Cut fruit turns brown because air reaches the flesh. A short soak in water with a little lemon juice slows this down. The advice below comes from university extension services in the United States, which publish it for home cooks. No Thai rule covers this, because it is a kitchen question and not a regulation.
Why fruit browns
Extension services call this enzymatic browning. An enzyme is a natural substance in the fruit that makes a chemical change happen. When a knife breaks open the cells of a fruit, enzymes in the flesh meet oxygen from the air and substances called phenolic compounds. Together they make brown pigments. University of Illinois Extension says that browning is caused by the fruit being exposed to air, and that it affects texture and appearance.
The University of California extension lists the fruits that brown most easily: apples, bananas, pears, peaches and avocados. It says these are the fruits with a higher amount of phenolic compounds.
A lemon juice soak
The simplest method needs only a lemon and water. University of Illinois Extension describes mixing 1 quart of water, which is a little under 1 litre, with 3 tablespoons of lemon juice, then soaking the cut fruit for 3 to 5 minutes. It notes that this may change the flavour or colour slightly.
The University of California extension gives a different recipe: fresh lemon juice diluted one to one with water, with the cut pieces submerged for about 10 minutes. The two pages do not agree on the amounts, so treat the numbers as a guide and follow one set at a time.
Some points from the sources:
- Treat the fruit soon after cutting. The California page says these treatments work best when applied straight away.
- Drain the fruit well afterwards.
- Some treatments may slightly affect flavour or texture.
What does not work as well
University of Illinois Extension states that citric acid alone is not enough to prevent browning. It also describes a vitamin C solution made from 1 teaspoon of powder, or six crushed 500 mg tablets, in 2 cups of water, with a soak of 3 to 5 minutes. It says this method does not change the flavour. Lemon juice is the option most kitchens already have.
Treated fruit still needs the refrigerator
The University of California page makes one point clearly. These treatments prevent browning, but they do not stop decay. Colour and food safety are separate matters. Treated fruit should go into the refrigerator promptly, and the page refers readers to the National Center for Home Food Preservation in the United States for food safety guidance.
Choosing a method
Both extension services treat browning as a question of how the fruit looks and feels, not a sign that something is wrong with it. If the plate will be eaten within a few minutes, a soak may not be needed at all. If the fruit has to wait, for example in a packed lunch, a brief soak in diluted lemon juice is the method both sources describe. Whichever amount you follow, keep to the stated time, drain the pieces and keep them cold.
In short
- Fruit browns when its cut surface meets air, and this changes appearance and texture.
- Illinois Extension describes a soak of 3 tablespoons of lemon juice in 1 quart of water for 3 to 5 minutes.
- California Extension describes lemon juice diluted one to one with water for about 10 minutes, so the sources differ.
- Citric acid alone is not enough to stop browning.
- Treatment slows colour change only. Put the cut fruit in the refrigerator soon after.
