It is relatively well known that peanut butter glows in the dark under UV light (if you don't believe me, see this short YouTube demo by NileRed).

I've tried it myself and it is clearly a real thing.

It is less clear whether the effect is phosphorescence or fluorescence though the glow persists for seconds after exposure suggesting the former.

Which chemicals cause this and, for bonus points, is it fluorescence or phosphorescence?

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    $\begingroup$ Aflatoxins are famous for their intense fluorescense. Raw peanuts are especially vulnerable in supporting growth of yellow feared mold Aspergillus flavus, producing these toxins. Fluorescence under UV is frequent screening procedure to detect problematic lots. Notes that the test is not specific and many substances or materials show fluorescence. Nature: Fluorescence spectroscopy and aflatoxin-B1 $\endgroup$
    – Poutnik
    Commented Oct 23, 2023 at 19:35
  • $\begingroup$ @Poutnik That would be a good suggestion were it not for the fact that food standards routinely test peanuts for aflatoxin levels and the glow effect seems to fairly universal in peanut butters. Unless the aflatoxin fluorescence is extremely bright at ppb levels. $\endgroup$
    – matt_black
    Commented Oct 23, 2023 at 20:50
  • $\begingroup$ I guess it is. It could be worthy to compare different lots. $\endgroup$
    – Poutnik
    Commented Oct 23, 2023 at 20:55
  • $\begingroup$ Also for the HNQ readers: chemistry.stackexchange.com/questions/72595/… $\endgroup$ Commented Oct 24, 2023 at 3:27
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    $\begingroup$ The reddit answers is I am afraid vague. By attributing luminescence to phenolics, he/she just suggested thousands of compounds. Black tea is rich in phenolics but it is non-fluorescent. In the middle of the answer trans-coumaric acid is proposed, which does not emit green light. $\endgroup$
    – ACR
    Commented Oct 24, 2023 at 13:19

1 Answer 1


Many natural oils from vegetable sources show intense fluorescence under UV. Olive oil glows brilliant red under UV. In the peanut butter case, my first guess was turmeric, the yellow spice, because it glows green under UV. When we look at the ingredients of this peanut butter which shows soybean oil, among others and turmeric is absent.


Ingredients: Select Roasted Peanuts, Soybean Oil, Corn Maltodextrin, Sugar, Hydrogenated Vegetable Oil (Cotton Seed And Rapeseed Oil), Salt, Mono- And Diglycerides.

From this article on ResearchGate (Monitoring the Transesterification Reaction of Vegetable Oil to Biodiesel by Fluorescence Spectroscopy with UV Excitation: Correlation with Viscosity January 2018Orbital - The Electronic Journal of Chemistry 10(1), DOI:10.17807/orbital.v10i1.1026), we can see the emission spectrum of soybean oil.

So, it is very likely the oils in the product are fluorescing green.


  • 1
    $\begingroup$ Did you mean to put in links for the ingredients of the peanut butter and for the article with the graph? $\endgroup$
    – quarague
    Commented Oct 24, 2023 at 2:44
  • $\begingroup$ Quick empirical test. My extra virgin olive oil has a red fluorescence under UV but my light olive oil seems to be blue. Neither shows any phosphorescence as seen in peanut butter (be interesting–if oils are the source–why it seems to achieve phosphorescence.) $\endgroup$
    – matt_black
    Commented Oct 24, 2023 at 12:53
  • $\begingroup$ Is there any reason why at 400nm wavelength all 3 of them had that dip ? Or is it just faulty/mini errors in the equipment $\endgroup$
    – Or4ng3h4t
    Commented Oct 24, 2023 at 13:11
  • $\begingroup$ It is very likely delayed fluorescence rather phosphorescence. Delayed fluorescence is common in plants too. It is hard to distinguish between phosphorescence and delayed luminescence until and unless someone does magnetic studies during photoirradiation. Light olive oil might be adulterated with other oils or it could be completely non-fluorescent. The apparently "blue light" could be Rayleigh scattering. Are you viewing oils at 90- degrees to the light source? $\endgroup$
    – ACR
    Commented Oct 24, 2023 at 13:14
  • 1
    $\begingroup$ @Or4ng3h4t, most likely a grating change. UV gratings might be different from Vis gratings. $\endgroup$
    – ACR
    Commented Oct 24, 2023 at 13:16

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