The heating program was performed in four successive steps. In the first step, the temperature was linearly increased to 120 C in 5 min with a maximum power of 1000 W; in the second step, the temperature was kept at 120 C for 2 min; the third step consisted of increasing the temperature linearly to 210 C in 10 min, whereas in the fourth step, the temperature was kept at 210 C for 15 min. After the end of the heating program, vessels were cooled down to room temperature and carefully open since they still might be pressurised.