Stroll down Portland, Maine’s wide brick-laid sidewalks these days, and you’re likely to see—and smell—the signs of the city’s culinary boom. In onetime factory buildings and warehouses just inland from the wharfs, young Turks of the food world are serving up flavors from far-flung corners of the globe—Indonesia, Polynesia, Mexico and Japan—in addition to modern American fare. Their tables are packed with epicureans who increasingly are making pilgrimages to the city just for the food.