Why does lemon juice stop apple slices turning brown?

Because browning is an enzyme-driven reaction, and lemon juice shuts that enzyme down in two separate ways at once.

When you cut an apple you break cells open and expose their contents to oxygen. An enzyme called polyphenol oxidase meets naturally occurring phenolic compounds in the fruit, and with oxygen present it converts them into quinones. Those quinones then link together into brown pigments called melanins. The same chemistry browns bananas, avocados, pears and freshly cut potatoes, and it is why the discoloured layer is always at the cut surface rather than throughout the fruit.

Lemon juice interferes twice. First, it is acidic. Polyphenol oxidase works best near neutral pH and slows dramatically as acidity increases, so lowering the pH of the cut surface effectively disables it. Second, lemon juice contains ascorbic acid, which is a reducing agent: it converts quinones back to their colourless precursors as fast as the enzyme produces them. The browning reaction keeps running, but it never gets far enough to produce visible pigment — until the ascorbic acid is used up, which is why protection is temporary rather than permanent.

This explains why the practical alternatives work. A salt-water soak slows the enzyme. Dropping slices into cold water limits oxygen contact. Blanching briefly in hot water denatures the enzyme outright, which is the most durable fix but changes the texture. Pineapple, orange and lime juice all work like lemon, with different flavour consequences.

Browning is cosmetic, not spoilage — a browned apple slice is perfectly edible. The flavour does change slightly, because the same phenolic compounds contribute bitterness, which is the real reason it is worth preventing.