Some beverages have been reported to cause more staining in composite or ceramic restorations in the literature. The knowledge that coffee is the most coloring beverage has been proven in many color stability studies on resin and hybrid ceramic materials [13–15]. In our study, in which coffee was accepted as the main coloring beverage, it was determined that the most staining was observed after 28 days for Lava (∆E = 12.68) after storing in coffee-tea (LG4) and for Enamic (∆E = 12.68) after storing in coffee-wine (EG6) among all groups.
Except for the EG2 group, the least color change was observed on the 1st day and the most color change was observed on the 28th day in all solution groups (p < 0.001). The first null hypothesis tested in this study was partially rejected because; the storage with different combinations of water, coffee, tea, coke, red wine, pomegranate juice, and turnip juice had a significant effect on color change in both hybrid ceramics due to staining over time.
The hybrid resin matrix falls into the dental ceramics category because they contain more than 50% inorganic particles . Ceramic is inert, but the organic structure is weak. Coffee may easily be diffused to the organic matrix of hybrid material dependent on the water absorption capacity. Also, the water absorption capacity of BIS-GMA was reported to be higher than those of UDMA, TEGDMA, and BIS-EMA [17, 18]. Lava contain BIS-GMA, as discint from Enamic. In the present study, in comparison of the color changes between Lava and Enamic samples (except for the samples in the LG6 and EG6 groups), more color changes were observed in the Lava samples than the Enamic samples in all groups and periods.
To better reflect the intraoral thermal exchange conditions, the samples were kept in the thermal cycle (10,000 cycles) in our study. Lava and Enamic samples were kept in coffee for 5000 thermal cycles in a study by Acar et al. . In the study, the highest color change has been observed in nanocomposite resin (Filtek Supreme Ultra Universal) followed by nanoceramic resin (Lava Ultimate), hybrid ceramic (Vita Enamic) and lithium disilicate ceramic (IPS e.max CAD) respectively, similar to our study. The study differed from our study in that they were polished the specimens with silicon carbide abrasive papers only and coffee was used for staining every 8 hours-time replaced with fresh coffee. Also, they have brushed the specimens with toothpaste after thermal aging.
Sagsoz et al. . investigated staining resistance of CAD/CAM blocks (including Lava Ultimate and Vita Enamic) surface polished with different polishing kits. The specimens were immersed in tea, Turkish coffee, fermented black carrot juice and distilled water for 1 day, 1 week, and 1 month. A month later unpolished Lava specimens were found dramatically higher ∆E values had than unpolished Enamic specimens. They claimed that this result could be attributed to the water absorption of monomers. Any thermal aging protocol was not applied in the study. Also, based on the study of Ertaş et al. , it was reported that the 1-month immersing period would be equivalent to 2.5 years of clinical aging in the study.
To the results of the present study, no significant color differences were observed in the LG1 and EG1 groups depending on the time in both hybrid ceramics. The second null hypothesis tested of the study was partially accepted because; the storage with different combinations of the beverages had a significant effect on color change in hybrid ceramics in overall storage time.
In our daily life, we consume various types of drinks in succession throughout the day that can cause discoloration on our teeth or restorations. However, a single type of colored beverage does not reflect the staining potential of human feeding behavior. In the literature, there are also studies conducted according to the pH of coloring beverages . It is thought that low-pH beverages will increase staining due to abrasion of the surface .
Arocha et al.  investigated color stainability of Lava Ultimate, Paradigm and two indirect laboratory-processed composites after being immersed in coffee, red wine, black tea, distilled water during 4 weeks period. Staining solutions were renewed every two days but without any thermal aging protocol. Similar to our study’s results, ∆E00 values of Lava Ultimate specimens immersed in red wine were higher than immersed in coffee, black tea and distilled water. Based on previous studies, they concluded that this result may facilitate staining by softening the resin matrix of alcohol. ∆E00 values (11.16) of Lava specimens in group LG6 (statistically not different from the specimens in LG4) after the 28th day were higher than the others in our study.
In our study, color measurements were made with the Easyshade device, which was reported most accurate color measurement device (92.6%) in the literature [24, 25]. CIEDE2000 color formula has been determined as being better to color differences which perceivable with the human eye than CIELab color formula . A ∆E color difference between 1.5-3 is determined as observable alteration (noticeable) on the National Bureau of Standards (NBS) system of expressing color differences . Color differences of more than 3.3 is acceptable easily detectable with laypeople but color differences as smaller than 3.3, is detectable only talented person is reported in a study .
It has been reported in the literature that surface roughness affects the color stability of ceramics. Ra > 0.2 µm is preferred for preventing biofilm retention to the restorations . A 0.3 µm-Ra is a generally accepted standard for creating a smooth restoration surface . Besides, existing a Ra > 0.3µm, it has been reported that patients have discomfort and feel by their tongue or lip contact on restoration surface . Hybrid ceramics were reported more resistant than conventional ceramics in surface roughness due to extrinsic coloring agents . In our study, the highest roughness value among the Lava samples; was observed in the measurements made on the 14th day in the LG8 group (0.37 ± 0.18 µm). The highest roughness values among Enamic samples were observed in the EG7 and EG8 groups as 0.36 ± 0.13 µm and 0.36 ± 0.17 µm, respectively, in the measurements made on the 14th day. Also, all recorded Ra values were under 0.4 µm in the present study.
The biggest limitation of our study is that it is an in-vitro study. Although temperature changes are imitated, a complete reflection is not possible due to the presence of many factors such as bacteria in the oral flora, the structure of saliva and hygiene habits. Therefore, more in-vivo studies should be conducted.