why does lemon juice stop apples from going brown?

Answer 1

The browning is caused by a reaction among three chemicals: phenols, enzymes and oxygen. The phenols reside in one part of the apples' cells, while the enzymes reside in another. When an apple is cut or bruised, cells are broken open. The phenols and the enzymes are then free to react with each other, assisted by the newly available oxygen at the surface. The result of these reactions is a hodgepodge of brown chemicals. All three -- phenols, enzymes and oxygen -- are needed to make the brown chemicals.

One way to inhibit these browning reactions is to knock out the enzymes. No enzymes, no browning. Enzymes are proteins, and acids will "kill," that is, destructively deform, them. So lemon juice, the strongest acid in your edible arsenal, is often recommended as a brown-blocker. Just toss the apple pieces with a teaspoon or so of lemon juice. Also, as soon as you assemble the salad, the apple surfaces become coated with mayonnaise or other dressing, which will protect them from the air. No oxygen, no browning.

But really, all of this is much ado about nothing. Most sliced apples just don't turn brown enough or they don't turn brown fast enough for us to worry about.

To test this idea, I cut three pieces each from a Honeycrisp apple and a Granny Smith. I coated one piece with lemon juice and another piece with mayonnaise. I left the third one untreated as a control. After about four hours of exposure to the air, none of the six pieces had taken on more than a hint of brown.

I then macerated samples of these two apples, plus a Jonafree apple, in a blender. All three apple slushes started turning brown within minutes, the Jonafree almost immediately, followed by the Honeycrisp and the Granny Smith.

Obviously, the blender had smashed open many more cells than the knife blade did, so more phenols met more enzymes and oxygen, creating more brownness. The Granny Smith browned the least, partly because its greenish flesh masks the brown color better than the Honeycrisp's yellow flesh does. But perhaps more significantly, the Granny Smith's innate acidity (I measured its pH at 3.2) was stronger than the Honeycrisp's or the Jonafree's (both pH 3.4), so its acids may have destroyed some of its own enzymes. Anyway, that's my hypothesis.

My conclusions: Different varieties of apples contain different amounts of phenols, enzymes and other acids, and therefore have different susceptibilities to browning. If your knife is real sharp, you can usually get away without any anti-tanning treatment. Just don't chop the apple into too many small pieces and don't let the pieces stand around for long before assembling the salad. And may all your Waldorfs be white.

Answer 2

its a mixture between the juice acting as a physical barrier between the air and the apple, and the acidity of the lemon juice

Answer 3

The acid prevents apples (and other things) from going brown

Answer 4

the citric acid in the lemon

Answer 5

citric acid in the lemon acts as a preservative.

Answer 6

Its the acidity level.
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