Inclusion Criteria:  Oral healthy adult volunteers who are able to give written consent  Exclusion Criteria:  Non-fulfillment of the inclusion criteria Denied access to the University Medical Center Goettingen at the Corona screening Exemption from wearing a face mask for medical reasons Medical reasons hindering participants from refraining eating and drinking prior (except for water) or during the visits Hyposalivation / xerostomia (unstimulated saliva < 0.3 mL/min, stimulated saliva < 0.7 mL/min) Refusal to use fluoridated toothpastes Smoking