Experimental Study On Continuous Ohmic Heating System For Liquid Food

Ohmic heating technology has been widely used and some new ohmic heating apparatus are being developed in USA, Japan, UK and other countries. But in our country, it is only in the starting phase. Up to now, there are no ohmic heating devices introduced to our country. In order to reduce the gap in the field, it is very urgent to develop an ohmic heating device of our own. The electrical conductivity of foods is one of the main factors which determines the production of heat in the foods, so the electricity conductivity of foods is very important to develop and research the ohmic heating equipment. In order to utilize ohmic heating technology, it is necessary to research the changes of the electrical conductivity of foods with temperature.Based on theories of Physics, Electrics, Principle of Foods Engineering, Fluid Mechanics, Physical Properties of Foods, Advanced Heat Transfer, Materials and Measurement Technology, there are two parts addressed in this thesis. One is to design and manufacture an ohmic heating system for liquid food. The other is the research of the electrical conductivity of tap water and soybean milk. The main results are as follows:1. A static ohmic heating system for liquid food was designed and manufactured and tap water and soybean milk was ohmic heated with the system. The results showed that the system could heat tap water and soybean milk uniformly and rapidly.2. The temperature distribution of tap water and soybean milk in the process of static ohmic heating was researched. The results showed that the temperature of tap water was uniform in the whole heater and the temperature of soybean milk was uniform in the same plane of the ohmic heating device, but not uniform in different planes. Except the manufacturing errors, the main reason was that the solids of soybean milk subsided in the effect of gravity. And then the density and viscidity of soybean milk in the lower plane increased and the temperature decreased. So a churning device should be used when using this kind of device. Using the static ohmic heating system for liquid food, the electrical conductivity of tap water and soybean milk with temperature was researched. The results showed that the changes of electrical conductivity of tap water and soybean milk with temperature were linear.3. A continuous ohmic heating system for liquid food was designed andmanufactured. Using this system, the tap water and the soybean milk were heated. The results showed that this system could heat tap water and soybean milk uniformly and rapidly. The heating process was stable and the condition of the system was well. Besides, using the data collector, the voltage, current and temperature in the ohmic heating process were measured. The data collecting process was stable.4. With the continuous ohmic heating system for liquid food, the temperature distribution of the tap water and the soybean milk in the process of continuous ohmic heating was researched. The results showed that the temperature distribution of the tap water was not uniform in the heater. As far as the soybean milk was concerned, the temperature was not uniform in the same plane, but it was uniform in different planes. The changes of electrical conductivity of the tap water and the soybean milk with temperature were also researched. The results showed that the electrical conductivity of the tap water and the soybean milk in the condition of flow was similar with that in the static state. The relationship between the electrical conductivity and temperature was linear. Because of the manufacturing error and the fouling surfaces of the electrodes, the electrical conductivity of the soybean milk in the continuous ohmic heating setup was lower than that in static state with the same solids content of the soybean milk at the same temperature point.5.When testing and verifying the three loads, the central current was not zero, but its numerical value was smaller. If eliminating the system error, three loads would be fundamental balance and the circuit model would be basically correct.Some proposals of further study of the optimization design of the continuous ohmic heating device were mentioned as well in the thesis.