Lipid (2): Trans Fat vs Cis Fat, where trans fats come from

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Most of the lipids in food are triglycerides that are classified into saturated fats and unsaturated fats according to the presence of double bonds. There are two types of unsaturated fats due to their double bonds conformations. Therefore, they are divided into trans fats and cis fats.

Structure of trans fats and cis fats

In cis fats, the hydrocarbon chains in fatty acid are located on the same side of the double bond giving the molecular a curved shape. They cannot arrange closely and result in lower melting points and excellent fluidity at room temperature. Cis fats are abundant in vegetable oils such as olive oil, rapeseed oil, and peanut oil. They are also abundant in fish oils.

In trans fats, the hydrocarbon chains in the fatty acids are on the opposite sides of the double bond. Because their structure is more similar to saturated fats, they exhibit similar properties: higher melting points makes them solid or semi-solid at room temperature.

Where do trans fats come from

Trans fats in the diet mainly come from industrial production and the biohydrogenation process in ruminant animals. They are respectively referred to as industrial trans fats (artificial trans fats) and ruminant trans fats (natural trans fats). High temperatures during the 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 trans fatty acids. The main component of industrial trans fatty acids (i-TFA) is Elaidic acid, while the main component of natural trans fatty acids (r-TFA) is vaccenic acid.

i-TFA: Vegetable Oils Hydrogenation

Due to the scarcity and expense of animal fats, people have long been seeking alternatives. The process of liquid oil being hydrogenated with the help of a catalyst was first discovered by French chemist Sabatier. German chemist Wilhelm Normann invented the industrial technology for liquid fats hydrogenation in 1901 based on his findings. Subsequently, partially hydrogenated oils (PHO) were introduced into the food industry as substitutes for expensive natural animal fats and vegetable fats, such as butter, lard, and cocoa butter.

Vegetable oils are heated together with nickel-based catalysts to temperatures ranging from 140 to 225°C, then mixed with high-pressure hydrogen gas. With the assistance of the catalyst, high temperature and high pressure, hydrogen gas converts the double bonds in vegetable oils into single bonds, causing the oils to become semi-solid or solid at room temperature. However, not all double bonds are converted 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, and are 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 the 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. They are heated above 200°C for 1-2 hours during deodorization. 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 do not regularly change the cooking oil in their fryers, resulting in oils being used for weeks or even months. Prolonged and repeated heating not only leads to abundant industrial trans fats in vegetable oils but also produces other carcinogens. Baked goods also contain trans fats because the high temperature of ovens causes conformational changes in fats just as frying does.

i-TFA from Irradiated Foods

Gamma rays can penetrate food packaging and kill microorganisms, thereby extending the shelf life of food without preservatives. Irradiation technology was once considered absolutely safe, but unexpectedly, foods exposed to radiation contain trans fats, and the higher doses the more these byproducts. High-energy radiation ionizes water molecules to creat free radicals that attack double bonds preferentially. When the new double bonds regenerate from the broken ones, they tend to form the trans conformation due to lower free energy.

Brito et al. 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, they increased by 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 the fats of ruminant animals and dairy products. This is because microorganisms in the stomach of cows or sheep convert linoleic acid from plants into conjugated linoleic acid (CLA, 18:2 9c,11t and 18:2 10t,12c). Conjugated linoleic acid is hydrogenated by microorganisms in the stomach 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 vaccenic acid comprising 80% to 90% of ruminant trans fatty acid.

The content of natural trans fats in animals varies depending on the 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.

Frequently Asked Questions

Why Hydrogenated Vegetable Oils Are Not Equivalent to Trans Fats

Hydrogenated oils can be divided into partially hydrogenated oils and fully hydrogenated oils based on the degree of hydrogenation. Partially hydrogenated oils undergo limited hydrogenation to retain some double bonds that experience a conversion from cis to a more stable trans structure, resulting in more trans fatty acids, typically around 20%. Fully hydrogenated oils are saturated fats, where all double bonds have been became single bonds. Consequently, their trans fatty acid content is less than 1%. It is incorrect to equate hydrogenated vegetable oils with trans fats in people's daily perception. Some factories have optimized their process so that vegetable oils are completely hydrogenated and contain almost no trans fatty acids.

Why Hydrogenated Vegetable Oils are used in the Food Industry?

Compared to liquid oils, solid hydrogenated oils are less prone to oxidation which gives them a longer shelf life. Desserts made with butter tend to melt at room temperature. To achieve a better appearance and softer texture, some vegetable shortening with higher melting points is added to butter. Additionally, it's more cost-effective. In the early 20th century, vegetable oils were perceived as healthier alternatives to animal fats due to their lack of cholesterol. Using cheap and "healthy" margarine instead of animal fats was considered progressive at that time. Despite health risks,they were deemed nearly perfect substitutes of butter and lard.

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