| The excitation current or rotating permanent magnet is induced to generate alternating flux in a closed magnetic circuit. According to its structural characteristics, the transformer can form a closed loop circuit for electro-conductive liquid food and produce an induced electric field, thus achieving the measuring and processing of food materials. Compared with existing electric field technology(that is, high-voltage pulsed electric field, medium-voltage electric field and Ohmic electric field), this electric field processing technology can avoid the use of metal electrode, adverse chemical reaction, electrode corrosion, heavy metal ion leakage and sample contamination through the conversion of electrical energy and magnetic energy. Therefore, excitation coil and O-core magnetic circuit are combined to design various systems, and the systems are applied to food engineering technology, to explore innovative green processing means and non-thermal technology.Firstly, an induction-based system was designed and built for the measurement of electrical parameters based on the magnetic circuit structure of the transformer. The system which takes O-core silicone steel as the magnetic circuit operates at a frequency of 50 to 400 Hz and has an adjustable coil turn ratio(primary coil to secondary coil). At room temperature, the terminal voltage of liquid food was measured to verify the existence of induced electric field, investigate changes in electric parameters induced(terminal voltage and ?-value), and characterize traditional physiochemical indexes of food, such as the soluble solid content, acidic value, peroxide value and salt content. Research results show that the terminal voltage of all tested samples increases with the increase of excitation voltage, and the coil turn ratio has a positive impact on the boost of terminal voltage. Such changes accord with the operation principle of the transformer. An increase of soluble solid content in watermelon juice and salt content in salted albumen can cause a drop in the terminal voltage of the system, while an increase in the acidic value and peroxide value of olive oil can result in a boost of terminal voltage. According to Ohm's law, induced electric parameters and these physicochemical indexes showed a significantly linear correlation(a?0.01), and the correlation coefficient R2>0.8. The terminal voltage model was used to predict the verified samples. Their standard error of prediction(SEP) was 0.893 Brix%, 0.057 mg/g, 0.26 mmol/kg and 0.93%, respectively, while their bias was respectively-0.238 Brix%, 0.036 mg/g, 0.023 mmol/kg, 0.17%. Electric parameters obtained by using alternating flux to excite liquid food as the secondary coil can be used as a potential technique for efficient measurement of the physiochemical indexes.Secondly, calcium ion intensification, anti-browning, salting and probiotics enrichment of different fresh-cut fruits and vegetables were implemented, under the condition that induced electric field is coupled with static magnetic field or alternating magnetic field. It was found that solute content increases with the improvement of excitation voltage and decreases with the enhancement of voltage frequency. A higher conductivity of the Ca2+ solution with the same ion concentration(200 mg/100 g) produces a better effect of impregnation. That is because the system impedance is alleviated, thus resulting in an increase in the load efficiency. In comparison to the control group, browning index, POD activity, and PPO activity of fresh-cut lettuces increased after treatment with alternating induced electric field in the anti-browning reagents solutions. 14 days after treated with 100 V, 200 V and 300 V at 50% duty ratio, the fresh-cut lettuces experienced a 20.6%, 34.4%, and 38.5% decline in the browning index, a 11.5%, 18.5% and 18.8% drop in the POD activity, and a 5.1%, 10.3% and 11.6% decrease in the PPO activity, respectively. The anti-browning effect of the impulse voltage with a 10% duty ratio was the best. The aggregation of ions in the surface space of the sample tissue subjected to an induced electric field and static magnetic field was observed. Along with an increase in the saline concentration of solution, the medium impedance declined, while the conversion efficiency of magnetic energy to electrical energy and the salting efficiency of the samples increased. A response surface analysis was made to build a regression model to estimate the salt content of vegetables. According to research findings, saline concentration, processing time and porosity are 3 factors affecting salt penetration of vegetables in saline processing. Processing time has the largest influence on salt penetration, followed by saline concentration and porosity. Experimental results have verified that this model can be used to estimate the salt content of vegetables with a porosity of 5.4~8.2%. In the case of probiotic enrichment, an induced alternating electric field and magnetic field were used to promote the penetration of lactobacillus rhamnosus to fresh-cut apples and strawberry samples. A higher excitation voltage and magnetic intensity were conducive to the diffusion and penetration of probiotics into sample tissues. When the system voltage was 3 V/cm and magnetic flux density was 1,300 Gs, the effect of the diffusion and penetration was the best, and the amount of probiotic in fresh-cut apples and strawberries increased to 9.23×108 CFU/g and1.22×108 CFU/g, respectively. Impregnation solutions with lower pH levels were conducive to the accumulation of probiotics in the samples, owing to the charge-carrying amount of probiotics. Since the strawberry has a lower porosity, the amount of probiotics of strawberry is smaller than apple in impregnation solution. When the environmental temperature in the impregnation solution is lower than 60oC, the amount of live probiotics showed a downward trend.Thirdly, the garlic polysaccharide and soluble calcium in fish bones were extracted by using the intermittent induced electric field. The extraction yield was improved with the increase of excitation voltage, while an increase of the electric field frequency has a negative effect on the extraction efficiency. A low pH value can ensure an efficient extraction of polysaccharide, because a decline of sample solution impedance is more conducive to the improvement of the energy-using efficiency. Meanwhile, garlic polysaccharides extracted by a high-frequency(20 kHz) induced electric field had good antioxidant activity. Calcium ions as divalent metal ions can be affected by the alternating electric field. To be specific, the alternating electric field has a positive effect on the mass transfer efficiency of calcium ions. According to the results of orthogonal test, the order of processing parameters according to their effects on the fish bone calcium-extracting rate from higher to lower is: excitation voltage > pH > ratio of solid-liquid>electric field frequency. With an excitation voltage of 160 V, electric field frequency of 450 Hz, pH value of 2.5, and solid-liquid ratio of 1:5 g/mL, the fish bone calcium-extracting rate reaches the highest(88.65% ± 4.6%), which is 224.25% higher than the conventional water-bathing extraction. In order to achieve massive production, a multi-series fluidic system was designed based on for electric-field-assisted extraction. The number of system reactors could be adjusted and arranged according to the actual yield. This research also observed a 16-series reaction system, and found that with the increase in the number of reactors, the induced voltage of the system was improved and the input power was also improved. With a 1000 V excitation voltage for each reactor, a voltage frequency of 20 kHz, an extraction temperature of 60oC, extraction time of 30 min, a solid-liquid ratio of 1:20 g/m L, and a pH value of 2.5, the mulberry polysaccharide-extracting yield by the 16 series-parallel system was the highest(8.45 ± 0.5%). The effect of series-parallel systems on the extraction of pectin from apple residues was investigated, and investigation results show that the pectin extraction follows the first-order kinetics equation. Moreover, induced-electric-field-assisted extraction can contributed to pectin release, without lowering the pectin quality. In the series system, the pectin yield increased with the increase of the number of reactor series. Various series-parallel modes significantly affected pectin yield under a fixed level of input power. Such changes are caused by the change of equivalent internal impedance, induced voltage, output power and power factor in the system.Lastly, a fluidic system with the field at 400~700 Hz and O-core silicone steel magnetic circuit was designed and applied. It was found that a low-frequency induced electric field could achieve a high-efficient hydrolysis of lignocellulose-bagasse. However, our previous study suggested that hydrolysis barely occurs at low temperatures(below 20oC) under the action of an induced electric field. The total reducing sugar(TRS) yield is positively affected by the excitation voltage, but negatively affected by the voltage frequency, because the input power and the effective free path of ions in the low-frequency system are improved. The small amount of reactors can be compactly connected to build a reaction system. Moreover, changes in the reactor-connecting mode would cause a change in the equivalent circuit parameters of the system. Series connection can be used as the most efficient reactor-connecting mode to build a system and achieve maximum energy efficiency and output power. The hydrolysis effect of the system accords with the operation principle of the transformer. When the metal ions are added into the reaction liquid, the TRS yield would increase. That is because metal icons can effectively reduce the liquid resistance, thus improving the induced current in the liquid system as well as the load efficiency of the secondary liquid circuit. Therefore, the use of alternating magnetic flux to achieve inter-conversion of magnetic to electric energy can promote the electric-field-assisted acid hydrolysis of lignocelluloses. Meanwhile, large-scale cyclic utilization of agricultural waste can be realized by increasing the number of fluidic reactors and flexibly using various connection modes. |
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