Why do sliced apples gradually turn yellow

The yellowing of sliced apples is mainly caused by the oxidation reaction between polyphenols in the flesh and oxygen. This process is influenced by factors such as enzymatic browning, environmental temperature, degree of air exposure, apple variety, pH, etc. Apple flesh contains abundant polyphenol oxidase and phenolic compounds. After the apple is cut open, 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. Phenolic substances are oxidized into quinone compounds, which further polymerize into brown pigments, causing the flesh color to gradually darken. The higher the temperature, the stronger the enzyme activity and the faster the browning rate. Soaking the sliced apple in saline or lemon water and using sodium ions or acidic environment to inhibit enzyme activity can delay the discoloration process. Some apple varieties, such as Red Fuji, have a higher polyphenol content and their color changes more noticeably when cut open. Apples that have undergone special processing, such as vacuum packaging and slicing, can maintain their color for a long time due to the isolation of oxygen. Fresh cut apples are often treated with food additives such as ascorbic acid in commerce, which block the oxidation reaction chain through reduction. When storing at home, wrap the incision tightly with cling film to reduce the contact area with air, and the refrigeration environment can also effectively slow down the browning rate. Although browning of apples affects their appearance, it does not necessarily indicate spoilage. Oxidative products are harmless to the human body. If you need to maintain the color of apple slices, it is recommended to cut them and eat them immediately or perform oxygen isolation treatment. Apples can be stored together with fruits containing natural antioxidants such as pineapples and citrus fruits in daily life, and the ethylene gas released can actually inhibit browning enzyme activity. understanding the principle of food oxidation helps us to scientifically preserve the nutrition and sensory quality of food ingredients.