Design and evaluation of a resonant sono-plate combined with infrared radiation to accelerate food drying at near-freezing temperatures

Abstract

This study designed and developed an ultrasonically vibrating sono-plate operated in conjunction with infrared radiation to accelerate the drying of solid foods at near-freezing temperatures. The prototype utilized resonance to generate ultrasonic standing waves to transmit acoustic energy from an aluminium plate to the food sample. The injected ultrasonic energy accelerates drying while maintaining near-freezing temperatures (−5 to 5 °C) to better preserve food quality. The prototype was designed by modelling the transmission of sound through the system. The efficacy of the system was evaluated through drying apple slices with different ultrasound (US) power levels (0, 50, 100 W), infrared radiation (IR) power (0, 100, 200 W), and drying temperatures (−5, 0, 5 °C) with nitrogen as the drying medium to reduce oxidative degradation. The application of US and IR reduced the drying time relative to the control by 31.6 % and 47.5 %, respectively. The combined application of US and IR resulted in a 54.7 % reduction in drying time. Moreover, the effect on the physico-chemical properties of dried apples was evaluated. Near-freezing temperature treatments with US, IR, and their combination did not significantly affect (p < 0.05) colour, shrinkage, antioxidant activity (AA), total polyphenol content (TPC), and textural properties of rehydrated slices relative to the control. Near-freezing US-IR drying was compared to hot air drying (40 °C) (HAD) and vacuum freeze drying (VFD). The colour of US and IR-assisted dried apple slices was preserved better than HAD and VFD. The study demonstrated that near-freezing drying can be accelerated with the assistance of US and IR without affecting quality relative to the control.

Industrial relevance of the research

Industries seek drying technologies that reduce temperature damage to dried foods while reducing drying times. In this study, US combined with IR reduced drying time by half (54.7 %) while preserving the quality attributes of dried apples due to the near-freezing temperatures. There were no significant (p < 0.05) differences in the measured quality attributes compared to drying without US and IR at the same temperature. Hence, this technology offers a viable industrial alternative to accelerate drying at low temperatures. Furthermore, the designed ultrasonic system achieved a high electrical efficiency of 93.5 %. This indicates that most of the electrical power used in the US was converted into ultrasonic vibrations. The better rehydration behaviour achieved at near-freezing temperatures can be applied to instant and ready-to-eat food formulations, targeting premium dried products, functional foods, and high-value ingredients.