Foods High in PPOs and How to Prevent Enzymatic Browning

January 4, 2026 •

3 min read

Polyphenol oxidase (PPO), the enzyme responsible for enzymatic browning, is widely distributed throughout the plant and animal kingdoms. This natural reaction causes freshly cut fruits and vegetables to turn brown when exposed to oxygen, impacting both their appearance and nutritional value. Understanding which foods are high in PPOs and what triggers their activity is key to preserving food quality at home and in the food industry.

Quick Summary

This article explores foods with high polyphenol oxidase (PPO) activity, including apples, bananas, potatoes, and avocados. It details the enzymatic browning process and provides practical techniques to inhibit it, preserving the aesthetic and nutritional value of produce.

Key Points

Foods High in PPOs: Common foods with high PPO activity include apples, avocados, bananas, potatoes, and lettuce, among others.
Cause of Browning: PPOs react with phenolic compounds in the presence of oxygen, leading to the formation of brown pigments (melanins) after plant tissue is damaged.
Inhibiting PPOs: Enzymatic browning can be inhibited using antioxidants like ascorbic acid (vitamin C) and acidulants like citric acid.
Physical Control: Methods such as blanching with heat, storing at low temperatures, and modified atmosphere packaging can also effectively reduce PPO activity.
Nutritional Impact: PPO activity can reduce the bioavailability of beneficial antioxidants in certain foods, highlighting the importance of proper food preparation.
Flavor Development: In specific food production processes like black tea and cocoa, controlled enzymatic browning is desirable for creating specific flavors and colors.

Understanding Polyphenol Oxidase (PPO) and Enzymatic Browning

Polyphenol oxidase (PPO) is an enzyme found in most fruits and vegetables. When plant tissue is damaged, PPOs mix with phenolic compounds and oxygen, catalyzing the oxidation of phenolic compounds into quinones. These quinones then form melanins, causing the brown discoloration known as enzymatic browning.

Enzymatic browning can lead to reduced consumer appeal and shorter shelf life for produce, resulting in economic losses. It can also diminish nutritional content in food. In certain processes like cocoa bean and tea leaf fermentation, controlled browning is desirable for flavor and color development.

Specific Foods High in PPOs

Many common fruits and vegetables exhibit high PPO activity, with browning rates varying based on cultivar and conditions.

Fruits with High PPO Content

Apples: Brown easily when cut due to PPO activity.
Avocados: High PPO content causes rapid browning when exposed to oxygen.
Bananas: Browning occurs after bruising or peeling. PPO in bananas can reduce antioxidant bioavailability from other fruits in smoothies.
Apricots and Peaches: Prone to browning after cutting, a concern for the fresh-cut industry.
Pears: Similar to apples, susceptible to browning after slicing.

Vegetables with High PPO Content

Potatoes: High PPO in the outer layers causes browning when damaged.
Lettuce: Cut lettuce can brown quickly due to PPO.
Mushrooms: Agaricus bisporus has high PPO, causing rapid browning post-harvest.
Eggplant: Shows enzymatic browning depending on PPO activity and phenolic content.

Inhibiting the PPO Reaction

Preventing browning involves chemical inhibitors, physical treatments, and natural methods.

Chemical Inhibitors

Ascorbic Acid (Vitamin C): An antioxidant that reverses quinone formation, preventing melanin production.
Citric Acid: Lowers pH, inactivating PPO which is sensitive to acidic conditions.
Sulfites: Inhibit PPO by binding to its active site. Use is restricted or banned for fresh produce in many regions due to health concerns.

Physical Treatments

Heat Treatment (Blanching): Brief heating denatures and inactivates the PPO enzyme. Common for frozen vegetables.
Modified Atmosphere Packaging: Reduces oxygen levels, suppressing PPO activity.
Low Temperatures: Refrigeration and freezing slow down the enzymatic browning rate.

Comparison of PPO Inhibition Methods


Method	Mechanism	Advantages	Disadvantages
Ascorbic Acid	Reducing agent.	Safe, natural, adds vitamin C.	Temporary effect, concentration-dependent.
Citric Acid	Lowers pH.	Safe, natural, can enhance flavor.	May alter taste, not effective for all produce.
Sulfites	Binds to active site.	Highly effective, inexpensive.	Potential allergic reactions, restricted use.
Blanching	Heat denaturation.	Highly effective, permanent.	Can alter texture, flavor, and nutrients.
Modified Atmosphere	Reduces oxygen.	Preserves fresh quality, no additives.	Requires special packaging, effectiveness varies.

Health Implications of PPOs

PPO activity and the resulting compounds are part of a plant's defense. PPO can also impact food's nutritional profile; the oxidation of phenolic compounds can reduce the bioavailability of antioxidants. Research shows blending high-PPO bananas with antioxidant-rich berries reduces antioxidant availability from berries.

However, controlled PPO reactions are beneficial in processes like black tea and coffee production for flavor. Consumers typically focus on minimizing undesired browning in fresh produce to preserve freshness and nutrients, with natural inhibitors like citrus juice being practical options.

Conclusion

Foods high in PPOs like apples, bananas, potatoes, and avocados are common. Enzymatic browning is a natural process caused by PPOs oxidizing phenolic compounds when damaged produce is exposed to oxygen. Understanding this mechanism allows for effective control using methods ranging from natural acids to heat treatments and specialized packaging. Choosing the right method helps preserve the visual appeal and nutritional value of produce for consumers and industry alike.

PPO-related resources

Frequently Asked Questions

Polyphenol oxidase (PPO) is an enzyme found in plants, microorganisms, and animals that catalyzes the oxidation of phenolic compounds into quinones, which then polymerize into dark pigments, causing browning.

Common foods with high PPO levels include apples, avocados, bananas, mushrooms, potatoes, peaches, apricots, and lettuce.

When plant cells are damaged, PPOs and phenolic compounds mix with oxygen. PPOs catalyze the oxidation of the phenolic compounds, which then polymerize into brown-colored melanins.

You can prevent browning by treating the cut surfaces with an acid like lemon or lime juice (citric acid) or by refrigerating the produce to slow down the enzymatic reaction.

Yes, cooking methods such as blanching, which involves a brief heat treatment, can denature the PPO enzyme and effectively halt the browning process.

No, PPOs are not inherently bad for your health. While they can reduce the bioavailability of some antioxidants when active, the primary concern is the cosmetic and aesthetic impact on food quality and appeal, not toxicity.

Yes, studies have shown that blending high-PPO foods like bananas with high-antioxidant foods like berries can reduce the bioavailability of the berry's antioxidants.