What is the principle of apple discoloration

The discoloration of apples is mainly caused by the oxidation reaction of polyphenols in the flesh under the action of oxygen. This process mainly involves factors such as polyphenol oxidase activity, oxygen contact area, environmental temperature, pH changes, and metal ion catalysis.

1. Polyphenol oxidase activity

Apple pulp cells contain a large amount of polyphenol oxidase. When the cell structure is disrupted, the enzyme comes into contact with the substrate and catalyzes the oxidation of colorless polyphenolic substances into quinone compounds. These enzymes have the highest activity in a neutral environment, and their activity is significantly inhibited in an acidic environment.

2. Oxygen Contact Area

The larger the surface area exposed after cutting an apple, the more complete the contact with oxygen, and the faster the color change rate. The intact fruit peel can effectively block oxygen permeation, while mechanical damage can accelerate the oxidation process. Vacuum packaging or soaking in saltwater can reduce oxygen exposure.

3. Environmental temperature effect

The enzymatic reaction rate can be increased several times for every 10 degrees Celsius increase in temperature. refrigerated storage can delay discoloration, but freezing can cause cell rupture and accelerate browning. Although high-temperature cooking can inactivate enzymes, it can damage the texture of apples.

4. Changes in acidity and alkalinity

Acidic substances such as lemon juice can lower the pH value, leading to a decrease in polyphenol oxidase activity. Vitamin C, as a reducing agent, can directly block the oxidative chain reaction. Alkaline environment promotes the polymerization of quinone substances to form dark pigments.

5. Metal ion catalysis

Metal ions such as iron and copper can accelerate the oxidation reaction process. Using stainless steel cutting tools is less likely to induce discoloration than carbon steel cutting tools. Aluminum foil wrapped apple slices can isolate metal catalysis. To delay the discoloration of apples, it is recommended to soak the cut pieces in an acidic solution containing vitamin C or wrap them tightly with plastic wrap to reduce air contact. Choose apples with moderate maturity, as their enzyme activity is relatively low. During daily storage, the fruit peel should be kept intact and placed in the refrigerator's vegetable and fruit compartment to effectively delay oxidation. Fresh cutting and eating can preserve nutrients and color to the maximum extent possible. It is recommended to use pasteurization or quick freezing processes to control enzyme activity in processed apple products. For apple slices that require long-term preservation, food grade antioxidants can be considered for treatment.