Salt water beats lemon juice for keeping apple slices white.

For apples, the best trick isn’t lemon juice — it’s a quick soak in lightly salted water.

Cut apples brown because the exposed flesh reacts with oxygen. Lemon juice works because the citric acid slows that reaction, but it changes the taste and can leave apples sour. Salt water works better: mix about half a teaspoon of salt into a cup of water, soak the slices for five minutes, then drain. The salt deactivates the browning enzyme and rinses off — apples taste normal, not salty. It’s the trick most cafeterias use.

Bananas are a different problem. They don’t brown from oxygen; they brown from bruising and ethylene gas. So keep them whole, wrap the stem in plastic wrap or a rubber band, and let the kid peel it at lunch. Cut bananas are a lost cause.

Either way, pack cut fruit in a tight container so it isn’t sitting in open air.

Or just send whole fruit and skip the science project.