| Pulsed electric field(PEF)technology is an emerging low-temperature food sterilization technology and is one of the technologies that is expected to replace traditional thermal sterilization methods.PEF processing technology treats liquid food by using high voltage pulses with high electric field strength,a certain pulse width and pulse frequency.A high-intensity electric field can inactivate microorganisms in food,and the temperature rise after the electric field treatment is low,with the natural flavor and nutrients of the food are retained with high quality.As a necessity for the daily life of people around the world,milk is rich in nutrients needed by the human body.However,the traditional thermal sterilization method has many adverse effects on the quality and nutrition of row milk while sterilizing it.On the one hand,after the high temperature,the color of row milk becomes darker and it will produce cooking flavor and loss of some vitamins.On the other hand,the row milk protein coagulated after high temperature treatment,which easily leads to scale accumulation inside the equipment,reduces the service life of the equipment,reduces efficiency,and has a secondary impact on product quality.Therefore,if the row milk is sterilized by PEF technology,it is expected to solve the above problems and better retain the nutritional components and original flavor in the row milk.In recent years,although domestic and foreign researches on PEF sterilization technology have been made a lot of progress in various aspects.In some aspects,there are still no clear conclusions and a mature theoretical system,such as the sterilization mechanism of PEF,the large-scale pulse electric field sterilization system,the optimal pulse electric field sterilization conditions,the specific impact of PEF treatment on the physical and chemical properties of specific liquid foods and so on.PEF technology has not reached industrial standards yet.This urgently requires scholars from various countries to further explore and improve PEF technology,to provide theoretical and practical support for unresolved problems,and to accelerate their industrialization process.Regarding the issue above,In this thesis,high-voltage pulsed electric field processing system is set up by studying a lot of relevant literature.After verifying the sterilization ability of the system,the sterilization effect of the system on milk under different operating parameters and the influence of some of its nutrients were explored.The main tasks are as follows:First,the high-pressure pulse sterilization system was designed,and the pulse waveform simulation was performed on Pspice software.According to the designed system,the overall circuit topology of the system was constructed,and important units were designed and configured,including the design and production of spark gap switch,the design and production of processing chamber,and the circuit construction of the entire system.Then,a high-pressure pulse sterilization device that can generate exponentially decaying waves was established.Secondly,the influence and law of electric field intensity,pulse number and treatment on sterilization effect were explored.The results confirmed that both the electric field strength and the number of pulses are positively related to the sterilization efficiency,and the temperature rise of the liquid and the increase in conductivity are positively related to the electric field strength and the number of pulses after PEF treatment.With an electric field strength of 40 k V/cm and 1000 pulses,it can reach a sterilization rate of 94.41% by using a static processing room,with a temperature rise of only 3.4℃,a conductivity increase of 15.9μs/cm,and energy consumption of only 1.29 k J.Finally,using raw milk as an experimental material,electric field strength and processing time as two factors,comparing the change of the colony number,the physical and chemical properties of raw milk after PEF treatment..The PEF was found to have a good sterilization effect on raw milk,and its sterilization efficiency showed a positive correlation with the number of pulses and electric field strength.After the treatment of PEF,the content of free amino acids in raw milk increased significantly,the protein content decreased slightly,the electrical conductivity also increased slightly,and the temperature rise was low.And with the increase of pulse number and electric field intensity,such changes became more obvious,while the p H value was hardly affected.Under the PEF treatment of 35 k V / cm and 2000 pulses,the sterilization efficiency of raw milk could reach 90.98%,the amino acid increase was 24.56mg/L,the protein reduction was 48.80mg/L,the conductivity increases was 0.05ms/cm,and the temperature rise was 5.9℃,the energy consumption was only 6.36 k J.The energy efficiency of the PEF system is analyzed,and the energy efficiency of the PEF system under each electrical parameter is higher than 90%,and the input energy density has a certain linear relationship with the sterilization efficiency of the raw milk and its physical and chemical properties.Compared with the physical and chemical indexes of raw milk after pasteurization,the changes of various indexes of PEF were lower than those of pasteurization.It is confirmed that the high-pressure pulse sterilization system does have low energy consumption for cold sterilization of row milk. |
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