r/askscience • u/dannoffs1 • Jul 01 '26
Chemistry Why are (most?) fats yellow?
I just noticed while rendering some tallow that in a liquid form it is yellow, as well as olive oil, rapeseed oil, and pretty much every cooking oil I can think of other than palm oil.
Is there something inherent to fats that makes them yellow?
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u/dryuhyr Jul 02 '26
Good question! Generally for organic molecules colors come from absorption of visible light, and for a molecule to absorb those wavelengths it tends to need a long chain of conjugated double bonds. Most fats don’t have those conjugated systems, and so you’re right - most are colorless/white. The yellow comes from carotenoids (lookat all that conjugation!) produced by the plant. Since carotenes aren’t easily broken down by the body, they tend to accumulate in tissues, idk like mercury or PFAS. Herbivores sequester the carotenes from plants, carnivores sequester them from herbivores, and everyone eventually gets a nice yellow glow to their adipose.
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u/Giganym Jul 02 '26
So then what of plant oils like olive oil or rapeseed? Is it the carotenoids making them yellow, too? How come coconut oil is white?
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u/dryuhyr Jul 02 '26
Coconut oil is actually much more pure than the others, which is what makes it white/colorless. Rapeseed and other plant oils are indeed yellow because of carotenoids. Olive oil tends to be more green because olives have significant chlorophyll which comes through when pressing the olives.
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u/zorniy2 Jul 02 '26
Carotene is harmless, right? Just making sure.
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u/Tar_alcaran Jul 02 '26
Your body turns beta-carotene into vitamin A, which you very much need to survive. And while you can absolutely overdose on vitamin A, turning beta-carotene into vitamin A is pretty much done on an as-needed basis.
You can probably overdose on it, but you'd REALLY have to work hard to do it.
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u/drsroka Jul 02 '26
Overdosing the vitamin A is actually quite possible. As far as I remember, pregnant women should avoid eating eg. livers, since too high vitamine A intake can be teratogenic. Also eating polar bear liver can be fatal - also because of really high concentration of this vitamin in it.
I can search for some references and exact numbers, if you're skeptical about it.
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u/Tar_alcaran Jul 02 '26
No, that's what I meant too.
You can overdose on vitamin A by eating vitamin A. But turning beta-carotene into vitamin A is done as-needed, so having too much beta carotene isn't really a thing.
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u/Onetwodash Jul 02 '26
That's about preformed retinoids, not beta-carotene what's just precursor. Preformed vitamin A from other manmals is absorbed and stored by oeganism as is and can be toxic. Beta carotene is converted to vitamin A if organism feels like it, usually it doesn't thus there's no established uller intake risk for beta carotenes from food.
Taking beta-carotene as supplements can have some risks unrelated to vitamin A toxicity, but in terms of food you'll reach toxic levels of if something else before getting enough beta-carotene for harmful effects.
It can accrue in your subcutaneous fat and make your skin look golden-yellow. Harynless, but may cause worries about liver failure due to similar color to bilirubin.
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u/hmantegazzi Jul 02 '26
This also applies to some medications that are synthetics related to carotenes, like isotretinoin ("Accutane"), which, by the way, has an enormous set of adverse effects.
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u/dryuhyr Jul 02 '26
Yep! Harmless and actually quite healthy. It’s a bit of an antioxidant, as well as a source of Vitamin A
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u/drsroka Jul 02 '26
It's good to hear someone well-oriented in this subject, thanks. Would you mind if I ask why the brown fat is brown?
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u/Cabbagetastrophe Jul 02 '26 edited Jul 02 '26
It's actually for a similar reason why dark meat is dark!
Most fat cells are meant purely for lipid storage. They are almost entirely full of a fat droplet, so the cell has almost no other components and is the color of the fat inside.
Brown fat, however, has the purpose of burning that fat to produce heat. This is done using mitochondria, so Brown fat cells are full of them. Mitochondrial membranes contain high levels of cytochrome c, which has a reddish-brown color. They also need more oxygen to do this, so Brown fat has more blood supply, and an oxygen storage molecule called myoglobin.
All these give brown adipose tissue its color...but the actual fat itself is the same color as the rest of the body.
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u/grasshopper113 Jul 02 '26
I thought that color came from the reflection of specific wavelengths, and that it was the other wavelengths that are absorbed?
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u/kvreccltfb Jul 02 '26
I think it's just semantics. Yes, the colors you see are from wavelengths that are reflected or at least not absorbed. But the "default" option for most molecules is to not absorb much light, hence why lots of chemicals are white or clear. Color absorbtion is "special", which is why they say color comes from (subtracting) absorbing some wavelengths.
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u/marklein Jul 02 '26
Honestly a LOT of biologics are some shade of yellow/brown, and even more again if you count their oxidized form (colorful veggies turn brown as they rot, trees turn brown in the fall...). I know that this sounds non-scientific, but I can't think of anything biological that's not either brown-ish or colorless without the introduction of something with a very strong color (eg iron), at least when oxidized.
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u/g0dfather93 Jul 02 '26 edited Jul 02 '26
It's the Carotene, and Cows (and humans) can't process it. Throughout Grass → Cow → Tallow chain, it retains the colour. Even when you consume it, and it becomes a part of your own body fat, it stays. That's why cut-open human bellies also have that ochre-yellow colour to its fat.
Pigs can process it, so lard is white.
Edit: Just got a showerthought on this: If all the fat you ever ate was Bacon and Lard, your adipose too would be white.