What is Polyphenol Oxidase (PPO)?
Polyphenol oxidase (PPO) is an enzyme found in many plant tissues, as well as in fungi and some animal species. Its primary role is to catalyze the oxidation of phenolic compounds into quinones. When the plant tissue is damaged, for instance by cutting, bruising, or pests, the PPO and phenolic compounds are exposed to oxygen. This exposure triggers a reaction that produces dark-colored pigments known as melanins, leading to the characteristic brown or black discoloration. While often considered undesirable in fresh produce due to its impact on appearance and nutritional value, enzymatic browning is a crucial step in the processing of certain foods like tea, coffee, and cocoa.
How Enzymatic Browning Affects Produce
The rate and intensity of browning are influenced by several factors, including the specific PPO isoenzymes present, the concentration of phenolic substrates, the pH level, and the temperature. High PPO activity and a rich supply of phenolic compounds in a fruit or vegetable lead to faster and more pronounced browning. This is why some produce turns brown almost instantly after being cut, while others remain relatively unaffected.
Fruits with High Polyphenol Oxidase Activity
Many fruits are known for their high PPO content, making them prone to browning when sliced or bruised. A few examples stand out for their particularly high activity:
- Apples: A classic example, apples contain significant amounts of PPO, especially in varieties like Fuji and Golden Delicious. The enzyme is released when the apple's cells are ruptured, causing the flesh to brown quickly upon exposure to air.
- Bananas: Bananas have very high PPO activity, which is responsible for the rapid browning of both the fruit's flesh and peel. This reaction is often exploited to make milkshakes but is a major concern for extending shelf life.
- Avocados: This fruit browns notoriously fast once sliced and exposed to oxygen. Avocado's high PPO activity can significantly reduce its aesthetic appeal in preparations like guacamole.
- Pears: Similar to apples, pears are susceptible to enzymatic browning due to their high PPO levels, with certain varieties browning more rapidly than others.
- Apricots: As a climacteric fruit, the PPO in apricots activates during ripening, contributing to browning once the skin is broken.
- Tropical Fruits: Mangosteen and lulo are two tropical fruits cited for their very high PPO activity, leading to rapid discoloration after crushing.
Vegetables with High Polyphenol Oxidase Activity
Just as with fruits, several vegetables are known to have high PPO levels, which can lead to rapid browning and spoilage after harvest or preparation.
- Potatoes: The PPO enzyme in potatoes is concentrated near the skin, contributing to browning when the potato is peeled or bruised. This is a defense mechanism against insects and pathogens.
- Mushrooms: Edible mushrooms, such as the common Agaricus bisporus, are rich in PPO, causing them to turn black when cut and exposed to air.
- Lettuce: Certain varieties of lettuce contain PPO that can cause the cut edges to turn brown. This is a common issue for pre-packaged salads and leafy greens.
- Eggplant: This vegetable, particularly when cut, experiences rapid browning due to its significant PPO content.
- Sweet Potatoes: Research has shown that sweet potatoes have high PPO activity, which is impacted by temperature during processing.
- Burdock: Studies have noted that burdock has a high PPO activity, which leads to a drastic decrease in total phenols during the cooking process.
Comparison of High PPO Produce
| Produce Item | Common PPO Location | Effect of Browning | Mitigation Strategy |
|---|---|---|---|
| Apple | Chloroplasts & Mitochondria | Loss of visual appeal | Soaking in acidulated water (lemon juice) |
| Banana | Pulp and Peel | Rapid discoloration; lower nutritional value | Mixing with low-PPO fruits in smoothies |
| Avocado | All parts, especially after mashing | Rapid browning; less marketable | Adding lime or lemon juice |
| Potato | Tuber peel and outer cortex | Browning upon cutting/bruising | Blanching in boiling water |
| Mushroom | Entire tissue | Blackening when sliced | Use of inhibitors like citric acid |
| Lettuce | Leaf tissue | Edge browning in fresh cuts | Modified atmosphere packaging |
| Eggplant | Fruit tissue | Quick discoloration | Blanching before cooking |
Controlling Enzymatic Browning
For home cooks and the food industry, controlling PPO activity is essential for maintaining product quality. Common methods include:
- Lowering pH: Acidic substances like lemon juice, pineapple juice, and vinegar are effective inhibitors of PPO because the enzyme functions best within a specific pH range, usually 4.0-8.0.
- Removing Oxygen: Since PPO requires oxygen to function, removing it from the produce's surface can prevent browning. Submerging items in water or using vacuum sealing are effective methods.
- Heat Inactivation: Heating or blanching produce can inactivate PPO, but this can also alter the texture.
- Adding Inhibitors: Salt and other chemical inhibitors can interfere with PPO's catalytic activity.
- Genetic Modification: In recent years, genetically modified apples (Arctic® apples) have been developed with suppressed PPO expression to prevent browning.
Conclusion
Polyphenol oxidase is a natural enzyme found in a wide range of fruits and vegetables, causing the common phenomenon of enzymatic browning. Items like apples, bananas, avocados, and potatoes are particularly rich in PPO, making them highly susceptible to discoloration when damaged. By understanding which produce has high levels of this enzyme, and applying simple scientific principles, it's possible to minimize browning and preserve the freshness, appearance, and nutritional value of your food. For more detailed scientific research on PPO activity in different plants, a comprehensive resource can be found on the National Institutes of Health website.
