McDonald's apple slices do not need refrigerated unless removed from the package.
McDonald's apple slices are treated with a solution of calcium ascorbate which is a blend of calcium and vitamin C to prevent browning and maintain freshness.
Although once opened the McDonald's apple slices should be kept refrigerated for optimal freshness and to prevent any browning once opened.
You can also soak the McDonald's apple slices or any other apple slices in some salt water solution for a couple of minutes and then rinse the apples, which is a very good and effective way of preventing browning of the apples, while also minimizing any impact on the taste of the apples.
Lemon juice can also be added to apple slices or other fruits to keep them from turning brown.
Lemon juice is a common liquid used on fruit salads to keep them from turning brown as well.
Apples turn brown as a result of a chemical reaction called enzymatic browning.
And when the flesh of the apples are exposed to oxygen, an enzyme called polyphenol oxidase reacts with polyphenols, which causes them to oxidize and turn brown.
Salt water helps to prevent the reaction by inhibiting the enzyme activity and also potentially create a barrier to the oxygen.
To use saltwater for preventing apples from turning brown, dissolve 1/8 teaspoon of salt in 1 cup of cold water and then soak the apple slices for around 5 minutes and then rinse and dry them.
Lemon juice is also effective but it can alter the taste of the apples, and salt water instead has a minimal impact on the flavor of the apples.