Fruits and Vegetables That Are High in PPO

Over 50% of food industry loss is attributed to enzymatic browning caused by polyphenol oxidase (PPO), an enzyme that reacts with oxygen to discolor produce after it is cut or bruised. The level of PPO activity varies significantly among different fruits and vegetables, influencing their browning rate and how they are best handled.

Quick Summary

This guide lists common fruits and vegetables with high polyphenol oxidase (PPO) activity. It explains how this enzyme causes browning and suggests practical methods to control the discoloration of produce for longer freshness and improved appearance.

Key Points

• High-PPO Fruits: Apples, bananas, avocados, pears, and apricots are notorious for rapid enzymatic browning due to high PPO content.

• High-PPO Vegetables: Watch for browning in vegetables like potatoes, eggplant, mushrooms, and artichokes, which also contain significant PPO levels.

• Inhibit with Acid: A simple squeeze of lemon or lime juice provides acid to lower the pH, inhibiting PPO activity and stopping discoloration.

• Use Heat to Deactivate: For processed or frozen produce, blanching with heat effectively denatures the PPO enzyme, permanently preventing browning.

• Limit Oxygen Exposure: Since oxygen is required for the PPO reaction, storing cut fruits and vegetables in water or airtight containers can prevent browning.

• Smoothie Blending Tip: When making smoothies, avoid combining high-PPO bananas with antioxidant-rich berries, as the banana's enzyme can degrade the beneficial polyphenols.

Understanding Polyphenol Oxidase (PPO)

Polyphenol oxidase, or PPO, is a naturally occurring enzyme found in many plants that serves a protective function. When plant tissues are damaged—for example, by cutting, slicing, or bruising—the PPO within the plant cells is exposed to oxygen. This contact triggers an oxidation reaction with phenolic compounds naturally present in the plant, converting them into quinones. These quinones then react further to form dark-colored pigments known as melanins, which are responsible for the brown color observed on the surface of many fruits and vegetables. While browning is undesirable for most fresh produce, it is a crucial process for developing the flavor and color of certain products, such as tea, coffee, and cocoa.

The level of PPO activity differs greatly depending on the type of fruit or vegetable, its ripeness, and environmental factors. For instance, certain varieties of apple or pear may brown more quickly than others due to variations in their PPO content and the availability of phenolic substrates. Knowledge of which produce is high in PPO can help in preventing unwanted discoloration during meal preparation and storage.

Fruits with High PPO Activity

Several popular fruits contain high levels of polyphenol oxidase, making them prone to browning when their flesh is exposed to air. Being aware of these can help you better prepare and preserve them.

• Apples: A classic example, apples brown rapidly once sliced. This is due to high concentrations of PPO found in all parts of the fruit, particularly just below the skin.
• Bananas: The browning of a banana's skin and flesh is a direct result of PPO activity. The enzyme is active throughout the fruit's ripening process.
• Avocados: Once cut, avocados turn brown quickly. This is caused by PPO oxidizing phenolic compounds in the avocado's flesh upon exposure to oxygen.
• Apricots: As a climacteric fruit, apricots have high PPO levels that can cause browning during post-harvest storage and when cut.
• Pears: Similar to apples, pears are highly susceptible to enzymatic browning after being sliced.
• Peaches: The flesh of peaches darkens due to the action of PPO when damaged or cut.
• Mangoes: The browning of mangoes, often seen as 'sap burn' on the skin or discoloration of the pulp, is caused by PPO activity.
• Lulo and Mangosteen: Some tropical fruits, including lulo and mangosteen, have particularly high PPO activity.

Vegetables with High PPO Activity

Vegetables can also be rich in PPO, especially starchy or leafy varieties. Proper handling is important to maintain their appearance and nutritional value.

• Potatoes: Browning in potatoes, particularly in the outer cortex tissue and peel, is a protective response to tissue damage triggered by PPO.
• Eggplant: This vegetable exhibits PPO activity when sliced, leading to a quick darkening of its flesh.
• Lettuce: Some varieties of lettuce can brown at their cut edges due to PPO.
• Sweet Potatoes: Certain cultivars of sweet potatoes have been shown to have high PPO activity.
• Mushrooms: High levels of PPO activity contribute to the rapid browning of sliced mushrooms.
• Artichokes: Globe artichokes contain significant levels of PPO, which can cause discoloration during processing.
• Spinach: High PPO activity in spinach can cause browning, particularly at cut or damaged spots.

Comparison Table: PPO Susceptibility in Common Produce

Produce Item	PPO Susceptibility	Browning Characteristics	Prevention Method Effectiveness
Apple	High	Rapid browning on sliced surfaces.	Very effective with acidulants like lemon juice; genetic modification is also used.
Banana	High	Quick darkening of flesh and peel once damaged.	Less effective with antioxidants; high PPO levels can interfere with other antioxidants.
Avocado	High	Flesh browns very quickly upon exposure to air.	Effective by limiting oxygen exposure; squeezing lemon or lime juice over the surface helps.
Mango	High	Browning can occur on skin (sap burn) and pulp.	High PPO activity can be reduced by heating.
Potato	High	Browning concentrated in the peel and outer layer.	Effective with acidulants, sulfites, or blanching.
Lettuce	Medium	Browning appears on cut edges.	Modified atmosphere packaging is effective.
Berries	Low*	Generally low PPO, but rich in polyphenols.	Not a primary concern for enzymatic browning.

Note: While berries are rich in beneficial polyphenols, they are often low in the PPO enzyme, meaning they don't brown readily. However, when combined with high-PPO fruits like bananas in a smoothie, the banana's PPO can degrade the berries' polyphenols.

Techniques to Prevent Enzymatic Browning

For those who wish to prevent or slow down enzymatic browning, especially in high-PPO fruits and vegetables, several effective techniques can be used.

1. Introduce an Acid: Acids like lemon juice, lime juice, and citric acid powder effectively lower the pH, inhibiting PPO activity and reducing browning. A common method is to sprinkle a few drops over cut surfaces.
2. Blanching: Brief exposure to boiling water or steam (blanching) denatures the PPO enzyme, permanently deactivating it. This is a common practice before freezing vegetables to maintain their color.
3. Reduce Oxygen Exposure: PPO requires oxygen to catalyze the browning reaction. Submerging cut produce in water, syrup, or brine can prevent this reaction from occurring.
4. Use Antioxidant Solutions: Ascorbic acid (Vitamin C) is a powerful antioxidant that can be dissolved in water to create an effective anti-browning soak.
5. Use Genetic Modification: A few varieties, like the Arctic apple, have been genetically modified to suppress the expression of PPO, making them resistant to browning when cut.

Conclusion

Polyphenol oxidase is a key enzyme that dictates how quickly fruits and vegetables brown after being cut or bruised. Foods like bananas, apples, avocados, and potatoes are well-known for their high PPO activity, while others, such as berries, contain more stable polyphenols and less PPO. Understanding which produce contains high levels of this enzyme is crucial for proper food handling and storage. By using simple techniques like acidification, blanching, or limiting oxygen exposure, it's possible to manage the browning process effectively, preserving the visual appeal and nutritional quality of your fresh produce.

Key Takeaways

• PPO Causes Browning: Polyphenol oxidase (PPO) is the enzyme responsible for the enzymatic browning that occurs in fruits and vegetables when they are cut or damaged.
• Common High-PPO Fruits: Apples, bananas, avocados, and pears are among the most common fruits with high PPO activity, leading to rapid browning.
• High-PPO Vegetables: Potatoes, eggplant, mushrooms, and artichokes are vegetables with significant PPO activity that can cause discoloration.
• Not All Browning is Bad: While often undesirable for fresh produce, browning caused by PPO is an essential process in the production of foods like black tea and cocoa.
• Preventive Measures Exist: Simple methods like adding acid (lemon juice), heat treatment (blanching), or reducing oxygen exposure can effectively inhibit PPO and prevent browning.
• Ingredient Combinations Matter: Mixing high-PPO fruits like bananas with flavonoid-rich fruits like berries in smoothies can reduce the availability of beneficial antioxidants.
• Genetic Solutions Are Available: Modern techniques like gene silencing have been used to create non-browning produce, such as the Arctic apple.

Further Reading

For a deeper dive into the science behind polyphenol oxidase and its role in food, consider exploring research articles like this one: Polyphenol Oxidases in Crops: Biochemical, Physiological and Genetic Aspects.

Frequently Asked Questions

What is polyphenol oxidase (PPO)?

Polyphenol oxidase (PPO) is a naturally occurring enzyme in plants that causes enzymatic browning when the plant's tissues are damaged and exposed to oxygen.

Why do some fruits brown faster than others?

The rate of browning is directly influenced by the amount of active PPO and phenolic compounds present in the fruit, which varies widely across different species and even cultivars.

How can you stop PPO from browning cut fruit?

You can prevent browning by inhibiting PPO activity using methods like adding an acid (lemon juice), applying heat (blanching), or reducing oxygen exposure by submerging the produce in water.

Can I prevent potatoes from browning after peeling them?

Yes, you can prevent peeled potatoes from browning by storing them in a bowl of cold water. This reduces their exposure to oxygen, which is necessary for the PPO-induced browning reaction.

Is enzymatic browning harmful?

Enzymatic browning is not harmful, but it can negatively affect the appearance, flavor, and nutritional quality of fresh produce, leading to decreased market value and consumer appeal.

Is it true that bananas can reduce the antioxidant benefits of berries in a smoothie?

Yes, research suggests that the high PPO activity in bananas can degrade the health-promoting flavanol polyphenols found in berries and cocoa when they are blended together.

Besides produce, where else is PPO relevant?

PPO plays a crucial role in the production of certain foods like tea, coffee, and cocoa, where the browning and resulting melanins are desirable for developing specific colors and flavors.