In this study, two hypotheses were rejected, because material/polishing method, BLE, beverages and PDP led to discoloration of the tested materials. The CIEDE2000 color difference formula (∆E00) is reported to be superior to the previously used CIELAB formula (∆Eab) in terms of perceptibility and acceptability of color difference. This formula, developed by the CIE in 2013, is considered the standard for color difference detection. The ∆Eab formula basically measures the distance between two points in color space, whereas the ∆E00 formula includes the luminance effect by adding SL [23]. Therefore, the ∆E00 color difference formula was chosen for this study. In previous studies testing CAD-CAM FC [22], ZLS [23] and HC [24], 0.8/1.8 was found to be appropriate for PT/AT values; therefore, 0.8/1.8 was chosen for PT/AT in this study.

Spectrophotometers, spectroradiometers, colorimeters and digital cameras are currently the most recommended instruments for color measurement in dentistry. Spectroradiometers designed for the measurement of radiometric values are the most recommended instruments for determination of the tooth color or translucency of the ceramic core structure [25]. In this study, we aimed to investigate color change on the surfaces of dental materials due to external factors. Spectroradiometers are extremely sensitive instruments, where even the slightest error in the position of the measuring device is difficult to tolerate and should be handled with extreme caution. In this study, since a total of 5 measurements were taken from 210 specimens, it was necessary to select a device more suitable for the working conditions. For these reasons, the Vita Easyshade V spectrophotometer device was used in this study [26].

Moreover, this instrument was used in previous studies for teeth [27], polymethyl methacrylates [28], resin composites [29] and ceramics [9]. Dozic et al. [30] stated that Vita Easyshade can make reliable measurements compared with some colorimeters and digital cameras and explained that these three different types of devices can be tested in in vitro studies. Igiel et al. [31] stated that given their accuracy (from 66.8 to 92.6%) and precision (from 87.4 to 99.0%), it is an excellent example among color determination methods. In this study, a custom-made color booth was prepared to ensure standardization and to prevent ambient light from causing errors [20].

According to the results, the color change exceeded the AT after 28 days in all specimens, except for FC-GLA and HC immersed in DIS and ZLS-GLA specimens immersed in ENE. Between day 1 and day 14, AT was exceeded in all other specimens, except for ZLS-GLA immersed in DIS, ENE or SAM. From day 14 to day 28, AT was exceeded only in FC-GLA immersed in DIS and not in HC immersed in DIS, ENE or SAM. When the color change values obtained from BAS to day 1, BAS to day 14 or BAS to day 28 were examined, no specimens, regardless of the beverage in which they were immersed, had a color change lower than PT.

In this study, a new HP formula (25% HPS) was used for in-office BLE. In this study, BLE was found to be influenced in color change of restorative materials, which was consistent with previous studies [8, 32, 33]. Conventional in-office BLE gels mostly contain 30–35% HP. It has been reported by the manufacturer that 25% HPS has a different content than conventional in-office BLE gels and therefore is more effective [34]. It was necessary to investigate this claim. In previous studies [8, 32] the 6% formulation of HPS used for at-home BLE was tested and the magnitude of the effect was found increased with time. In this study, 25% HPS produced a similarly noticeable effect on the materials tested. According to the results, at the end of 28 days, the color change did not exceed AT in FC-GLA, ZLS-POL and HC specimens without BLE, but exceeded it in specimens with BLE (p = 0.0239, p = 0.0091, p = 0.0273, respectively). These results were possibly due to the amount of content and concentration. In contrast to conventional BLE gels, HPS contains CP and vinyl pyrrolidone peroxide in combination, as well as heat-reversible poloxamer to adjust the viscosity. When the gel comes into contact with the tooth surface, an exothermic reaction cycle occurs, resulting in an increase in the magnitude of the effect.

When the BLE agent contacts the surface of a ceramic material for a long time, it may cause surface deterioration as in resin composites. Free radicals such as H+ and H3O+ produced by alkali ions infiltrate into the material matrix and cause dissolution of ceramic glass networks, disintegration of SiO2 and K2O2 components, abrasion of the surface, destruction of chromogens and formation of a less light-reflecting surface. The quality of surface polishability reduces the magnitude of exposure to external factors [35]. Alshali et al. [36] reported that the polishability quality may change depending on the material structure. In this study, when the results of BLE treated ceramic specimens after 28 days were analyzed, it was found that mechanically polished FC was more influenced than glazed FC, mechanically polished ZLS was more influenced than glazed FC and mechanically polished ceramics were more influenced than hybrid ceramic (p < 0.0001).

Previous studies [35, 37] had reported that the application time is significant in the effect of BLE on ceramics. Similar to this study, the number of studies in which glazed and mechanically polished ceramics were examined together is insufficient, and studies [14, 33] found that glazed surfaces were less affected than mechanically polished surfaces in parallel with this study. The researchers reported that the magnitude of the effect of BLE is related to the application time.

FCs have microstructurally a single-phase and multiporous structure. In ZLSs, lithium disilicate crystals have a large, rod-like appearance and cerium, the equalizing material of zirconium oxide and tetragonal zirconium, is properly dispersed throughout the structure. In this study, with BLE or ENE, higher resistance to color change after 28 days was obtained for glazed FCs and ZLSs than for mechanically polished ones. In a study by Ramos et al. [38] using energy dispersive spectroscopy (EDS), it was found that the needle-like rods on the surface of ZLS were so covered that they were invisible. This may be an indicates that the glaze layer is able to protect the material well against external factors.

Furthermore, Ramos et al. [38] stated that according to the results of the study, the water permeability of HC and the resulting degradation of its structure should be investigated, and it was surprising that the calculated density of HC was higher than FC and ZLS. As can be seen, HC is a suitable material for testability and therefore hybrid ceramics were used for comparison with ceramics. In this study, the components such as hydrophobic urethane dimethacrylate (UDMA) and hydrophilic triethylene glycol dimethacrylate (TEGDMA) may also be effective in the color change on the surface of HC. In the study by Asthiani et al. [39], HC was more affected by beverages than ZLS and reported that this was due to TEGDMA absorbing staining pigments into the material.

The focus of this study was to investigate three materials with different structures and the selection of materials was structurally based on the fact that all three materials were ceramic based. The first material was a feldspathic ceramic, which has been used in the majority of studies. The other was zirconia-reinforced lithium disilicate ceramic, which is a newly developed product. Since it is a newly developed product, hybrid ceramic, which is a polymer infiltrated ceramic, was selected as the third material [38]. The selection of material during the design of the study was in accordance with the study by Ramos et al. [38]. In HC, it is claimed that the cracks in the polymer network are stopped by the ceramic phase and therefore the structural reliability is high, but it was necessary to determine to what extent the superiority of this ceramic over color-changing agents was different [38].

Resin-nanoceramics are alternative to HCs. Lava Ultimate (3 M ESPE), one of these, contains 80% nanofiller by weight and CeraSmart (GC Dental), another product, 71%. These products are probably produced at high pressure and temperature [40]. Unlike resin composites, HCs contain two interconnected networks, polymer and ceramic. In a previous study [41], a crystalline structure with leucite and secondary zirconium surrounded by polymer was identified in HCs. HCs are more similar to ceramics than resin-nanoceramic materials in terms of causing wear on opposing teeth and elastic modulus value. Moreover, Lava Ultimate (3 M ESPE), a resin-nanoceramic product, is not recommended by the manufacturer for full crowns because it produces micro-segregation at the interface between the crown and tooth under occlusal load [40]. In this study, we wanted to evaluate the more commonly recommended materials for full crowns; therefore, Lava Ultimate was eliminated. For comparison with ceramics, HCs, which are closer in structural properties to ceramics, were preferred compared to resin-nanoceramics.

Dental materials can be discolored over time by saliva or acidic beverages [42]. In this study, RedBull energy beverage with a pH of 3.18 was used [11]. Silva et al. [11] reported that the surface degrading effect of RedBull varied with consumption frequency and time. Elhamid and Mosallam [43] found that even when carbon dioxide evaporated from energy beverages, the pH level remained low. Dos Santos et al. [44] reported that citric acid in carbonated beverages is the main factor for material surface degradation. Acidic beverages for dental ceramics cause selective extraction of alkali metal ions with low stabilisation in the glass matrix. Insufficient polymerisation of the material, water absorption and the pigment type of the beverage for resin-containing ceramics are important factors [45]. ENE and SAM used in this study, contain citric acid and staining pigments. The staining pigments contained in these beverages possibly caused the discoloration and over time the acid caused the surface of the material to deteriorate, facilitating the retention of the pigment.

In this study, FC-POL immersed in ENE had the highest color change after 28 days (∆E00 = 5.4, p = 0.0002) and HC immersed in DIS had the lowest color change (∆E00 = 1.3, p = 0.0001). According to the BLE-beverage interaction, at the end of 28 days, the highest color change was found in the specimens with BLE and immersed in ENE (∆E00 = 3.5, p = 0.0496), and the lowest in the specimens without BLE and immersed in SAM (∆E00 = 1.9, p = 0.0074). When the literature was reviewed, no study on the effect of energy beverages on HC was found, but a few studies [12, 46] on the effect of energy beverages on ZLS were found and in these studies, it was reported that low pH and dietary habits affected on the effect of energy beverages on discoloration. Since energy beverage consumption increased especially in young people after the Covid-19 pandemic, this type of study was needed [12].

There are many studies investigating the effect of beverages on the color of restorative materials. These studies mostly include beverages such as tea, coffee, cola, and wine [19, 46]. There is no study in the literature on the effect of immune-boosting beverage with black elderberry, whose consumption has increased since the Covid-19 pandemic due to its antiviral properties, on the color of ceramic [47]. Tokuc and Sukur [48] evaluated resin composites for white spot lesions in their study on children’s teeth. Consistent with this study, Sambucus Nigra caused clinically unacceptable color change. In this study, after 28 days, SAM tended to produce a color change similar to DIS. The fact that the specimens had been subjected to BLE increased the magnitude of color change due to SAM.

PDP is a polishing procedure that the majority of clinicians perform after prosthetic treatment to remove discoloration and plaque deposits [13]. Nowadays, a new generation of PDP pastes with lower dentin abrasiveness has been developed and their effects on dental materials should be investigated. This study is in agreement with the study of Elhamid and Mosallam [43] who investigated the effect of PDP on resin composites. Depending on the type of paste, the effect of PDP on the surface may differ [49]. In this study, a paste containing fine silica particles with a RDA of 7 was used. According to the findings of this study, PDP is effective for restoring the color of materials discolored by beverages. This restoring effect was most successful in ZLS-GLA and HC for ENE, FC-GLA, ZLS-GLA and HC for DIS, and FC-POL and HC for SAM. Regardless of the beverage, when BLE and non-BLE specimens were compared, color recovery was more successful in non-BLE specimens; HC and ZLS-GLA were superior to other materials in this respect. Regardless of the material, when evaluated in terms of BLE-beverage, color recovery was better in all specimens without BLE and immersed in DIS, ENE or SAM compared to those with BLE.

Ceramic stains contain pigments composed of raw metal oxides. The color of ceramics after firing may vary depending on the type of ceramic or stain. In a previous study [50], yellow stain applied to body porcelain increased the gloss of the porcelain, whereas blue, purple and red stains decreased it. In another study [51], it was reported that yellow and orange stains exhibited low color stability during firing. Color differences may result due to the addition of metal oxides under different oxidation conditions [52]. The magnitude of the porosities in the ceramic structure, number of firings and temperature can also cause changes in the color effect of stain pigments [53]. In this study, in order to standardize the glazing conditions, only the glazing powder-liquid set suitable for the glazing process was used, without the use of external stains.

A limitation of this study was that intraoral conditions cannot be fully simulated. Exposing both sides of the specimens to beverages does not fully represent clinical scenarios where only one side is mostly exposed to colorants. This study adds to the literature in many ways, but future in vivo and in vitro studies are needed to investigate the effects of immune-boosting beverages, energy beverages, bleaching and new PDP paste on the surface properties of newly developed CAD-CAM materials.