Theoretical And Experimental Study On Heat Treatments Of Postharvest Fruits And Vegetables

Fruits and vegetables are rich in high nutritional value and low in fat,which are of great significance to strengthen functions of the human body.As a large agricultural country,the yields of fruits and vegetables production and consumption have occupied the first place in the world,and the backward of fresh keeping technology and method for fruits and vegetables is the important reason for postharvest losses.Besides,with the increasing concern over the presence of chemical residues on plant,to explore the physical environmental-friendly method for fruits and vegetables has become an important task.In this paper,based on multi-subject approaches,the heat treatment equipment and process for postharvest fruits and vegetables were theoretical and experimental researched.The main contents of the dissertation are as following:(1)In order to improve energy efficiency,a heat treatment system was designed and built based on thermodynamic inverse cycle,which was auto-controlled by Siemens programmable logic controller S7-200 and Wincc system of configurable software.The operation features of heat pump compressor under fixed frequency?PID frequency conversion control or with feedforward PID control were studied,such as coefficient of performance ? temperature of heat treatment medium ? energy consumption,etc.(2)Cucumbers(Fire phoenix/Jinyou 12)were selected as heat treatment materials,and the heat treatment technique was optimized through response surface methodology(RSM).Heat treatment time and medium temperature were picked as independent variables,and experiment scheme was central composite designed(CCD).The quadratic models for quality parameters of cucumber were obtained.Finally,the optimum temperature–time combinations were obtained for different varieties of cucumber heat treatment process.(3)An unsteady heat transfer model of heat treatment process was proposed based on the structural features of cylindrical fruits and vegetables,and the precision of the model were verify by test.The temperature change of different type cucumber(Fire phoenix/Netherlands cucumber)and various type of heat treatment(Continuous/Intermittent)were simulated by Fluent.The internal relationship of fresh keeping effect and heat treasfer peocess were analyzed based on the optimum heat treatment process results of response surface test,and the heat treasfer characteristic difference between continuous and intermittent heat treatment were also studied.(4)The conventional heat treatment process was changed for different heat transfer parameters.Cucumbers(Fire phoenix/Jinyou 12)were selected as materials to continuous and intermittent heat treatment.Selective measurement the quality and physiological and biochemical indexes during storage,such as weight loss,firmness,color,antioxidant enzyme activity(External/Internal),respiration intensity,etc.The effect of heat transfer characteristics on fruit and vegetable physiological and biochemical indexes was analyzed,and obtain the primary reason for heat treatment in fresh keeping effect.(5)Cucumbers(Jinyou 12)were selected as materials to thermal damage and chilling injury test under continuous and intermittent heat treatment.and the quality and physiological and biochemical indexes were measured during the storage above or below the critical temperature of chilling injury,respectively.The internal relationships between the heat transfer characteristics and heat treatment produce irreversible thermal damage and inhibit chilling injury were deeply analyzes.