Bob Pitts knows doughnuts. He fried his first one in 1961 at the original Dunkin' Donuts shop in Quincy, Mass. Just by looking at the lumps and cracks on a misshapen doughnut, he can tell if the frying oil is too cool or the batter too warm. But Mr. Pitts, the company's doughnut specialist, cannot find a way to make one that tastes good without using partially hydrogenated oil, now considered the worst fat in the American diet. An artificial fat once embraced as a cheap and seemingly healthy alternative to saturated fats like butter or tropical oils, partially hydrogenated...