2.3. Freezing and thawing
Samples were frozen at 80 C in a cryogenic freezer, and
at 40 C in air-blast and jet-impingement freezers. The latter two
freezing methods were conducted at air velocities of 4 and 35 m/s,
respectively. The distance between the jet nozzle end to the
sample surface (H) and the jet nozzle diameter (D) was fixed at 6
(D ¼ 1 cm, H ¼ 6 cm) as the best position for high heat transfer on
the sample surface (Van Heiningen, 1982). The initial temperature
of the samples was controlled at 22 3 C. Freezing was terminated
once the thermal center of the samples reached 18 C.
Then, the frozen samples were thawed at 6 C overnight before
quality analysis.