Coating pretreatment improved drying efficiency and quality attributes of ultrasonic assisted heat pump dried scallop adductors

Abstract: Heat pump drying (HPD) is especially suitable for the drying of aquatic products, owing to the mild conditions and high energy efficiency. However, the low drying temperature can result in a long drying time for the quality degradation in aquatic products processing. This study aims to enhance the drying efficiency of scallop adductors and quality attributes of dried products, particularly for better HPD application. The scallop adductors were firstly pretreated with ultrasonic (US) pulses. Then, three kinds of edible coating were selected, including sodium alginate (SA), low-methoxy pectin (LMP), and gum ghatti (GG). Subsequently, the HPD was carried out for the samples with different coating pretreatment. The pretreatment without coating (i.e., merely pretreated with US) was set as the control group (CK). Finally, an investigation was made to determine the effects of three kinds of edible coating pretreatment on the drying kinetics and quality attributes of scallop adductors during US-assisted HPD. The low-field nuclear magnetic resonance (LF-NMR) was utilized to monitor the drying process of scallop adductors. The univariate linear (UL) and partial least squares regression (PLSR) models were employed to correlate the LF-NMR parameters with the moisture content (MC) and water activity (aw). The effective moisture diffusivity (Deff) of scallop adductors increased with the edible coating pretreatment in the range of 1.32%-8.41%, compared with the CK. The SA and GG pretreatment slowed down the migration rate of immobilized water towards bound water. As such, the flow of free water increased during drying, thus enhancing the drying efficiency of scallop adductors. More importantly, the SA and GG pretreatment increased the hardness, elasticity, cohesion, and chewiness of dried scallop adductors, whereas, decreased the rehydration ratio, shrinkage rate, and total color difference of dried products. There was a decrease in the hardness and total color difference of LMP pretreated samples. In summary, the US-SA was the most suitable pretreatment for the US-assisted HPD of scallop adductors. There was a significant (P<0.05) correlation between the LF-NMR transverse relaxation time (T2) and most quality attributes of dried scallop adductors. The end point of drying was determined in the LF-NMR spectra of samples, indicating the main peak of transverse relaxation time T2 <10 ms and only one peak. The UL model showed that the MC and aw were significantly (P<0.05) positively correlated with the peak area of immobilized water and total peak area, but negatively (P<0.05) correlated with the peak area of bound water. The PLSR model revealed that the regression model with four LF-NMR parameters as the variables accurately predicted the MC and aw of scallop adductors during HPD. This finding can provide a novel non-thermal pretreatment for the drying of scallop adductors, particularly with the enhanced drying efficiency and quality attributes of the dried products. Furthermore, the LF-NMR technology can also provide the theoretical basis and practical reference for the rapid and non-destructive monitoring of scallop adductors during US-assisted HPD.