Mangoes, like other fruits, darken when cut surfaces are exposed to air. This happens when the oxygen reacts with the fruit and oxidizes it, creating the darker color. There are many ways to keep this from happening, not just with mangoes but other fruits and vegetables such as bananas, apples and potatoes. Additives to keep the mangoes bright after cutting are safe to eat and minimally affect the flavor of the fruit.
Better known as vitamin C, ascorbic acid is an antioxidant that prevents mangoes from browning. Harold McGee, in "Keys to Good Cooking," recommends adding a crushed 250 milligram vitamin C tablet to 1 cup of ice water for treating cut mangoes to prevent browning.
Keep cut mango frozen if you will not use it immediately to slow the rate of browning. Stored at temperatures above 0 degrees Fahrenheit, cut or pureed mangoes will darken, according to "Processing Fruits: Science and Technology" by Diane M. Barrett. Freezing alone will not stop browning of the fruit. Before putting it in the freezer, mango puree or cut fruit should be briefly cooked to 210 degrees Fahrenheit for one minute to stop any enzymatic browning in storage. Combining freezing with an acid treatment instead of heating will also prevent browning during long-term freezer storage.
Lemon juice is naturally full of the antioxidant vitamin C and it does not need to be combined with water to slow the browning rate of cut mangoes. Just toss the mango slices with lemon juice to completely coat all the exposed surfaces of the fruit. The acid in the lemon juice will slow the rate of browning in the cut mango. This technique also works with other fruits prone to darkening.
To keep the cut surfaces of a mango from darkening through oxidization, keep them from becoming exposed to air in the first place. As soon as you cut the mango, submerge the pieces into a sugar syrup or fruit juice as recommended in "Essentials of Professional Cooking, Volume 1" by Wayne Gisslen. This protects the cells of the fruit from oxygen. Use mango juice or another tropical juice to avoid dramatically changing the flavor of the fruit.