| In recent years, The vacuum freeze drying technology (FD) has been applied to the processing of fruits and vegetables, due to vacuum freeze drying can not only fully retain nutrients and active substances in fruits and vegetables,but also can keep the original color,flavor and shape in the greatest degree,so the vacuum freeze drying technology has broad application prospect. But because of high energy consumption, high processing cost of the vacuum freeze drying,so that the drying technology is not very good popularization and application in drying process of fruits and vegetables and agricultural products.In recent years, combined with the actual production of fruit and vegetable freeze-drying process, inconducted a systematic study of low energy consumption of fruits and vegetables freeze drying processing technology and process,in the process of optimization of low energy consumption of the freeze drying process,we explored several fruits and vegetables freeze-dried new technique for detection of moisture content online,using the dielectric parameters of fruits and vegetables in the freeze-drying process to express the moisture content of fruits and vegetables at the same time is one of research contents.But the conventional dielectric characteristic test device is not online measurement of electrical properties of fruit and vegetable in freeze-drying chamber.Therefore, based on the bridge principle,this paper designs the adjustable needle-typle probe electrode dielectric properties measurement system,with apple as the test materials, experiments were made to study the online measurement of the dielectric properties using the system.The main contents include:(1) Adjustable2needle and4needle-typle probe electrode what were innovative designed constitute the fruits and vegetables dielectric property testing device.The adjustable needle-typle probe electrode dielectric properties measurement system is composed of the needle-typle probe electrode and the LCR tester, based on bridge theory.the system can measure the dielectric parameters of fruits and vegetables such as impedance,capacitance and resistance etc.(2) I have studied the effects of needle-typle probe electrode parameters and high voltage pulsed electric field pretreatment on the dielectric properties of apple. Effect of needle-typle probe electrode parameters on the dielectric parameters of apple is significant, with the increase of the probe spacing and the decreases of probe insertion depth, impedance, inductance and resistance of apple increased, capacitance decrease.The dielectric parameters of different needle-typle probe electrode measurement was conducted regression analysis,there is linear correlation between probe Spacing of the2needle-typle probe electrode and dielectric parameters. probe spacing of the4needle-typle probe electrode had reciprocal relationship with dielectric parameters, there are linear correlation between probe insertion depth of2needle and4needle-type probe electrode and dielectric parameters, The regression model is significant, the fitting accuracy is relatively high.With the increase of frequency, the dielectric parameters decrease,there are power function relationship between them. Impedance, inductance and resistance of HPEF pretreated apple is reduce, the capacitance increases,impedance, resistance and inductance of HPEF pretreated apple are linear correlation with the probe spacing, the fitting precision is very high, there is a reciprocal relationship and high correlation between capacitance and probe spacing. Resistance, impedance, capacitance and inductance of HPEF pretreated apple are linear correlation with Probe insertion depth, the coefficient of determination was more than0.8.(3) The apple hot air drying process under different temperature have online monitored by2needle-style probe electrode dielectric property testing device,and experimental research on the apple dehydration, dielectric properties and correlation.In the process of the hot air drying, the drying rate and moisture content decreases.At the start of the hot air drying, change range of impedance, inductance and resistance is very small,the capacitance decreases rapidly,the dielectric parameters have turning point in a moment,impedance,resistance and inductance increases quickly, capacitance almost remain the same.After a period of time, impedance, inductance and resistance changes in the vicinity of a value.The drying temperature is higher, the turning point appeared in a earlier moment.(4) Changes of dielectric properties of apple in the vacuum freeze dried process have online monitored by2needle and4needle-style probe electrode, in order to study the relevance of dehydration and dielectric properties of apple, inductance have experienced the adjustment period, the buffer period, the rising period and the steady period. Resistance only experienced the buffer period and rising period, capacitance experienced the adjustment period, rising period, attenuation period and steady period. In the adjustment period, dielectric properties is disorder and no rules,in the buffer period and stable period, dielectric properties almost remained unchanged, in the rising period, dielectric properties increases rapidly, in the attenuation period, dielectric properties decreased rapidly. the characteristic interval is the rising period and attenuation period,in the characteristic interval, I conducted a polynomial regression of different probe electrode measuring dielectric properties during the hot air drying process, the determination coefficient is greater than0.9, the regression model has high fitting accuracy.(5) The chapter compare dehydration and dielectric properties of apple.during hot air drying and vacuum freeze drying process.resistance, inductance and impedance of Vacuum freeze-dried Apple are more than resistance, inductance and impedance of hot air dried Apple, capacitance of Vacuum freeze-dried Apple is less than capacitance of hot air dried apples.When the moisture content is40%～80%, The chapter respectively conducted the regression analysis of resistance and capacitance of80℃hot air dried apple and vacuum freeze-dried apple.Based on the same moisture content,, the equations can be established,, the result showed that the resistance of hot air dried apple and vacuum freeze dried apple is a power relationship, capacitance is logarithmic relationshipIn this paper, the research content lays the foundation for moisture on-line measurement of vacuum freeze dried fruit and vegetable using dielectric property... |
PDF Full Text Request