# Why do vegetable oils go rancid quickly, but not animal fats? **URL:** \ **Category:** Factual Questions\ **Created:** [April 3, 2016, 1:50am UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016 "2016-04-03T01:50:42Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 3, 2016, 1:50am UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/1 "2016-04-03T01:50:42Z")
Let’s assume that someone in my household has left two pans out for a couple of weeks: one with some butter residue; the other with vegetable oil residue. The butter residue appears to have not degraded in any way. It may even be edible, though I wouldn’t test it. It cleans up easily with soap and water. The vegetable oil has turned into a revolting, sticky goo. It smells rancid. Furthermore, it’s virtually impossible to clean–soap and water do almost nothing, and it actively ruins scrubber sponges (the sticky goo gets into the sponge, where it remains impossible to clean). An enzymatic cleaner helps slightly, as does a pass through the dishwasher, but not completely. Only after many passes does the residue disappear. What’s going on here, chemically? It seems that the vegetable oil has turned into something that’s no longer oil. I believe that going rancid is an oxidation process, but what about this makes the resulting goo not behave like oil? And why does animal fat not appear to be subject to the same thing, or at least not at the same rate? Am I right that this is a vegetable-vs-animal thing or is it actually a different axis? Finally, is there some cleaner that works on rancid oil? ---
**Author:** ![flatlined](https://avatars.discourse-cdn.com/v4/letter/f/eb9ed0/32.png) [@flatlined](https://boards.straightdope.com/u/flatlined)\ **Post date:** [April 3, 2016, 3:08am UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/2 "2016-04-03T03:08:09Z")
The first thing I’d do is find the person who left the dirty pans to sit for a couple of weeks and beat him or her about the head and shoulders with the rancid vegetable oil covered pan. This would both stop the problem from happening again and would also render the pan unusable so you could throw it away. I don’t know what the difference is between animal fats and vegetable fats are, I’m sure that there will be more knowledgeable folks who will tell you. I do know that butter is more water soluble than margarine. Put a pat of butter in the sink and run water over it and it goes away. Do the same thing with margarine and you will be scrubbing the sink with Dawn. Hot water makes no difference for vegetable fats. Now to the real answer if you really want to save the pan. Simple Green or Dawn dishwashing detergent. Both cut the grease and can safely be used on animals or eating surfaces. ---
**Author:** ![WhyNot](https://avatars.discourse-cdn.com/v4/letter/w/c77e96/32.png) [@WhyNot](https://boards.straightdope.com/u/WhyNot)\ **Post date:** [April 3, 2016, 3:11am UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/3 "2016-04-03T03:11:33Z")
Saturated vs. unsaturated fats. Unsaturated fats have more open spots for chemical bonds that can receive yucky molecules, like sulfur and iron oxide, than saturated fats do. Alton Brown explains it very well in this video clip: [Fats Explained - YouTube](https://www.youtube.com/watch?v=ukaaesfbO1k) ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 3, 2016, 4:01am UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/4 "2016-04-03T04:01:50Z")
Thanks; I had a feeling it was something to do with saturated vs. unsaturated, but wasn’t sure about the details. Still curious about a few things. I’m not sure about Simple Green, but I can say that normal dishwashing soap did not touch the stuff. I suspect I need something a bit more vigorous, like something hydrocarbon based (goo remover, etc.). The oil has seemingly turned into something approaching plastic. I guess that monounsaturated fats are somewhere halfway between saturated and polyunsaturated in terms of shelf life and smoke point. My vegetable oil is either soy or canola–soy in particular has a high polyunsaturated oil content. Would I get a noticably better shelf life out of, say, sunflower oil, which (seemingly) has a high monounsaturated content and low polyunsaturated content? ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [April 3, 2016, 1:10pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/5 "2016-04-03T13:10:30Z")
> [@Dr.Strangelove](#): > > Still curious about a few things. I’m not sure about Simple Green, but I can say that normal dishwashing soap did not touch the stuff. I suspect I need something a bit more vigorous, like something hydrocarbon based (goo remover, etc.). The oil has seemingly turned into something approaching plastic. Oxidative cross-linking. See this article on [drying oils](https://en.wikipedia.org/wiki/Drying_oil). Note that they’re not “drying” in the sense that a liquid (e.g. water) is evaporating. But it turns from a liquid to a solid. > [@Dr.Strangelove](#): > > I guess that monounsaturated fats are somewhere halfway between saturated and polyunsaturated in terms of shelf life and smoke point. My vegetable oil is either soy or canola–soy in particular has a high polyunsaturated oil content. Would I get a noticably better shelf life out of, say, sunflower oil, which (seemingly) has a high monounsaturated content and low polyunsaturated content? Probably. It’s not just the gunking; you’ll also get nasty flavors. And those may vary with the different fatty acids involved. And if you throw in some palladium and add hydrogen, you could saturate all the double bonds and make margarine- or shortening-like product that might be shelf-stable. ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [April 3, 2016, 1:16pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/6 "2016-04-03T13:16:20Z")
Here are some compositions of some animal fats, in case anyone else is tempted to fall down the wiki rabbithole with me. They’re not as saturated as I thought: > **[Butterfat](https://en.wikipedia.org/wiki/Butterfat)** > > Butterfat or milkfat is the fatty portion of milk. Milk and cream are often sold according to the amount of butterfat they contain. > Butterfat is mainly composed of triglycerides. Each triglyceride contains three fatty acids. Butterfat triglycerides contain the following amounts of fatty acids (by mass fraction): > Butterfat contains about 3% trans fat, which is slightly less than 0.5 grams per US tablespoon. Trans fats occur naturally in meat and milk from ruminants. The predominant kind of tra... > **[Tallow | Composition](https://en.wikipedia.org/wiki/Tallow#Composition)** > > The composition of the fatty acids is typically as follows: > 1.Saturated fatty acids: > Palmitic acid (C16:0): 26% > Stearic acid (C18:0): 14% > Myristic acid (C14:0): 3% | > 2.Palmitic acid (C16:0): 26% | > 3.Stearic acid (C18:0): 14% | > 4.Myristic acid (C14:0): 3% | > 5.Monounsaturated fatty acids: > Oleic acid (C18-1, ω-9): 47% > Palmitoleic acid (C16:1): 3% | > 6.Oleic acid (C18-1, ω-9): 47% | > 7.Palmitoleic acid (C16:1): 3% | > 8.Polyunsaturated fatty acids: > Linoleic acid: 3% > Linolenic acid: 1% | > 9.Linole... > **[Lard](https://en.wikipedia.org/wiki/Lard)** > > Lard is a semi-solid white fat product obtained by rendering the fatty tissue of a pig. It is distinguished from tallow, a similar product derived from fat of cattle or sheep. > Lard can be rendered by steaming, boiling, or dry heat. The culinary qualities of lard vary somewhat depending on the origin and processing method; if properly rendered, it may be nearly odorless and tasteless. It has a high saturated fatty acid content and no trans fat. At retail, refined lard is usually sold as paper... Note that lard on the grocery shelf is hydrogenated. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 3, 2016, 6:13pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/7 "2016-04-03T18:13:25Z")
Cool. Another factor: I keep my cooking oil in the fridge, in a perhaps misguided attempt to reduce oxidation and other chemical changes. One thing that happens, though, is that the oil partially separates into a liquid upper portion and a solid lower portion. Is this separation happening across the degree of saturation? If I use the liquid upper portion in my cooking, is it more liable to later oxidative cross-linking, because it has a greater concentration of polyunsaturated fats? ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [April 3, 2016, 6:48pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/8 "2016-04-03T18:48:50Z")
> [@Dr.Strangelove](#): > > Cool. Another factor: I keep my cooking oil in the fridge, in a perhaps misguided attempt to reduce oxidation and other chemical changes. > > One thing that happens, though, is that the oil partially separates into a liquid upper portion and a solid lower portion. Is this separation happening across the degree of saturation? If I use the liquid upper portion in my cooking, is it more liable to later oxidative cross-linking, because it has a greater concentration of polyunsaturated fats? Those sound like reasonable hypotheses to me. The only way to be sure is to cook us lots of food. It’s for science. Most chemical reactions are slower at lower temperature, so the fridge is reasonable. You could also increase the shelf life by sparging with an inert gas every time you open the container, or using some sort of oxygen getter. That seems like overkill, but I know of at least one…enthusiast who stores his vermouth under nitrogen. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 3, 2016, 7:39pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/9 "2016-04-03T19:39:38Z")
Sadly, my cooking skills are somewhat… limited. As you might imagine if I can do without two pans for weeks at a time. But I can fry up a mean batch of frozen potstickers. I do not as of yet have a nitrogen sparging system. I do have a vacuum chamber, though so far the only food I’ve put in it is marshmallow Peeps. Reading around some more, I see that the new thing when it comes to cooking oils is “high oleic” sunflower/soy oils. Essentially, they’re close to 100% monounsaturated. They don’t have the health problems of saturated fats, and avoid most of the shelf-life and rancidity issues of polyunsaturated. They have a high smoke point and are suitable for frying. The stuff is available; I’ll pick some up and see how it goes. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [April 3, 2016, 8:07pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/10 "2016-04-03T20:07:33Z")
The following was explained to me by a friend who was a career microbiologist, so I grant him some credibility: The processes of artificially hydrogenating vegetable oil (causing it to be solid rather than liquid at room temperature, and to have better shelf life) was invented by the makers of Crisco, late in the 19th century. Each individual molecule of fat thus artificially saturated has a 50-50 random chance of becoming either a _cis_ molecule or a _trans_ molecule, resulting in a product that is 50% _trans_ fat, unless the manufacturer takes additional heroic steps to separate out the _trans_ fat. In contrast, naturally occurring saturated fat produced by animals is done by enzymes, which are very specifically oriented to producing only _cis_ molecules. Thus, saturated animal fat is essentially 0% _trans_, having only the occasional aberrant _trans_ molecule. Conversely, animals who eat fat also use enzymes to metabolize it, and those enzymes are also specifically oriented to work only on _cis_ molecules. Thus, we can burn _cis_ fat for energy (or build new fatty tissue with it), but we have no way to metabolize or otherwise eliminate the _trans_ fat that gets into us. So it just accumulates. Furthermore, it’s sticky – so it forms a sticky gloppy coating on the insides of our blood vessels. Here is what I infer from what I’ve heard and read: Saturated fat may be more unhealthful than unsaturated fat, but it’s really the _trans_ fat that is by far the most deadly. From this, we can further infer that _butter_, being made of natural animal fat, is more healthful than _margarine_, being made of artificially partially hydrogenated vegetable oil – quite the opposite of what we’ve all heard all our lives. ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [April 3, 2016, 8:31pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/11 "2016-04-03T20:31:23Z")
> [@Senegoid](#): > > The following was explained to me by a friend who was a career microbiologist, so I grant him some credibility: > > The processes of artificially hydrogenating vegetable oil (causing it to be solid rather than liquid at room temperature, and to have better shelf life) was invented by the makers of Crisco, late in the 19th century. Each individual molecule of fat thus artificially saturated has a 50-50 random chance of becoming either a _cis_ molecule or a _trans_ molecule, resulting in a product that is 50% _trans_ fat, unless the manufacturer takes additional heroic steps to separate out the _trans_ fat. > > In contrast, naturally occurring saturated fat produced by animals is done by enzymes, which are very specifically oriented to producing only _cis_ molecules. Thus, saturated animal fat is essentially 0% _trans_, having only the occasional aberrant _trans_ molecule. > > Conversely, animals who eat fat also use enzymes to metabolize it, and those enzymes are also specifically oriented to work only on _cis_ molecules. Thus, we can burn _cis_ fat for energy (or build new fatty tissue with it), but we have no way to metabolize or otherwise eliminate the _trans_ fat that gets into us. So it just accumulates. Furthermore, it’s sticky – so it forms a sticky gloppy coating on the insides of our blood vessels. > > Here is what I infer from what I’ve heard and read: Saturated fat may be more unhealthful than unsaturated fat, but it’s really the _trans_ fat that is by far the most deadly. From this, we can further infer that _butter_, being made of natural animal fat, is more healthful than _margarine_, being made of artificially partially hydrogenated vegetable oil – quite the opposite of what we’ve all heard all our lives. Agree. In addition, trans fats raises your LDL cholesterol and lowers your HDL cholesterol. BTW, I use extra virgin olive oil and keep it in the fridge. After a while, the entire amount will freeze, but leave it out for a few minutes and enough will melt to use for that meal. Or run hot water over it. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [April 3, 2016, 8:34pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/12 "2016-04-03T20:34:07Z")
Lard is “healthier” still, in some important areas. But that’s for a different thread. But I like that the perennial bad guy saturated fat can do such a simple, demonstrable in-your-face to the fairest oils of them all. ETA: Originally to **Senegoid** , re butter. ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [April 3, 2016, 9:21pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/13 "2016-04-03T21:21:00Z")
> [@Senegoid](#): > > The processes of artificially hydrogenating vegetable oil (causing it to be solid rather than liquid at room temperature, and to have better shelf life) was invented by the makers of Crisco, late in the 19th century. Each individual molecule of fat thus artificially saturated has a 50-50 random chance of becoming either a _cis_ molecule or a _trans_ molecule, resulting in a product that is 50% _trans_ fat, unless the manufacturer takes additional heroic steps to separate out the _trans_ fat. This is close, but the cis/trans label only applies to double bonds – the unsaturated sites. Fully saturated fatty acids have no carbon-carbon double bonds. So it’s _partial_ hydrogenation that causes the problems. Trans double bonds tend to be more stable in linear hydrocarbons, so it may actually be more than 50-50, but I’d have to look into that. ---
**Author:** ![Anaglyph](https://avatars.discourse-cdn.com/v4/letter/a/ed8c4c/32.png) [@Anaglyph](https://boards.straightdope.com/u/Anaglyph)\ **Post date:** [April 4, 2016, 4:47pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/14 "2016-04-04T16:47:13Z")
> [@Dr.Strangelove](#): > > Cool. Another factor: I keep my cooking oil in the fridge, in a perhaps misguided attempt to reduce oxidation and other chemical changes. > > One thing that happens, though, is that the oil partially separates into a liquid upper portion and a solid lower portion. Is this separation happening across the degree of saturation? If I use the liquid upper portion in my cooking, is it more liable to later oxidative cross-linking, because it has a greater concentration of polyunsaturated fats? The “freezing point” of oils depends on the chain length and the proportion of unsaturated fatty acids: Peanut oil and olive oil usually solidify in the refrigerator, while sunflower oil and canola don’t. However, this is reversible, the solidified oils will melt again as soon as you warm them up to room temperature. Yes, it is possible that the solid and the liquid phase represent some separation between the different components. Most important for the shelf life of cooking oils is to keep them in the dark - UV light will catalyze oxydation of the double bonds in unsaturated fatty acids. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [April 4, 2016, 5:28pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/15 "2016-04-04T17:28:41Z")
> [@Senegoid](#): > > Here is what I infer from what I’ve heard and read: Saturated fat may be more unhealthful than unsaturated fat, but it’s really the _trans_ fat that is by far the most deadly. From this, we can further infer that _butter_, being made of natural animal fat, is more healthful than _margarine_, being made of artificially partially hydrogenated vegetable oil – quite the opposite of what we’ve all heard all our lives. > [@Ruken](#): > > This is close, but the cis/trans label only applies to double bonds – the unsaturated sites. Fully saturated fatty acids have no carbon-carbon double bonds. So it’s _partial_ hydrogenation that causes the problems. And modern production of margarine has changed from “hydrogenate until it’s saturated to a degree that makes it the right consistency”, which left you with a lot of trans fat, to “do full hydrogenation and then mix with unsaturated fat until you have the right consistency”, which doesn’t. Margarine made from partially hydrogenated vegetable oil is indeed not healthful, but you’d have to live in some really backwards country for those to still be available in this day and age … ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [April 4, 2016, 10:31pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/16 "2016-04-04T22:31:33Z")
I, for one, keep a squeeze bottle of 50% clarified butter and 50% canola oil by the stove at all times. So I’m running a different kind of experiment/demonstration for OP. Over I don’t know how many years, nothing to report. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 4, 2016, 11:39pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/17 "2016-04-04T23:39:32Z")
Yeah, culinarily, trans fats served the same purpose as saturated fats. Back when people thought that saturated fats were the worst, this meant that you saw a lot of trans fats instead. But now that we’ve discovered that trans fats are even worse than saturated, all those recipes just switched back to saturated. So while it’s good to avoid trans fats, you don’t really need to expend any effort on it, because there’s no reason for trans fats to even still be on the marketplace. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [April 5, 2016, 6:32am UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/18 "2016-04-05T06:32:19Z")
I also think this is why advanced Western civilizations, for the last several generations, have had such a notably high rate of heart attacks and other cardiovascular diseases, whereas more primitive people living in villages in the jungles haven’t. We’ve been poisoned by about 100 years of Crisco that we’ve grown up eating, until fairly recently. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 5, 2016, 1:34pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/19 "2016-04-05T13:34:02Z")
I’m sure that’s a contributor, but the biggest factor is that we just plain eat too much, of everything, and don’t get enough exercise, of any sort, to compensate. ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [April 5, 2016, 3:38pm UTC](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016/20 "2016-04-05T15:38:58Z")
Is the cis/trans thing a coincidence, or is it the source of the words for gender status? [Next page](https://boards.straightdope.com/t/why-do-vegetable-oils-go-rancid-quickly-but-not-animal-fats/751016.md?page=2)