Spray-freeze-drying of whey proteins at sub-atmospheric pressures

Spray-freeze-drying (SFD) involves spraying a solution into a cold medium, and freeze-drying the resultant frozen particles, which can be performed by contacting the particles with a cold, dry gas stream in a fluidized bed, typically at atmospheric pressure. This enables much faster drying rates than are usually possible by conventional freeze-drying, due to the small particle sizes involved. However, the quantities of gas required for atmospheric fluidized bed freeze-drying are prohibitively expensive. This has led to a process modification whereby fluidization is performed at sub-atmospheric pressures, which still allows rapid freeze-drying, but using much less gas. This study demonstrates the fluidized bed spray-freeze-drying technique at sub-atmospheric pressures (0.1 bar) using whey protein isolate solution (20% w/w solids) at gas inlet drying temperatures ranging from -10°C to -30°C. The process yields a powder consisting of highly porous particles and shows little loss of solubility for β-lactoglobulin and a-lactalbumin, the principal proteins in the isolate. A wet basis moisture content of 8.1% was achieved after freeze drying at -10 °C for only 1 hour, whilst at 30 °C a longer drying time (100 minutes) produced a wetter product (14% w.b.).