EXPLORING AIRBORNE SOUND IN A NONVACUUM FREEZE‐DRYING PROCESS

Abstract

SUMMARY– A study was made to determine if sound energy could be used to enhance freeze‐drying rates of foods and to analyze the kinetics in such a system. A prototype pilot freeze‐dryer with airborne sound was designed and built to freeze‐dry liquid foods, e.g., tea and coffee extracts. The sound source was a stem‐jet whistle producing 10.8–12.2 kcps with a maximum sound intensity level of 149 db. Desiccated compressed air at 70° to 90°F served as the driving force of the whistle as well as the carrier of the sublimed moisture. Experimental results showed this to be a feasible process. Sample temperature, inlet air pressure, the freeze‐dryer tube size, and the sound source, all had significant effects on the freeze‐drying rates. Freeze‐drying rates were 11–100% higher in the stem‐jet whistle runs than in the dummy whistle runs; 7% of this improvement was attributed to the thermal effect on the air resulting from friction and adiabatic compression. The remaining increase was due to the sound pressure energy, the increase in heat and mass transfer coefficients, and the “reduced‐pressure” effect in the sweeping air stream during the rarefaction cycle of the sonic vibration. The flaking action of the sound waves on near‐dried tea and coffee was considered a processing advantage because of the ease in product recovery and because a new surface was continuously being exposed for sublimation.