Studies On The Vacuum-Microwave Drying Of Scallop

Scallop culture has been increased rapidly in China in recent years for its high nutrition and being delicious taste. Sun drying is the most commonly used method for scallop drying. As the demands for high quality dried scallop are rising and particularly as China's entry to WTO, it is critically important to develop more efficient technique for scallop drying. In this study, microwave vacuum drying(MVD) was firstly introduced for the drying of scallop. Detailed research was conducted on drying theory, quality changes and optimal drying parameters of MVD scallop. Comparisons were made between MVD sun drying(SD) ,hot air drying(AD) and the combined MVD/AD process of scallop.Experiments showed that MVD resulted in substantial saving of processing time, lower of drying temperature, and improved quality of scallop. Three drying periods, namely accelerated drying, constant drying and decelerated drying, were found in MVD scallop. Consequently, temperature changes of MVD scallop were categorized into pre-heating, constant heating, and temperature increasing stages. Moisture transferring process of MVD scallop was governed by three different mechanisms, immigration under moisture gradient, vapor diffusion forced by pressure and temperature differences and direct removal of liquid water . Moisture changing of MVD scallop could be described by Page equation, MR = exp(-k tn),where the drying constant K and n were closely depended on the power of microwave, vacuum pressure and the on/Off ratio.The quality of MVD scallop was influenced by both the drying parameters and the physical properties of scallop. Lower shrinkage and higher rehrydration capacity could be achieved by increasing vacuum pressure and microwave power, but it would shorten the constant heating stage and resulted in worse appearance quality at extremely high microwave power level. Increased on/Off ratio and salt content of scallop had adverse effect to the quality of MVD scallop.The results showed that on/Off ratio had the greater effect on the drying rate and the quality of scallop than microwave power and vacuum pressure. As the drying rate increased considerably with increased on/Off ratio, the shrinkage and appearance quality of dried scallop would adversely changed. Considering the drying rate and quality of MVD scallop, the operation parameters of vacuum pressure, power of microwave and on/Off ratio should be kept at >0.088 MPa, 3.0 -3.6 w/g and 0.25-0.33, respectively.It was found that the drying rate of MVD scallop was 9 -10 times and 150 times higher than that of AD and SD, respectively. The shrinkage and redydration rate were similar to that of SD. Furthermore, MVD scallop had higher crush resistibility and good appearance quality.The drying time of scallop at a combination of MVD/AD was reduced 50% or more than that at AD. Scallop dried by MVD/AD also had lower shrinkage than that of MVD. The suggested optimal combination of MVD/AD should be MV5+AD120+MV and MV5+AD180+MV.