2.2.2. Freezing and centrifugation
Juice solutions (12 ml) contained in plastic centrifugal tubes (internal
diameter D = 15 mm) were frozen in a static freezer at −20 °C
for 12 h. As shown in Fig. 2, the external surface of the plastic tubes
was covered with thermal insulation made of foamed polystyrene
(8 mm thickness, thermal conductivity K = 0.035 W m−1 K−1)
so that heat transfer during freezing mainly occurred either unidirectionally.
During freezing, the temperature in the sampleswasmeasured
using needle-type copper-constantan thermocouples (Ellab A/S,
Rοdovre, Denmark) at the geometric center of samples. Thermocouples
were connected to a data acquisition systemmodel CTF84-S8 (Ellab A/S,
Copenhagen, Denmark) and registered continuously. The freezing rate
(mm min−1) was calculated as the thickness divided by the freezing
time (assuming that freezing occurs from one side)