Macrobrachium rosenbergii is commonly recognized as the giant freshwater prawn or Malaysian prawn. M. rosenbergii has also gained much popularity with the extremely important commercial value, due to its rich nutrition and delicious flavor. Nevertheless, M. rosenbergii is highly susceptible to spoilage at room temperature, due to the high moisture content rich in various endogenous proteases and plentiful microorganisms. Among them, drying can be expected to serve as an important way for the high-value processing and utilization of aquatic products. Vacuum freeze drying (VFD) has been commonly used for high-quality dried aquatic products. However, the application of VFD has been limited in the field of aquatic products drying, including the prolonged processing duration and high energy demand. Microwave vacuum drying (MVD) has promising potential in the food drying field, due to its high heat transfer rate and controllability. This study aimed to fully utilize the merits of the high heat and mass transfer efficiency of MVD. The high quality of dried M. rosenbergii products was achieved by improving the high drying efficiency and low energy consumption of a single VFD. Macrobrachium rosenbergii meat was used as the experimental raw material. A systematic investigation was made to explore the effects of different conversion point moisture content (MC) on a wet basis (viz., 70%, 60%, 50%, 40%, and 30%) of M. rosenbergii meat on the drying rate and specific energy consumption, microstructure properties, and quality. The dehydration of M. rosenbergii meat with VFD alone was also performed for comparison. It was regarded as the control check (CK). The results demonstrated that the specific energy consumption of the VFD-MVD combined drying groups was reduced significantly by 35.20%-79.11% (P<0.05), compared with the CK. The average drying rate of the CK was 0.28 kg/kg/h. But there was a significant increase with the ranging from 0.43 kg/kg/h to 1.39 kg/kg/h, in the combined drying groups. Compared with the rest VFD-MVD in the drying treatment groups, the structure of the dried M. rosenbergii meat was much looser and porous, when the conversion point MC was 40% (wet basis). The retention rate of the astaxanthin content was the highest measurement of 2.40 μg/g. There were the lower oxidation degrees of protein and lipid, where the free hydrosulfuryl and thiobarbituric acid values were (TBARS) measured to be 0.91 μmol/g and 0.53 mg/kg (TBARS is expressed on terms of malondialdehyde content), respectively. Moreover, the combined VFD-MVD drying treatment group with the conversion point MC of 40% also produced the dried products with the superior total color difference (ΔE 4.43), porosity (10.91%), shrinkage rate (ranging from 0.52% to1.89%), rehydration ratio (5.30), and texture properties. In conclusion, the combined VFD-MVD drying was achieved in the dried products with the quality attributes resembling those of the products fabricated by VFD alone, when the conversion point MC of M. rosenbergii meat was 40%. The drying efficiency was significantly enhanced for the lowest energy consumption of the drying system. This finding can provide a theoretical basis for the "energy-saving, high efficiency and superior-quality" drying technology of M. rosenbergii meat.

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. Lowfield 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.

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.