Abstract: The effect of ultrasonic (US) pretreatment time (0, 10, 20, 30 min) on the drying kinetics and quality attributes of heat pump-dried scallop adductors were investigated to enhance the drying efficiency and improve the product quality. Low-field nuclear magnetic resonance (LF-NMR) spectroscopy was used to explore the water state and distribution of scallop adductors during the drying process. The results showed that the drying process took place in the falling rate period and was controlled by internal moisture diffusion. The drying rate and the effective moisture diffusion coefficient were increased by US pretreatment. Compared with the untreated control group, US pretreatment reduced the total color difference and improved the toughness of dried scallop adductors. The fitting results of the drying mathematical model showed that Midilli et al. model is the most suitable model for describing the drying process. LF-NMR spectra revealed that immobilized water determined the change in the water content of scallop adductors during the drying process. The migration rate of free and immobilized water was increased with increasing US time. The relaxation time of immobilized water T22 had a strong correlation with the moisture content. We concluded that US pretreatment increases the drying efficiency and affects the color, shrinkage rate, rehydrate rate and texture properties of dried scallop adductors. LF-NMR spectroscopy can be employed to predict the change in the moisture content of scallop adductors during the drying process. This study provides a theoretical basis and practical reference for the heat pump drying of aquatic products, especially scallop adductors.

Key words: scallop adductors; ultrasonic; heat pump drying; mathematical modeling; low-field nuclear magnetic resonance; water status and distribution