| Tilapia is an ideal animal protein source in aquatic food,rich in nutritional value.The production of tilapia in China has been increased in the last year.At present,freezing treatment is main processing of tilapia and which exists many problems,such as single processing and low added value,so it can’t meet the needs of social development.As a traditional processing technology,drying has wide range of applications.A new drying method,osmotic dehydration vacuum-microwave drying（OD-VMD）,vacuum-microwave at low temperature combined with dehydration of osmotic treatment was developed in recent years.It is expected to provide a theoretical basis for the development of diversification and new technologies of tilapia processing.Tilapia fillets were processed with osmotic pretreatment and vacuum-microwave drying,and changes of moisture and qualities during fillet drying were investigated in this study.The OD-VMD kinetic model was established based on the law of moisture variation.Based on the above research,the differences quality of tilapia fillet were discussed of drying methods including hot-air.vacuum microwave,osmosis-microwave and OD-VMD.The main contents and results are as follows:1.The effects of different osmotic compositions（0%～20%sucrose + 20%salt）,temperature（10-40 ℃）,time（1～4 h）and slice size（300～1100 mm2）on the moisture and quality of tilapia fillet OD-VMD were studied.The result showed that the moisture of dried fillet declined with the decrease of slice size and increase of sucrose concentration in osmotic solution.However,the quality of fish fillet is not ideal（such as the high level of shrinkage rate and low level of rehydration rate）.Extending osmotic time within 2 h and rising temperature below 30℃ can accelerated the dehydration.The drying effect of fish fillet was improved as the reduce of moisture content and water activity,small change of color and other good qualities became.High temperature and long time of osmotic treatment caused deterioration for tissue structure of fish fillet,which was not conducive to dehydration.The osmotic conditions of tilapia fillets were determined by orthogonal test.The osmotic composition was 20%salt + 15%sucrose,treatment time was 2 h,osmotic temperature was 30℃ and slice size was 500 mm2.2.The varies of tilapia fillet’s qualities index after osmosis under different microwave gap ratio（1～5）,microwave time（4～12 min）,power density（10～30 W/g）and vacuum degree（0～0.08 MPa）were investigated.It was found that the shrinkage rate,rehydration rate and chromatic aberration of tilapia fillet were all rose with the increase of microwave time and power density.Promoting the vacuum degree was beneficial to improve quality of product,which rehydration rate and chromatic aberration were raised,but the shrinkage rate was decreased.The rehydration rate and chromatic aberration were rose first and then fall with the increasing of microwave gap ratio,however,the shrinkage rate increased gradually during the process.Therefore,it was possible to reduce the deterioration of the fish fillet quality by increasing the microwave gap ratio appropriately within a certain range.3.The OD-VMD characteristics and drying kinetics of fish fillet were studied.The effect of microwave gap ratio,power density and vacuum degree on vacuum-microwave drying process of fish fillets were obtained.The results showed that the moisture of fish fillet was reduced gradually with the increase of microwave time,power density and vacuum degree.At the same time,the drying rate was speed up.The moisture index was decreased first and then raised with the increase of microwave gap ratio,and the lowest value at ratio of 2.Thus,improving the microwave gap ratio appropriately was conductive to accelerate the drying process.In addition,the whole drying process was divided into two stages with different conditions,including the accelerated drying and falling rate drying.The accelerated drying was short and the main process was falling rate drying.A kinetics model was established based on moisture change during tilapia fillet drying.It was found that the drying kinetics met Midilli model and it suitable for describing OD-VMD process of tilapia fillet.The model expression was:MR=(6.353×10-2P-1.6097×10-Q+2.7241V+9.979×10-1)exp[-(1.3237×10-1P+1.3928×10-2Q+2.5160V-4.9675×10-1)t-0.08482P-0.005373Q-1.7101V+1.3909]+(2.07×10-4P+6.0×10-6Q+3.7844x 10-2V-4.670×10-3)t4.The changes of shrinkage rate,rehydration rate,nutrient composition,texture and microstructure of tilapia fillets under hot-air,vacuum microwave,osmosis-microwave and OD-VMD were compared and the energy consumption was evaluated,respectively.The results showed that the color and nutrient composition were changed a lot,compacted structure,hard texture and poor quality of tilapia fillet after hot air drying,and this method consumed most energy.The fish fillets after vacuum-microwave drying had the lowest shrinkage rate,hardness and nutrient loss,the highest rehydration rate and better quality compared with hot air drying.However,the color changed a lot with low lightness,and high energy consumed of vacuum-microwave drying.The osmosis-microwave drying and OD-VMD tilapia fillets had good quality and structure compared with hot air drying.The OD-VMD was better than osmosis-microwave drying in some aspects,such as microstructure,nutrient composition,hardness and rehydration rate of fish fillets.In addition,the energy consumption of OD-VMD was lower than the vacuum-microwave drying. |
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