Preservation Mechanism And Experimental Research On Combined Hot Water And Alternating Magnetic Field Treatment For Postharvest Fruits And Vegetables

People can ingest most of nutrients from fresh fruits and vegetables.With the development of the economy,fruits and vegetables have become an indispensable food in people's lives.In order to adapt to the growing market demand,China's fruit and vegetable post-harvest storage industry is developing rapidly,but most of them still use mechanical low-temperature storage and chemical technology.Therefore,the development of an efficient,environmentally friendly and residue-free physical preservation method has become an important topic in the field of post-harvest preservation of fruits and vegetables.In order to improve the preservation quality of fruits and vegetables during storage and to extend the shelf life,this paper studies the combined treatment of heat shock and alternating magnetic field after fruit and vegetable harvesting,and provides new ideas for the innovation of postharvest preservation of fruits and vegetables.The main contents of this paper are as follows:From the perspective of biomagnetic effect,this paper explored the influence of alternating magnetic field on thermal conductivity and specific thermal capacity of fruits and vegetables in a theoretical analysis,and the influence of the number of dimensionless criteria such as Reynolds Number,Prandtl Number and Nusselt Number during the heat shock treatment.Based on the knowledge of heat transfer,the heat transfer process of fruits and vegetables had been simplified in this paper,and the spherical fruit and vegetable heat transfer model and the columnar fruit and vegetable layering model had been initially established.The heating rate of "Fire Phoenix" cucumber in the process of combined magnetic field and heat shock had been studied.The results showed that the alternating magnetic field could affect the heat transfer performance of cucumber in heat shock treatment.With the increase of the magnetic field frequency,the heating rate was not simply expressed as monotonously increasing or decreasing.There was an optimal value,which could effectively accelerate the rate of cucumber heating within a certain range.In this paper,the "Fire Phoenix" cucumber was used as a test material to study the changes of sensory index,biochemical index and microstructure during storage by combined alternating magnetic field and heat shock treatment at different frequencies(50Hz,100 Hz,150Hz and 200Hz).The results showed that combined treatment could reduce weight loss rate,chromatic aberration and decay,increase antioxidant enzyme activity,and reduce malondialdehyde(MDA)content,thereby reducing relative conductivity,inhibiting cell membrane structure changes,reducing the water loss and preventing cell atrophy.In addition,the alternating magnetic field with a frequency of 50 Hz,which combined with heat shock treatment,had the best performance on the fresh-keeping effect of cucumber.In this paper,the "Jiu Feng" grape was used as the test material to study the changes of sensory and biochemical indexes during the storage by combined alternating magnetic field and heat shock treatment at different intensity(15Gs,25 Gs and 35Gs).The results showed that the combined treatment could maintain a stable pH environment,inhibit the activity of polyphenol oxidase,thereby reducing the color difference of the grapes during storage,reducing the malondialdehyde content and cell membrane permeability.In addition,the alternating magnetic field with a strength of 35 Gs,which combined with heat shock treatment,had the best performance on the fresh-keeping effect of grape.