What makes the Homer Simpsons of the world slaver for a doughnut? The key lies in the frying oil, says chemist Gerald DeMenna of Chem-Chek, a consulting company in New Jersey. After 50 to 75 hours of frying, heat and steam make parts of cooking-oil molecules break off into free fatty acids. The acids combine with the minerals and salts in dough to form alkalines— soap, in other words— that allow the hot oil to permeate a thin outer layer of dough. That layer quickly cooks into a crispy crust that seals the interior so that it steam-cooks and remains moist. New oil lacks the crucial breakdown products, so it leaves doughnuts pasty white and dry. "But in an abused oil, there's so much soap that the doughnut gets saturated with oil," DeMenna says. To get things just right, doughnut makers can seed new oil with samples taken from aged batches, or purchase broken-in oil previously used to fry foods such as potato chips.