Why Do Mashed Potatoes Stay White?
Potatoes are rich in starch. The starch will turn black when exposed to air. This process of turning black is through oxidation. The raw potatoes will have purely starch as its component while most of the starch will get removed when it is cooked. Cooked potatoes can be mashed easily and hence mashed potatoes can be represented by boiled or cooked potatoes. Moreover, cooked potatoes will be added with salt, and other ingredients. The remaining starch in the potatoes will take long time to get oxidized. So, cooked potatoes will not turn black and stay white.
The enzyme that oxidizes the raw potatoes and turns them black is Polyphenol Oxidase. This enzyme helps the raw potatoes to turn black when exposed to air. When the potatoes are cooked and mashed, the enzyme Polyphenol Oxidase gets denatured and hence will not oxidize the potatoes. So, mashed potatoes stay white.