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
- When I make chocolate chip cookies, why don’t the chocolate chips melt in the oven?
- Why do certain types of potato make better mash than others?
- Why is rice such a food-poisoning culprit?
- Why do eggs taste so different, depending on how they’re cooked?
To submit your questions email us at email@example.com (don’t forget to include your name and location)