Effect Of High Hydrostatic Pressure On Quality And Microorganism Of Fresh Cut Lettuce And Its Mechanism

The control of quality and safety of fresh cut lettuce is directly related to the health of consumers,as one of new nonthermal technologies, high pressure has many advantages in the processing of food,whether high pressure can be used in the fresh cut lettuce industry or not depends on the effect of highpressure on the quality and microorganisms of fresh cut lettuce. This study aimed to investigate theeffect of high pressure on the sensory, nutrient, protective enzymes, antioxidant power, microorganismand cell ultrastructure of fresh cut iceberg lettuce by biochemical, UPLC/MS/MS, electron microscopyand microorganism analysis. The findings from this study are expected to direct the utilization of highpressure in the fresh cut lettuce industry as follows.For the quality part, firstly, high pressure was found to improve the sensory quality of fresh cutlettuce, especially for50MPa with5min, which was tasted more sweet following with the originalcrisp and aroma of lettuce, and the data showed that the glucose and fructose content of lettuceincreased after high pressure processing. However, the sweetness can not be tasted with increasing ofpressure, and the disagreeable bitter was tasted obviously, which was attributed to the taste of tongue tobitter substance from lettuce or the possibly increasing of bitters substances induced by high pressure.Secondly, with the increasing of pressure, the firmness showed a decreasing trend, and the toughnesscan be tasted, while, it was not related to the synthesization of lignin, because UPLC-MS-MS findingsshowed that the lignin synthetic substances like p-coumaric acid and ferulic acid were not existed orincreasing or decreasing in experimental samples of lettuce whether high pressure processing or not, andthe ultrastructure image by Scanning Electron Microscop(ySEM)also showed the shapes of lettuce cellschanged from bulge to concave and flat, so the crispness was lost and the taste of toughness presented.Thirdly, the browning and blank taste of fresh cut lettuce were induced when pressure reached100MPawith20min, while200MPa made the lettuce disagreeable bitter taste, high toughness, serious wateryand browning. The ultrastructure image by SEM and Transmitting Electron Microscopy (TEM) showedthat the substances in the cells was squeezed out and the cell membranes was broken when the pressurewas over100MPa with20min, which can induced relative biochemical reactions and consequently thequality deteriorated. Fourthly, the content of chlorogenic acid and caffeic acid and total phenols were allnot affected by high pressure treatments used in this study (p>0.05). High pressure did not induce thebrowning of lettuce (p>0.05). The phenylalanine ammonialase (PAL) activity increased when lettucewas treated by high pressure at100MPa (p<0.05), while the total phenols content did not increasesignificantly (p>0.05). During shelf life of fresh cut lettuce processed by high pressure, PAL activityincreased significantly (p<0.05), but the total phenols content showed the decreasing trend, andperoxidase (POD) activity involved in phenols polymerization was observed to increase significantly(p<0.05), suggesting that maybe the synthetic rate of phenols was lower than the utilization rate ofphenols, which resulted in the increasing of browning. Polyphenol oxidase (PPO) activity appeared tohave no relationship with browning during the shelf life of fresh cut lettuce, maybe the browning was mainly induced by POD passway. Fifthly, during shelf life, vacuum packaging reduced the loss of totalphenols and inhibited the browning. Vacuum package increased PAL activity significantly (p<0.05),and kept the PPO activity, but inhibited the increasing of POD activity significantly(p<0.05),whichfurther supported that the PPO activity is not key factor during the browning of fresh cut lettuce。For the enzymes of lettuce, the findings showed that the protective antioxidant enzymes were notaffected by high pressure processing (p>0.05), and the susceptibility of enzymes in lettuce to highpressure(200MPa with20min at ambient temperature)were PPO, ascorbate oxidase, glutathioneperoxidase, PAL, catalase, ascorbic peroxidase and POD in turn. Although the protective enzymesactivities were kept well under high pressure over100MPa with20min, the ultrastructure imagesshowed that the membrane of cells was broken and substances were out of cells, which will induce themetabolism disorder of cells and resulted in the deterioration of quality actually, suggesting the highpressure treatment over100MPa with20min can not be used in fresh cut lettuce industry.For the antioxidant capacity of fresh cut lettuce, high pressure had no effect on DPPH scavengingactivity (p>0.05), while it can improve the ferric reducing/antioxidant power (p<0.05). During shelflife, the DPPH scavenging activity was found to be increased (p<0.05) whether high pressuretreatments or not, while the ferric reducing/antioxidant power decreased with the decreasing of totalphenols (p<0.05).For the microorganisms of lettuce, high pressure can reduced aerobic plate counts, coliform andfungi counts remarkably significantly (p<0.01). Temperature (5-25℃) had no synergism with highpressure on reduction of microorganisms of lettuce. Coliform was more sensitive to high pressurecompared to fungi, during shelf life at4℃, coliform counts of fresh cut lettuce treated by high pressuredecreased rapidly compared to the control, which suggested high pressure would help to controlfood-borne pathogen like E.Coli O157:H7.At the cell level, lettuce cell was susceptible to the high pressure,200MPa can change the cellshapes obviously, and result in the collapse of cell membrane and cellular organelle membrane andbreak of cell wall by the observation of SEM and TEM.For the application of high pressure in the fresh cut lettuce processing, high pressure faced thechallenges against assuring the quality and control of microorganisms of fresh cut lettucesimultaneously. The better high pressure treatment like50MPa with5min at5℃can produce moredelicious quality, keep high total antioxidant power and realize the significant reduction of coliformwithin3days' shelf life at4℃. Like other processing technologies, high pressure processing also needother factors to realize the synergism on assuring the quality and safety of fresh cut lettuce, like pressureresistant varieties, and conducting HACCP plan to reduce the original microbes counts and prevent thepollution from food borne pathogen etc.. Under above premise, high pressure is a new technology withimproving the quality and assuring the safety of fresh cut products.