Although it plans to keep its Longmont, Colo., turkey hot dog processing plant, Butterball LLC, Garner, N.C., sold its Longmont brand to Sigma Alimentos, one of Mexico's largest refrigerated and frozen food processors and distributors. Terms were not disclosed. . . Encouraged by its success with SUBWAY Restaurants, vending sales and growing interest in its single-serve milks, Shamrock Farms, Phoenix, said it plans to more widely distribute its flavored, on-the-go products and become a true national brand.