Effects of ultrasonic pre-treatment on the porosity, shrinkage behaviors, and heat pump drying kinetics of scallop adductors considering shrinkage correction

Shrinkage has a negative impact on drying efficiency and quality. However, insufficient information about the effect of shrinkage on drying kinetics hampers the comprehension of the underlying mechanism. This study aims to investigate the effects of ultrasonic (US) pre-treatment and drying conditions on volume ratio (VR), apparent density, and porosity, as well as drying kinetics with and without shrinkage correction of scallop adductors during heat pump drying. The VR and apparent density decreased, while porosity increased with higher drying temperatures and longer drying times. US pre-treatment resulted in decreased apparent density, but enhanced VR and porosity, demonstrating the effective role of US in alleviating shrinkage and promoting pore formation during drying. Ignoring shrinkage phenomenon caused a weak underestimation in drying efficiency but a significant overestimation in the effective moisture diffusivity coefficient. Therefore, shrinkage effect must be considered to properly comprehend drying behaviors. The mechanism of shrinkage was closely related to the glass transition theory. The critical moisture ratio at which the samples underwent a glass transition was increased by 15 % with US pre-treatment in comparison to the control. US-pretreated samples could achieve a glassy state more quickly than the control under the same drying conditions, leading to reduced volumetric shrinkage.