Effect Of Ultrasound Combined With Calcium Salt On Quality Of Fresh-cut Iceberg Lettuce And Its Mechanism

Iceberg lettuce is rich in nutrients and high in water content,so it is widely used in the field of fresh-cut fruits and vegetables.However,after processing,fresh-cut lettuce will lose nutrients,brown tissues and soften,and is extremely vulnerable to microbial infection,which will lead to a rapid decline in flavor and shortened shelf life.Therefore,the development of feasible fresh-keeping processing technology is of great significance to maintaining the nutritional quality of fresh-cut lettuce.In this paper,the effects of ultrasonic combined with calcium ascorbate treatment on quality microorganism physiology,biochemistry and flavor of fresh-cut iceberg lettuce were studied,which provided basis for the application of this combined treatment in fresh-cut lettuce industry.The effects of calcium chloride,calcium ascorbate and calcium lactate on the quality of fresh-cut iceberg lettuce during storage were studied in order to screen the calcium salt varieties with the best freshness effect at the same concentration.Results showed that the three kinds of calcium salt treatment can effectively inhibit the increase in weight loss rate of fresh-cut iceberg lettuce during storage,delay the degradation of chlorophyll,vitamin C(VC),total soluble solids(TSS)and total phenols(TP),maintain the hardness value and color value,and reduce The activity of polyphenol oxidase(PPO)and peroxidase(POD)is reduced,while the growth of microorganisms is inhibited,and its sensory quality is effectively maintained.But under the condition of the same concentration(1%),the fresh-keeping effect of calcium ascorbate is the most significant(P<0.05),so the variety of calcium ascorbate is selected for follow-up experiments.In order to ensure that ultrasound combined with calcium ascorbate can have the best bactericidal effect on fresh-cut iceberg lettuce,single factor test was used to determine the ultrasonic power treatment time Three levels of calcium salt concentration and response surface design were used to optimize the process conditions for the sterilization effect of ultrasonic combined calcium salt on fresh cut west lettuce,and the process parameters were optimized according to the feasibility of actual operation.Finally,the optimal conditions for the combined treatment were obtained: ultrasonic power of 180 W,calcium salt concentration of 2.5%,and treatment time of 10 min,the total colony of fresh cut western lettuce could be reduced to 3.086 log CFU/g by using this conditionThen,the effects of ultrasonic combined with calcium ascorbate treatment on physiological and biochemical antioxidant capacity and flavor of fresh-cut iceberg lettuce during storage were investigated.The results showed that combined treatment could significantly reduce the weight loss rate,electrical conductivity and malondialdehyde(MDA)of fresh-cut iceberg lettuce,inhibit the degradation rate of chlorophyll,VC,and TSS content,and delay the decrease in hardness and color value.It has a good bactericidal effect and can effectively maintain the sensory quality during storage(P<0.05).By increasing the total phenol content,inhibiting the activity of PPO and POD,enhancing the activities of catalase(CAT)and superoxide dismutase(SOD),improving the scavenging ability of DPPH free radical,FRAP iron reduction antioxidant ability and ABTS cation free radical scavenging ability,thus enhancing the antioxidant ability of fresh-cut iceberg lettuce.In the whole storage process,the increase of sour,bitter and astringency of fresh-cut iceberg lettuce was inhibited,the decrease of sweet and umami taste was delayed,and there was no adverse effect on other flavors.The comprehensive score of flavor data was ranked by principal component analysis.The results were as follows: single calcium ascorbate treatment group > combined treatment group >single ultrasonic treatment group > control group.Liquid chromatography-tandem mass spectrometry(LC-MS/MS)was used to qualitatively and quantitatively analyze the metabolites of fresh-cut iceberg lettuce treated with ultrasound combined with calcium ascorbate at the initial stage of storage.The results showed that ultrasound combined with calcium ascorbate treatment could effectively maintain the cell membrane function,inhibit the browning reaction,improve the antioxidant capacity,enhance the stress resistance,and improve the quality of fresh-cut iceberg lettuce by changing the metabolism of sugar,organic acid,amino acid and lipid.