The control of ice nucleation temperature during the freezing stage has significant influences on quality uniformity of freeze-dried products and drying rate of later stages. For active depressurization to induce ice nucleation of supercooled water in vials

the effects of depressurization rate

final pressure level and supercooling degree on the nucleation phenomenon were studied experimentally and theoretically here. The following results were obtained: under atmospheric pressure the range of releasing supercooling degree(RSD) was 9-17 ℃

while during depressurization the range of RSD became 8-11 ℃; in the range of 5-20 kPa/s the depressurization rate had little effect on the nucleation of supercooled water; in the range of 100-400 Pa

lower final pressure led to lower RSD. Through numerical calculation and analysis it was indicated that the lowest temperature of surface water during depressurization maybe the key factor influencing ice nucleation.