Experimental groups
The specimens were randomly distributed into 6 main experimental groups (n = 30/group), according to the test substances. Four groups were plant extracts: Açaí extract (Bio-Açaí Pulver, ZeinPharma, Germany; 33 g polyphenol per 100 g powder), Blueberry extract (Heidelbeer-Extrakt mit Anthocyanen, Fairvital B.V., Germany; 125 mg polyphenol per capsule), Green tea extract (Green Tea Deluxe, ZeinPharma, Germany; 250 mg polyphenol, namely EGCG, per capsule), and Grape seed extract (OPC, Fairvital B.V., Germany; 300 mg polyphenol, namely OPC, per capsule). As negative control (Control), deionized water was used. As positive control, a commercial stannous- and fluoride-containing mouthrinse (SnTo test the dual action of the solutions, each group was further divided into two subgroups (n = 15), where the solutions were either applied directly onto the dentine surface (D) or they were used to modify the salivary pellicle (P) formed on the specimen.