[Physics] Why is condensed milk glowing green under UV


Teeth, nails and also сгущёнка (condensed milk) are glowing green when exposed to black light. What do they have in common? Is it a specific luminophore substance?

Best Answer

Short answer: autofluorescent proteins, most likely.

Many biological structures emit light after absorption of light at a different wavelength; this is called autofluorescence. "Auto" indicates that the fluorescence occurs naturally, i.e. without adding artificial sources of fluorescence. For example, melanin (pigment which determines skin color) absorbs light between 340-400 nm and emits between 360-560 nm while collagen (structural protein in bones, teeth, cartilage, skin, etc.) absorbs 270-370 nm and emits 305-450 nm.$^{1,2}$ As for the specific materials you list:

Teeth: under the hard inorganic enamel coating of teeth is the tissue called dentin and “The main organic matrix in dentin is the fibrous collagen [4], which generates intense second harmonic and autofluorescence (AF) signals [5–8].”$^3$ However, “The origins of autofluorescence from dental tissue may be quite diverse, and include both organic and inorganic components [4,29].”$^3$

Nails: they are mostly made of the structural protein keratin and “Strong keratin fluorescence with excitation and emission characteristics similar to collagen were observed in the topmost layer of the keratinized squamous epithelium.”$^4$

Milk: see below a figure from a study$^5$ which measured the autofluorescence of a number of proteins found in milk.Fluorescence spectra of milk proteins in water

Please note that the autofluorescent proteins I've mentioned may not be the only sources of fluorescence in these materials. Check the references for more detail.

$^1$ James M. Gallas and Melvin Eisner (May 1987). "Fluorescence of Melanin-Dependence upon Excitation Wavelength and Concentration." Photochem. And Photobiol. 45 (5): 595–600. doi:10.1111/j.1751-1097.1987.tb07385.x

$^2$ Georgakoudi et al. (2002). "NAD(P)H and collagen as in vivo quantitative fluorescent biomarkers of epithelial precancerous changes." Cancer Res. 62 (3): 682–687.

$^3$ Lin et al. "Imaging carious dental tissues with multiphoton fluorescence lifetime imaging microscopy." Biomedical Optics Express. 2011;2(1):149-158. doi:10.1364/BOE.2.000149.

$^4$ Wu Y. and Qu J. Y. “Autofluorescence spectroscopy of epithelial tissues,” J. Biomed. Opt. 11(5), 054023 (2006).10.1117/1.2362741

$^5$ Gallier et al. "Using confocal laser scanning microscopy to probe the milk fat globule membrane and associated proteins." Journal of Agricultural and Food Chemistry, 58 (2010), pp. 4250–4257