their intake of foods high in tyrosine, the substrate of tyrosinase.

Besides browning, what else does tyrosinase do?

In addition to causing enzymatic browning in foods, tyrosinase is involved in melanin biosynthesis in mammals, defense mechanisms in plants against pests, and contributes to the flavor and color development in products like black tea and chocolate.