The reason why apples turn brown

The browning of apples is mainly due to the catalytic oxidation of phenolic substances by polyphenol oxidase. The main factors include damage to fruit pulp cells, oxygen exposure, temperature effects, changes in acidity and alkalinity, and metal ion catalysis.

1. Damage to fruit pulp cells

When an apple is cut open or mechanically damaged, the cell structure is disrupted, and polyphenol oxidase, which was originally separated in different areas of the cell, comes into contact with phenolic substances and undergoes enzymatic browning reaction with the participation of oxygen. This is the main reason for the rapid discoloration of sliced apples in daily life, and the intact apple skin can effectively block oxygen permeation.

2. Oxygen Contact

Phenolic substances are catalyzed by polyphenol oxidase in an oxygen environment to form quinone compounds, which then polymerize to form brown pigments. Vacuum packaging or soaking in salt water can reduce oxygen contact, and steam blanching is commonly used in canned apple processing to kill enzymes and store them in the air.

3. Temperature Effect

Low temperature can reduce enzyme activity but cannot completely inhibit browning. Apple slices refrigerated at 4 ℃ will still slowly change color. High temperature treatment, such as above 80 ℃, can completely inactivate polyphenol oxidase, which is also a key step in blanching before making dried apples.

4. Changes in pH

Polyphenol oxidase exhibits strong activity within the pH range of 4-8, and acidic environments can delay browning. Adding vitamin C to freshly squeezed apple juice can not only lower the pH value, but also reduce oxidized quinones and maintain the color of the juice.

5. Metal ion catalysis

Metal ions such as iron and copper accelerate non enzymatic browning reactions, and using stainless steel cutting tools to cut apples is less likely to induce discoloration than ordinary carbon steel cutting tools. The selection of contact surface materials in processing equipment should also avoid the leaching of metal ions.

In daily life, cut apples can be soaked in light salt water or lemon water to slow down browning, and stored in refrigeration for no more than 4 hours. Commercial processing uses comprehensive methods such as vacuum degassing, pasteurization, and adding antioxidants to control browning. understanding the mechanism of browning can help choose the appropriate way to preserve fruits. Although the abundant polyphenols in apples are prone to oxidation and discoloration, these natural antioxidants are beneficial to human health and there is no need to worry excessively about nutrient loss due to appearance changes.