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Why does flaxseed oil go rancid faster than olive oil?

Because its fat chains mostly carry three double bonds, versus just one for olive oil. Each carbon caught between two double bonds is a weak spot where oxygen attacks, and flaxseed oil has about ten times more of them. These same kinks keep omega-3s liquid and flexible.

The chains, the weak spot, omega-3s, then kitchen oils: pick a fat, change the step.

Why is butter solid and oil liquid?

A fatty acid is a chain of carbons with an acid head. With no double bond, the chain is straight and the chains pack tightly: it is solid. Each double bond adds a kink, so the chains pack poorly: it is liquid.

Fatty acid:
single bonddouble bond (a kink)weak spot: carbon between two double bondsacid head

    Good to know: this page explains the chemistry of fats; it does not recommend any oil. An oil that oxidizes more slowly is not necessarily “healthier”: the effect of fats on the heart depends on the whole meal and is still debated for some of them.

    Frequently asked questions about the double bonds of fats

    What is a saturated or unsaturated fat?

    Saturated: the chain has no double bond, and each carbon carries all the hydrogens it can (it is “saturated” with hydrogen). Unsaturated: it is missing two per double bond. Monounsaturated = one double bond (olive oil), polyunsaturated = two or more (sunflower, flaxseed, fish).

    Why does olive oil turn cloudy in the fridge?

    It contains about 14% saturated chains (USDA), and its oleic-rich triglycerides set at a few degrees Celsius. In the cold, part of it crystallizes: the oil turns cloudy, then pasty, and clears up again as it warms.

    What is a “trans” fat?

    A double bond can be cis (both ends of the chain on the same side, which makes a kink) or trans (on opposite sides: the chain stays almost straight). The natural fats in oils are cis. Trans fats mostly come from the partial hydrogenation of oils and, in small amounts, from the milk and meat of ruminants.

    Does the smoke point tell you whether an oil holds up to cooking?

    Not really. In a test of 10 oils heated for 6 h at 180 °C, the smoke point did not predict the amount of damaged compounds formed; the share of polyunsaturated chains and the oxidative stability predicted it better. Extra virgin olive oil formed among the fewest: 10.5%, versus 21.8% for sunflower (De Alzaa, 2018, a single study, run by a lab linked to the olive industry).

    Why does fish oil smell fishy as it ages?

    Its EPA and DHA chains have 4 and 5 weak spots. As they oxidize, they break into small odorous molecules, aldehydes: propanal comes mostly from omega-3s, hexanal from omega-6s (Shahidi, 2001).

    See also

    Where do omega-3s go in the body? How does vitamin E protect against oxidation? Where do the fats we eat go? Why doesn't Lp(a) go down with diet?

    Sources

    1. Cosgrove JP, Church DF, Pryor WA. The kinetics of the autoxidation of polyunsaturated fatty acids. Lipids, 1987;22(5):299-304. doi:10.1007/BF02533996
    2. Xu L, Davis TA, Porter NA. Rate constants for peroxidation of polyunsaturated fatty acids and sterols in solution and in liposomes. J Am Chem Soc, 2009;131(36):13037-44. PMC3656724
    3. Juita, Dlugogorski BZ, Kennedy EM, Mackie JC. Low temperature oxidation of linseed oil: a review. Fire Science Reviews, 2012;1:3 (table 2, after Gunstone 1996). doi:10.1186/2193-0414-1-3
    4. Gardner HW. Oxygen radical chemistry of polyunsaturated fatty acids. Free Radic Biol Med, 1989;7(1):65-86. doi:10.1016/0891-5849(89)90102-0
    5. Blanksby SJ, Ellison GB. Bond dissociation energies of organic molecules. Acc Chem Res, 2003;36(4):255-63. doi:10.1021/ar020230d
    6. PubChem: stearic acid (CID 5281), oleic (CID 445639), linoleic (CID 5280450), alpha-linolenic (CID 5280934). EPA: Thermo Scientific data sheet (VWR) and LGC. DHA: TCI data sheet D2226 (Fisher Scientific)
    7. Li Y, Li Y, Yan G, et al. Dry fractionation efficiency of milk fats from different sources and the characteristics of their fractions in chemical composition, thermal property, and crystal morphology. Food Chem X, 2024;22:101350. PMC11063380
    8. USDA FoodData Central: olive oil 171413, canola 172336, sunflower 171025, flaxseed 167702, coconut 171412, salmon oil 172343, butter 173410
    9. Codex Alimentarius. Standard for olive oils and olive pomace oils, CXS 33-1981, revised 2015. PDF
    10. Psomiadou E, Tsimidou M. Stability of virgin olive oil. 2. Photo-oxidation studies. J Agric Food Chem, 2002;50(4):722-7. doi:10.1021/jf010847u
    11. Aparicio R, Roda L, Albi MA, Gutiérrez F. Effect of various compounds on virgin olive oil stability measured by Rancimat. J Agric Food Chem, 1999;47(10):4150-5. doi:10.1021/jf9812230
    12. De Alzaa F, Guillaume C, Ravetti L. Evaluation of chemical and physical changes in different commercial oils during heating. Acta Scientific Nutritional Health, 2018;2(6):02-11. PDF
    13. Nakamura MT, Nara TY. Structure, function, and dietary regulation of delta6, delta5, and delta9 desaturases. Annu Rev Nutr, 2004;24:345-76. PMID 15189125
    14. Lee JM, Lee H, Kang S, Park WJ. Fatty acid desaturases, polyunsaturated fatty acid regulation, and biotechnological advances. Nutrients, 2016;8(1):23. PMID 26742061
    15. Park HG, Park WJ, Kothapalli KS, Brenna JT. The fatty acid desaturase 2 (FADS2) gene product catalyzes delta4 desaturation. FASEB J, 2015;29(9):3911-9. PMID 26065859
    16. Sugasini D, Yalagala PCR, Subbaiah PV. Efficient enrichment of retinal DHA with dietary lysophosphatidylcholine-DHA: potential application for retinopathies. Nutrients, 2020;12(10):3114. PMC7601701
    17. Agbaga MP et al. Differential composition of DHA and very-long-chain PUFAs in rod and cone photoreceptors. J Lipid Res, 2018;59(9):1586-96. PMC6121944
    18. Shahidi F. Headspace volatile aldehydes as indicators of lipid oxidation in foods. Adv Exp Med Biol, 2001;488:113-23. PMID 11548150

    True or false?

    The more double bonds a fat has, the more liquid it is.

    True. Each cis double bond kinks the chain, which then packs less well: stearic acid melts at 69 °C, oleic at about 13 to 16 °C, DHA at about -45 °C.

    The body can make omega-3s from sugar.

    False. It can place double bonds only near the head of the chain. The first omega-3, alpha-linolenic acid, has to come from food: we lack the enzymes that place double bonds far from the head.

    The oleic acid in olive oil has a weak spot.

    False. It has a single double bond, so no carbon caught between two double bonds. It oxidizes too, but in air about 27 times more slowly than linoleic acid.

    The steps as text

    The texts shown during the animation, to read without it.

    1. The caught carbon
      In linoleic acid, the 11th carbon is caught between two double bonds. Its two hydrogens hold on much less tightly than those of an ordinary carbon: that is the weak spot.
    2. A radical pulls off a hydrogen
      Heat, light or a little iron create radicals, molecules that are missing an electron. One of them pulls a hydrogen off the weak spot: the chain in turn becomes a radical.
    3. Oxygen latches on
      An oxygen molecule (O₂) binds right away to the damaged chain. It forms a peroxyl radical, even hungrier for hydrogen.
    4. The neighbor is attacked
      The peroxyl radical steals the hydrogen from the weak spot of the neighboring chain. It calms down, but the neighbor becomes a radical, which grabs an oxygen, which attacks another neighbor... The broken chains give off the small molecules that smell rancid.
    5. Vitamin E stops the chain
      Vitamin E gives up its hydrogen to the peroxyl radical. It becomes a radical too stable to attack another chain: the reaction stops. Seed oils contain it, and virgin olive oil also has its polyphenols.
    1. Counting from the tail
      Omega is the last letter of the Greek alphabet: carbons are counted from the tail. The first kink comes at the 9th carbon in olive oil (omega-9), the 6th in sunflower (omega-6), and the 3rd in flaxseed and fish (omega-3).
    2. What the body can do
      Our enzymes can place a double bond only near the head, up to the 9th carbon. So the body makes oleic acid, but not omega-6 or omega-3: their kinks are too far away. Plants can. These are called “essential”: we have to eat them.
    3. Lengthening into EPA and DHA
      From the plant omega-3, the body lengthens the chain at the head end and adds kinks where it can: EPA (20 carbons, 5 double bonds), then DHA (22 carbons, 6). The tail does not move: they stay omega-3s. It makes little (see the omega-3 page).
    4. Flexible in the retina, and fragile
      DHA coils and moves constantly: it makes flexible the membrane of the retinal cells that capture light, where it makes up as much as 60% of fatty acids in animals. The vision pigment changes shape faster there. The price: 5 weak spots per chain, which the body protects with vitamin E.
    1. Mostly saturated chains (two thirds) and oleic ones: almost no weak spots.
    2. Almost entirely saturated, solid at 20 °C: almost no weak spots, so it keeps a very long time.
    3. Three quarters of its chains are oleic, with no weak spot. Virgin olive oil also keeps phenols, which account for about half of its stability. Its sensitive point: light. Its chlorophyll captures light and starts oxidation, whereas in the dark it slows it down.
    4. Mostly oleic, but with some omega-6 and omega-3: about three times as many weak spots as olive oil.
    5. Two thirds of its chains are linoleic acid, one weak spot each. Heated to 110 °C under a stream of air, it held up for 6 h, versus 33 h for extra virgin olive oil. There are also “high-oleic” sunflowers, close to olive.
    6. More than half of its chains are alpha-linolenic. It has about ten times as many weak spots as olive oil.
    7. EPA and DHA carry 4 and 5 weak spots each and make up more than a third of its chains: about 15 times as many weak spots as olive oil.

    Other texts from the animation

    • Each carbon carries all its hydrogens: the chain is straight. The chains line up tightly, like pencils in their box, and hold onto each other: you have to heat them to 69 °C to pull them apart.
    • A double bond in the middle (between the 9th and 10th carbons) kinks the chain. The chains pack less well: oleic acid is already liquid at 13 °C. It makes up about three quarters of the fat in olive oil (55 to 83% depending on the oil).
    • Two double bonds, two kinks: it melts at about -7 °C. Between them, one carbon is caught: that is the first weak spot, where oxygen attacks (“The weak spot” tab). It is the main fatty acid in sunflower oil.
    • ALA (alpha-linolenic acid) has three double bonds: it melts between -16 and -11 °C depending on the measurement, and it has two weak spots. It is the plant omega-3: more than half the fat in flaxseed oil (56%), about a tenth of the fat in canola (9%).
    • Melting points of pure fatty acids: stearic 69 °C, oleic 13 to 16 °C, linoleic about -7 °C, alpha-linolenic -16 to -11 °C depending on the measurement (PubChem); EPA about -54 °C (supplier data sheets from Thermo Scientific and LGC, -54 and -53 °C) and DHA about -45 °C (supplier data sheet from TCI), for lack of a measurement published in PubChem.
    • In food , fatty acids are attached in threes to glycerol (triglycerides) and mixed: the hundreds of fatty acids in milk form a mix that melts over a wide range, from about -40 to 40 °C (Li, 2024): so butter has no sharp melting point, it softens little by little; olive oil turns cloudy in the fridge.
    • Kinks : the double bonds in natural fats are “cis”: both ends of the chain point to the same side, which kinks the chain.
    • Weak spots : in the lab, the oxidation rate of polyunsaturated fatty acids follows almost a straight line with their number of carbons caught between two double bonds: in a solvent at 37 °C, arachidonic acid (3 weak spots) oxidizes 2.8 times faster than linoleic (1 weak spot), and DHA (5 weak spots) 5 times faster (Cosgrove, 1987, cited by Xu, 2009).
    • Oleic : with no weak spot, it oxidizes in air about 27 times more slowly than linoleic and 77 times more slowly than alpha-linolenic (Gunstone, 1996, cited by Juita, 2012). C-H bond : about 75 kcal/mol on the caught carbon (Gardner, 1989), 98 to 101 kcal/mol on an ordinary carbon (Blanksby, 2003).
    • Desaturases : in humans, the enzymes that place double bonds act near the head (positions 9, 6, 5 and 4 counting from the head); those for positions 12 and 15 are missing, which is why linoleic and alpha-linolenic acids are essential (Nakamura, 2004; Lee, 2016; Park, 2015).
    • Retinal DHA : up to 60% of fatty acids in rod membranes in rats (Sugasini, 2020), 33 to 35% of those in the whole retina in mice (Agbaga, 2018).
    • Composition : USDA FoodData Central, in % of fatty acids (entries 173410 butter, 171412 coconut, 171413 olive, 172336 canola, 171025 high-linoleic sunflower, 167702 flaxseed, 172343 salmon oil; 18:2 and 18:3 counted in full).
    • Weak spots per 100 chains : our calculation, 1 per omega-6 chain, 2 per alpha-linolenic, 4 per EPA or DPA, 5 per DHA. Since the oxidation rate follows this number (Cosgrove, 1987), it gives the order of magnitude of fragility, without counting each oil's antioxidants.
    • Animation : oxygen falls at the same pace in every bottle; only the target changes. The pace is illustrative; it is not a measure of shelf life.
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    To cite this page in a paper, an article or a presentation:

    Metabolive. "Why does flaxseed oil go rancid so fast?" Updated November 7, 2026. https://metabolive.com/en/everyday/double-bonds.