Optimization Design On Continuous Ohmic Heating Process And Device For Soymilk

Heat treatment has a very important role in the food processing, which not only affect the quality of the final products, but also related to the energy consumption and production cost. Ohmic heating, also known as the joule heating, the basic principle is food materials as a part of the electric circuit, when the electric current passes through the food material, due to the inherent impedance characteristics of the food materials electricity directly within the food is converted into heat energy, so as to achieve the purpose of heating food. The advantages of Ohmic heating technology are uniform heating, without the heating surface, rapid, easy to control, electric energy conversion rate is 95%, environmental friendly, and can achieve solid food and liquid-solid mixing foods containing particles sterilization at high temperature. This technology is considered as one of the most development potential of heat treatment technology of food processing. Ohmic heating technique research began relatively early in abroad, and continuous Ohmic heating apparatus are being used successfully in USA, UK, and Japan. At present, the study of continuous Ohmic heating devices of our country is in the laboratory explore stage. The effect of Ohmic heating technology on food quality also needs further research.This thesis mainly contains two aspects:The first part is to study the effects of Ohmic heating and conventional heating on antinutritional factors (trypsin inhibitor and urease) of soymilk; the second part is to design a continuous Ohmic heating device for liquid food. The main results are as follows:(1) To verify the effects of Ohmic heating and conventional heating on the activation of urease and trypsin inhibitor of soymilk, guarantee the thermal history of the sample treated with the ohmic heating process and the conventional heating process in order to eliminate the influence of thermal effects. The results showed that the activity of both enzymes treated by ohmically heated was significantly lower than that of the sample conventionally heated at the same temperature and holding time.(2) To verify the effect of the ohmic heating on the urease and trypsin inhibitor activity in the soymilk, the ohmic heating methods with the different electrical field conditions were employed. The results show that the urease activity of the sample at the end of heating decreased significantly with the increase of the frequency of the power with the same heating process; the urease activity of the sample at the end of heating increased with the increase of the voltage with the same frequency. However, the activities of trypsin inhibitor in soymilk increased with the increase of power frequency with the same heating process; the activities of trypsin inhibitor in soymilk decreases with the increase of the voltage with the same frequency. It is mainly because urease enzyme is a kind of nickel-dependent metalloenzyme.(3) The inactivation kinetics of the urease and trypsin inhibitor in the soymilk could be described with a biphasic model during holding time at a target temperature during both heat methods. The results show that all of the coefficients of determination R2 were higher than 0.990, which proved that the biphasic model fit the urease inactivation kinetics curve well. Thus, it was concluded that the urease in the soymilk would contain isoenzymes; one is the thermolabile fraction, the other the thermostable fraction. The presence of electric field enhanced the heat inactivation of the thermolabile fraction of urease, and had no significant effect on the thermostable fraction of urease with the analyse of kinetic parameters. As R2 were all higher than 0.994 the biphasic model was proved to be suitable to describe the kinetics of trypsin inhibitor inactivation treated by both heating methods. However, the inactivation rate constant of the thermolabile fraction and thermostable fraction of trypsin inhibitor during ohmic heating is higher than conventional heating. In heat preservation phase, the value of activation energy Ea. for KTI and BBI during conventional heating are about 155.62 kJ/mol and 168.42 kJ/mol, respectively, and the value of activation energy Ea for KTI and BBI during Ohmic heating are about 153.33 kJ/mol and 168.42 kJ/mol, respectively. It can be concluded that trypsin inhibitor in soymilk with electric field conditions is easier for deactivation.(4) Continuous Ohmic heating device for liquid food is designed. In order to avoid pollution to food materials, the annular titanium electrode has used in the device. The heat exchanger has used to preheat the soymilk and improve the energy conversion rate.