Mechanism Of Quality Changes Of Scallop Adductors During Heat Pump Drying Based On Microstructure Evolution Induced By Mechanical Properties

Scallop adductors contains abundant nutrients and active substances,but they have high moisture content and water activity,a wide variety of endogenous enzymes and high activity,which would lead to accelerated spoilage.Drying is one of the main ways of deep processing of scallop adductors.The application of heat pump drying（HPD）technology in aquatic products is limited by its low energy efficiency and unclear quality change mechanism.During the drying process of aquatic products,the imbalance of internal and external pressure produces shrinkage stress and causes volume shrinkage,which affects drying efficiency and dried product quality.Based on this,this paper focuses on the mechanical viscoelastic characteristics of scallop adductors,discusses the influence of ultrasonic（US）pretreatment on the mechanical characteristics and volume shrinkage of scallop adductors,and illustrates the change mechanism of heat pump drying quality of scallop adductors based on the thermodynamic characteristics and protein denaturation dynamics.The main research contents and conclusions are as follows:1.The effects of hot air drying（HAD）,heat pump drying（HPD）and vacuum freeze-drying（VFD）on drying kinetics,water composition and distribution,physicochemical and microstructural properties of scallop adductors were compared.The results show that the drying rate was HAD>VFD>HPD.Low field nuclear magnetic resonance（LF-NMR）results show that the immobile water is the main component controlling the drying process.Drying results in increased hardness and chewiness and the texture properties of HPD are better than those of VFD and HAD.Total color difference was VFD>HPD>HAD;Crude protein content was VFD>HPD>HAD.The rehydration rate of VFD products was higher than that of HPD and HAD.Compared with the drying process,the relaxation properties of scallop adductors during the rehydration process were opposite.The degree of protein denaturation and degradation was HAD>HPD>VFD.Considering drying efficiency and quality attributes,HPD technology can be the best drying method for scallop adductors.2.The effects of ultrasound（US）pretreatment on volume shrinkage,drying kinetics and effective water diffusion coefficient of the heat pump were investigated experimentally.The results showed that the US pretreatment effectively alleviated the shrinkage behavior of scallop adductors during drying.Compared with the control check（CK）,the shrinkage ratio of scallop adductors in the US treatment group decreased by 2.04%on average,and the porosity increased by 5.88%.The shrinkage behavior of scallop adductors during drying can be described by Quadratic equation.Under the same water content,the drying rate and effective water diffusion coefficient of scallop adductors treated by US were significantly higher than those in CK.By considering shrinkage in drying data processing to correct drying characteristics,it is found that it is necessary to consider the effect of shrinkage on drying rate during drying of scallop adductors.3.The water desorption isotherms and thermodynamic characteristics of scallop adductors at 15,30 and 45℃were studied by static weighing method.The glass transition temperature（T_g）of HPD scallop adductors was determined by differential scanning calorimeter（DSC）,and the influence of US pretreatment on thermodynamic characteristics and T_gof scallop adductors was investigated.The results show that the desorption isotherm of scallop adductors conforms to type III.The net equivalent adsorption heat and differential entropy of scallop adductors decreased with the increase of moisture content of dry base and tended to be stable in the later stage.The desorption of water process of scallop adductors was enthalpy driven.Under the same conditions,US pretreatment significantly reduced the interaction between water and solid substrate,resulting in easier removal of water in the drying process.The T_gof pretreatment scallop adductors in US was higher than that of CK,which means that pretreatment with US can reduce the rate and degree of quality deterioration in the drying process of scallop adductors at the same drying temperature.4.In order to reveal the quality change mechanism of scallop adductors during drying,the effects of US pretreatment,drying temperature and drying time on the water state and mechanical viscoelastic properties of scallop adductors were studied,and the correlation between mechanical properties and water state was revealed.LF-NMR results show that the relaxation component（T₂）of free and immobile water moves to the left with the increase of US pretreatment,drying temperature and drying time（i.e.the decrease of moisture content）,and the creep flexibility increases,but the relaxation modulus decreases.Burger’s model and ternary generalized Maxwell model are the best models to describe creep behavior and stress relaxation.Frequency and temperature scans showed that storage modulus（G’）and loss modulus（G"）decreased with the increase of US pretreatment,drying temperature and moisture content.Pearson correlation analysis and principal component analysis showed that the water state variation of scallop adductors was closely related to the variation of viscoelastic properties.US pretreatment had positive effects on water state and viscoelastic properties of scallop adductors,but drying temperature and drying time were opposite.LF-NMR can accurately and nondestructive feedback the variation of viscoelastic properties of scallop adductors during drying by monitoring the moisture composition and distribution of scallop adductors.5.The kinetic parameters of protein denaturation in scallop adductors were obtained by DSC,and the thermal denaturation of protein in scallop adductors was numerically simulated based on temperature distribution and water state distribution.The results showed that the initial denaturation temperature of myosin was 32.5℃,while the complete denaturation temperature was 55℃,The initial temperature of action denaturation was51℃,but the complete denaturation required 77.5℃.After US pretreatment,the myosin degeneration temperature and action degeneration temperature were increased by 3.75℃.US pretreatment delayed protein denaturation of scallop adductors,inhibited volume shrinkage of scallop adductors,improved microstructure and improved product quality.LF-NMR analysis showed that with the increase of protein denaturation degree,the changes of tightly and loosely bound water were not significant,the peak time of immobile water decreased,and the free water remained in a dynamic stable state.