What is PPO and How Does it Affect Mangoes?
Polyphenol oxidase, or PPO, is a metalloproteinase enzyme found in most fruits and vegetables. In plants, PPO is kept separate from its phenolic substrates within the cell's compartments. However, when the fruit is damaged by cutting, bruising, or chilling injury, the cell compartments break down, allowing PPO and phenolic compounds to mix in the presence of oxygen. This interaction initiates a series of chemical reactions known as enzymatic browning, where the colorless phenolic compounds are oxidized into dark, brownish pigments called melanins.
In mangoes, PPO activity is most concentrated in the skin, sap, and pulp. A specific catechol oxidase-type PPO is located in the chloroplasts of the skin cells. This is why conditions like “sap burn,” which damage the skin's cells, lead to noticeable skin browning. PPO is also involved in the fruit’s ripening process, with its activity levels increasing as the fruit matures and ripens.
The Function of PPO Beyond Enzymatic Browning
While its most visible effect is browning, PPO serves several important functions for the mango tree itself. These roles are primarily defensive and protective.
- Defense Against Pests and Pathogens: When a mango tree is damaged by herbivores or pathogens, PPO is released, producing quinones. These compounds have antimicrobial properties and can cross-link with proteins to form a physical barrier, which protects the plant from further invasion.
- Wound Healing: The browning reaction serves as a form of wound healing for the plant. The melanins and other polymers form a protective layer over the damaged tissue, similar to a scab, to prevent further damage or infection.
- Oxygen Regulation: PPO may play a role in regulating oxygen levels in the plant's photosynthetic systems, particularly during environmental stress such as water deprivation.
Comparison of PPO in Mangoes vs. Other Fruits
To understand PPO's activity in mangoes, it is helpful to compare it with other common fruits. While the enzyme's fundamental function is the same, its specific properties (e.g., optimal pH and temperature) and impact on quality can differ significantly.
| Feature | Mango PPO | Apple PPO | Avocado PPO |
|---|---|---|---|
| Optimal pH | Ranges from 5.6 to 8.0, depending on the fruit part and variety. | Generally has an optimal pH around 4–7. | Acts efficiently within the pH range typical for avocados. |
| Optimal Temperature | Can range from 30°C to 40°C depending on the tissue source (skin vs. pulp). | Active across a wide temperature range, similar to other plant PPOs. | Optimal temperature can be similar, but overall activity is highly visible and rapid. |
| Browning Trigger | Caused by cellular damage, such as bruising, cutting, and especially sap burn. | Triggered by cutting and exposing the flesh to oxygen. | Famous for extremely rapid browning after exposure to air. |
| Processing Effects | Heat inactivation is possible; for Haden mango, half-activity was lost after short treatment at high temperatures (80-85°C). | Significant research exists on inhibiting apple PPO for pre-sliced products. | The high-fat content can influence browning and anti-browning strategies. |
| Storage Challenges | Susceptible to chilling injury at low temperatures, which can increase PPO activity and browning. | Commercial varieties like Arctic Apples have been genetically engineered to reduce PPO activity. | Can be inhibited by ascorbic acid and other sulfur-containing compounds. |
How to Inhibit PPO Activity in Mangoes
For consumers and the food industry, controlling PPO activity is crucial for extending shelf life and maintaining aesthetic quality. Several methods can be used to inhibit enzymatic browning:
- Acidification: PPO activity is reduced in acidic environments. Applying a solution of lemon juice, lime juice, or citric acid to cut mango surfaces can effectively prevent browning.
- Heat Treatment: Blanching or pasteurizing fruit, often through hot water treatment, can inactivate PPO. However, this is more common for processed products than for fresh fruit.
- Excluding Oxygen: The browning reaction requires oxygen. Vacuum sealing, modified atmosphere packaging, or simply submerging cut fruit in water can prevent the enzyme from coming into contact with air.
- Chemical Inhibitors: For industrial use, compounds like ascorbic acid (Vitamin C) or sulfites are used to inhibit PPO. Ascorbic acid is a powerful antioxidant that effectively inhibits the reaction.
- Low-Temperature Storage: While chilling-sensitive fruits like mango can suffer from chilling injury, proper low-temperature storage (around 10-12°C for mature mangoes) can slow down the overall metabolic processes, including PPO activity.
The Impact of PPO on Mango Ripening and Quality
Polyphenol oxidase activity is closely linked to the overall quality of mangoes. During the ripening process, a series of physiological changes occur, and PPO is one of the key enzymes involved. Proper ripening is essential for developing the characteristic flavor, aroma, and texture that consumers expect.
As the fruit ripens, the activity of PPO can contribute to changes in skin and flesh color. However, unwanted browning can significantly reduce the marketability of fresh fruit. For example, chilling injury can lead to accelerated PPO activity, causing graying or browning of the peel, which is aesthetically unappealing and indicates a reduction in overall quality. The concentration of PPO varies depending on the mango variety and maturity level, affecting how it responds to post-harvest handling and storage. Understanding and controlling PPO activity is therefore a major focus of post-harvest technology to ensure fruit quality is maintained during transportation and storage.
Conclusion
In summary, mangoes do contain polyphenol oxidase (PPO), a natural enzyme integral to the fruit's life cycle. While serving a defensive purpose for the plant, PPO is the primary cause of enzymatic browning in mangoes when the fruit is damaged or exposed to air. Its activity is closely tied to the ripening process and can be affected by storage conditions. Fortunately, simple methods such as adding acid or excluding oxygen can effectively control PPO's browning effects. For the food industry, managing PPO is essential for preserving the quality and aesthetic appeal of mango products. Consumers can use this knowledge to properly handle and store mangoes to ensure a better eating experience.
Can I eat a mango with brown spots from PPO?
Yes, you can eat a mango with brown spots caused by PPO, as the browning is only a cosmetic issue resulting from an enzymatic reaction. The fruit is still safe to consume, though the texture and flavor of the affected area might be slightly altered.
Does cooking mango destroy PPO?
Yes, cooking mango, for example by boiling or baking, will inactivate the PPO enzyme. This is why processed mango products like jams or purees do not turn brown, because the heat treatment effectively stops the enzymatic reaction.
How does PPO relate to mango allergies?
PPO is not the direct cause of mango allergies, but some individuals may have a contact dermatitis reaction to urushiol, an oil in the mango's peel. Other mango allergies are often related to proteins in the pulp or cross-reactivity with pollen, not the browning enzyme itself.
What is the difference between PPO and an antioxidant in mangoes?
Antioxidants in mangoes, such as beta-carotene and vitamin C, protect the body's cells from damage caused by free radicals. PPO, on the other hand, is an enzyme that causes the oxidation of other compounds within the fruit itself, leading to browning, though it also has antioxidant roles within the plant.
Is PPO the only thing that makes mangoes ripen?
No, PPO is not the only factor. Mangoes are a climacteric fruit, meaning they continue to ripen after being harvested, a process regulated primarily by the plant hormone ethylene. PPO is just one of many enzymes involved in the complex cascade of ripening.
Are PPO levels different in different types of mangoes?
Yes, studies have shown that PPO activity levels can differ among various mango varieties, which affects their storage life and susceptibility to enzymatic browning. This variation can be due to genetics, maturity at harvest, and growing conditions.
Do dried mangoes still have active PPO?
No, dried mangoes do not have active PPO because the drying process, which involves heat, inactivates the enzyme. This is why dried mango maintains its color and does not turn brown after processing.
