Polyphenol oxidase, often abbreviated as PPO, is a naturally occurring enzyme widely distributed in the plant kingdom. This enzyme plays a significant role in various biological processes, most notably causing fruits and vegetables to brown when their tissues are damaged and exposed to air. This common phenomenon, known as enzymatic browning, affects the appearance and sometimes the flavor of many fresh produce items. Understanding PPO’s function helps explain why apples turn brown after being sliced or avocados darken after being cut.
What is Polyphenol Oxidase?
Polyphenol oxidase is a type of protein that functions as a catalyst, speeding up specific chemical reactions without being consumed. These enzymes are commonly found in plants, fungi, and bacteria. Within plant cells, PPO is typically compartmentalized, often located within chloroplasts or other plastids.
The enzyme contains copper atoms at its active site, essential for its catalytic function. PPO acts upon phenolic compounds, organic molecules naturally present in plants. These phenolic compounds are stored separately from PPO within the plant cell’s vacuoles. This separation prevents the browning reaction from occurring in intact plant tissues.
The Science of Enzymatic Browning
Enzymatic browning begins when plant tissues are damaged, such as by slicing, bruising, or peeling. This damage breaks down the cellular compartments, allowing polyphenol oxidase to come into contact with its phenolic substrates and oxygen. Once PPO interacts with these components, it catalyzes the oxidation of phenolic compounds into highly reactive molecules called quinones.
These quinones are unstable and readily polymerize to form larger, complex brown pigments known as melanins. Melanins are responsible for the dark discoloration observed in cut surfaces of fruits like apples, bananas, and avocados. The intensity of the browning depends on factors like the amount of PPO and phenolic compounds present, as well as the duration of oxygen exposure.
Preventing Food Browning
Several practical methods can inhibit polyphenol oxidase activity and prevent enzymatic browning in foods. One effective strategy involves excluding oxygen. Submerging cut fruits or vegetables in water or using vacuum sealing can limit their exposure to air.
Another common method is acidification, which involves lowering the pH of the food environment. PPO activity is significantly reduced in acidic conditions; for example, adding lemon juice or vinegar to cut apples prevents browning. Heat treatment, such as blanching, can also prevent browning by denaturing the PPO enzyme. Boiling vegetables for a short period before freezing is a common application of this method.
Antioxidants, particularly vitamin C (ascorbic acid), are also widely used. Ascorbic acid works by reducing the quinones back to their original phenolic compounds before they can polymerize into brown pigments. This intercepts the browning pathway. Storing foods at colder temperatures, such as refrigeration, slows down the rate of PPO activity.
Polyphenol Oxidase Beyond the Kitchen
Beyond its role in food browning, polyphenol oxidase serves broader functions within plants, particularly in defense mechanisms. When plant tissues are damaged by pests or pathogens, PPO activity increases, leading to the rapid production of quinones. These quinones can be toxic to invading organisms or can cross-link with proteins, forming a protective barrier that deters herbivores and inhibits microbial growth. This response is a natural part of a plant’s immune system.
Polyphenol oxidase also plays a significant role in various industrial processes where its activity is either intentionally desired or carefully controlled. In tea production, PPO enzymes oxidize phenolic compounds in tea leaves, contributing to the characteristic color and flavor of black tea. In contrast, in winemaking, PPO activity is often undesirable and carefully managed to prevent unwanted browning.