Trans fat is made primarily through an industrial process called partial hydrogenation, which forces hydrogen gas into liquid vegetable oil under high heat and pressure, transforming it into a solid or semi-solid fat. This process rearranges the molecular geometry of the oil’s fatty acids, flipping some of them from their natural “cis” shape into an unnatural “trans” configuration. The result is a fat that’s firmer, more shelf-stable, and cheaper than butter, but significantly worse for your health.
The Partial Hydrogenation Process
The manufacturing starts with a common liquid vegetable oil, such as soybean or cottonseed oil. The oil is pumped into a large pressurized reactor, where it’s heated to between 120 and 250°C (roughly 250 to 480°F) and exposed to hydrogen gas at pressures of 1 to 5 bars. A powdered nickel catalyst is mixed in to speed up the chemical reaction between the hydrogen and the oil.
What happens inside the reactor is straightforward in principle: hydrogen atoms attach to the carbon chain of the oil’s fatty acids, filling in some of the gaps at the double bonds that make the oil liquid and flexible. The more double bonds that get filled (or “saturated” with hydrogen), the firmer the fat becomes. If you saturate every double bond, you get a fully hydrogenated oil, which is extremely hard and waxy. Manufacturers don’t usually want that. They stop the reaction partway, which is why the product is called “partially” hydrogenated oil.
Stopping partway is exactly what creates trans fats. During the reaction, the nickel catalyst doesn’t just add hydrogen to double bonds. It also rearranges some of them. Double bonds that weren’t saturated get flipped from their original cis configuration into a trans configuration. On average, partially hydrogenated oil contains 25 to 45% trans fat by weight, according to the World Health Organization.
What “Cis” and “Trans” Actually Mean
Every unsaturated fatty acid has at least one double bond along its carbon chain, and at each double bond, a hydrogen atom sticks off to the side. In a cis fatty acid, both hydrogen atoms sit on the same side of the chain. This creates a kink or bend in the molecule, which is why natural vegetable oils are liquid at room temperature: the bent molecules can’t pack tightly together.
In a trans fatty acid, the hydrogen atoms sit on opposite sides of the chain. This straightens the molecule out, letting it stack neatly alongside other molecules, much like saturated fat does. That tight packing is what gives trans fat its solid texture and higher melting point. It’s also what makes it behave differently in your arteries than natural unsaturated fats.
Why Manufacturers Wanted Trans Fat
Partial hydrogenation solved two expensive problems at once. First, it made fats more resistant to rancidity. Rancidity happens when oxygen attacks the double bonds in unsaturated oils, breaking them down and producing off-flavors. By reducing the number of double bonds, hydrogenation dramatically extended shelf life. Crackers, cookies, and frozen foods could sit in stores for months without tasting stale.
Second, it raised the melting point of cheap vegetable oils, turning them into solid fats that could replace butter and lard in baking. Partially hydrogenated oils gave pie crusts their flakiness, made frosting hold its shape, and gave fried foods a satisfying crispness. The technique dates back to 1901, when German chemist Wilhelm Normann first successfully hydrogenated fat, patenting the process in 1903. By the mid-20th century, partially hydrogenated oils had become a staple of processed food worldwide.
The Role of the Catalyst
The metal catalyst is the key variable in how much trans fat the reaction produces. Nickel is the standard industrial choice because it’s inexpensive and effective. But nickel is not particularly selective. It readily flips cis bonds into trans bonds as a side reaction during hydrogenation.
Noble metals like rhodium, palladium, and platinum are more selective toward producing cis-monounsaturated fatty acids and can reduce trans fat formation, but they’re far too expensive for commercial food production. Among the options studied, platinum produces the least trans fat, while rhodium and palladium produce the most. Nickel falls in the middle, which made it an acceptable tradeoff for manufacturers who, for decades, didn’t consider trans fat a health concern.
Fully Hydrogenated vs. Partially Hydrogenated Oil
These two products are fundamentally different in trans fat content. Fully hydrogenated oil has had all its double bonds saturated with hydrogen, leaving virtually no trans fat. The result is an extremely hard, waxy fat that’s essentially pure saturated fat. It doesn’t work well on its own in food, but manufacturers sometimes blend it with liquid oils to achieve a desired texture without the trans fat.
Partially hydrogenated oil, by contrast, is where the trans fat problem lives. Because the reaction was stopped before completion, many double bonds remain, and a large fraction of those have been flipped into the trans configuration. That 25 to 45% trans fat concentration is what made partially hydrogenated oils so harmful at population scale, since they were added to thousands of everyday products.
Trans Fat From Cooking at High Heat
Partial hydrogenation isn’t the only way trans fats form. Heating regular cooking oils at very high temperatures can also produce small amounts. Below 200°C (about 390°F), the effect is negligible. Above that threshold, trans fat levels start climbing, especially with prolonged heating.
Between 200 and 240°C (390 to 465°F), total trans fat increased by about 0.38% for every 10°C rise in temperature. Duration matters too: heating oil for six hours in that range increased trans fat by 0.86% per 10°C rise, compared to virtually no change after just 15 or 45 minutes. Still, even under harsh frying conditions, the trans fat generated tops out around 2 to 3%, a fraction of what’s found in partially hydrogenated oils. For home cooking at normal temperatures and times, the amount produced is insignificant.
Natural Trans Fats in Meat and Dairy
Trans fats also occur naturally in small amounts in beef, lamb, and dairy products. Bacteria living in the stomachs of ruminant animals (cattle, sheep, goats) partially convert polyunsaturated fats from the animal’s feed through a process called biohydrogenation, which is biochemically similar to industrial hydrogenation but happens at body temperature inside the animal’s gut. This produces specific trans fats, primarily trans-vaccenic acid and rumenic acid.
The amounts are small compared to what industrial processing created. A glass of whole milk or a serving of beef contains a few tenths of a gram of natural trans fat. These ruminant trans fats are the reason trans fat can never be fully eliminated from the human diet, even with industrial bans in place.
Current Regulatory Status
The FDA determined in 2015 that partially hydrogenated oils are not generally recognized as safe. Manufacturers were prohibited from adding them to foods starting June 18, 2018, with extended compliance dates through January 2020 for products already in the supply chain and January 2021 for a handful of specific petitioned uses that were ultimately denied. Partially hydrogenated oils are now effectively banned as food additives in the United States. The WHO has similarly pushed for global elimination, and many countries have followed with their own restrictions or outright bans.
Food labels in the U.S. can still list 0 grams of trans fat if a serving contains less than 0.5 grams. If you want to verify that a product is truly free of industrial trans fat, check the ingredient list for “partially hydrogenated” oil. Fully hydrogenated oil, while high in saturated fat, does not carry the same trans fat concern.