## 4 Discussion
That explains why red wine has a stronger discoloring effect than Coke(r), which is more acidic (pH-2.0) and contains citric acid. This behavior is confirmed by the results of this study (Table 5,Figure 9). Coke(r) had values similar to distilled water and coffee, with no significant differences during all periods (Table 5, Figure 9). There is a general assumption by many studies [17; 30; 31; 32; 33; 34; 35; 40] that coffee has a strong discoloring effect in dental resins compared with other staining agents. Coffee contains tannin and chlorogenic acids with a mild acidic pH (\(\sim\)5) [19]. The abovementioned hydrolytic action of water combined with these characteristics leads to a surface discoloration process, as these pigments are deposited on the resin's surface. [PERSON] et al. [17] mention in their review that coffee is the worst of all common beverages. In the present study, coffee exhibited a much lower discoloring effect than tea and red wine. It exhibited values that were not significantly different to distilled water and Coke(r). This agrees with [PERSON] et al. 's [19] findings that coffee has a low discoloration potential compared with lemon juice and Coke(r). The possible explanation they gave was that after the coffee layers have reached a certain thickness on the specimen surface, the layers are dislodged and dissolved in the water. To summarize, in the present study there was clear evidence that tea and red wine have the strongest discoloring effect in all materials. All materials in the present study showed similar behavior in all staining media during all time periods. All three types of resins exhibited color changes over time (Figure 3) at a statistically significant level (Table 4), thus presenting a time-dependent pattern of \(\Delta\)E changes. The highest values in the 30-day period were measured for the 3D-printed soft resin. The two PMMA resins (heat-polymerized and 3D-printed) exhibited a similar behavior. There are, however, contradictory findings in the literature. [PERSON] et al. [19] found that 3D-printed denture resins demonstrated better color stability than heat-polymerized resins, which presented the highest \(\Delta\)E. [PERSON] et al. [30] estimated that a 3D-printed resin performed better in color stability than a PMMA heat-polymerized resin, showing similar results with another study [11]. Moreover, [PERSON] et al. [32] found inferior color stability behavior in 3D-printed resins as compared to conventional heat-polymerized and CAD/CAM-milled PMMA resins. [PERSON] et al. [22] reported that 3D-printed resins demonstrated color differences above acceptable clinical thresholds compared to CAD/CAM denture resins when exposed to curry, grape juice, coffee, and water. [PERSON] et al. [21] showed that, during a coffee thermocycling test, CAD/CAM-milled PMMA showed better coloring behavior. After proper polishing, all materials presented clinically acceptable changes. Additively manufactured denture base resin had the highest color difference, and prepolymerized PMMA had the lowest color difference after coffee thermocycling; however, the first exhibited a perceivable color change. Significant differences in surface roughness were observed among tested materials within each time interval and among different time intervals. Also, [PERSON] et al. [20] found that 3D-printed acrylic teeth had no color differences from conventional acrylic teeth in different solutions (coffee, Coke(r), erythrosine, and water). [PERSON] et al. [29], in a systematic review of the optical properties and color stability of different types of denture teeth, concluded that 3D-printed teeth showed contradictory findings, needing more research for improving the materials. On the other side, CAD/CAM teeth had similar properties to conventional heat-polymerized teeth. Coffee was the worst among common beverages. In the present study, there was no clear differentiation between the denture base resin materials as, largely, no significant differences were detected.