Hydrogenation process


  • Hydrogenated fats are cheap alternatives to naturally obtained animal fats and have optimal properties for culinary purposes including: long shelf life, desirable texture of baked products and stability when reheating.
  • They have been used in the food industry for the last 100 years, but there is insufficient evidence that they are safe.
  • Partial hydrogenation damages the fats’ molecular structure, producing trans fats, which have been associated with cardiovascular diseases and cancer.
  • Hydrogenated products are also made of GMOs and contain a large proportion of pro-inflammatory omega-6 fatty acids.


What is hydrogenation of oils?

Many products, such as margarine, shortenings, biscuits, pastries and other baked products, contain hydrogenated vegetable oils.

But what is hydrogenation? Should we be concerned? What is the difference between fully hydrogenated and partially hydrogenated oils?

Find out why hydrogenated oils are unhealthy, how they are made and some history about why we started to use it in our food.

Hydrogenation definition

Hydrogenation is a chemical process where hydrogen reacts to an organic compound.

In the context of food processing, it refers to the saturation of unsaturated liquid oils with hydrogen atoms.

Molecules in unsaturated fatty acids are bent (have kinks in them), not allowing them to pack together closely.

This limits the intramolecular attractive forces and makes their melting point lower, which means that they are liquid at room temperature.

The hydrogenation process removes those kinks and makes the fatty acids straight, either by making them fully saturated or altering them to trans fatty acids.

Both saturated and trans fatty acid chains are straight. Therefore, they can stack up closely together. This increases their melting point.

Note: for more detailed (chemical) information on the hydrogenation process, please see the “Triglycerides – basic chemistry” article.

The most common oils that undergo hydrogenation processing are soy (about 60% of hydrogenated oils), sunflower, safflower, peanut, palm, cotton seed and corn.

Fully and partially hydrogenated oils differ considerably in their physical and chemical properties and in their impact on health, as explained in sections below.

The main purposes of hydrogenation oils to solid or semi-solid fats are (1) :

  • They are a cheap substitute of saturated fats from animal origins, such as lard or butter;
  • Hydrogenated oils are more stable than unsaturated oils. The process of hydrogenation transforms fatty acids that are prone to oxidative rancidity, especially in high temperatures during cooking;
  • Hydrogenated oils are also more stable than saturated products, such as butter.

    Although butter is considered as a saturated fat food, it contains some short chain fatty acids and unsaturated fats, both of which are more prone to rancidity than long chain saturated fatty acids present in hydrogenated products.

    Vegetable oils used in hydrogenation are usually more uniform in length with medium to long chains and after saturation are more stable.

  • The physical properties, such as the melting point, are easily manipulated by changing the degree of saturation. For instance, the final product may be hardened or softened by adjusting how much hydrogen is added during the process;
  • They have increased tolerance to constant reheating (such as in deep frying).

    This means that in countries where the legislation allows for the usage of partially hydrogenated oils, fried foods are often fried in multiple reheated PHOs.

  • They give a more appealing texture to baked products (e.g. a flaky, crispy texture of the pastry), due to the way they mix with flour.

Health concerns related to the hydrogenation process of oils

This point relates to hydrogenation (whether partial or full). The concerns specific to partial hydrogenation are in the point below.

  • Nickel

    The hydrogenation process involves nickel as a catalyst, which is then mostly removed via special filters. (2)

    The safe upper limit of nickel is established at 1mg/day for adults.  The absorption of dietary nickel is less than 10%.

    Adults consume on average between 207mcg and 406mcg of nickel daily from all different food sources. The nickel intake from the residual of hydrogenated oils was estimated in 1979 to be 30mcg per day, which is much lower than from other dietary sources.

    If checking the FDA page I am referencing here, please note that at the time of writing of this article, this page contained an error: mg should be mcg. (3, 4, 5)

    Although the amount of nickel and other metal catalysts residues in hydrogenated oils is within the safe range, it only adds to the concoction of these dangerous substances from diet.

    For better health, avoid highly processed foods such as hydrogenated oils.

  • GMOs

    Most hydrogenated oils are made of genetically modified plants, such as soy or corn. The long-term health impact of the consumption of genetically modified foods has not been well studied and it will be more apparent in the future generations.

  • Modification of foods on a molecular level

    The hydrogenation process modifies the fatty acid molecules. However, there are no studies showing the health consequences of consuming foods manipulated on that level.

    The health consequences are still not well researched.

  • Other additives

    Since they are highly processed, hydrogenated products contain other additives, many of which are displayed in the nutrition facts of the product label in codes, numbers or technical names which are not familiar to the end users.

    The long-term health impacts from this combination of additives and chemicals have not been well studied.

Full hydrogenation (full saturation)

The full hydrogenation process saturates all carbon atoms with the maximum possible number of hydrogen atoms, making the oil fully saturated (no more hydrogen atoms can be attached and also no trans fatty acids exist).

The process of full saturation transforms unsaturated into fully saturated fatty acids. All of the kinks/bends are removed from the structure.

The shape of the chains becomes straight. Therefore, when they stack up together, they form a solid fat in room temperature.

What are the health risks of fully hydrogenated oils?

Full hydrogenation undergoes heavy processing using high temperatures, high pressure and other processes, such as inter-esterification, emulsification and mixing with other oils, that alter the chemical structure of the fatty acids.

Whether these alterations on a molecular level have a long-term impact on human health is not known.

Since fully hydrogenated oils are hard and not very practical in some food preparation methods, they are usually mixed with unsaturated oils. Most of these oils are genetically modified (e.g. soybean oils) or may be even partially hydrogenated.

Note: Trans fatty acids are only a concern in partially hydrogenated oils (see the section on partially hydrogenated oils below).

Fully hydrogenated oils don’t contain trans fatty acids because there are no carbon double bonds and each carbon within a chain is connected to two hydrogen atoms.

What is partial hydrogenation (partial saturation)

Note: for more detailed (chemical) information on the process of partial hydrogenation, please see the relevant section in the “Triglycerides – basic chemistry” article.

Partial hydrogenation is an incomplete process. This means that the hydrogenation process stopped before completing a full saturation of carbons with hydrogen atoms. 

As a result, there are still some double bonds present. Some of these double bonds are of “cis” configuration (normal configuration characteristic for unsaturated fatty acids) and some have been damaged in the process forming a “trans” configuration, where one of the hydrogens at the carbon double bond has been removed and another placed on the opposite side of the chain.

The purpose of partial hydrogenation of oils is to make their properties optimal for food preparation. Partial hydrogenation results in a product that is more suitable for culinary purposes than fully hydrogenated products. (6, 7)

Why are oils only partially hydrogenated?

In addition to the common purposes described above, under the hydrogenation section, there are some specific purposes of partially hydrogenated fats, such as:

  • Partially hydrogenated oils can give a wide range of consistencies of the finished fat product;
  • Fully hydrogenating oils would make the oils hard at room temperature and not as versatile in food preparation;
  • They increase the stability and the shelf life of fats in processed foods;
  • They are extremely cost efficient and preferred for their usability in most commercial baked goods;
  • Since saturated fats are still improperly considered “unhealthy” (read more..), products such as margarines are still promoted as a “healthy” alternative to butter.

    A fully saturated product would not sound good. That is why the process of hydrogenation usually doesn’t go all the way through. It retains a large portion of unsaturated fats to preserve the supposedly “healthy” properties.

Negative health consequences of partially saturated products

  • Trans fats

    By far the biggest problem with partially hydrogenated oils, is the damage to the fat molecules that occurs during the process. The damaged molecules are called Trans-Fats or Trans Fatty Acids.

    There is strong evidence linking trans fats with heart disease, obesity, diabetes, cancer and other serious medical conditions. (read more..)

  • Omega-6

    Oils used during the partial hydrogenation process contain polyunsaturated fats omega 6, which cause inflammation, if consumed in high omega-6 to omega-3 proportions.

    Our diets today already provide us with a high ratio of omega-6 to omega-3 and partially hydrogenated oils only increase that ratio.

  • GMOs

    The majority of partially hydrogenated products are made of soybean and corn. Most of the crops of these two plants are genetically modified.


You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.


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