Maintaining white teeth and a bright smile are widely regarded as symbols of youth and beauty, driving a strong consumer demand for effective tooth-whitening solutions. While there is a high interest in specialized tooth-whitening products, many consumers feel burdened by their complexity or cost. As a result, there is significant interest in whitening toothpastes that not only promote oral health, but also offer noticeable whitening benefits. This demand underscores the need for products with enhanced whitening effects beyond those currently available.

Whitening is a broad concept that encompasses both the removal of discoloration and the enhancement of tooth gloss. Even when the discoloration is removed, teeth without a glossy finish may appear unappealing and unhealthy. Conversely, teeth that are glossy but retain a yellowish hue due to insufficient discoloration treatment also appear to be less appealing. Discoloration typically occurs when staining substances adhere to tooth enamel, whereas loss of gloss results from surface roughness caused by daily wear and staining, leading to increased diffuse reflection (Figure 1). Therefore, restoring a healthy and attractive smile requires not only the removal of discoloration but also the recovery of gloss.

Figure 1. Schematic representation of tooth gloss improvement through surface roughness reduction.

Enhancement of gloss plays a crucial role in tooth whitening, particularly as teeth naturally lose gloss with age [1]. In cosmetic dentistry, the concept of tooth gloss can be encompassed by terms such as radiant, brilliant, sparkling, and lustrous. Current whitening toothpastes on the market primarily focus on discoloration-related effects, often quantified by the delta E value. However, in the field of aesthetic dentistry, significant research has been conducted on the importance of gloss [2-5]. For example, when damaged enamel is replaced with resin, the resulting surface often fails to replicate the natural gloss of tooth enamel, prompting ongoing research to improve the gloss of restorative materials to match that of natural enamel [2,3]. Tooth gloss is increasingly used as a metric to evaluate enamel damage caused by toothpaste [2,5].

Gloss is not only a cosmetic concern but also an important factor in dental health. A rough tooth surface can facilitate the attachment of bacteria and staining substances, leading to plaque formation, discoloration, and increased risk of caries and periodontal diseases [6-8]. Therefore, smoothing rough surfaces to restore gloss may help prevent these issues and maintain the initial healthy appearance of teeth [9-12].

This study focused on developing a premium whitening toothpaste capable of delivering superior outcomes in terms of both stain reduction and surface brilliant appearance, thereby meeting and exceeding consumer expectations for a more confident and attractive smile. To achieve this, we developed a novel evaluation method that quantitatively assesses gloss enhancement, moving beyond qualitative expressions of the effects typically used in the market for whitening toothpaste.

1. Materials

Hydroxyapatite (HAP) powder was obtained from OCI. Black instant coffee served as a staining agent for the teeth. Phytic acid was obtained from Sigma–Aldrich. All the other chemicals were of analytical or reagent grade.

2. Method for evaluating color change

The color of the samples was quantified using the CIELAB color space, where each color is represented by the coordinates (L*, a*, b*). These values were measured using a chroma meter (NF555; Nippon Denshoku Industries Co., Ltd., Tokyo, Japan). For the powder samples, approximately 0.1 g was placed on a glass slide and covered with cover glass. The color change was measured directly using a chroma meter.

3. Stain prevention efficacy test

To assess stain prevention efficacy, HAP powder was pretreated with aqueous solutions of various whitening agents, each at a fixed concentration of 1.0% by weight, as detailed in previous studies [13-16]. A 0.5 g portion of the HAP powder was transferred to conical tubes, followed by the addition of 20 ml of the solution, which was vortexed for 1 minute. The mixture was then centrifuged at 4,000 rpm for 15 minutes and the supernatant was discarded. The remaining HAP powder was thoroughly washed with distilled water to remove residual solution. Finally, the resulting powder was vortexed and centrifuged. The pretreated powders were then stained with a coffee solution. For staining, 20 ml of hot coffee solution was added to a conical tube containing 0.5 g of pre-treated HAP powders, and the mixture was vortexed for 2 minutes to ensure even dispersion of the powders. The coffee solution was centrifuged for 15 minutes at 4,000 rpm, followed by two washes with 20 ml of water. After the final centrifugation, the supernatant was removed, and the powders were dried overnight at 50°C. For the quantitative analysis, the L*, a*, and b* values of the powdered samples were measured using a chroma meter. A control experiment was performed using the same procedure but with water instead of whitening agents.

The efficacy of stain prevention was calculated as the percentage reduction in overall color change (ΔE) compared to the control, using the following equation:

Average reduction percentage of ΔE (%)=[ΔE_water−ΔE_sample]×100/ΔE_water

4. Stain removal efficacy tests

HAP discs were prepared and stained according to a procedure described previous studies [15,17]. The stain removal efficacy of the toothpaste was assessed using a modified pellicle cleaning ratio (PCR) method [15]. The HAP specimens were brushed with 5,400 strokes using a brushing machine, and a constant load of 250 g was applied to each specimen. This simulated 10 days of brushing, assuming 90 strokes per minute, 2 minutes per session, and 3 sessions per day. After brushing, the HAP specimens were dried and the color change was measured using a chroma meter (NF555, Nippon Denshoku Industries Co., Ltd., Tokyo, Japan). The color change (ΔE) was then analyzed by comparing it with the initial values.

5. Preparation of bovine enamel specimens and measurement of the initial gloss

Bovine incisors were used in this study. The roots were separated from the crowns and the teeth were sectioned through the pulp chamber using a water-cooled diamond saw to obtain enamel slices. The enamel surface of each slice was mounted with epoxy putty (Repairit Quik; Polymeric Systems, INC) for brushing and gloss measurements. As the initial analysis indicated variations in gloss among the bovine enamel specimens, a baseline adjustment was necessary. The enamel surfaces of each slice were ground flat using 400-, 600-, 800-, and 1200-grit SiC papers to achieve a gloss unit close to 100, mimicking the original gloss of the unaffected teeth. The test specimens were then exposed to coke as an erosive challenge to simulate the damage and discoloration caused by acidic foods and beverages in daily life. After washing with distilled water and drying with an air blower, the gloss of the enamel surface was measured. This process was repeated until all the test teeth had an adjusted gloss unit between 30 and 40.

Gloss analysis was performed using a gloss meter (ZG 8000; Zehntner Testing Instruments, Sissach, Switzerland) with a readout geometry of 60° to measure the light reflected from the surface. The gloss was calculated based on the ratio of the light reflected from the surface fragment to the incident light at a 60° angle. Measurements were expressed in gloss units (GU). The change in gloss (ΔGU) was calculated by subtracting the mean initial gloss values from the mean final gloss values (ΔGU=GU_f−GU_i) [3,10].

6. Measurement of gloss after brushing

The test teeth were fixed in the slurry bath of a brushing machine. A slurry consisting of 15 g toothpaste and 45 g distilled water was poured into the bath for tooth immersion. They were then brushed at 90 strokes/min for 10, 20, and 30 min. Thirty minutes simulated five days of brushing, assuming 90 strokes per min, 2 minutes per session, and three sessions per day, and 10 minutes simulated less than two days. After brushing, the teeth were rinsed with distilled water, dried with an air blower, and their surface glosses were measured using a gloss meter.

1. Stain prevention efficacy of whitening agents

We assessed the stain prevention efficacy of various agents, including sodium hexametaphosphate (SHMP), sodium phytate, phytic acid, tripolyphosphate, calcium pyrophosphate (CPP), potassium oxalate, potassium sodium (K/Na) tartrate, malic acid, and tartaric acid at a fixed concentration of 1.0% by weight (Figure 2). SHMP, sodium phytate, and phytic acid demonstrated excellent stain-prevention efficacy against coffee, as shown in Figure 3.

Figure 2. Chemical structures of toothpaste additives: SHMP, STP, sodium phytate, phytic acid, CPP, potassium oxalate, malic acid, potassium sodium tartrate, and tartaric acid.

Figure 3. Efficacy of various whitening agents in stain prevention, measured as percent reduction against water.

2. Stain removal efficacy of the whitening toothpaste

To evaluate the stain removal efficacy, several representative whitening toothpastes (WT) were chosen and one regular anti-cavity toothpaste (RT), as listed in Table 1. WT-1 toothpaste and WT-2 toothpaste derive their unique qualities from the inclusion of highly concentrated sodium hexametaphosphate (SHMP) in their formulated composition. A modified pellicle cleaning ratio (PCR) method based on Stookey’s approach was employed for HAP specimens before and after brushing [15,17]. As shown in Figure 4, SHMP-containing toothpaste, such as WT-1 and WT-2, and phytic acid-containing toothpaste, such as WT-3 exhibited higher whitening efficacy than the others.

Table 1 . Comparison of toothpastes by company and whitening agent(s)

Sample toothpaste	Company (country)	Whitening agent
WT-1	P (US)	SHMP
WT-2	L (Korea)	SHMP
WT-3	K (Japan)	Phytic acid
WT-4	C (China)	Arginine
WT-5	O (Korea)	Hydrogen peroxide
RT	L (Korea)	-

WT: whitening toothpaste, RT: regular toothpaste.

Figure 4. Comparative efficacy of five whitening toothpastes in stain removal, measured by ΔE.

3. Effects of toothpaste on gloss

The changes in gloss units (GU) of the bovine enamel specimens before and after brushing with different toothpastes are shown in Figure 5. SHMP toothpaste containing phytic acid demonstrated superior gloss enhancement, effectively countering the damage caused by a representative staining and etching beverage, coke (Figure 6).

Figure 5. Gloss enhancement performance of six toothpastes over time, measured by ΔGU at 10 (white), 20 (grey) and 30 (black) minutes of brushing.

Figure 6. Gloss enhancement of SHMP toothpaste with various concentrations of phytic acid, measured by ΔGU at 10 (white), 20 (grey) and 30 (black) minutes of brushing.

SHMP demonstrated the highest stain prevention efficacy against coffee (Figure 3). Several studies have reported that SHMP are effective in preventing and eliminating discoloration [13-16,18]. In this study, we tested dicarboxylates and dicarboxylic acids with chelating effects, including potassium oxalate, potassium sodium (K/Na) tartrate, malic acid, and tartaric acid, which are known to be effective whitening agents. However, their stain prevention efficacy is insuffi-cient. Recently, the whitening effect of sodium phytate has been reported [19,20]. Several review articles have highlighted the potential application of phytic acid, known as inositol hexakisphosphate (IP6), as a whitening agent in oral care [21,22]. In this study, we found that phytic acid and sodium phytate exhibited excellent stain prevention efficacy comparable to that of SHMP.

In a previous study, the stain-prevention mechanism of SHMP was attributed to its extensive and robust coverage of the enamel surface, facilitated by its polydentate and polymeric structure, which enables a strong chelating effect [15]. The exceptional stain prevention efficacies of phytic acid and sodium phytate can be attributed to their unique structures with six phosphate groups. Enamel, the thin outer layer of the tooth, is primarily composed of hydroxyapatite (HAP), which constitutes 96 wt% of the enamel. HAP, with the formula Ca₁₀(PO₄)₆(OH)₂, is rich in calcium ions. Similar to SHMP, the phosphate groups of phytic acid and sodium phytate, which act as bidentate ligands, can strongly interact with the calcium ions on the enamel surface, inhibiting the binding of stains.

Commercial toothpastes containing SHMP and the toothpaste containing phytic acid demonstrated excellent stain removal efficacy in a modified PCR evaluation, an in vitro whitening effect assessment related to their ability to address common staining issues. SHMP-containing toothpastes showed much higher efficacy than phytic acid-containing toothpastes in terms of the whitening effect (Figure 4). In our study, toothpastes incorporating SHMP and the toothpaste incorporating phytic acid showed noteworthy amplification in gloss, registering a remarkable boost of 1.5-fold or more and 1.7-fold, respectively, compared to the regular anti-cavity toothpaste following a mere 10-min brushing session, as depicted in Figure 5. The peroxide-containing toothpaste, WT-5, did not show any improvement in gloss. Notably, tooth-bleaching agents such as hydrogen peroxide can decrease the gloss of stained restorative materials [23,24].

Our goal was to develop a superior whitening toothpaste that effectively addresses discoloration while enhancing gloss and ensuring consumer satisfaction. Figure 6 shows the gloss-enhancing effects of toothpaste with varying concentrations of phytic acid added to the same SHMP toothpaste formula as the WT-2, as listed in Table 1. We labeled these new whitening toothpastes NT (new toothpaste) with added phytic acid concentrations, such as NT (PA 0.3%). Figure 6 shows that SHMP toothpastes containing 0.3%, 0.5%, and 1.0% phytic acid exhibited significantly higher gloss-enhancing effects than commercial SHMP toothpastes without phytic acid of WT-2. Comparatively, after a 10-min brushing session, the SHMP toothpaste with 0.3% phytic acid displayed a significant increase in the ΔGU value. Specifically, the increase was measured to be 7.9 units higher than that of WT-1 10.0 units higher and WT-2 both of which solely contain SHMP. Additionally, when compared to the regular anti-cavity toothpaste (RT), the ΔGU value experienced a substantial rise of 17.1 units. The results are shown in Figures 5 and 6. Moreover, the observed enhancement in gloss was remarkable, exemplified by a ΔGU value surpassing 6.4—a threshold acknowledged in the realm of aesthetic dentistry as perceptible for surface gloss variation [4]. P company prides itself in promoting its whitening toothpaste (WT-1) formulated with saturated SHMP as the epitome of whitening toothpaste globally, claiming to eliminate 100% of stains within only three days while providing round-the-clock protection against further staining. Nevertheless, our findings substantiate the superiority of our novel toothpaste formulation featuring the incorporation of SHMP and 0.3% phytic acid, surpassing the esteemed WT-1 as the premier whitening toothpaste in the global market. However, the addition of 4% phytic acid to the toothpaste reduced the gloss compared to that of the original SHMP formulation. This may be attributed to the fact that phytic acid itself has a color and can cause discoloration at higher concentrations. Additionally, because of its acidic properties, phytic acid may induce surface damage when used at elevated concentrations. Overall, we conclude that SHMP toothpaste with a low concentration of phytic acid (below 1.0% by weight) provides a higher gloss-enhancing effect than toothpaste without phytic acid.

The newly developed SHMP-whitening toothpaste, containing a low concentration of phytic acid additive (≤1.0% by weight), demonstrated an outstanding improvement in tooth gloss. We anticipate that this new toothpaste will meet the expectations of consumers seeking effective whitening and enhanced tooth health. Furthermore, we believe that the approaches outlined in this study will contribute to the design and thorough evaluation of tooth-whitening products, particularly in terms of gloss enhancement.

No potential conflict of interest relevant to this article was reported.