Appropriate conversion point enhancing drying rate and quality attributes of Macrobrachium rosenbergii meat during vacuum freezing-microwave vacuum combined drying

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.