Chat GPT 4 Explains Why Drops of Lemon Juice Keep Sliced Apples Fresh

I have noticed that when I cut apples, they turn brown quickly.   However, if I dip them in a bowl of water with a few drops of either lime juice or lemon juice, they stay fresh looking and don't turn brown.  

Can you explain the chemistry of this in as much detail as possible?

ChatGPT 4:

When apples are cut or bruised, they undergo a series of chemical reactions that cause them to turn brown. This browning process, known as enzymatic browning, is due to the action of an enzyme called polyphenol oxidase (PPO), also known as tyrosinase, and the presence of phenolic compounds.

Here's a detailed explanation of the chemistry behind this process:

1. When apples are cut, the cell walls are disrupted, allowing polyphenol oxidase enzymes and phenolic compounds (substrates) to come into contact with each other.

2. The polyphenol oxidase enzyme catalyzes the oxidation of phenolic compounds to o-quinones, which are highly reactive intermediates.

3. The o-quinones then react with various amino acids and proteins present in the apple, leading to the formation of a complex mixture of brown-colored compounds called melanins.

Lime or lemon juice helps prevent the browning of cut apples because they are acidic and contain ascorbic acid (vitamin C). The acidity and ascorbic acid work in a couple of ways to inhibit the browning process:

1. Lowering pH: The citric acid present in lime or lemon juice lowers the pH of the apple surface, which creates an unfavorable environment for the polyphenol oxidase enzyme. Enzymes have an optimal pH range in which they function most effectively, and in the case of PPO, it is more active in a neutral to slightly acidic environment (pH 5-7). By lowering the pH, the enzyme activity is reduced, slowing down the enzymatic browning process.

2. Ascorbic acid: Vitamin C is a powerful antioxidant that can reduce the o-quinones back to their original phenolic form, preventing them from reacting with amino acids and proteins to form melanins. This reduction helps maintain the fresh appearance of the apples.

So, by dipping the apples in a bowl of water with a few drops of lime or lemon juice, you're effectively creating an acidic environment and providing antioxidants to inhibit the activity of the polyphenol oxidase enzyme and slow down the formation of brown melanin pigments.