FOOD CHEMISTRY: FATS AND FATTY ACIDS
Diagram
Same chain, different bend
Cis fatty acid
The two hydrogens sit on the same side of the double bond. The chain bends, so the fat stays liquid.
↓
Trans fatty acid
The two hydrogens sit on opposite sides of the double bond. The chain stays straight, so the fat packs tightly and behaves more like a solid fat.
↓
The key point
Both are unsaturated fatty acids. The shape of the double bond, not the number of carbons, is the difference.
Figure 1: Cis and trans fatty acids differ in the shape of the double bond. Drawn in our own style from Coultate, Food: The Chemistry of Its Components, chapter on fatty acids, and Akoh and Min, Food Lipids, chapter on methods for trans fatty acid analysis. This pair is distinct from the built article on saturated versus unsaturated fat, which compares saturation, not double bond shape.
Introduction
Cis and trans fatty acids are both unsaturated fatty acids, and both have at least one double bond in the carbon chain. The difference is the shape of that double bond. In a cis fatty acid the two hydrogen atoms sit on the same side of the double bond, so the chain bends. In a trans fatty acid they sit on opposite sides, so the chain stays straight. That bend, or lack of it, changes how the fat packs, how it melts, and how it behaves in processing and in the body.
Coultate covers fatty acid structure and distribution, including the elimination of trans fatty acids, and Akoh and Min give a chapter to methods for trans fatty acid analysis. This article follows those two books.
Quick comparison
Table 1: Cis fatty acids compared with trans fatty acids
| Aspect | Cis fatty acids | Trans fatty acids |
|---|---|---|
| Double bond shape | Hydrogens on the same side, chain bends | Hydrogens on opposite sides, chain stays straight |
| Chain packing | Bent chains pack loosely | Straight chains pack tightly, more like a saturated fat |
| Physical state | Usually liquid oils at room temperature | More solid, or firmer, than the matching cis fat |
| Melting point | Lower, because loose packing melts easily | Higher, because tight packing needs more heat to melt |
| Natural source | Most natural unsaturated fats in plants and fish | Small amounts in ruminant fat, from bacteria in the rumen |
| How they are made in processing | Present in the oil as it comes from the plant | Formed during partial hydrogenation of oils |
| Effect on texture | Give a liquid oil, good for pouring and frying | Give a firmer fat, good for spreads and pastry |
| Stability | More easily oxidised, because the bent chain is open to attack | More stable to oxidation than the matching cis fat |
| Health view in the books | The normal form of unsaturated fat in food | Coultate discusses the elimination of trans fatty acids from food |
| Measurement | Separated and measured by gas chromatography, as in Akoh and Min | Separated and measured by gas chromatography, as in Akoh and Min |
Scientific explanation
Definition
A cis fatty acid is an unsaturated fatty acid in which the two hydrogen atoms on the carbons of a double bond sit on the same side, so the carbon chain bends at the double bond. A trans fatty acid is an unsaturated fatty acid in which the two hydrogen atoms sit on opposite sides, so the carbon chain stays straight at the double bond. Both are unsaturated, and both have the same number of carbons in the matching pair: only the geometry differs.
Working principle
The bend in a cis chain stops the chains from packing tightly, so the fat stays liquid and melts at a lower temperature. The straight trans chain packs tightly, like a saturated fat chain, so it is firmer and melts at a higher temperature. Partial hydrogenation, which adds hydrogen to an oil to make it firmer, can flip some double bonds from cis to trans, which is how most trans fatty acids in processed food are made. In nature, most unsaturated fatty acids are cis, and only small amounts of trans fatty acids occur, mainly in ruminant fat.
Apparatus
The books used here name gas chromatography for the analysis of trans fatty acids, in the chapter on methods for trans fatty acid analysis in Akoh and Min. No other apparatus is named for this comparison.
Measurement procedure
Akoh and Min give a chapter to methods for trans fatty acid analysis, based on gas chromatography of the fatty acid methyl esters, which separates the cis and trans forms. The books used here give no worked food example with numbers for this comparison, so none is given.
Factors affecting readings
The degree of hydrogenation changes how many trans fatty acids form. The catalyst, temperature and pressure of hydrogenation change the cis to trans ratio. Storage and heating can change the fat, but the cis or trans shape is fixed when the fat is made, not by storage.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Cis fatty acids are the main unsaturated fats of vegetable oils used for cooking and frying.
- Trans fatty acids were used in spreads and shortenings, where a firm fat was wanted from a liquid oil.
- Cis oils are used in salad dressings and mayonnaise, where a liquid oil is wanted.
- Partially hydrogenated oils, with trans fatty acids, were used in pastry and biscuits for texture.
- Food makers now remove trans fatty acids, as Coultate discusses, and use other fats instead.
Advantages and limitations
- Cis fatty acids: advantage: the normal liquid form of unsaturated fat, good for pouring and for frying. Limitation: more easily oxidised than trans fats, so the oil goes rancid sooner.
- Trans fatty acids: advantage: firmer and more stable to oxidation, so they gave spreads a good texture and a long shelf life. Limitation: Coultate discusses their elimination from food, and makers now avoid them.
- Cis fatty acids: advantage: measured easily by gas chromatography. Limitation: their bent shape makes the oil less stable in long storage.
FAQ
Are trans fats saturated fats?
No. Trans fatty acids are unsaturated, because they still have a double bond. They only behave a little like saturated fats because their straight chain packs tightly.
Where do trans fats come from?
Mostly from partial hydrogenation of oils in processing, and in small amounts from ruminant fat.
Why are trans fats being removed?
Coultate discusses the elimination of trans fatty acids from food, and makers now use other fats to get firmness without them.
Is this the same as saturated versus unsaturated fat?
No. That built article compares saturation. This article compares the shape of the double bond in unsaturated fats.
