Why do apples change color after being stored for a long time

The discoloration of apples after prolonged storage is mainly caused by the oxidation reaction between polyphenols in the flesh and oxygen. This phenomenon is called enzymatic browning, and the main influencing factors include phenolic content, oxygen concentration, temperature, pH, enzyme activity, etc.

1. Phenolic Substance Oxidation

Apple flesh contains abundant polyphenolic substances and phenolic oxidase. When apples are cut open or their skin is damaged, the cellular structure is disrupted, and phenolic substances react with phenolic oxidase under the action of oxygen to produce brown quinone compounds. The content of phenolic compounds varies greatly among different varieties of apples, and varieties such as Red Fuji usually change color faster than green apples.

2. Oxygen contact

browning reaction requires the participation of oxygen. The intact apple skin can block most of the oxygen, and once the skin is damaged or cut open, exposing the flesh to air will accelerate oxidation. Soaking sliced apples in saltwater or lemon water can reduce oxygen exposure, and refrigeration can also lower oxygen solubility.

3. Temperature Effect

An increase in temperature will accelerate enzymatic browning reaction. At room temperature, apple slices may change color within a few minutes, while refrigeration can significantly delay this process. High temperature cooking can inactivate phenoloxidase, which is why cooked apples will not continue to turn brown.

4. Changes in pH

Acidic environment can inhibit the activity of phenol oxidase. Acidic substances such as lemon juice and vinegar can lower the pH value and slow down the browning rate. Alkaline environment will promote the reaction, and apple slices treated with soda water will change color faster.

5. Metal ion catalysis

Metal ions such as iron and copper can catalyze oxidation reactions. Using stainless steel cutting tools to cut apples causes a slight brown change compared to ordinary carbon steel cutting tools. Avoiding prolonged contact between apples and metal containers can also slow down discoloration.

To maintain the fresh color of apples, it is recommended to cut and eat them immediately or soak them in acidic liquid immediately. Wrap the cut tightly with cling film during storage to reduce oxygen contact. Choose apple varieties with lower phenolic content, such as Galaguo and Huangshuai, which change color relatively slowly. In daily life, uncut apples can be placed in the refrigerator compartment with a temperature controlled at around 4 degrees Celsius to effectively delay browning. If slices need to be stored for a long time, vacuum packaging or treatment with food grade antioxidants can be considered.