Why do apples turn brown after you cut them?

The short version: cutting into an apple exposes the flesh to oxygen, and an enzyme the fruit was carrying all along reacts with it to produce a brown pigment within minutes. It is the same broad category of chemistry as a bruise darkening under the skin, not spoilage, and the colour on its own says nothing about whether the apple is still good to eat.

What is actually happening inside the apple

As long as an apple's cells stay intact, they keep two things apart: an enzyme called polyphenol oxidase, and a group of natural plant compounds called phenolics. A knife breaks that separation open along every surface it touches. The enzyme and the phenolics mix, oxygen from the air joins in, and the reaction produces melanin — the same broad family of pigment behind a banana's brown speckles, a bruise darkening under unbroken skin, and, further afield, a suntan. None of those are decay. They are all oxygen reacting with something a cell used to keep sealed away.

The reaction starts within minutes of cutting and is visible well before an hour is up, which is fast enough that it reads as alarming even though nothing dangerous is happening. It is also strictly a surface event — the flesh a few millimetres in in an uncut slice is untouched by it, which is why cutting a browned slice in half reveals pale, unaffected apple underneath.

Why some apples brown faster than others

Variety matters more than almost anything else you might try to control. Granny Smith and other tart, high-acid apples naturally carry lower enzyme activity and brown slowly; sweeter varieties like Golden Delicious and Fuji tend to brown noticeably faster. That is a genuine varietal trait, not something you are doing wrong with one apple and right with another.

How you cut it plays a part too. A dull blade crushes cells along the cut rather than slicing cleanly through them, and crushed cells release their contents over a wider area than a clean cut does — part of why fruit chopped with a blunt knife often looks worse, sooner, than the same apple cut with a sharp one. Ripeness and temperature shift it as well: a very ripe apple, or one that has been sitting out somewhere warm, tends to brown faster than a firm, cold one, simply because the enzyme itself works faster at higher temperatures.

Browning is not spoiling

This is the part worth actually holding onto: the reaction that browns a cut apple has nothing to do with the bacteria and mould that make food genuinely unsafe. A slice that has browned but has not dried out, gone slimy or started to smell is still the same apple you cut an hour ago — changed in appearance, and little else. Any difference in taste is a very slight loss of crispness at the surface, not a safety concern.

The signs that actually mean something has gone wrong are different from browning, and usually show up much later: a sour or fermented smell, a sticky or slimy film that was not there when you cut it, or visible mould. Those are worth acting on. Colour by itself, on apple slices that have sat in the fridge for a normal day or so, is not.

What actually slows it down

Every method that works does one of two things: it keeps oxygen away from the cut surface, or it interferes with the enzyme directly.

Acid. A splash of lemon, lime or pineapple juice lowers the surface pH enough to slow the enzyme sharply, which is why lemon juice is the standard trick and why a light squeeze is enough — you do not need to coat every slice.
Cold water. Submerging cut slices in plain water cuts off their oxygen supply almost entirely. It works with no added flavour, though the apple loses a little crispness once drained.
Salt water or honey water. A very light brine, or a spoon of honey dissolved in water, slows the reaction in much the same way as acid does, at a concentration too low to taste.
Heat. Cooking or blanching denatures the enzyme completely, which is why cooked apple — in a pie, a sauce, dried into chips — never browns the way raw slices do, no matter how long it sits afterward.
An airtight wrap pressed onto the cut surface itself, rather than draped loosely over the container, works for the same reason cold water does: no gap means no oxygen reaching the flesh.

None of this needs to be precise. A quick dip in a bowl of water with a squeeze of lemon in it, before slices go into a lunchbox or onto a platter, is normally enough to keep them looking freshly cut for hours rather than minutes.

The same reaction, in other foods

Apples are simply the most commonly cited example of a much more general reaction. Cut pears and bananas brown the same way and for the same reason. Peeled potatoes turn a similar grey within minutes, which is why recipes often say to hold them in a bowl of water before cooking. Avocado flesh browns fastest of all, and the classic fix there — press plastic wrap directly onto the surface, or leave the pit in — is really the same "keep oxygen out" logic in a different shape. Once you recognise the pattern in one fruit, it explains what you are looking at almost everywhere else you see it.

This is close to what Munch is built to reduce without you having to think about it: marking an item opened switches it onto its own countdown from that day, so a bowl of apple slices you cut this morning is judged by how long it has actually been sitting, not by how brown it happens to look under the kitchen light — and if it is close to that window, it shows up among tonight's recipes rather than being judged by eye and binned early.

None of this needs a rule to memorise. It needs one habit: judge a cut apple by smell and texture rather than by colour, and treat browning as the ordinary, cosmetic thing it is.