Most of the lipids in food are triglycerides that classified into saturated and unsaturated fats according to the presence of double bonds. There are two types of unsaturated fats due to their double bond conformations.
Structure of trans fats and cis fats
In cis fats, the hydrocarbon chains in fatty acid are located on the same side of double bond to give itself a curved shape. Cis fats can’t arrange closely and result in lower melting points, therefore they have excellent fluidity at room temperature. Vegetable oils such as olive oil, rapeseed oil, and peanut oil are all plentiful in cis fat. They’re also found largely in fish oils.
The hydrocarbon chains are on the opposite sides of double bond in trans fats. Because their straight structure is more similar to saturated fats, they exhibit similar properties. The higher melting points make them solid or semi-solid at room temperature.
Where do trans fats come from
Dietary trans fats mainly come from industrially processed food or biohydrogenation in ruminant animal gut. They’re respectively referred to as industrial trans fats (artificial trans fats) and ruminant trans fats (natural trans fats). High temperatures during edible oils deodorization are also one of the reasons for trans fatty acids creation. In addition, processes such as frying, baking, and irradiation also produce them. The main component of industrial trans fatty acids (i-TFA) is Elaidic acid, while vaccenic acid is main content in natural trans fatty acids (r-TFA) .
i-TFA: Vegetable Oils Hydrogenation
People have been looking for alternatives to scarce and expensive animal fats for a long time. Liquid oil hydrogenation was first discovered by French chemist Paul Sabatier. German chemist Wilhelm Normann invented the industrial technology for in 1901 based on Sabatier’s researching. Subsequently, partially hydrogenated oils (PHO) were introduced into food industry as substitutes for expensive natural animal and vegetable fats, such as butter, lard and cocoa butter.
Vegetable oils and nickel-based catalysts are heated together to high temperatures from 140 to 225°C. Then, high-pressure hydrogen gas is pumped into them to converts oil double bonds into single bonds. The final product is a semi-solid or solid at room temperature. However, not all double bonds are transformed to single bonds during this process. Some double bonds undergo isomerization to become industrial trans fatty acids. Partially hydrogenated oils appear on the market as margarine and shortening. They’re extensively used in foods such as cakes, cookies, ice cream, fried chicken and french fries.
i-TFA: Refined Vegetable Oils, Frying and Baking
Another factor that turns vegetable oil into artificial trans fats is high temperature above 200°C. Generally, the higher temperature and longer heating time, the more trans fats are produced. Cold-pressed vegetable oils are predominantly cis unsaturated fats and contain almost no trans fats, but they emit a characteristic odor of plant stems and leaves. This unpleasant smell is removed above 200°C for 1-2 hours. Refined vegetable oils have lighter colors and milder odors but contain a few industrial trans fats.
Another commonly overlooked everyday activity is frying and baking. Research has shown that in soybean oil at 180°C, 0.3% of cis fatty acids are converted to trans fatty acids every 10 hours. Some unscrupulous merchants don’t regularly update their cooking oil. Sometimes, they are used for weeks or even months. Prolonged and repeated heating not only leads to abundant industrial trans fats but also produces other carcinogens in oils. Baked goods also contain trans fats because the high temperature in ovens changes fat conformation just as frying does.
i-TFA from Irradiated Foods
Gamma rays can penetrate food packaging to kill microorganisms, thereby extending food shelf life without preservatives. Irradiation technology was once considered absolutely safe, but unexpectedly, foods exposed to radiation will contaminate trans fats, and the higher doses the more these byproducts. The free radicals created from ionized water molecules will attack double bonds preferentially. When new double bonds regenerate from the broken ones, they tend to form trans conformation due to lower free energy.
Scientists evaluated the effect of gamma radiation on trans fatty acids in fresh beef. They observed an 80.4% increase in trans fatty acids at doses between 1-5 kGy. Doses between 6 and 7 kGy resulted in a 106.5% increase. At 8 kGy, the value is 139.1%, equivalent to about 11% of the total fatty acids in beef.
Ruminant trans fats (r-TFA)
Trans fats are not only synthesized artificially, but also present in small amounts in ruminant animals fats and dairy products. This’s because microorganisms in cows or sheep gut convert plant linoleic acid into conjugated linoleic acid (CLA, 18:2 9c,11t and 18:2 10t,12c). Then, conjugated linoleic acid is hydrogenated by microorganisms to form vaccenic acid (trans-18:1), which ultimately becomes stearic acid. Biohydrogenation and absorption occur simultaneously, so a portion of natural trans fatty acids is used by animals to synthesize fats present in milk, organs and muscles. Natural trans fats account for 2-5% of total fat approximately. The 80% to 90% of ruminant trans fatty acid is vaccenic acid.
The content of natural trans fats in animals varies depending on breed and feed. Beef fat contains more trans fatty acids than sheep fat. Ruminant animals fed on grass have more trans fatty acids compared to those fed on grains.
Cis and trans fats and health
Cis fats contain antioxidative unsaturated fatty acids that eliminate free radicals, alleviate inflammation, and resist aging. Trans fats, on the other hand, are considered hazardous rubbish, especially artificial trans fats. They can cause high level lipid in blood, cardiovascular diseases, inflammation, and disruptions in fatty acid metabolism.