Why Brinjal and Apples Turn Dark After Cutting

The darkening of brinjal (eggplant) and apple surfaces after cutting is a common phenomenon caused by a biochemical reaction known as enzymatic browning. This process involves the interaction of certain plant enzymes with oxygen in the air, resulting in the formation of brown-coloured compounds. It is a natural defence mechanism of plants and occurs in many fruits and vegetables when their tissues are damaged or exposed.

The Biochemical Basis

When an apple or brinjal is cut, peeled, or bruised, the plant cells are disrupted, allowing the contents of different cellular compartments to mix. Normally, these components are kept separate within intact cells:

• Enzymes such as polyphenol oxidase (PPO) or tyrosinase are located in specialised cell structures.
• Phenolic compounds, which serve as substrates for these enzymes, are stored in vacuoles.

Once the cells are damaged, the enzymes come into contact with the phenolic compounds in the presence of oxygen from the air. This initiates a chain of oxidation reactions that lead to browning.

The Chemical Reaction

The main steps of enzymatic browning can be summarised as follows:

1. Oxidation of Phenols: The enzyme polyphenol oxidase (PPO) catalyses the oxidation of phenolic compounds (such as catechol, tyrosine, or chlorogenic acid) to form quinones.

   Phenol+O2→PPOQuinone+H2OPhenol + O_2 \xrightarrow{PPO} Quinone + H_2OPhenol+O2​PPO​Quinone+H2​O

2. Polymerisation of Quinones: The reactive quinones subsequently undergo non-enzymatic polymerisation, combining with amino acids and proteins to produce melanin-like brown pigments.

   Quinone→Melanin(brownpigment)Quinone \rightarrow Melanin (brown pigment)Quinone→Melanin(brownpigment)

These pigments are responsible for the dark or brown discolouration observed on the cut surfaces of apples, brinjals, potatoes, bananas, and other similar produce.

Role of Enzymes and Oxygen

The key enzyme responsible for this process is polyphenol oxidase (PPO), also known as catechol oxidase or tyrosinase, depending on the plant species. The rate of browning depends on:

• Enzyme concentration: Higher levels of PPO accelerate the reaction.
• Availability of oxygen: The reaction ceases in the absence of air.
• Amount of phenolic substrate: Some fruits (like apples) and vegetables (like brinjal) naturally contain more phenolic compounds, making them more prone to browning.

Factors Affecting the Rate of Browning

Several environmental and physiological factors influence how quickly browning occurs:

• pH Level: PPO activity is optimal in slightly acidic to neutral conditions (pH 5–7). Lowering the pH (for instance, using lemon juice) reduces browning.
• Temperature: Higher temperatures initially increase enzyme activity but extreme heat can denature the enzyme, halting the reaction.
• Metal Ions: Certain metal ions, particularly copper, act as cofactors for PPO and enhance its activity.
• Water Availability: Excess moisture and air exposure promote oxidation.

Prevention and Control of Browning

Although enzymatic browning is harmless, it affects the visual and nutritional quality of foods. Several methods are used in households and food industries to control it:

1. Use of Acids: Dipping cut fruits or vegetables in lemon juice, vinegar, or other acidic solutions lowers the pH, thereby reducing enzyme activity.
2. Exclusion of Oxygen:
   • Storing cut pieces in water or airtight containers prevents contact with air.
   • Vacuum packing and modified atmosphere packaging are used industrially.
3. Thermal Treatment: Brief heating or blanching (immersing in hot water or steam) denatures PPO enzymes and halts the reaction.
4. Use of Antioxidants: Substances like ascorbic acid (vitamin C), citric acid, or sulphites prevent oxidation by reducing quinones back to phenols.
5. Cold Storage: Lowering temperature slows down enzyme activity and oxidation rates.
6. Genetic Modification: In modern agriculture, genetically modified apple varieties such as the Arctic Apple have been developed with reduced PPO levels to resist browning.

Biological Significance

From a biological perspective, enzymatic browning may act as a defence mechanism for plants. The melanin-like compounds formed can serve as protective barriers against microbial invasion and further tissue damage. These compounds are also thought to help seal the wounded tissue and prevent excessive moisture loss.

Examples of Enzymatic Browning

Besides apples and brinjal, enzymatic browning occurs in:

• Potatoes when peeled or cut.
• Bananas when exposed to air.
• Avocados after slicing.
• Mushrooms and pears during handling.

This process differs from non-enzymatic browning, such as the Maillard reaction or caramelisation, which occur during cooking and involve sugars and amino acids rather than enzymes.