Comparative evaluation of the effect of whey protein powder and plant-based protein powder on the microhardness, sorption, and solubility of a nano-hybrid giomer restorative composite: an in vitro study

Whey and plant protein powders are becoming common additions of people concerned with fitness in their diet. Consumed repeatedly as reconstituted shakes, these products remain in contact with the dentition for extended periods and introduce a chemically heterogeneous mixture of organic acids, fermentable carbohydrates, and bioactive phytochemicals capable of altering the oral environment. Acidic beverages and sports drinks are well documented to compromise resin-based restorative materials, yet comparative data on whey versus plant-based protein powders are lacking. An investigation using an in vitro method of analysis evaluated the effect of the whey protein powder and plant protein powder on the water sorption and solubility of the nano-hybrid giomer restorative composite. Fifty-six disc-shaped Beautifil II a nano-hybrid universal restorative composite incorporating Giomer technology and S-PRG fillers specimens were fabricated and allocated to three immersion groups: WPP, PBP, and distilled water control, with an additional independent baseline group for pre-immersion microhardness assessment. The specimens were exposed to freshly prepared solutions for 10 min at 37 °C daily for 15 days. Vickers microhardness was evaluated using a standardized indentation protocol, while water sorption and water solubility were assessed using a gravimetric mass-difference procedure based on ISO 4049. Specimen-level Wsp and Wsl values were calculated in µg/mm³. Normality was measured by using the Shapiro–Wilk test. The Kruskal-Wallis test followed by Dunn-Bonferroni post-hoc analysis was used for intergroup comparisons of microhardness. Water solubility was compared among the three immersion groups using the Kruskal–Wallis test followed by Bonferroni-adjusted pairwise comparisons, whereas water sorption was reported descriptively because of measurement-resolution limitations. Post-immersion microhardness was highest in the PBP group (86.33 ± 9.54 VHN), followed by WPP (83.93 ± 7.25 VHN), the independent pre-immersion baseline group (80.33 ± 9.46 VHN), and the distilled-water control (80.14 ± 9.48 VHN). After Bonferroni correction, PBP showed significantly higher microhardness than distilled water (adjusted p = 0.041), but its difference from the independent baseline group did not remain significant after adjustment; WPP showed no significant change relative to any comparator. Water solubility differed significantly among the three-immersion media (Kruskal–Wallis, p = 0.001). Mean Wsl was 371.36 ± 87.13 µg/mm³ for PBP, 148.54 ± 108.80 µg/mm³ for WPP, and 137.93 ± 92.85 µg/mm³ for distilled water. PBP demonstrated significantly higher Wsl than WPP (adjusted p = 0.0069) and distilled water (adjusted p = 0.0025), whereas WPP did not differ significantly from distilled water. Mean Wsp was 159.15 ± 150.05 µg/mm³ for PBP, 0.00 ± 115.68 µg/mm³ for WPP, and 26.53 ± 166.61 µg/mm³ for distilled water. Plant-based protein powder produced significantly higher microhardness than distilled water and significantly higher water solubility than both whey protein powder and distilled water, while whey protein powder showed no significant effect on either outcome relative to the neutral control. These preliminary findings are limited to the tested products and experimental conditions. They should not be extrapolated directly to clinical deterioration or used to support specific patient-management recommendations without confirmation from broader laboratory and clinical studies.