It’s mostly to do with the fatty, starchy crust that acts like an insulating layer. First, pre-boiling a potato causes its starch granules to absorb water and swell until carbohydrate molecules seep out to create a kind of thick gel.
The high temperature in the oven then drives off moisture, causing the gelatinised starch on the outside of the potato chunks to form a crispy crust, trapping the heat inside. The fat from the baking tray also collects in cracks and crevices and strengthens the heat-keeping structure.
Asked by: Sarah Fullerton, London
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