Whitening Efficacy of Toothpastes on Coffee-Stained Teeth: An Enamel Surface Analysis

Abstract

Aim

This study aimed to identify the optimal toothpaste for removing coffee-induced stains while also evaluating its impact on tooth enamel through roughness and abraded depth parameters, providing a comprehensive understanding of their effects.

Materials and methods

Three whitening toothpastes and 2 conventional toothpastes were prepared for a simulated brushing procedure on coffee-stained bovine enamel tooth specimens. Using a toothbrushing machine, up to 10,000 brushstrokes were completed, while spectrophotometric readings were taken at designated intervals. A mixed-effects model for statistical analysis determined the effects of toothpaste and brushing on colour change, roughness, and abraded depth.

Results

Whitening toothpastes significantly deviated from the control (P < .001, P < .001, and P < .003, respectively), whereas the conventional toothpaste did not exhibit a significant contrast (P < .081). Regarding colour restoration following coffee staining, whitening toothpastes showed higher restoration than conventional toothpastes. Surface roughness and abraded depth parameters increased with accumulated brushing.

Conclusions

Sodium hexametaphosphate-containing toothpaste demonstrated the highest efficacy in removing coffee-induced stains and restoring tooth colour. Nevertheless, this stronger whitening effect was associated with increased abrasion. While conventional toothpastes exhibited some whitening effects, the most substantial improvement in lightness was consistently observed with whitening toothpastes.

Clinical Relevance

Understanding how whitening toothpaste affects enamel integrity is crucial for refining formulations and advancing dental care. This knowledge lays the groundwork for more effective oral care products and improved whitening procedures, ultimately enhancing the overall quality of dental treatments.

Introduction

Extrinsic tooth stains, induced by everyday habits such as smoking or consumption of beverages such as coffee or red wine, present a common challenge.¹ These stains, often adhering to the outer enamel layer, can compromise the aesthetic appeal of teeth and lead individuals to seek effective solutions. At present, the oral care market offers a diverse choice of dental products tailored to meet the needs of consumers. Despite the availability of over-the-counter tooth whitening solutions, such as whitening strips and LED whitening kits, the most traditional and readily accessible methods remains the use of whitening toothpastes.²

The efficacy of whitening toothpastes depends on several factors, including its specific formulation, active ingredients, abrasiveness, and compatibility with the type of stain that it targets.^3,4 The findings from a previous study, which compared whitening toothpastes with conventional ones, revealed the need for a more comprehensive understanding of the outcomes associated with both types of toothpaste.⁵ While hydrogen peroxide-containing whitening toothpastes exhibited the anticipated superior whitening effects, it caused less abrasion along the enamel surface than conventional alternatives. This highlights the need for a closer examination of toothpaste ingredients and their potential effect on the enamel surface.

Coffee contains various chemicals that induce staining. Chromogens, tannic acids, and chlorogenic acids found in coffee are linked to tooth discoloration.^6,7 These chemicals have different staining mechanisms – the acids erode the enamel surface and chromogens bind to pellicles on the enamel surface.^8,9 While toothpastes with abrasives may contribute to whitening to some degree, the inherent irregularities of the enamel surface may necessitate chemical bleaching.¹⁰ Therefore, to determine the most optimal toothpaste for addressing coffee-induced tooth staining, comparing a broader range of toothpastes is necessary.

This study used commercially available toothpastes to conduct an in vitro analysis of tooth colour changes resulting from the use of different toothpaste formulations. Three types of whitening toothpastes and 2 types of conventional toothpastes were used to determine the long-term effects of using such toothpastes on teeth colour. Tooth lightness was assessed based on the Commission Internationale de l'Éclairage Lab* colour space using a spectrophotometer.

This study aimed to ascertain which type of whitening toothpastes proves most effective in removing extrinsic stains caused by coffee and determine potential disparities among these toothpastes. In addition, this study considered roughness and abraded depth values as indicators of surface abrasion along the enamel surface resulting from brushing. By integrating the colour and abrasion results, this study also aimed to provide readers with a comprehensive understanding of the effects of various toothpastes on tooth enamel.

Methods

Central and second incisors from the bovine mandibular arch were extracted to create enamel specimens (n = 50; Figure 1). A bench drilling machine (YDM-13 mm; Yongsoo Precision) with a diamond core (diameter: ⌀10 × ⌀̸8 mm) was used to obtain an 8-mm diameter portion at the teeth's centre. Teeth were affixed to acrylic rings (diameter ⌀30 × ⌀̸12 × 4 mm) using self-cure resin (Vertex-Dental). Enamel preservation was ensured through meticulous polishing (LaboPol-5; Struers) with silicon carbide papers (#220, #600, and #1200 SiC paper; R&B). Thickness was measured between each polishing stage using a digital micrometre (CD67-S15PM; Mitutoyo) to ensure consistent thickness.

Figure 1.

Bovine tooth enamel specimen with a brushing window (20 × 5 mm) established using polyester tape. The specimen measures 4 mm in thickness.

Before immersion in coffee, specimens were selected based on hardness and microroughness criteria. A Vickers hardness tester (HM-220; Mitutoyo) was used to select specimens with hardness >300, and a 3D surface profiler (NV-E1000, NanoSystem) was used to select specimens with roughness of <0.3 µm.

For colour restoration assessment, specimens were immersed in coffee (Dunkin’ instant black coffee) for a total of 72 hours. A water bath was used to maintain consistent temperature of 40°C (WB-6; Daihan Scientific). The CIELAB colour space was employed for colour representation and analysis. Lightness (L*), green to red (a*), and blue to yellow (b*) were measured using a spectrophotometer in reflectance mode (Ci7600, X-rite Pantone). Colour difference (ΔE*₀₀) was calculated using the following formula:

$$\Delta {E^{*}}_{00}=\sqrt{{\left(\frac{\Delta L^{\prime }}{k_L{S}_L}\right)}^2+{\left(\frac{\Delta C^{\prime }}{k_C{S}_C}\right)}^2+{\left(\frac{\Delta H^{\prime }}{k_H{S}_H}\right)}^2+R_T\left(\frac{\Delta C^{\prime }}{k_C{S}_C}\right)\left(\frac{\Delta H^{\prime }}{k_H{S}_H}\right)}$$

After measuring the lightness and roughness of specimens, 3 types of whitening toothpastes and 2 types of conventional toothpastes were prepared for brushing simulation (Table 1). The pH of the toothpastes was assessed using a pH meter (F-71; HORIBA). Abrasive particle sizes were measured through field emission scanning electron microscopy at 30,000× magnification (FE-SEM; Apreo S LoVac, Thermo Fisher Scientific). The RDAs for each toothpaste were determined through the RDA-PE (Relative Dentin Abrasion – Profilometry Equivalent) method, following the instructions outlined in ISO 11609. Dentin specimens underwent identical preparation as the enamel specimens, as previously described. To ensure the specimens were dentin, Vickers hardness values ranging from 30 to 70 were selected. To create toothpaste slurries, 25 g of toothpaste was homogenized with 40 mL of distilled water. A reference slurry was formed by blending 10 g of calcium pyrophosphate with 50 mL of reference dilution. Subsequently, the specimens underwent brushing for 10,000 strokes in a toothbrushing machine (RB118, R&B). The RDA-PE was calculated by dividing the abraded depth value resulting from using the whitening toothpaste by the abraded depth value resulting from the use of the reference slurry. Profilometer measurements were employed to determine the abraded depths (Table 1).

Table 1.

Detailed information of the toothpastes used.

Type	Toothpaste	Active ingredients	pH	Abrasives v/w%	Particle size (μm)	RDA
Whitening	Pinksalt Whitening*	Sodium hexametaphosphate, Sodium fluoride, tocopheryl acetate, silicon dioxide	6.08	33	4-20	106
	Vussen 28†	Hydrogen peroxide (2.8%), sodium metaphosphate, citric acid, colloidal silicon dioxide	4.81	37	4-20	46
	White Sparkle‡	Sodium fluoride, sodium bicarbonate (20%), tetrasodium pyrophosphate, dental-type silica	8.25	42	4-20	70
Conventional	Pinksalt Regular*	Sodium fluoride, sodium pyrophosphate, dental-type silica, supercritical carbon dioxide extraction	7.40	25	4-20	10
	Enamel Care‡	Sodium fluoride, sodium bicarbonate (35%), dental-type silica	8.25	29	4-20	43

^⁎LG Household & Health Care.

^†Osstem Oral Care Company.

^‡Arm & Hammer, Church & Dwight.

To assess the efficacy of whitening toothpaste on enamel specimens, toothpaste slurries were prepared by mixing 25 g of toothpaste with 40 mL of distilled water. Utilizing a toothbrushing machine, a series of up to 10,000 strokes were executed, and measurements via spectrophotometer and profilometer were recorded at intervals of every 1000, 2000, 3000, 5000, and 10,000 strokes. Implementing 10,000 strokes aimed to evaluate the whitening effectiveness of toothpaste over a 1-year period. This decision was based on the approximation that brushing teeth 10,000 times is approximately equivalent to one year of tooth brushing for an individual.^11,12

To assess the impact of whitening toothpastes on teeth, 3D surface profiler was used to measure both surface roughness and abraded depth of the enamel surface following each brushing session. Before initiating the brushing process, a consistent reference surface was established by applying polyester tape to protect specific areas of the teeth. This tape aids in differentiating between the altered and unaltered regions of the teeth by creating a brushing window measuring 20 × 5 mm (Figure 1). The unaltered surface functions as a baseline for evaluating the abraded depth of the enamel surface.

For the statistical analysis, a mixed-effects model was used to address repeated measurements and assess the effects of toothpaste and brushing on colour change, surface roughness, and abraded depth. For the purpose of this particular statistical analysis, Enamel Care group was selected as the control. Data analysis and visualization were conducted using Python 3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0].

Results

The mixed-effects model analysis comparing the 5 groups revealed significant disparities in specimen lightness. Notably, Pinksalt Whitening, Vussen, and White Sparkle toothpastes exhibited substantial variations from the control – Enamel Care (P < .001, P < .001, and P < .003, respectively). Conversely, Pinksalt Regular toothpaste did not display a significant difference from Enamel Care (P < .081). Among the toothpaste variants, Pinksalt Whitening demonstrated the highest colour restoration at 80.83% following coffee staining, followed by White Sparkle (71.67%), Vussen (70.83%), Pinksalt Regular (54.00%), and Enamel Care (14.25%) (Figure 2). ΔE*₀₀ values also exhibited a consistent trend where Pinksalt Whitening presented the highest value after 10,000 strokes of brushing (Table 2).

Figure 2.

Mean L* at initial state, after immersion in coffee, and after each cycle of brushing. Pinksalt (W): Pinksalt Whitening; Pinksalt (R): Pinksalt Regular.

Table 2.

Mean L*, a*, b* at original state, after coffee immersion, and after each brushing cycle.

Group	Initial			Stained			X1000				X2000				X3000				X5000				X10000
	L*	a*	b*	L*	a*	b*	L*	a*	b*	ΔE*00	L*	a*	b*	ΔE*00	L*	a*	b*	ΔE*00	L*	a*	b*	ΔE*00	L*	a*	b*	ΔE*00
Pinksalt (W)	74.69 ± 2.32	−1.16 ± 0.32	7.17 ± 2.32	66.03 ± 2.90	2.71 ± 1.67	17.19 ± 5.26	70.30 ± 1.76	0.69 ± 0.95	12.33 ± 4.17	6.14 ± 2.36	72.00 ± 1.34	0.13 ± 0.62	10.42 ± 3.27	7.88 ± 3.07	72.19 ± 1.85	−0.02 ± 0.58	9.85 ± 3.09	8.29 ± 3.50	72.47 ± 1.97	−0.24 ± 0.51	9.72 ± 2.49	8.56 ± 3.37	72.94 ± 1.58	−0.13 ± 0.42	9.24 ± 2.90	9.04 ± 3.49
Pinksalt (R)	73.52 ± 1.98	−1.13 ± 0.43	6.34 ± 3.53	65.15 ± 3.40	2.23 ± 1.55	17.18 ± 3.46	66.89 ± 2.20	1.34 ± 1.39	13.15 ± 5.16	3.96 ± 1.42	67.60 ± 2.86	0.81 ± 1.17	11.15 ± 4.80	5.09 ± 1.95	68.71 ± 2.38	0.62 ± 1.12	10.34 ± 5.07	5.86 ± 2.16	68.98 ± 2.48	0.22 ± 1.12	9.71 ± 4.79	6.24 ± 2.04	69.53 ± 2.29	−0.09 ± 0.84	8.89 ± 3.93	6.91 ± 2.23
Vussen	74.01 ± 3.06	−1.11 ± 0.41	8.30 ± 1.80	65.42 ± 3.21	2.52 ± 1.77	18.63 ± 4.45	69.61 ± 2.79	0.53 ± 1.46	12.16 ± 4.82	6.09 ± 2.72	70.52 ± 2.80	0.04 ± 1.00	10.72 ±3.97	7.42 ± 2.87	70.96 ± 2.54	−0.04 ± 1.07	10.13 ± 4.64	7.83 ± 3.07	71.24 ± 2.59	−0.23 ± 0.91	10.27 ± 4.18	7.97 ± 2.95	71.35 ± 2.28	−0.45 ± 0.73	9.43 ± 4.07	8.44 ± 3.07
White Sparkle	74.45 ± 1.43	−1.30 ± 0.29	7.87 ± 2.22	65.68 ± 2.95	2.68 ± 2.56	18.86 ± 4.40	68.84 ± 1.87	0.91 ± 1.44	13.07 ± 4.35	5.24 ± 2.51	70.56 ± 1.13	0.13 ± 1.00	10.68 ± 4.32	7.20 ± 3.49	71.08 ± 1.39	−0.01 ± 0.89	10.24 ± 3.69	7.72 ± 3.56	71.47 ± 2.20	−0.21 ± 0.71	10.88 ± 3.83	8.07 ± 4.14	71.61 ± 2.23	−0.41 ± 0.63	10.33 ± 2.76	8.29 ± 4.09
Enamel Care	74.80 ± 0.69	−1.50 ± 0.55	6.09 ± 2.32	65.71 ± 2.59	2.76 ± 1.94	20.22 ± 3.76	65.75 ± 2.23	2.28 ± 1.36	17.20 ± 3.27	3.18 ± 0.82	65.86 ± 2.36	2.47 ± 1.57	16.03 ± 3.51	3.71 ± 1.11	66.03 ± 2.33	2.45 ± 1.56	15.72 ± 3.99	3.98 ± 1.27	66.22 ± 2.19	2.24 ± 1.82	15.40 ± 4.35	4.15 ± 1.26	66.76 ± 2.12	1.90 ± 1.54	14.50 ± 4.22	4.85 ± 1.99

Colour changes (ΔE*00) were determined by calculating differences in lightness (ΔL*), chroma (ΔC*), and hue (ΔH*) between postcoffee immersion and after each brushing cycle.

The surface roughness and abraded depth of all specimens increased as the number of brushing accumulated (Figure 3, Figure 4). The mixed-effects model analysis was also performed on these data, and only Pinksalt Whitening toothpaste exhibited a significant difference from Enamel Care (P = .001). All the other groups did not show significant difference from Enamel Care.

Figure 3.

The change in roughness after each cycle of brushstrokes. ΔRa = surface roughness after discoloration – surface roughness after each cycle of brushstrokes. Colour-shaded regions indicate standard deviations.

Figure 4.

The change in abraded depth after each cycle of brushstrokes. ΔDepth = abraded depth after discoloration – abraded depth after each cycle of brushstrokes. Colour-shaded regions indicate standard deviations.

Discussion

Sodium hexametaphosphate (SHMP) emerges as a noteworthy whitening agent, alongside hydrogen peroxide and sodium bicarbonate.^13,14 Its classification as an active or inactive ingredient may vary depending on perspectives; however, studies have consistently demonstrated the effectiveness of SHMP in both removing and preventing tooth stains.^15,16 This is attributed to the chelating properties of SHMPs, which play a pivotal role in preventing metal ions, frequently found in staining materials, from binding to the tooth surface.¹⁷ In this study, the SHMP-containing whitening toothpaste (Pinksalt Whitening) demonstrated the highest colour restoration, exhibiting an 80.83% recovery in specimen lightness (L*) following 10,000 brushing cycles. In comparison, hydrogen peroxide-containing toothpastes showed 70.83% recovery, and sodium bicarbonate-containing toothpastes demonstrated 71.67% recovery. This highlights SHMP's proficiency in coffee stain removal, although its potential effects on the enamel surface must be considered.

The findings of this study indicate that for individuals seeking immediate teeth whitening, short-term use of toothpastes containing hydrogen peroxide may be the most effective choice. Hydrogen peroxide (Vussen)- and SHMP (Pinksalt Whitening)-containing toothpastes demonstrated a noticeable increase in brightness after the initial brushing session. However, specimens brushed with the SHMP-containing toothpaste exhibited greater surface damage. Specifically, only the SHMP-containing toothpaste showed a significant difference from conventional toothpaste in terms of surface abrasion, whereas other whitening toothpastes (Vussen and White Sparkle) did not yield a significant distinction.

Furthermore, the use of hydrogen peroxide-containing toothpastes over an extended period may not be advisable because of the potential increase in tooth abrasion compared with other toothpastes. The slightly acidic nature of hydrogen peroxide-containing toothpastes, which tend to have pH < 5.0, might contribute to the degree of abrasion subsequent to erosion of the enamel surface.^11,18 While the concentration of hydrogen peroxide in this study (2.8%) was not considered high, exposure to higher concentrations and prolonged use of hydrogen peroxide have resulted in the demineralization of the tooth enamel.^19,20 The tooth specimens in this study experienced repetitive exposure to hydrogen peroxide, suggesting that even lower concentrations could lead to demineralization when exposed continuously. In addition, the inclusion of citric acid in hydrogen peroxide-containing whitening toothpastes, intended to regulate the pH of the formulation, was found to exacerbate the erosion of the enamel surface.¹¹ However, the duration of hydrogen peroxide and citric acid exposure in this study far exceeded that in real-life situations. Moreover, this discrepancy is relative, as none of the specimens displayed dentin exposure, given that the enamel thickness of bovine incisors is approximately 1.5 mm.^21,22 Therefore, transitioning back to conventional toothpaste after using whitening toothpastes for a short period may be a viable option for those seeking rapid whitening effects while preserving the enamel surface.

One of the most notable findings of this study was the disparity in whitening effectiveness between White Sparkle and Enamel Care toothpastes. As their names imply, White Sparkle demonstrated superior brightness enhancement, whereas Enamel Care outperformed in preserving the enamel surface. The only distinction between these 2 toothpastes lies in the composition, specifically the proportion of sodium bicarbonate and the presence of tetrasodium pyrophosphate. White Sparkle incorporates 20% sodium bicarbonate along with tetrasodium pyrophosphate; however, their exact quantities remain undisclosed. Conversely, Enamel Care contains 35% sodium bicarbonate along with other active ingredients, excluding tetrasodium pyrophosphate. Given the noteworthy disparities in colour recovery and extent of surface abrasion between White Sparkle and Enamel Care, sodium bicarbonate content may not significantly affect tooth colour restoration. This suggests a likelihood of a potential synergistic effect of tetrasodium pyrophosphate in conjunction with sodium bicarbonate or other ingredients. However, the lack of precise knowledge regarding the exact amounts or proportions of each ingredient suggests the importance of this information in making conclusive determinations about why White Sparkle and Enamel Care toothpastes yielded such contrasting results. While pyrophosphates in toothpastes target the removal of dental calculus, this study's findings highlight the crucial role of tetrasodium pyrophosphate in tooth whitening .²³

Notably, this study focuses on coffee-induced tooth discoloration and does not address other substances that may lead to tooth discoloration. In addition, the process of tooth discoloration from coffee involves a complex interaction of various chemicals as mentioned earlier. Therefore, further research is needed to assess whether the present findings are consistent with those obtained when the specimens are exposed to a different staining substance and to understand how the specific chemicals in toothpastes interact with those in staining substances.

Abrasion patterns observed in the specimens present a noteworthy area for future research. In a previous study, distinctions were apparent in the abrasion patterns of tooth specimens subjected to brushing with hydrogen peroxide-whitening toothpastes compared with a conventional toothpaste.²⁴ Specimens brushed with hydrogen peroxide toothpastes displayed a relatively uniform abrasion pattern, resembling a “U” shape, whereas those brushed with conventional toothpastes exhibited irregular, jagged patterns reminiscent of brushstrokes. The profilometric findings of this study mirrored such patterns where hydrogen peroxide-containing toothpaste exhibited the similar “U” pattern while the conventional toothpastes exhibited non-uniform jagged patterns. These abrasion patterns may have a substantial effect on processes related to re-staining and re-whitening of teeth. Given that imperfections and the uneven topography of tooth enamel can promote staining, the utilization of abrasive whitening toothpastes, which can generate additional crevices for chromogens to adhere to, could lead to more noticeable discoloration when exposed to staining agents.²⁴ The question of which teeth surfaces are more prone to staining and the subsequent effect on the efficacy of whitening require further investigation.

Conclusions

The results of this study revealed that SHMP-containing toothpastes emerged as the most effective in removing coffee-induced stains and restoring tooth colour. Both hydrogen peroxide- and sodium bicarbonate-containing toothpastes showed promising results, whereas SHMP-containing toothpastes outperformed them by approximately 10%. Furthermore, whitening toothpastes demonstrated significantly greater effectiveness in tooth whitening than conventional ones. While conventional toothpastes did exhibit some whitening effects, particularly the Pinksalt Regular toothpaste, dramatic restoration in lightness was only observed in whitening toothpaste specimens.

CRediT authorship contribution statement

Soyeon Kim: Data curation, Methodology, Formal analysis, Investigation, Writing – original draft. Chang-Ha Lee: Data curation, Formal analysis, Investigation, Writing – original draft. Sunyoung Ma: Data curation, Formal analysis, Investigation, Writing – original draft. Young-Seok Park: Supervision, Funding acquisition, Project administration, Writing – review & editing.

Conflict of interest

None disclosed.