What Is the Hydrogenation Process?
Hydrogenation is a chemical reaction that converts unsaturated fatty acids into saturated fatty acids. This is done by adding hydrogen atoms to the carbon-carbon double bonds found in unsaturated oils, such as vegetable oils, in the presence of a metal catalyst like nickel. The result is a change in the oil's molecular structure, which in turn alters its physical properties.
Key steps in the process include:
- Heating: The oil is heated in a pressure vessel.
- Catalyst Addition: A small amount of a metal catalyst, typically nickel, is added.
- Hydrogenation: Hydrogen gas is introduced under high pressure, reacting with the oil.
- Saturation: This reaction breaks the double bonds between carbon atoms and replaces them with single bonds linked to new hydrogen atoms.
The Purpose of Hydrogenating Oil
Food manufacturers initially adopted hydrogenation for several practical reasons:
- Longer Shelf Life: By reducing the number of double bonds, the oil becomes less susceptible to oxidation, which is what causes rancidity. This extends the product's lifespan significantly.
- Improved Texture and Consistency: Hydrogenation allows manufacturers to control the fat's consistency, from a semi-solid margarine to a solid shortening. This is crucial for baked goods, where solid fats produce a flaky, desirable texture.
- Cost-Effectiveness: Fully or partially hydrogenated vegetable oils were a cheaper alternative to animal fats like butter and lard for many decades.
Partial vs. Full Hydrogenation: The Health Difference
The health impacts of hydrogenated oils depend heavily on the extent of the hydrogenation process. The two main types are partial and full hydrogenation.
Partial Hydrogenation and Trans Fats
Partial hydrogenation, as the name suggests, does not fully saturate all the double bonds. A major problem with this process is that the high heat and catalytic action can reconfigure some of the remaining double bonds, flipping their molecular shape from cis to trans. These newly formed fatty acids are known as artificial trans fats.
Artificial trans fats are especially detrimental to health because they:
- Raise levels of LDL (bad) cholesterol.
- Lower levels of HDL (good) cholesterol.
- Increase the risk of heart disease and stroke.
It is because of these risks that the US Food and Drug Administration (FDA) determined in 2015 that partially hydrogenated oils (PHOs) were no longer "Generally Recognized as Safe" (GRAS) for use in food products. This effectively led to a ban on their use in most processed foods.
Full Hydrogenation and Saturated Fats
Full, or complete, hydrogenation saturates all double bonds, so no trans fatty acids are formed. This process produces a very hard, solid fat at room temperature. For food applications, this solid fat is then often blended with unhydrogenated liquid oil to achieve the desired consistency, without the formation of trans fats. The end product of full hydrogenation is a saturated fat, which, while not as harmful as trans fats, should still be consumed in moderation as part of a balanced diet.
Foods That Historically Contained Hydrogenated Oils
Prior to the 2015 FDA ban on PHOs, many common food items contained partially hydrogenated oils due to their functionality and low cost:
- Baked goods: Cakes, cookies, and pie crusts often used PHOs to achieve a desirable texture and extend shelf life.
- Snack foods: Crackers, chips, and microwave popcorn frequently contained PHOs for flavor stability.
- Fried foods: Many fast-food restaurants used PHO-based frying oils because they could be reused longer than other oils before becoming rancid.
- Spreads: Stick margarines and some vegetable shortenings were prime examples of partially hydrogenated products designed to mimic animal fats.
- Processed foods: Refrigerated dough, frozen pizzas, and coffee creamers were also common sources.
After the ban, manufacturers largely transitioned to using fully hydrogenated oils, palm oil, or other naturally solid fats to replace PHOs. This shift was a major public health victory, but it still means processed foods may contain higher levels of saturated fat.
What to Look For on Food Labels
Even after the FDA ruling, consumers should remain vigilant when reading ingredient lists. A product can be labeled as having "0g trans fat" per serving if it contains less than 0.5g. This means that small amounts of trans fat may still exist, especially in foods that list "partially hydrogenated oil" from before the ban took full effect, or in older stock. Always check the ingredient list for the specific term "partially hydrogenated oil." You can see the history of this public health issue in this timeline published by the Center for Science in the Public Interest.
Conclusion
To be hydrogenated means to have liquid, unsaturated fats converted into solid or semi-solid saturated fats through a chemical process involving hydrogen and a metal catalyst. While this process has been utilized to create a variety of processed foods with desirable textures and extended shelf lives, its most concerning legacy is the creation of artificial trans fats from partial hydrogenation. With bans on PHOs in place, the focus has shifted to fully hydrogenated oils, which are essentially saturated fats. For consumers, the key takeaway is to continue reading food labels carefully and to prioritize healthier, whole food options when possible to minimize intake of industrially altered fats, regardless of the hydrogenation level.
