The Chemistry of Hydrogenation and Its Products
Hydrogenation is a chemical process that adds hydrogen atoms to unsaturated compounds, such as liquid vegetable oils, in the presence of a metal catalyst like nickel. The primary purpose in the food industry is to convert liquid oils into solid or semi-solid fats, a process that increases their stability and shelf life. The outcome depends on whether the process is partial or complete.
Partial vs. Complete Hydrogenation
During complete hydrogenation, all the carbon-carbon double bonds in the fatty acid chains are converted to single bonds, resulting in a fully saturated fat. These fully hydrogenated fats, like those found in some shortening and hard margarine, contain virtually no trans fats. Conversely, partial hydrogenation is a carefully controlled process where only some of the double bonds are saturated. It is this partial process that is responsible for producing artificial trans fatty acids.
The Origin of Trans Fatty Acids in Food Processing
The creation of trans fatty acids during partial hydrogenation is a major and often undesirable side effect. The double bonds in natural unsaturated fatty acids typically have a cis configuration, which creates a 'kink' in the molecule's chain. The high temperatures and the metal catalyst used in the partial hydrogenation process cause some of the remaining unsaturated bonds to undergo cis-to-trans isomerization. In the trans configuration, the hydrogen atoms are on opposite sides of the double bond, resulting in a straighter, more rigid molecular shape that behaves similarly to a saturated fat. A well-known example is the isomerization of cis-oleic acid to trans-elaidic acid.
Commercial Appeal vs. Health Concerns
Before the health risks were widely known, partially hydrogenated oils were favored by food manufacturers for several reasons. Their higher melting point and semi-solid state at room temperature made them ideal for baking and producing spreads like margarine. They also offered a longer shelf life and were less expensive than many other fats.
However, research has since revealed significant negative health implications. Industrially produced trans fats have been shown to raise low-density lipoprotein (LDL) cholesterol (the 'bad' kind) and lower high-density lipoprotein (HDL) cholesterol (the 'good' kind). This detrimental combination increases the risk of heart disease and stroke. Due to these findings, health organizations like the World Health Organization (WHO) and regulatory bodies like the FDA have taken action to limit or eliminate the use of partially hydrogenated oils in the food supply.
Comparing Cis and Trans Fatty Acids
| Feature | Cis Unsaturated Fatty Acids | Trans Unsaturated Fatty Acids |
|---|---|---|
| Molecular Shape | Kinked or bent chain | Straighter, more rigid chain |
| Melting Point | Lower, typically liquid at room temperature | Higher, often solid or semi-solid at room temperature |
| Origin | Found naturally in most plants and animals | Primarily formed during partial hydrogenation (small amounts naturally occur in ruminant meat/dairy) |
| Health Effects | Generally considered healthy and beneficial | Strongly linked to increased risk of heart disease |
Chemical Changes During Partial Hydrogenation
When a liquid vegetable oil is partially hydrogenated, the following chemical changes occur:
- Reduction of double bonds: Hydrogen atoms are added across some of the carbon-carbon double bonds, reducing the oil's overall unsaturation.
- Isomerization: The high-temperature, catalyst-driven process causes some of the remaining cis double bonds to convert into a more thermodynamically stable trans configuration.
- Shifted double bonds: The position of the double bonds along the fatty acid chain can also move during the reaction.
- Increased melting point: The formation of straight-chained trans fatty acids and the reduction of total double bonds increase the melting point of the oil, resulting in a solid or semi-solid fat.
Conclusion
In summary, the specific type of unsaturated fatty acids produced due to the industrial partial hydrogenation process are trans fatty acids. This occurs through a side reaction that converts the naturally occurring cis configuration of double bonds to a trans configuration, which mimics the straight shape and physical properties of saturated fats. These artificial trans fats were once widely used but are now known to pose significant health risks, leading to a move towards healthier, non-hydrogenated alternatives. As a result, food manufacturers have significantly reduced their use, opting for different processing methods or ingredients to achieve the desired food characteristics.
For more information on the health risks associated with trans fats, consult the World Health Organization's fact sheet: Trans fat - World Health Organization (WHO).
