Enhancing the heat pump drying rate and quality attributes of scallop adductors using ethanol coupled with other pre-treatments

Low drying rate and high energy consumption in the later stage of heat pump drying (HPD) have restricted its application in aquatic products drying. Non-thermal pretreatment has exhibited great potential for the drying of perishable food with enhanced drying rate and quality performances. This study aims to investigate the effects of ethanol (E) alone and in combination with other pretreatments, viz., vacuum (VC+E), ultrasonic (US+E), and ultrasonic-assisted vacuum (USVC+E) on the drying kinetics and quality of dried scallop adductors (SA). Moreover, the SA without pretreatment (control group, CK) was also performed for comparison. The water state and distribution in the SA during HPD were also explored using low-field nuclear magnetic resonance (LF-NMR). The results showed that the drying behavior of SA was typical in a falling rate stage, where in the internal moisture diffusion dominated the enrire drying. The Weibull model perfectly described the HPD process. Ethanol alone and combined with US and/or vacuum pretreatment enhanced the effective moisture diffusivity (D_eff) and drying rate, compared with the CK. LF-NMR analysis revealed that the immobilized water was dominated in the SA. The transverse relaxation time of all the components was shifted left, whereas the proportion of the immobilized water decreased, but the tightly and loosely bound water and free water increased, as the drying progressed. The conversion of the immobilized water into the free one was more favored with the respective pretreatment, compared with the CK. There was a decrease in the yellowness, total color difference (ΔE), shrinkage rate (S_R), hardness, springiness, and chewiness; but an increase in the lightness, redness, and rehydration ratio (R_R) of the dried SA with ethanol alone and in combination with other pretreatment in comparison to the CK. Among them, the SA with US+E and USVC+E achieved the best color parameters (ΔE (7.40±0.22)and (6.99±0.16)for US+E and USVC+E, respectively), the lowest S_R (35.97% ± 1.29% and 34.43% ± 1.24% for US+E and USVC+E, respectively) and hardness ((25.20±1.08) N and (26.68±0.61) N for US+E and USVC+E, respectively); whereas the US+E performed the lowest springiness (0.55±0.01) and chewiness (7.27±0.30) N, but with the highest R_R (1.753±0.022). Compared with the CK, the US+E significantly decreased ΔE by 58.24%, SR by 32.75%, hardness by 23.17%, springiness by 15.38%, and chewiness by 38.91%, whereas, there was an increase in the R_R by 9.975% after 180-min rehydration. Overall, the US+E was recommended as the suitable non-thermal and green pretreatment of SA with a high D_R and quality performances.