Effect of pretreatment on water migration and volatile components of heat pump dried tilapia fillets

In this study, ultrasonic assisted osmotic pretreatment and pulsed vacuum assisted osmotic pretreatment were applied to investigate their effects on water migration and volatile components of heat pump dried Tilapia fillets. To achieve that, some effective parameters including sample drying rate, water diffusivity, microstructure, water morphology, water distribution, and volatile components were compared and analyzed with some advanced measurement devices. The water diffusivity, water distribution characteristics, and composition of volatile components were obtained after different pretreatment methods. As the drying process progresses, the sample moisture content decreases. Meanwhile, the high-degree-of-freedom water migrates to the low-degree-of-freedom water and the water-solid bond strength increases. Subsequently, the effective water diffusion coefficients of control group (without pretreatment samples), ultrasonic assisted osmosis pretreatment group and pulsed vacuum assisted osmosis pretreatment group were measured as 4.304 × 10−7m2/s, 6.109 × 10−7m2/s, and 5.003 × 10−7m2/s, respectively. In addition, the control group, ultrasonic assisted osmosis group, and pulse vacuum assisted osmosis group contained 52, 59, and 41 volatile compounds, respectively. Compared to the results from the control group, the water diffusion coefficients of ultrasonic osmotic pretreatment and pulse vacuum osmotic pretreatment increased by 41.94% and 16.24%, respectively. From the point of view of increasing drying rate, the ultrasonic penetration pretreatment provided better improvement, which was exactly consistent with the results of microstructure. On the other hand, the ultrasonic assisted osmotic pretreatment group had more types of volatile compounds, which could stimulate more flavored substances to be released. Evidently, the samples with ultrasonic assisted osmotic pretreatment showed less drying time and more aromatic substances whereas the samples from the pulsed vacuum assisted osmotic pretreatment had better protein protection feature. Although the dried samples had higher ratio of bound water and better storage stability after these two pretreatment methods, from the point of view of increasing drying rate and stimulating flavor substances, the ultrasonic assisted osmosis pretreatment method had more advantages. The research outcomes can contribute to optimize better pretreatment methods for the process of heat pump dried Tilapia fillets.