| After sterilized by high temperature, the quality of some thermosensitive food was influenced seriously for nutrition losing and smell of burnt food creating. So, people have focused on cold sterilization technique, such as pulsed magnetic field sterilization. It was reported the pulsed magnetic field was good for sterilization without nutrition loss. The influence of magnetic intensity and pulsed number of magnetic field on the sterilization of microbes, the activity of enzymes and nutrient components of strawberry juice, were investigated, at the same time, the mechanism of sterilization and inactivation of enzyme were also discussed. The main contents and results of experiments are as following.(1) The effect of high-intensity pulsed magnetic field on the sterilization of strawberry juice. The influence of some pulsed magnetic filed factors, such as magnetic intensity and pulsed number, on the sterilization of TPC (total plate count), molds, yeasts and colibacillus, was investigated systemically. It was confirmed that the effect was improved with the rising of magnetic intensity and pulsed number. When magnetic intensity up to over 16.2 tesla and pulsed number up to over 6 under room temperature(28±1℃), the number of total plate count decreased from 1.8×104cfu/ml to 80cfu/ml, below to standard of food safety, 100cfu/ml; When magnetic intensity up to over 17.3 tesla and pulsed number up to over 12, molds and yeasts were killed completely from 4.0×102 cfu/ml,2.3×102 cfu/ml separately; When magnetic intensity was up to over 13.4 tesla and pulsed number up to over 9, the number of colibacillus decreased to 6cfu/100ml, which have satisfied requirements of food business. When magnetic intensity and pulsed number increase continuously, the number of colibacillus will be killed completely. Molds and yeasts have strong resistance to the effect of pulsed magnetic field, which mean that higher magnetic field and more number of pulses would be used to kill them. The effect of sterilization to strawberry juice by pulsed magnetic field is as follow: colibacillus> total plate count> yeasts> molds.(2) The effect of high intensity pulsed magnetic field to the enzyme of strawberry juice. The influence of high intensive magnetic field to polyphenol oxidase and peroxidase was investigated. It was found that the high pulsed magnetic field had a weak inactivation effect on the polyphenol oxidase and strong inactivation effect to peroxidase. Under the best inactivation condition, the intensity of magnetic field up to 16.2 tesla and and pulsed number up to 12, the activation of strawberry polyphenol oxidase decreased to 40%, and the activation of strawberry peroxidase decreased to 8% with magnetic field up to over 17.3 tesla and pulsed number up to over 9.(3) The effect of high intensive pulsed magnetic field to the quality of strawberry juice. The nutrition of strawberry juice treated by pulsed magnetic field was determined. It was proved that content of total sugar, reducing sugar, solubility solid matter and pH were constant.The loss ratio of Vc is 28% when the intensity of magnetic field up to 19.0 tesla and and pulsed number up to 9. Compared to heating sterilization, the loss of Vc is much less.(4) The mechanism of effect of high intensive pulsed magnetic field to microbe and enzyme. The activation of circular dichroism (CD) spectra and fluorescence chromatography of horse radish peroxidase were determined. It was found that there was a obvious change to its three level of structure but hardly change to its two level structure of horse radish peroxidase under the inactivation effect of high intensity pulsed magnetic field. The fluorescence of aspergillus niger was determined. The result showed that after treated by different magnetic field ( pulsed number 10), the fluorescence of aspergillus niger was becoming stronger with the magnetic induction increasing, that is to say, the aspergillus niger was killed more. Also, the mechanism of effect of pulsed magnetic field on the microbe was discussed from the factors, such as faradic current, Lorentz force, the destroy of cell membrane, and the mechanism of the pulsed magnetic field to the enzyme by changing the configuration of protein. |
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