Effects Of Magnetic Field On Quality And Physiological Metabolism Of Strawberry During Cold Storage

Strawberries are popular among consumers due to their bright color,unique aroma and soft texture,rich juice,sweet and sour taste,and abundant nutrients such as vitamins,polyphenols,and minerals.After picking,strawberries have a strong respiratory metabolism,and because of their soft and thin skin,they are prone to quality deterioration such as dehydration,shrinkage,loss of nutrition and flavor,and microbial attack caused by spoilage and other adverse changes.Magnetic field,as an emerging physical field-assisted preservation technology,has received increasing attention in improving the storage quality of fruits and vegetables due to its non-contact,non-polluting,mild action and biocompatibility characteristics.In this study,different intensities of magnetic field（0 m T,2 m T,4 m T,6 m T,and 8 m T）were applied to strawberry during 4℃ storage.The effects of magnetic field on strawberry apparent quality,the changes of nutrients and flavor substances were investigated.Furthermore,the mechanism of magnetic fields delaying senescence and improving storage quality of strawberry was expounded from the perspectives of respiration and reactive oxygen species metabolism.The main findings of the research are as follows:The effects of magnetic field on the apparent quality of strawberry were investigated by measuring the appearance,surface color,decay rate,microorganism,hardness,microstructure and comprehensive sensory score during 4℃ storage.After 15 days of storage,the brightness（L*）of the control strawberry（0 m T）was decreased by 53.01%,and L*of the strawberry treated with 2 m T,4 m T,6 m T,and 8 m T magnetic fields was decreased slowly（45.40%,33.15%,36.84%,and 32.47%,respectively）.The decay rate of strawberry in the control group was 85.83%,while the magnetic field treatment significantly reduced that of strawberry.The reduction depended on the magnetic field strength,which was 70.83%（2 m T）,60.83%（4 m T）,57.78%（6 m T）and 51.94%（8 m T）,respectively.The total number of bacterial colonies,mold and yeast was also significantly decreased compared with the control group.The firmness and cell integrity of strawberry were also well-maintained under the magnetic field.The sensory scores of strawberries treated with 4 m T,6 m T,and 8 m T magnetic field were significantly higher than those of other groups（0 m T and 2 m T）,but there was no significant difference among the three groups.The effects of magnetic field on the nutrient composition and flavor substances of strawberries were investigated using weight loss rate,water distribution,total soluble solids,titratable acid and p H,sugar-acid ratio,ascorbic acid,and volatile compounds as indicators during 4℃ storage.After 15 days of storage,2 m T,4 m T,6 m T,and 8 m T magnetic field significantly slowed down the weight loss of strawberry fruits during storage,but the differences in weight loss rates between the 4 m T,6 m T,and 8 m T were not significant.The analysis of the low-field NMR results showed that the magnetic field inhibited the conversion of cell wall water and cytoplasmic water to free water during storage.The free water relaxation time(T₂₄)of strawberries stored at 2 m T,4 m T,6 m T,and 8 m T for 15 days was decreased by48.03 ms,137.57 ms,137.57 ms,and 260.11 ms,respectively,compared to the control group（1383.46 ms）.The migration of water from the center to the outside during refrigeration was observed by NMR imaging,but the magnetic field effect limited this water migration.After 15days of storage,the total soluble solids content of control strawberries decreased from 11.53%to 8.77%,while the 2 m T,4 m T,6 m T,and 8 m T magnetic field groups remained at 9.43%,9.70%,9.73%,and 10.03%.The titratable acid content of control strawberries decreased from1.25%to 0.69%and the p H increased from 3.49 to 3.88,while those of the magnetic field treatment maintained by 0.77%～0.82%and 3.57～3.61 at the end of storage.The ascorbic acid content of the magnetic field group was higher than that of control group by 6.90%（2 m T）,12.90%（4 m T）,10.00%（6 m T）,and 10.00%（8 m T）.The magnetic field（4 m T）treatment significantly delayed the beneficial ester compounds content such as methyl butyrate,methyl hexanoate,ethyl hexanoate in strawberries,inhibited the accumulation of volatile compounds such as furaneol and styrene of strawberries during storage,and the effect became more obvious with the extension of storage time,but it was not necessarily linearly correlated with the magnetic field intensity.The mechanism of the effect of magnetic field on the storage quality of strawberry was investigated using respiration rate,energy substances,respiratory metabolism substrates,energy metabolizing enzymes,targeted energy metabolites,reactive oxygen species,DPPH scavenging,rate,malondialdehyde（MDA）and cell membrane permeability as indicators.The results showed that the strawberry treated by magnetic field（4 m T）showed lower respiratory rate than the control strawberries throughout the storage period.After 15 days of storage,the energy substance in the control group such as ATP content decreased from 41.97 mg/g to 19.15mg/g,the ADP content increased from 5.35 mg/g to 9.36 mg/g,and the energy charge level decreased from 0.93 to 0.79.The magnetic field group maintained significantly higher ATP content,lower ADP content and higher energy charge level,which were 26.24 mg/g,8.26 mg/g and 0.83 at the end of storage.The activity of H⁺-ATPase decreased from 9.56 U/g to 3.82 U/g,and that of Ca²⁺-ATPase decreased from 10.54 U/g to 1.79 U/g under conventional cold storage.However,the magnetic field significantly delayed the decrease of the two enzymes’activities,maintaining at 6.36 U/g and 3.41 U/g,respectively.The activities of succinate dehydrogenase and cytochrome oxidase in the magnetic field group were 39.41%and 21.72%higher than those in the control group.The magnetic field treatment reduced the consumption of glucose and citric acid,both of which were 40.64%and 32.71%higher than the control at the end of storage.Meanwhile,the magnetic field inhibited the accumulation of reactive oxygen species including hydrogen peroxide and superoxide anion content in strawberries,which maintained the high DPPH scavenging rate and the content of endogenous antioxidants such as anthocyanins,flavonoids and polyphenols,and effectively alleviated the generation of lipid peroxide MDA and the alteration of cell membrane relative permeability.