Evaluation of color changes

The color of the samples was measured according to the CIELab and CIEDE2000 systems using a non-contact spectrophotometer (SpectroShade Micro, MHT, Milan, Italy) against a white background, under daylight conditions in air, and at the same time of day to obtain initial values. The device was calibrated at the beginning of every sixth measurement according to the manufacturer’s recommendations using the white and dark calibration standard provided. The positioning of the device on the sample was achieved by an angle control system of the device, which calculates the optimal angle of incidence between the optic handpiece and a target sample. This angle was verified by a horizontal green line representing the accurate geometry. The spectral data obtained were translated into CIELab and CIEDE2000 coordinates by software using the standard D65 illuminant and 2° observer angle as a reference. Three consecutive measurements were taken on the samples, and the mean value was recorded as the final value (ΔE and ΔE₀₀).

According to Ertas et al. [34], the average consumption time for one cup of coffee is 15 min, and the quantity is 3.2 cups/day. Therefore, 24-hour storage time simulates about one month of coffee consumption. The samples were immersed in a coffee solution for seven days to simulate approximately 6-month of consumption. For this purpose, 2 g of instant coffee (Nescafe Classic, Nestle, Switzerland) was mixed with 200 ml of hot water to obtain the solution. The samples were immersed in individual containers perpendicularly to achieve uniform staining on each surface (Fig. 3). The coffee solution was refreshed every day.

The specimens were perpendicularly immersed in coffee solution

After one week of storage, the samples were rinsed in distilled water for 1 min to remove excess colorant and were subsequently dried. The color of the samples was measured again using the same spectrophotometer and converted to the formula of the CIELab and the CIEDE2000 systems. These measurements were made by a single operator at the same time of day and under similar lighting conditions to provide a standard evaluation.

Color measurements were repeated three times for each specimen, and averages were recorded for L1*, a1*, and b1* using the CIELab color system. L* indicates 0 to 100 (lightness to darkness), a* represents chroma; green and red content ranking from -a (green) to + a (green), and b* represents hue ranking from -b (blue) to + b (yellow). The following formula was used to determine the ΔE values between color measurements according to the CIELab system [4, 28]:

Values according to the CIEDE2000 were also recorded as a second color evaluation. For each pair of samples, ΔL^′, ΔC^′, and ΔH^′ indicate the differences in lightness, chroma, and hue, respectively, according to the CIEDE2000 metric system. The weighting functions S_L, S_C, and S_H were used to adjust for the location of the color differences in the L^′, a^′, and b^′ values. The parametric factors K_L, K_C, and K_H served as correction terms for the experimental conditions. In addition, R_T was used as a rotation function that accounts for how chroma and hue variations interact in the blue region. The CIEDE2000 color formula was as follows [8, 28]:

After these color measurements, the discolored surfaces were repolished using the same systems (TWD or NOV) as those used at the beginning to restore the original color. The color recovery of the samples was measured as ΔE values using the CIELab and as ΔE00 values using the CIEDE2000 systems, as for the baseline and coffee immersion.