Dental caries is a dynamic disease characterized by cycles of demineralization and remineralization at the tooth surface. When remineralization phases predominate, the carious process is arrested [15]. Being the main constituent of hydroxyapatite crystals, the bioavailability of phosphate and calcium ions is crucial to enhance the remineralization process [16]. The current work was conducted to examine the remineralization capability of goat milk compared to cow milk and artificial saliva on early enamel lesions via evaluating their effects on surface roughness and microhardness of demineralized enamel.

In this study, a freshly prepared citric acid solution (pH 3) served as the demineralizing agent. Citric acid causes enamel demineralization by chelating calcium ions or promoting ligand dissolution, resulting in enamel weakening and the release of hydrogen ions. Demineralization begins with the prism sheath, then the prism core, and the damage may extend to the interprismatic region, eroding the enamel structure [12]. The pH value was selected in an attempt to mimic several daily consumed drinks, such as fruit juices and carbonated drinks, that have a pH range around 3 [17]. The teeth of all study groups in the present study were subjected to 3 cycles of demineralization (10 min each), followed by a single longer cycle (30 min), for a total demineralization time of 1 h to mimic cumulative acid attacks that take place in the oral cavity, which are usually short in duration but occur repeatedly. Similarly, remineralization with the 3 tested solutions (artificial saliva, cow milk, and goat milk) was performed in 3 cycles (1 h each). This cyclic protocol also simulates the prolonged remineralizing effect of saliva under in-vivo conditions where remineralization is a slower process that requires longer exposure to calcium- and phosphate-rich environments for efficient repair [12].

The findings of the current work demonstrated a demineralizing effect of citric acid on enamel of the teeth in all studied groups, as evidenced by increased surface roughness and decreased microhardness values. This was in agreement with previous studies [12, 18], which showed that surface roughness increased, and hardness decreased progressively with the demineralization process of enamel. In contrast, the roughness of the demineralized enamel surface significantly decreased, and the microhardness significantly increased after immersion in the three remineralization solutions. These findings could be explained by Larnani et al. [12] who stated that cow milk is widely proven to operate as a remineralization agent and is known as a component that helps strengthen teeth. Furthermore, previous studies [19, 20] denoted that cow milk is supersaturated with calcium and phosphate, where approximately one-third of the calcium and half of the phosphate are free ions distributed in the serum phase, and the remainder are associated with casein protein forming micelles. These micelles dissolve slowly in the oral cavity whenever needed, thus acting as a reservoir for ions necessary to rebuild the demineralized hydroxyapatite crystals in enamel.

In the same context, Huanambal et al. [21] reported that artificial saliva has remineralizing properties due to its mineral content, such as phosphate and calcium, which are essential in the remineralization process of dental enamel, thus encouraging the creation of hydroxyapatite crystals and enhancing the resistance of enamel to acid decomposition. The artificial saliva with the formulation (20 mmol/l Hepes, 30 mmol/l KCl, 4 mmol/l KH2PO4, 0.7 mmol/l CaCl2, and 0.2 mmol/l MgCl2 / pH 7) was chosen in the current study as it offered the highest percentage of enamel hardness gain and superior remineralization results in previous studies, compared to other tested artificial saliva formulations [6, 14].

The findings of the current work also showed that after the remineralization phase, no significant variation in surface roughness or microhardness was noted among the cow milk and artificial saliva groups. However, previous studies [7, 22] have reported different results, where cow milk showed a significant remineralization effect on demineralized enamel compared to artificial saliva. These discrepancies may arise from methodological differences, where the authors used different techniques for demineralization and remineralization. Additionally, since there is no standardization in saliva formulations, the varying compositions of artificial saliva could have a major impact on the results, as different formulations can result in different remineralization degrees depending on their saturation with calcium phosphates. Furthermore, additives added to improve the viscoelastic properties of artificial saliva, such as carboxymethyl cellulose, are known to decrease the remineralization effect of artificial saliva by increasing viscosity, thus decreasing the diffusion rate of minerals into the demineralized enamel surface. These additives can also form complexes with calcium and phosphate ions, leading to their unavailability for remineralization [6].

In the herein study, the goat milk group showed a statistically significant difference compared to both artificial saliva and cow milk groups (P = 0.00), having the least surface roughness (0.19 ± 0.02) and the highest microhardness (298.37 ± 1.63) values (Tables 1and 2; Figs. 1 and 2).This could be explained by the fact that goat milk contains significantly higher concentrations of calcium ions compared to cow milk, in addition to its lower allergenic potential [23]. Additionally, goat milk casein was previously tested by Hartami et al. [10] on demineralized enamel of extracted premolars in a paste form of variable concentrations (10, 20 and 30%), where it was found to be significantly effective in increasing enamel hardness. The paste group with the highest concentration (30%) revealed the greatest increase in enamel hardness. These findings highlight the potential advantages of goat milk over cow milk and artificial saliva in caries prevention. Although numerous studies reported the higher nutritional value of goat milk over cow milk, further in vitro and in vivo research on the direct effect of goat milk on tooth remineralization is needed to support these observations.

In the present study, a strong negative correlation was observed between surface roughness and microhardness following the remineralization phase in all treated groups (Fig. 3). This finding is consistent with the well-established fact that rougher surfaces are less hard and more liable to abrasion [24]. On the other hand, the remineralized enamel surface may be more susceptible to abrasion during function, when consuming hard food, or by aggressive brushing [12]. This increased susceptibility might be caused by residual surface roughness and porosity, lower mechanical strength of the freshly formed mineral deposits, and deficient incorporation of the minerals into the underlying enamel prisms, resulting in a less resistant surface.

The SEM examination demonstrated that all treated groups exhibited improved surface morphology compared to the demineralization group, indicating an enhanced remineralization potential. Goat milk produced comparable results to both cow milk and artificial saliva. However, further studies are needed to confirm these findings.