Study On Process Optimization And Mechanism Of Freezing-Heat Pump Combined Drying Tilapia Fillets

Tilapia has high water content,and tilapia is apt to be highly spoilage when it is affected by enzymes and microorganisms.Drying is an important processing method to prolong the storage life of tilapia.In recent years,the combined drying has become a research hotspot,because it can achieve complementary advantages.This research is based on the idea of combined drying and the combination of physical and chemical pretreatment process to design a new process idea,and a series of related experimental studies were carried out after obtaining optimized process parameters.The tilapia was used as the experimental material in this study.The effects of different drying methods on the quality of tilapia fillets were studied,The optimum pretreatment conditions of ultrasonic assisted polyglucose permeation heat pump drying and ultrasonic assisted polyglucose permeation vacuum freeze-heat pump combined drying were obtained by orthogonal experiments.The effects of optimum pretreatment process of UAPOHPD,optimum pretreatment process of UAPOVFHPCD,heat pump drying（HPD）and vacuum freeze-drying（VFD）on the quality of tilapia fillets were compared.The low-field NMR studies the change in cellular water and the mechanism of drying rate promotion in tilapia fillet tissue during the drying process of the optimum pretreatment in ultrasonic assisted polyglucose permeation heat pump drying and ultrasonic assisted polyglucose permeation vacuum freeze-heat pump combined drying.The effects of different drying methods on volatile flavor components and the law of flavor variation of tilapia fillets were compared by electronic nose combined with headspace solid phase microextraction combined with gas chromatography-mass spectrometry.Finally,the storage model and the design of storage conditions of the desorption-adsorption isotherms and thermodynamic characteristics of dried fish fillets of vacuum freeze-heat pump combined drying were studied.The main research contents and research results including:（1）The optimum pretreatment conditions of ultrasonic assisted polyglucose permeation heat pump drying and ultrasonic assisted polyglucose permeation vacuum freeze-heat pump combined drying were obtained by orthogonal experiments.The different ultrasonic power（300,350,400,450 and 500 w）,ultrasonic time（55,65,75,85 and 95 min）,and 30 g/L sodium chloride solution combined with different concentrations of polyglucose（20,40,60,80 and 100 g/L）were used as single factors.The rehydration rate,whiteness,hardness and Ca²⁺-ATPase activity were used as quality indicators.The optimum pretreatment conditions of ultrasonic assisted polyglucose permeation heat pump drying were ultrasonic power 400 w,ultrasonic time 65 min,polydextrose concentration 60 g/L,and the effects of ultrasonic time was the largest.The optimum conditions of ultrasound-assisted polyglucose permeation vacuum freeze-heat pump combined drying were ultrasonic power 450 w,ultrasonic time 65 min,polydextrose concentration 80 g/L by the single factor and orthogonal experiments.Then,the quality of tilapia fillets in ultrasonic assisted polyglucose permeation heat pump drying,ultrasonic assisted polyglucose permeation vacuum freeze-heat pump combined drying and heat pump drying and vacuum freeze drying were compared.The results show that the quality of dried tilapia fillets in ultrasonic assisted polydextrose osmotic vacuum freeze heat pump combined drying is close to that of vacuum freeze-drying products,in addition,its drying time is shorter than that of vacuum freeze drying,therefore,it should be applied in the drying processing of similar aquatic products.（2）The low-field NMR studies the change in cellular water in tilapia fillet tissue during different drying process of ultrasonic assisted polyglucose permeation heat pump drying and ultrasonic assisted polyglucose permeation vacuum freeze-heat pump combined drying.The results show that the cell membrane of tilapia fillets tissue is ruptured during different drying processes,the free water is the highest when cell membranes started to rupture.In addition,it also observed that the cell membrane ruptures at different stages of drying.The mechanism of increasing speed in the design of drying process is solved,which can provide a good theoretical basis for the improvement design of drying process of similar aquatic products.（3）The volatile flavor components of tilapia fillet by ultrasound-assisted polydextrose osmotic heat pump drying（UAPOHPD）,heat pump drying（HPD）,ultrasound-assisted polydextrose osmotic vacuum freeze-heat pump combined drying（UAPOVFHPCD）,vacuum freeze drying（VFD）,respectively,were analyzed by electronic nose combined with headspace solid phase microextraction combined with gas chromatography-mass spectrometry.The main components of flavor compounds of different dried products were analysed by the PCA and the key flavor components of different dried products were determined by the relative odor activity value.The results show that the electronic nose could better distinguish different dried tilapia products.The key flavor components of fresh tilapia fillets are 1-octen-3-ol,1-dodecanol and decanal.The key flavor components of UAPOHPD products are nonanal,1-octen-3-ol,decanal,3-hydroxy-2-butanone,and 1-dodecanol.The key flavor components of HPD products are nonanal,1-dodecanol,2,6-dimethylpyrazine,isobutyl acetate,and 3-hydroxy-2-butanone;The key flavor components of UAPOVFHPCD products are nonanal,heptanal,1-dodecanol,isovaleric aldehyde;The key flavor components of VFD products are isovaleric aldehyde,nonanal,decanal,1-dodecanol,3-hydroxy-2-butanone.The flavor of HPD products is the best,following by UAPOHPD products,VFD products,and UAPOVFHPCD products,The results showed that the formation of PCA is correlated with the length of drying time,which can provide reference for the design of processing technology for tilapia fillets and similar aquatic products.（4）The thestatic method was adopted to obtain vacuum freeze-heat pump combined dried tilapia fillets’desorption-adsorption isothermal data at 25℃,35℃,and 45℃,with the water activity between 0.1116 and 0.8434.The desorption-adsorption isotherms’experimental results were fitted by 8 common mathematical models.The optimal model used to describe the desorption-sorption isotherms of dried tilapia fillets was determined by comparing the models’coefficient of determination（R²）and root mean square error.According to the desorption-adsorption isotherm data of dried tilapia fillets at different temperatures,the thermodynamic properties of dried tilapia fillets were obtained.The results showed that:the desorption-sorption isotherm of dried tilapia fillets belonged to type III,the Peleg model is the best model to describe the desorption-adsorption isotherm of tilapia fillets of heat pump combined drying,at the same time,the absolute safe moisture content of dried tilapia fillets was 16.30%,13.68%,and 13.64%and the relative safe moisture content was 22.89%,19.21%,and19.77%respectively at 25℃,35℃,and 45℃.The net adsorption heat decreased and the differential entropy of dried tilapia fillets decreased exponentially with the increase of the equilibrium water content.The desorption and adsorption processes were driven by enthalpy that were non-spontaneous.Through the study of the vacuum freeze-heat pump combined dried tilapia fillets water desorption-adsorption isotherm which can provide the theoretical basis for the process optimization and storage stability setting of the vacuum freeze-heat pump combined dried tilapia fillets.It provides the basis for the design of storage technology of dried tilapia fillets and similar aquatic products.