This study demonstrated significant differences in the staining potential of various sauces, with distinct factors influencing the extent of tooth discoloration. The Yangjo, followed by the Sashimi and Jin soy sauces, exhibited the highest staining potential, which can be attributed to the dense concentrations of chromogens, particularly melanoidin, known to strongly bind to enamel surfaces and cause pronounced discoloration [6]. Melanoidins, also found in coffee, are produced during the Maillard reaction, which occurs during fermentation and heating, involving interactions between reducing sugars and free amino acids [16,17,18]. This reaction gives soy sauce its characteristic brown color and flavor and enhances its staining potential. Melanoidins contribute to the darker appearance of the tooth by binding to metal ions, such as calcium and proteins, on the enamel surface [19].

Previous studies have consistently highlighted the staining potential of soy sauces. Kang et al. [20] demonstrated that soy sauce caused more severe discoloration than other Korean fermented pastes, such as red pepper and soybean pastes, with the staining intensity increasing over time. Similarly, Chen et al. [21] reported a strong association between frequent soy sauce consumption and black tooth stains in preschool children. Their study revealed that children who regularly consumed soy sauce had 1.33 times more black-stained teeth than those who rarely or never consumed it. This finding underscores the critical role of dietary habits in extrinsic tooth discoloration and highlights soy sauce as a significant contributor to staining.

Acidity and pigment concentration emerged as key factors influencing the extent of discoloration. Soy sauces are mildly acidic, with pH levels conducive to enamel surface softening, which facilitates chromogen binding. The tooth enamel is primarily composed of hydroxyapatite crystals that are stable under neutral pH conditions. When the oral environment becomes acidic, these crystals begin to dissolve in a process known as demineralization due to the increased hydrogen ion concentration (H⁺); the hydroxyapatite lattice is disrupted in acidic conditions, leading to the release of calcium and phosphate ions from the enamel structure [22]. Previous studies have established that prolonged exposure to low pH or acidic solutions can enhance tooth discoloration by increasing enamel porosity, allowing pigments to adhere more effectively [13, 23].

The high absorbance values of soy sauces, indicative of their dense chromogen content, further reinforce their staining potential. The soy sauces demonstrated significantly higher absorbance levels than the fish sauce, which showed relatively lower staining potential in the current study. However, it is evident from the findings that acidity and absorbance alone do not fully explain the staining outcomes. Ketchup and sriracha exhibited lower pH and higher absorbance than soy sauces. However, they caused significantly less discoloration, indicating that other factors, such as the molecular composition of pigments, chemical interactions with enamel, and sauce viscosity, contribute to the complexity of tooth staining mechanisms.

The physical properties of the sauces, particularly their consistency and viscosity, may have played a role in the observed staining differences. Low-viscosity sauces, such as soy sauces and fish sauce, spread more uniformly over the tooth surface, allowing for greater contact and interaction with the enamel. This extended exposure time enhances the deposition and penetration of chromogens, leading to more pronounced discoloration. In contrast, thicker sauces like ketchup, sriracha, and curry exhibit reduced flow and less uniform surface coverage, which may limit their staining potential.

Among the soy sauces, Sashimi soy sauce, which has a slightly thicker consistency than other soy sauces, may better adhere to enamel surfaces, further increasing its staining potential. The prolonged contact time associated with higher viscosity can enhance the accumulation of pigments, promoting deeper and more intense discoloration. This raises important considerations regarding the extent of discoloration—whether it is limited to surface-level (extrinsic) staining or involves deeper penetration into the enamel matrix, potentially transitioning to intrinsic discoloration.

The staining potential of the sauces varied significantly depending on their composition, physical properties, and chemical characteristics. Among the soy sauces, Tsuyu soy sauce, which is typically diluted before consumption, exhibited the lowest absorbance due to its concentrated nature. Nonetheless, it caused noticeable staining over extended exposure periods, suggesting that sauces with low chromogen concentrations can lead to discoloration with sufficient contact time, thus highlighting the cumulative nature of chromogen interactions with the enamel.

In contrast, despite its vivid yellow color and high absorbance, curry showed remarkable resistance to staining, possibly due to its relatively neutral pH, which helped preserve enamel integrity by reducing erosion and preventing chromogen adherence. Curcumin, the primary pigment in curry, has been shown to stain materials such as polyoxymethylene brackets [24]. In addition, another study reported that orthodontic elastomeric chains made of polyurethane were more heavily stained by curry than coffee and wine, highlighting a strong interaction between curcumin and specific orthodontic dental materials [25]. This selective interaction may account for the minimal staining observed on the enamel in the current study. Similarly, with their vibrant red hues and low pH, ketchup and sriracha displayed surprisingly low staining potential, challenging the assumption that red-colored substances inherently cause tooth discoloration. The lack of a significant increase in redness compared to soy sauces suggests that staining is not solely dependent on color intensity but on the chemical behavior of chromogens and their interactions with enamel surfaces. Carotenoids, for example, are hydrophobic molecules that dissolve in lipids rather than binding strongly to hydrophilic surfaces, such as the enamel [26, 27]. Curcumin also binds to the tooth surface, but the inherent brown color of melanoidin likely plays a key role in darkening the tooth surface [28]. It should be noted that the absence of statistically significant staining in some groups may have been influenced by limited sample size, potentially masking smaller effects that the study was underpowered to detect.The findings of this study have important clinical implications, particularly for dietary recommendations aimed at minimizing tooth discoloration. While soy sauces, especially Yangjo, Sashimi, and Jin, pose a significant risk for tooth staining, sauces like curry, ketchup, and sriracha may offer alternative options for individuals concerned with esthetic dental outcomes. For patients undergoing tooth whitening or other esthetic dental treatments, avoiding highly pigmented and acidic sauces, such as soy sauces, may help maintain treatment results and prevent rapid discoloration.

In addition, the results of this study suggest that viscosity and contact time should be considered when assessing the staining potential of foods and beverages. Understanding the interplay between the physical properties, chemical composition, and enamel interactions can aid in developing strategies to manage and mitigate extrinsic discoloration. These insights are particularly relevant for the formulation of preventive care protocols and dietary guidelines tailored to individual esthetic needs.

While this study provides valuable insights into the staining potential of various sauces, certain aspects warrant further exploration. The experiments were conducted under in vitro conditions, which may not fully capture the complexity of the oral environment, including factors such as saliva flow, biofilm formation, and individual oral hygiene habits. Saliva plays a crucial role in modulating tooth discoloration through its natural buffering capacity, while the salivary pellicle serves as a protective barrier, preventing enamel demineralization and minimizing pigment deposition on the tooth surface [29]. Salivary proteins, conversely, may promote staining, as previous studies have shown that the interaction between salivary components and dietary substances can enhance discoloration [30, 31]. Therefore, the presence of saliva may substantially impact the discoloration outcomes.

Polished bovine enamel differs from natural human enamel because it lacks a surface texture and an intact pellicle layer, which may affect stain adhesion. In addition, the sauces were tested in their undiluted forms, whereas in real-world settings, they are typically diluted or consumed with other foods. Dietary habits, such as consumption frequency, and the balance between staining and tooth brushing, were also not considered. Lastly, the sample size was determined based on practical feasibility rather than strictly adhering to calculated value. While meaningful differences were still observed between several groups, this limitation should be considered when interpreting comparisons that did not reach statistical significance. Future research should address these factors for a more comprehensive understanding of the tooth discoloration mechanisms.