Theoretical And Experimental Study On Low Temperature Preservation Of Postharvest Fruits And Vegetables Assisted By Pulsed Magnetic Field

China is not only a big producer of fruits and vegetables(F&V),but also a big consumer of F&V.However,the imperfect cold chain system and lack of advanced preservation technology,result in serious postharvest loss of F&V and economic losses of 100 billion yuan every year.As the improvement of people’s living standards and the attention to health and food safety,it is of great significance to develop a new green physical preservation technology to further reduce the postharvest loss of F&V and improve preservation quality.Based on the multidisciplinary theory,the application of pulsed magnetic field(PMF)in the field of F&V preservation was researched in this paper.The main points of the paper are as follows:(1)Based on the Helmholtz coil,a low temperature pretreatment system assisted by PMF for postharvest F&V was constructed.The distribution and uniformity of the magnetic field were analyzed.In a large radial direction,the magnetic flux density was relatively uniform,and there was basically no attenuation,which was helpful to improve the accuracy of the experimental results.(2)Based on the biological effect of magnetic field and cucumber as experimental material,the low temperature PMF pretreatment process of postharvest F&V was studied and optimized by the central composite design method.The response surface model and comprehensive evaluation method were used to optimize the low temperature PMF pretreatment process of postharvest F&V using sensory indexes such as weight loss rate,color difference,decay rate and hardness change rate as response values.The result shows that under the condition of 0 ℃ cold water treatment for 40 min,the optimal processing parameters of PMF treatment were the frequency of 25 Hz,the duty ratio of 50 % and the magnetic flux density of 2.83 m T.(3)The mechanism of cold water pretreatment assisted by PMF to inhibit the chilling injury of postharvest F&V was studied under different magnetic flux density.The result shows that PMF treatment could inhibit the respiration of F&V during storage.It could improve the activity of antioxidant enzyme system,reduce the damage of cell membrane caused by the accumulation of harmful substances,thus reduce the occurrence of chilling injury and improve the storage quality of F&V.The best effect could be achieved when the magnetic flux density of PMF was 4 m T.(4)Based on the theory of cryodynamics,the effects of PMF on the freezing properties of distilled water,cucumber tissue fluid and apple tissues were studied.The results showed that PMF treatment weakened the hydrogen bond between water molecules,reduced the volume of water cluster,and thus reduced the initial nucleation temperature and freezing point temperature.The lowest average nucleation temperatures were-11.20 ℃ and-7.41 ℃ for distilled water and cucumber tissue fluid,respectively,when the magnetic flux density of PMF was 4m T.When the magnetic flux density of PMF was 2 m T,the lowest average nucleation temperature of apple tissues was-2.99 ℃.However,the effects of PMF treatment on the freezing time of distilled water,cucumber tissue liquid and apple tissues were slightly different,and PMF treatment increased the latent heat of phase transition of apple tissues.