Once an apple (& other veggies) is cut, enzymes are being released. The enzymes then interact with oxygen and turn the apple brown.
Try sprinkle a few drops of lemon juice, lime juice, or your favourite vinegar on cut apple to prevent browning. Soaking cut apples in water also help. The acidity in juices and vinegar destroy the enzymes. Soaking cut apples in water inhibites contact between the apples and oxygen in air.
Let me know if you like your new lemony apple
Judy