What darkens teeth: chromogens bind the pellicle, not the mineral

The color shift people notice after a morning coffee or an evening glass of red wine is rarely a change inside the enamel crystal itself. Most everyday darkening is extrinsic: colored molecules from the diet settle into the thin protein film that covers the tooth almost as soon as saliva touches it.

That film is the acquired salivary pellicle. As soon as saliva contacts the teeth surface, salivary proteins adhere to the tooth surface to form acquired salivary pellicle. The formation of this acquired salivary pellicle is a dynamic and selective process of macromolecular adsorption and desorption. Although acquired salivary pellicle contains proteins and peptides, it also contains lipids, and other macro-molecules, all of which contribute to its protective properties.[1] It reaches a rough equilibrium within a couple of hours. It lubricates, it buffers, and it is the surface where bacteria later attach. It is also the surface where dietary color sticks.

Tea polyphenols, coffee melanoidins, and wine anthocyanins are chromogens—molecules that absorb visible light. They do not need to etch the mineral to darken the appearance of a tooth. They adsorb onto the pellicle proteins. Controlled laboratory work that formed a pellicle-like layer from whole saliva on hydroxyapatite discs showed the pattern clearly. Components from black tea readily adsorbed to the pellicle. Subsequent exposure to saliva led to further adsorption of salivary components to give an overall increase in the amounts adsorbed. The amounts adsorbed increased still further following a third tea and saliva exposure. Components of red wine gave significantly greater amounts of adsorption to the pellicle than black tea. The adsorption of components of black tea gave a concomitant increase in colour or stain as measured by a reflectance chromameter. In all cases, the black tea- and red wine-modified pellicles were not eluted by either phosphate buffer or sodium dodecyl sulphate (SDS) rinses. Thus, black tea and red wine components have been shown to have a profound effect on in vitro pellicle maturation, causing thickened layers of stained material to build up, which are not readily removed.[2]

The same laboratory later looked at purified tea polyphenols. Epicatechin-3-gallate, epigallocatechin-3-gallate, and theaflavin all adsorbed readily onto the salivary pellicle under the same conditions, and the modified films were not stripped by buffer or detergent rinses.[3] The practical reading is straightforward: the color is held in a thickened, protein-plus-polyphenol layer sitting on the mineral. Brushing and flossing remove loose debris and some of the outer film. They do not oxidize the chromogens that have already bound more tightly.

Whitening products that contain hydrogen peroxide or carbamide peroxide work by oxidation. The active species that form when peroxide decomposes can break conjugated double bonds in organic chromogen molecules, converting them into smaller, less colored fragments. A 2015 review that integrated the literature on diffusion, stain interaction, and optical change put the mechanism in plain terms: Tooth whitening is a dynamic process that involves diffusion of the whitening material to interact with stain molecules and also involves micromorphologic alterations on the surface and changes within the tooth that affect its optical properties. The interaction seems not to be limited to stain molecules, but rather an affinity-based interaction process that also accompanies effects on sound enamel and dentin structures.[4] Diffusion matters because the gel has to reach the chromogens wherever they sit—on the surface film and, to a lesser degree, in the outer enamel pores. The primary optical effect that people see is the loss of those bound dietary pigments.

This is why restorations stay the same shade. Porcelain, ceramic, composite, and zirconia do not have a pellicle that binds dietary chromogens the same way enamel does, and they do not contain the organic chromogens that peroxide is oxidizing. The mineral of the natural tooth is also not being bleached in the sense of dissolving the crystal lattice. Peroxide’s job is the organic color molecules. That distinction is the reason a new shade can look brighter and then drift as the pellicle reforms and new chromogens adsorb.

The pellicle’s protective roles are real. It limits direct acid contact with enamel and is part of the natural defense against erosion and the early stages of caries. Removing it completely is neither possible nor desirable; saliva rebuilds it continuously. The goal of a whitening session is not to strip the film permanently. It is to oxidize the chromogens currently bound in it, then let the next film form on a lighter starting point. Habits that reduce how much new chromogen reaches that film—rinsing after coffee, finishing the glass rather than sipping for an hour, brushing the outer surfaces—extend the time before the shade drifts back.

Sensitivity that some people feel during or after a session is a separate event. It is usually a pulp response to peroxide that has diffused through the enamel tubules, not a sign that the mineral is being dissolved. Short, completed contact times limit how much peroxide reaches the pulp. The American Dental Association notes that whitening products contain hydrogen peroxide or carbamide peroxide and that these bleaches break stains into smaller pieces, which makes the color less concentrated.[5] It also notes that people should discuss options with a dentist and look for products that have been tested for safety and effectiveness.

None of this changes the fact that active decay, open sores, or unexplained dark spots that are not surface film belong with a dentist before any whitening gel is applied. The chromogen-and-pellicle story explains the everyday coffee stain and the way a peroxide session lightens it. It does not explain every dark tooth, and it does not replace a clinical exam when something looks or feels wrong.

A ten-minute session is long enough for peroxide to begin oxidizing the chromogens already present in the pellicle and short enough that most people can finish it. The shade that results is a starting position, not a permanent state. The pellicle will reform. New chromogens will adsorb. The maintenance loop is biological: oxidize, then reduce the next load of color molecules that reach the film.

Sources

1. Chawhuaveang DD, Yu OY, Yin IX, Lam WY, Mei ML, Chu CH. Acquired salivary pellicle and oral diseases: a literature review. J Dent Sci. 2021;16(1):523-529. doi:10.1016/j.jds.2020.10.007. https://pubmed.ncbi.nlm.nih.gov/33384841/

2. Joiner A, Muller D, Elofsson UM, Malmsten M, Arnebrant T. Adsorption from black tea and red wine onto in vitro salivary pellicles studied by ellipsometry. Eur J Oral Sci. 2003;111(5):417-422. doi:10.1034/j.1600-0722.2003.00073.x. https://pubmed.ncbi.nlm.nih.gov/12974686/

3. Joiner A, Muller D, Elofsson UM, Arnebrant T. Ellipsometry analysis of the in vitro adsorption of tea polyphenols onto salivary pellicles. Eur J Oral Sci. 2004;112(6):510-515. doi:10.1111/j.1600-0722.2004.00166.x. https://pubmed.ncbi.nlm.nih.gov/15560834/

4. Kwon SR, Wertz PW. Review of the mechanism of tooth whitening. J Esthet Restor Dent. 2015;27(5):240-257. doi:10.1111/jerd.12152. https://pubmed.ncbi.nlm.nih.gov/25969131/

5. American Dental Association. Teeth whitening. MouthHealthy.org. https://www.mouthhealthy.org/all-topics-a-z/teeth-whitening