Foods High in Polyphenol Oxidase Enzyme

January 12, 2026 •

4 min read

Over 50% of global fruit and vegetable production is lost due to enzymatic browning, a reaction catalyzed by the polyphenol oxidase (PPO) enzyme. This enzyme, widely distributed in plants, animals, and fungi, causes cut fruits and vegetables to turn brown and can reduce the nutritional value of foods. Knowing what foods contain high levels of the polyphenol oxidase enzyme is crucial for home cooks and food manufacturers alike to preserve product appearance and nutritional content.

Quick Summary

This article details which common fruits and vegetables are naturally rich in the polyphenol oxidase (PPO) enzyme. It explains how this enzyme causes browning and affects nutritional content, particularly in smoothies. It also provides practical advice on handling these foods and controlling the browning process during preparation.

Key Points

Enzymatic Browning: The polyphenol oxidase (PPO) enzyme is the primary cause of fruits and vegetables turning brown when cut or damaged due to a reaction involving oxygen.
PPO Activity in Fruits: Common fruits with high PPO levels include bananas, apples, avocados, pears, apricots, and peaches.
PPO Activity in Vegetables: Key vegetables with high PPO include mushrooms, potatoes, lettuce, and eggplant, which darken when their cellular structure is disrupted.
Smoothie Caution: Blending high-PPO foods like bananas with antioxidant-rich ingredients like berries can significantly reduce the bioavailability of polyphenols.
Mitigation Techniques: Methods to inhibit PPO activity and prevent browning include adding acids (e.g., lemon juice), blanching with heat, limiting oxygen exposure (vacuum packing), and using commercial anti-browning agents.
Nutritional Impact: PPO's activity can lead to a loss of nutritional value, particularly antioxidants, affecting the quality and health benefits of food.
Food Preservation: Controlling PPO is a major focus in the food industry to extend the shelf life and preserve the aesthetic and nutritional quality of fresh produce.
Natural Role: In plants, PPO serves a protective function by producing antimicrobial compounds in response to damage.

What is the Polyphenol Oxidase (PPO) Enzyme?

Polyphenol oxidase (PPO) is a copper-containing enzyme found in many plants that catalyzes the oxidation of phenolic compounds, leading to browning. This process occurs when plant cells are damaged—for example, by cutting or bruising—and the PPO comes into contact with oxygen and phenolic substrates stored separately within the cell. The reaction produces quinones, which then polymerize to form brown, red, or black pigments known as melanins.

While this reaction can be undesirable for food aesthetics, in nature, it plays a vital role in protecting plants from pests and pathogens by producing antimicrobial compounds. However, when preparing food, especially fruits and vegetables, controlling PPO activity is key to maintaining a desirable appearance and preserving nutrients, particularly antioxidants.

Fruits with High Polyphenol Oxidase Activity

Many fruits are well-known for their rapid browning, a direct consequence of high PPO activity. Understanding which fruits have the highest concentrations can help in planning food preparation to minimize browning and preserve nutritional value.

Bananas: Bananas contain very high levels of PPO, which is why they turn brown quickly when bruised or peeled. A recent study highlighted that adding bananas to smoothies with polyphenol-rich berries can significantly reduce the bioavailability of beneficial compounds like flavan-3-ols. The banana's PPO can degrade these antioxidants even after blending, and potentially continue its activity in the stomach.
Apples: Apples are perhaps the most classic example of PPO activity. When sliced, the enzyme reacts with the apple's phenolic compounds and oxygen, causing the flesh to brown. Some genetically modified 'Arctic apples' have been developed with silenced PPO genes to prevent this browning.
Avocados: Much like apples, avocados brown rapidly when cut and exposed to air due to high PPO content. The enzymatic browning in avocados is a multi-step process that eventually leads to the formation of brown polymeric pigments.
Pears: Pears are another common fruit that exhibits significant enzymatic browning after being cut or damaged. The rate of browning can vary depending on the pear variety and its ripeness.
Apricots: Apricots, being a climacteric fruit, experience fast maturation after harvest, during which latent PPO can be activated, accelerating browning.
Peaches: Similar to apricots, peaches have high PPO activity that leads to discoloration upon tissue damage.

Vegetables Rich in the PPO Enzyme

While fruit browning is more commonly discussed, many vegetables also contain substantial amounts of the PPO enzyme, which is responsible for their post-harvest discoloration.

Mushrooms: Mushrooms, particularly species like Agaricus bisporus (the common button mushroom), are known for having high PPO activity. This causes them to darken quickly, a process that is often managed in the food industry to extend shelf life.
Potatoes: PPO is concentrated in the peel and outer cortex of potato tubers. When a potato is peeled or bruised, the damage triggers the PPO reaction, resulting in visible browning.
Lettuce: Cut lettuce, especially shredded varieties like iceberg lettuce, is prone to browning at the edges. The PPO enzyme is a primary driver of this discoloration, known as 'pinking' or browning, which reduces the product's market value. Research has identified specific PPO genes responsible for this browning and strategies to mitigate it.
Eggplant: Eggplant is another vegetable known for its rapid browning when cut. The PPO activity is a significant factor in its post-harvest quality.

Comparison of High-PPO Foods


Food Item	Primary PPO Effect	Common Usage Context	How to Mitigate Browning
Banana	Rapid browning when bruised/peeled; degrades antioxidants in smoothies	Smoothies, fresh consumption, baking	Freezing or mixing with high-acidity ingredients (e.g., lemon juice)
Apple	Browning on cut surfaces	Sliced snacks, salads, juices	Acidulants like lemon juice; commercial anti-browning agents
Mushroom	Darkening, especially post-harvest	Fresh use in salads, cooked dishes	Storing at low temperatures; blanching
Avocado	Quick browning after cutting	Guacamole, salads, toast	Acid (lemon or lime juice); sealing surface with air-tight wrap
Lettuce	Pinking or browning on cut edges	Packaged salads	Use of modified atmosphere packaging (MAP) in commercial products
Potato	Browning after peeling or bruising	Chips, fries, peeled vegetables	Storing peeled potatoes in cold water

How to Control PPO Activity

Controlling the polyphenol oxidase enzyme's activity is a primary goal in food preservation and preparation. The strategies revolve around inhibiting the enzyme or preventing its access to oxygen or its phenolic substrates.

Using Acids

Acidic conditions inhibit the function of PPO by altering its optimal pH range. Adding an acid to cut fruit or vegetables is a simple and effective method. Lemon juice, containing citric acid, is a common home remedy for preventing apple and avocado slices from browning.

Blanching

Blanching involves briefly immersing food in boiling water, which denatures and deactivates the heat-sensitive PPO enzyme. This is a standard practice for preserving vegetables before freezing to maintain their color and quality.

Vacuum Packaging

Removing oxygen from the food's environment is another effective method. PPO requires oxygen to catalyze the browning reaction. Storing cut produce in a vacuum-sealed bag or submerged in water can prevent browning by limiting oxygen exposure.

Anti-browning Agents

For commercial food processing, various anti-browning agents are used, including reducing agents like ascorbic acid (Vitamin C), chelating agents that bind the copper in the enzyme's active site, and acidulants. Research is also focused on finding natural, food-based inhibitors from spices and herbs.

Conclusion

Polyphenol oxidase is a natural enzyme found in a wide variety of plants, and its activity is the primary cause of enzymatic browning in foods like bananas, apples, mushrooms, and lettuce. While the enzyme is beneficial to the plant for defense, it can degrade the nutritional content and visual appeal of food after harvesting or processing. By understanding which foods are high in PPO, consumers and industry professionals can employ simple, effective techniques like using acids, blanching, or limiting oxygen exposure to control browning and preserve food quality. Knowledge of this enzyme's behavior, particularly its effect on antioxidant-rich foods in preparations like smoothies, is key to maximizing nutritional benefits.

Frequently Asked Questions

The primary cause of browning is the polyphenol oxidase (PPO) enzyme, which reacts with oxygen and phenolic compounds in the plant's cells after they are damaged, producing brown pigments.

Bananas, apples, avocados, and pears are among the fruits with the highest concentration of the polyphenol oxidase enzyme, causing them to brown quickly when exposed to air.

Yes, adding bananas to smoothies can significantly reduce the bioavailability of beneficial antioxidants, such as those from berries, because the banana's PPO enzyme breaks down the polyphenols.

To prevent browning in cut apples, you can add an acid like lemon juice, which inhibits the PPO enzyme. Another option is storing the slices in cold water or an airtight container to limit oxygen exposure.

Mushrooms darken over time due to high levels of the PPO enzyme, which continues to cause browning during storage, especially after handling or processing.

Enzymatic browning is not harmful to eat. However, it can indicate a loss of some nutritional value, particularly antioxidants, and is often considered undesirable for aesthetic reasons.

To preserve the color of cut potatoes, the best method is to submerge them in cold water immediately after peeling to prevent the PPO from reacting with oxygen.

Yes, cooking methods such as blanching (briefly boiling) effectively denature and deactivate the PPO enzyme, preventing further enzymatic browning.