Short answer
Cutting breaks cells and lets polyphenol oxidase (PPO), an enzyme kept apart from the fruit's phenolic compounds, meet them in the presence of oxygen. PPO oxidises the phenols into quinones, which polymerise into brown pigments. It needs no heat. You slow it by removing oxygen, lowering pH with acid, adding a reducing agent such as ascorbic acid, chilling, or destroying the enzyme with heat.
In an intact apple the phenolic compounds sit in the vacuole and polyphenol oxidase sits in the plastids, separated by membranes. Cutting, bruising or freezing breaks those barriers. With oxygen present, PPO converts the phenols into highly reactive quinones, which react with each other and with proteins to form brown, melanin-like polymers. Nothing about this needs heat: the reaction runs at room temperature and continues, more slowly, in the fridge. The browning is the plant's wound response, not decay.
The speed and depth of browning depend on how much PPO a variety has and how much of its phenolic substrates. Some apple and potato varieties brown within minutes; others stay pale for hours. Breeders have produced low-browning varieties by reducing PPO activity, including by gene silencing and genome editing. The same enzyme family is wanted in some foods: tea leaves, cocoa beans, dates and raisins get part of their brown colour and flavour from deliberate PPO oxidation.
Every practical anti-browning method removes one of the three ingredients. Lemon juice lowers the pH below the range where PPO works well. Ascorbic acid (vitamin C) goes further: it reduces quinones back to colourless phenols before they can polymerise, so it delays browning until it is used up. Cold slows the enzyme; blanching destroys it, which is why fruit and vegetables are blanched before freezing. Covering the cut surface with water, syrup, cling film pressed onto the surface or a vacuum pack keeps oxygen out. Sulfites were once widely used for the same purpose but are now restricted in many foods.
Keeping the stone in guacamole only protects the patch it covers; pressing film onto the whole surface does the same job everywhere.
A cut apple browns through PPO; a baked apple browns through Maillard reactions and caramelisation, which need heat and sugar. The two can be told apart by what stops them. Acid and cold slow enzymatic browning sharply but do little to a pan that is already hot; drying and heat drive Maillard browning but destroy PPO. Red wine and white wine also brown by non-enzymatic oxidation of phenolics over years in bottle, without any enzyme activity, so not every oxidative browning is enzymatic.
| Method | Removes or blocks | Limitation |
|---|---|---|
| Lemon juice, citric acid | Enzyme activity (lower pH) | Adds sourness; weaker on low-acid fruit |
| Ascorbic acid | Quinones (reduced back to phenols) | Protects only until used up |
| Chilling | Enzyme rate | Slows, does not stop |
| Blanching | The enzyme (heat inactivation) | Cooks the surface; not for fresh use |
| Water, syrup, film pressed on, vacuum | Oxygen | Must cover the whole cut surface |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Enzymatic browning results from polyphenol oxidase oxidising phenolic compounds to quinones in the presence of oxygen, followed by polymerisation into brown pigments.
Source: Prevention of Enzymatic Browning by Natural Extracts and Genome-Editing: A Review on Recent Progress; Maillard Reaction: Mechanism, Influencing Parameters, Advantages, Disadvantages, and Food Industrial Applications: A Review
FactStrong evidence
Anti-browning treatments work by lowering pH, reducing quinones (for example with ascorbic acid), inactivating the enzyme with heat, chilling, or excluding oxygen; plant breeding and genome editing have also produced low-browning varieties by reducing PPO.
Source: Prevention of Enzymatic Browning by Natural Extracts and Genome-Editing: A Review on Recent Progress
Colourwise interpretationModerate evidence
Brown discolouration on a freshly cut surface is a wound response of the tissue and on its own indicates neither spoilage nor safety.
Based on: Follows from the enzymatic mechanism in the cited review, which proceeds in sound tissue within minutes of cutting.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.