If you cut an avocado or burdock in advance, the cross section will change color.
This is because polyphenols in tissues are oxidized by enzymes and oxygen to form a brown pigment (melanin).
Foods that often brown: Burdock, lotus root, eggplant, apple, banana, pear, avocado
Foods that do not easily brown: Daikon, carrots, green onions, cucumbers, tomatoes
[How to prevent browning]
(1) Soak in water: Cut off contact with oxygen.
(2) Add vinegar or lemon juice: Lowering the pH to 4 or less suppresses enzyme activity.
(3) Add salt: The copper ion at the active center of polyphenol oxidase is removed, and the enzyme activity is suppressed.
(4) Heating: Polyphenol oxidase is inactivated at 60-70℃. However, since the reaction is accelerated at 40-50°C, browning may progress during heating.
(5) Use a reducing agent: Applying vitamin C (ascorbic acid) can prevent discoloration.
From here on it's subjective.
In (1), contact with oxygen can be prevented by applying dressing or mayonnaise (oil).
In (2) and (5), lemon juice corresponds to this.
Also, it is important to cook quickly to prevent browning of "avocados" and "apples" when making salads.