Study On The Quality And Mechanism Of Ultrasound/Coating Combined With Modified Atmosphere Treatment On Fresh-cut Lettuce And Cucumber During Cold Storage

With the change of people's consumption concepts and the acceleration of the pace of life,convenient,fresh,and nutritious fresh-cut fruit and vegetable products are gradually being loved by consumers.However,fresh fruits and vegetables are prone to cell tissue browning,nutrient loss,texture softening,water loss and microbial infection after fresh-cut processing,which accelerates the quality deterioration of fresh-cut fruits and vegetables and shortens shelf life of the products.Therefore,it is necessary to develop efficient and safe preservation methods for maintaining the quality of fresh-cut fruits and vegetables and extending their shelf life.In this paper,fresh-cut lettuce and cucumber were taken as the research objects,and the effects of ultrasound and/or carbon dots/chitosan coating combined with modified atmosphere on the physiological characteristics,quality and microorganisms of fresh-cut vegetables during cold storage are deeply studied,which provides theoretical basis for the preservation of fresh-cut vegetables,and has guiding significance for the application of ultrasound and carbon dots/chitosan coating treatment in the preservation of fresh-cut vegetables.In order to reveal the effect of ultrasound treatment on the preservation of fresh-cut vegetables,the effect and mechanism of ultrasound combined with ordinary modified atmosphere on the quality and shelf life of fresh-cut lettuce and cucumber during cold storage was studied.The results showed that the ultrasonic combined with modified atmosphere treatment during cold storage reduced the weight loss of fresh-cut lettuce and cucumber,inhibited the decrease of ascorbic acid and color change of fresh-cut lettuce and cucumber,reduced the chlorophyll degradation of fresh-cut lettuce,and inhibited the increase of polyphenol oxidase?PPO?and peroxidases?POD?activities fresh-cut lettuce and the increase of malondialdehyde content of fresh-cut cucumber.At the same time,the water mobility of fresh-cut lettuce and cucumber were reduced,the structural integrity of fresh-cut cucumber cell was maintained.In addition,the growth of microorganisms of fresh-cut lettuce and cucumber during cold storage is inhibited.Compared with ultrasound treatment for 5 and 15 min treatments,ultrasound treatment for 10 min combined with modified atmosphere treatment has better quality on fresh-cut lettuce and cucumber.And the shelf life of fresh-cut lettuce and cucumber were all extended to 12 days.In addition,ultrasound treatment for 10 min combined with modified atmosphere treatment inhibited production rate of superoxide anion(O₂^·—),lipoxygenase?LOX?,catalase?CAT?and ascorbate peroxidase?APX?activities,improved the antioxidant capacity of DPPH and ABTS in fresh-cut lettuce and cucumber.The bacteriostatic effect and shelf life extension of fresh-cut lettuce by ultrasonic treatment alone are limited,the effect and mechanism of ultrasound and?-polylysine combined with modified atmosphere on the quality and shelf life of fresh-cut lettuce during cold storage were studied.The results showed that as the concentration of?-polylysine?0-0.5 g/L?increased,the inhibited effect of microbial inhibition increased.When the concentration of?-polylysine increased from 0.4 g/L to 0.5 g/L,?-Polylysine had no significant difference in the total number of colonies,the number of molds and yeasts during storage of fresh-cut lettuce.Considering the use cost and antibacterial effect comprehensively,0.4 g/L was selected as the optimum concentration.Ultrasound,?-polylysine,especially the combined treatment,can significantly slow down the increase in weight loss,respiration rate and color difference,delay the degradation of total phenols,ascorbic acid and chlorophyll,inhibit the increase of PPO and POD activities,and reduce the water mobility of fresh-cut lettuce during cold storage.In addition,the growth of microorganisms in fresh-cut lettuce during cold storage was inhibited.Ultrasound and?-polylysine combined with modified atmosphere treatment improved the quality of fresh-cut lettuce during cold storage,and the shelf life of fresh-cut lettuce was extended to 15 days.At the same time,ultrasound and?-polylysine combined with modified atmosphere can delay the membrane lipid peroxidation and maintain the antioxidant capacity of fresh-cut lettuce during cold storage.Further control the microbial growth of fresh cut vegetables during cold storage,thereby improving the quality of fresh cut vegetables,ultrasound and carbon dots/chitosan coating combined with modified atmosphere on the preservation effect and mechanism of fresh-cut lettuce and cucumber were studied.The results showed that the particle size of the prepared carbon dots is about 0.54-0.83 nm,which is a typical carbon dots size range.The fourier transform infrared spectra and X-ray diffraction patterns of carbon dots showed that the surface of carbon dots is rich in functional groups?such as-OH,-COOH,etc.?,thus exhibiting good hydrophilicity and water solubility.The antibacterial activity of carbon dots/chitosan coating increased with the increase of the mass fraction of carbon dots?0-4.5%?.Compared with other coating treatments,the 4.5%carbon dots/chitosan coating had better microbial inhibition effect,and was more conducive to the preservation of fresh-cut lettuce and cucumber.Ultrasound,carbon dots/chitosan coating treatment,especially the combined treatment,can significantly inhibit the increase in weight loss,respiration rate,PPO and POD activities,delay the decrease of ascorbic acid content of fresh-cut lettuce and cucumber during cold storage.At the same time,it slowed down the decrease of total soluble solids and hardness in fresh-cut cucumber,reduced the chlorophyll degradation of fresh-cut lettuce,inhibited the increase of malondialdehyde content of fresh-cut cucumber,preserved the smell and taste,and limited moisture mobility of fresh-cut lettuce and cucumber during cold storage.In addition,ultrasound and carbon dots/chitosan coating combined with modified atmosphere significantly inhibited the total number of colonies,molds and yeasts of fresh-cut lettuce and cucumber during cold storage,resulting in the reduction of the spoilage.And the shelf life of fresh-cut lettuce and cucumber was extended to 18 and 15 days,respectively.Through the mechanism analysis of membrane lipid peroxidation,protective enzyme activity and antioxidant ability of fresh cut lettuce and cucumber during cold storage,compared with ultrasound or carbon dots/chitosan coating,ultrasound and carbon dots/chitosan coating combined with modified atmosphere maintained the high antioxidant capacity of fresh-cut lettuce and cucumber.Based on the problems that anoxic state occur in ordinary modified atmosphere packaging during storage of fresh-cut vegetables and the gas barrier properties of commercial polymer films limit the applicability of modified atmosphere packaging,ultrasound and carbon dots/chitosan coating combined with laser-induced microporous modified atmosphere on the preservation effect and mechanism of fresh-cut cucumber was studied.The results showed that ultrasound and carbon dots/chitosan coating combined with 100?m microporous modified atmosphere packaging can provide suitable O₂ and CO₂ gas concentrations.Ultrasound and carbon dots/chitosan coating combined with 100?m microporous modified atmosphere packaging suppressed the increase in weight loss and MDA content,slowed the decrease of hardness and ascorbic acid content,and retained the main flavor substances such as alcohols,aldehydes and ketones,limited the water mobility of fresh-cut cucumber during cold storage.Compared with other treatments,ultrasound and carbon dots/chitosan coating combined with 4micropores of modified atmosphere packaging had a better preservation effect on fresh-cut cucumber during cold storage.In addition,ultrasound and carbon dots/chitosan coating combined with 4 micropores of modified atmosphere packaging was helpful to delay the membrane lipid peroxidation and reduce free radical accumulation of fresh-cut cucumber during cold storage.