When liquid oils are hydrogenated, hydrogen gas is forced into the oil under heat and pressure, converting some or all of the unsaturated fatty acids into saturated ones. This transforms the oil from a liquid at room temperature into a semi-solid or solid fat. The process was once central to the food industry, producing everything from margarine to baking shortening, but it also creates trans fats, which carry serious cardiovascular risks.
What Happens at the Molecular Level
Vegetable oils like soybean, canola, and sunflower oil are liquid because their fatty acid chains contain double bonds, which create kinks in the molecule. These kinks prevent the chains from packing tightly together, keeping the fat fluid. Hydrogenation adds hydrogen atoms to those double bonds, straightening out the kinks and allowing the chains to stack more closely. The result is a fat that behaves more like butter or lard: firm, spreadable, and stable.
The reaction requires a metal catalyst, typically nickel supported on silica or alumina, and temperatures in the range of 120 to 180°C. At lower temperatures (around 60°C), the catalyst barely works. At 180°C, polyunsaturated fatty acids like linolenic and linoleic acid convert rapidly, with over 84% of linoleic acid and 90% of linolenic acid reacting within a few hours under optimized conditions.
The reaction follows a stepwise path. A fatty acid with three double bonds (linolenic acid) first loses one to become linoleic acid (two double bonds), which then loses another to become oleic acid (one double bond). If the process continues, oleic acid converts to stearic acid, which has no double bonds at all and is fully saturated.
How Trans Fats Form
In nature, the double bonds in vegetable oils almost always have a “cis” configuration, meaning the hydrogen atoms sit on the same side of the bond. This is what creates the characteristic kink. During hydrogenation, the energy of the reaction can flip one of those hydrogen atoms to the opposite side, producing a “trans” configuration. The double bond remains, but the kink disappears, and the chain straightens out almost as if it were fully saturated.
This matters because partial hydrogenation, where the process is stopped before all double bonds are filled, inevitably produces a mix of cis and trans bonds. The oil becomes semi-solid, which is useful for food manufacturing, but it now contains artificially created trans fatty acids that don’t exist in significant quantities in nature.
Partial vs. Full Hydrogenation
The distinction between partial and full hydrogenation is critical. Partial hydrogenation stops the reaction partway through, leaving some double bonds intact. This produces a soft, spreadable fat ideal for margarine and shortening, but it also generates trans fats as a byproduct. Partially hydrogenated oils (PHOs) were the primary dietary source of artificial trans fat in processed foods for decades.
Full hydrogenation, by contrast, saturates every double bond. The result is a very hard, waxy fat with no trans fats at all, because no double bonds remain to have a cis or trans configuration. Fully hydrogenated oils are sometimes blended with liquid oils to achieve the desired consistency without the trans fat problem.
Why the Food Industry Used Hydrogenation
Hydrogenated fats solved several practical problems at once. Liquid oils go rancid relatively quickly because their double bonds react with oxygen. Hydrogenation reduces this sensitivity to oxidation, dramatically extending shelf life. The resulting semi-solid fats also have higher melting points, so products made with them hold their shape at room temperature.
In baking, the benefits are especially pronounced. Solid or semi-solid fats coat flour particles during mixing, preventing water from reaching the proteins that form gluten. This “shortening” effect produces tender, crumbly baked goods rather than tough, chewy ones. When liquid oil is used instead, it disperses into tiny globules that are far less effective at blocking gluten formation, resulting in softer, less structured dough.
Semi-solid fats also trap air better than liquid oils. Fat crystals attach to air bubbles during mixing, and as they melt during baking, a protein membrane forms around the expanding bubbles, giving baked goods their lift and light texture. Biscuits made with hydrogenated shortening consistently showed better surface characteristics and higher crispness compared to those made with liquid oils.
Cardiovascular Risks of Trans Fats
The health consequences of industrial trans fats are well established. A large meta-analysis published in The BMJ found that a 2% increase in energy from trans fats is associated with a 25% increased risk of coronary heart disease and a 31% increase in death from heart disease. Total trans fat intake was linked to a 34% higher risk of death from all causes.
Industrial trans fats, the kind created by partial hydrogenation, are significantly more dangerous than the small amounts of trans fats that occur naturally in meat and dairy. Industrial trans fats were associated with a 42% increased risk of coronary heart disease, while naturally occurring trans fats from ruminant animals showed no significant association. The damage works through multiple pathways: trans fats raise LDL (“bad”) cholesterol, lower HDL (“good”) cholesterol, and promote inflammatory processes in blood vessels.
Regulatory Action and the PHO Ban
The FDA declared partially hydrogenated oils no longer “Generally Recognized as Safe” for any human food in June 2015. Manufacturers were given years to reformulate their products, with a final compliance date of January 1, 2021. The last administrative actions formally revoking all approved uses of PHOs in food took effect on December 22, 2023.
Trans fat hasn’t disappeared entirely from the food supply. It occurs naturally in small amounts in meat and dairy, and trace levels exist in other edible oils. But the dominant industrial source, partially hydrogenated oils in processed foods, has been effectively eliminated from the U.S. market.
What Replaced Hydrogenated Oils
Food manufacturers have turned to several alternatives. One of the most common is interesterification, a process that rearranges the fatty acid groups on a fat molecule without creating trans bonds. This can be done chemically or with enzymes, and it allows manufacturers to blend oils (particularly palm oil) into semi-solid fats with the right melting profile for baking, spreads, and confections. Palm oil itself, which is naturally semi-solid at room temperature, has also seen increased use, though it raises separate environmental concerns. Fully hydrogenated oils blended with liquid oils offer another route to achieving the right texture without trans fats.