If you have already cut apples which you will use later, the pieces will get brown. This process is called oxidization. It can cause a loss in taste, moisture, and nutritional value. To save your apple from this, it is advisable to use them immediately after cutting.
One method to deal with oxidization is to spray or brush the juices of citrus fruits such as Lime, orange or pineapple on apple slices. You can prolong the oxidization of apple slices but can not avoid it. So consume/use apple as soon as you cut it.