How could a food be made with hydrogenated oils, but say it has zero trans fat?

2 reasons: First, items that list partially hydrogenated oils in the ingredients but contain less than 0.5g of trans fats per serving are considered by the government to be trans-fat free. A good example of this would be commercial peanut butter, which contains a tiny amount of partially hydrogenated oil to prevent separation. Second, products that contain fully hydrogenated oils are trans-fat free.