Model 600

Commercially available zirconia blocks (Ivoclar IPS e.max® ZirCAD MT A2, Lot:VM9002) of 98.5 mm in diameter and 14 mm in thickness were cut into discs of 25 mm in diameter and 1.2 mm in thickness by a high-speed linear precision saw (Isomet® 5000, BUEHLER, USA) with a diamond blade under running water. The samples were then polished with SiC abrasive papers of 1000-grit and 2000-grit in sequence on a polishing machine, and reached a surface roughness value of 0.5 µm. The polished zirconia blocks were sintered following the manufacturer’s protocol. The size of the sintered samples turned into 20.0 ± 0.5 mm in diameter and 1.0 mm in thickness due to the phase transformation during sintering. After the above treatments, the samples were ultrasonically cleaned with a 70% ethanol solution for 15 min, rinsed with double deionized water, and dried in clean ambient air for four hours before the samples were blown with clean high-pressure gas to remove any possible residue on the surface. The samples were finally sterilized with a dry heat method in 160 °C for two hours in a heating and drying oven (Model 600, MEMMERT, Germany).
A total of 24 cylindrical zirconia samples were randomly divided into six groups. A control group of four specimens was taken without any further treatment.

Humulus lupulus leaves from five hop genotypes (Table 1) grown under organic farming conditions were collected at harvest at the farm I Vizi del Luppolo (Cori, Italy; 41°63′46″ N-12°87′18″ E) and cold-transported to the Food Chemistry and Biotechnology laboratory at CREA Research Centre for Olive, Fruit and Citrus Crops (Rome, Italy). An aliquot of leaves (300 g) of each genotype was subjected to oven drying (OD) at 45 °C (air velocity: 0.6 ms⁻¹, relative humidity < 0.5%, system power: 1.4 kW/h; model 600, Memmert GmbH + Co.KG, Schwabach, Germany). This type of drying and the temperature were chosen considering the possibility of exploiting the drying systems for hop cones that are generally present in these farms. The remaining part (300 g) was freeze-dried at −54 °C and 0.075 mbar (model Modulyo 4 K, Edwards, UK). Sample dehydration using all of the methods mentioned above was continued until about 8–10% final moisture content was reached. At the end of each drying treatment, samples were finely milled (sieve 0.5 mm), stored under vacuum and kept protected from light and moisture until analysis. Four replicates for each treatment were carried out.