All fats can develop a rancid taste and smell when exposed to oxygen. Manufacturers have three imperfect ways to prevent this: using an airtight seal combined with refrigeration, adding antioxidants to compete for the oxygen or by hydrogenation, which stabilizes the oil but also creates trans fats, according to Eleanor Whitney and Sharon Rolfes in “Understanding Nutrition.”
Hydrogenated oils have undergone a chemical process that adds hydrogen atoms to monounsaturated or polyunsaturated fats to reduce the number of double bonds. This process increases their shelf life by making the fats more saturated and therefore less prone to become rancid.
Some trans fats result when hydrogen is added to vegetable oil through the process of hydrogenation. Trans fats are more solid than oil and less likely to spoil, so the food has a longer shelf life.
Most fats contain a mixture of saturated, monounsaturated and polyunsaturated fatty acids. When vegetable oils are partially hydrogenated, they become spreadable margarines. Fatty acids that are fully loaded with hydrogen atoms are saturated and usually solid at room temperature. Animal fats, such as butter, beef tallow and lard, as well as tropical oils, such as coconut and palm, consist mostly of saturated fatty acids.
Commercially baked goods, such as cakes and cookies, as well as fried foods including french fries and doughnuts may contain trans fats. Shortenings and margarines can also be high in trans fat, notes Harvard School of Public Health. The use of trans fats in commercial foods has decreased in recent years because of health concerns. Less than 0.5 grams may be rounded down to zero grams trans fat on nutrition labels, so if you consume several servings of those foods, you may exceed recommended limits.
Hydrogenation has two advantages. It prolongs shelf life and alters the texture of foods. Hydrogenated fats make flaky pie crusts and creamy puddings. A disadvantage of hydrogenation is that it makes polyunsaturated fats more saturated and lowers their health advantages. Another disadvantage is that some of the molecules that remain unsaturated change shape and become trans fatty acids. This different shape affects their function in your body. Trans fatty acids behave more like saturated fats than like unsaturated fats and they may have similar adverse effects on your health.
Hydrogenated fats raise cholesterol and therefore increase your risk of heart disease, according to the American Heart Association. Trans fats are bad on two counts, notes Harvard School of Public Health. They raise your low-density lipoprotein, “bad” cholesterol, and lower your high-density lipoprotein, “good” cholesterol, so this combination of low HDL and high LDL increases your risk of heart disease in two ways. Cardiovascular disease is the leading cause of death in both men and women.