Effcet Of Ultra-High Pressure Treatment On Sterilization, Enzyme Inactivation And Aroma Of Strawberry Juice

Strawberry is thermal sensitivity fruit and is suited to make juice. Heating can sterilize and blunt enzymes, but also can induce a poorer juice quality. The Ultra High Pressure (UHP) is a kind of cold sterilization method. The effect of different UHP treatments on strawberry juice total bacterial count, Pectin methylesterase enzyme (PME) activity, polyphenol oxidase enzyme (PPO) activity, protein secondary structure, aroma and sensory evaluation were investigated. The results were as follows:1. Study on the total bacteria count of UHP treated strawberry juice displayed that the treatment of200MPa for20min could not corresponded to commercial sterile (<100cfu/mL). Higher pressure, resulted in more killed for the total bacteria count. When treated under400MPa, higher thermal synergy UHP, longer treatment time and more times resulted more killed for the total bacteria count. After30d storage under10℃, the total bacteria count in the juice of UHP (≥400MPa) treatments had no change. Treated by UHT were reproduced significantly (was from59cfu/mL to85cfu/mL).2. The PME activity of all treatments were reduced significantly in juice compared with the control (p<0.05). When treated under30℃about20min,200MPa (80.7%) and400MPa (73.0%) had the same effect on inactivation of PME enzyme with UHT treated (p>0.05). Higher pressure (600MPa,52.2%) treatment resulted more inactivated for the PME enzyme.When treated under400MPa, longer treatment time resulted more inactivated for the PME enzyme and treated after60min PME residual activity was63.1%. When treated for two repeated cycles or treated under30～60℃, the activity of PME did not chang obviously.After30d storage under10℃, the PME activity of all treatments were remained the same with0d respectively in juice.3. All the treatments could Significantly reduce the PPO activity compared with the control (p<0.05). When treated under30℃about20min,200MPa and400MPa had the same effect on inactivation of PPO enzyme (p>0.05). The PPO residual activity were higher than600MPa treated (p<0.05), which was stay the same with UHT treatment.When treated under400MPa about30℃,20min and40min treated had the same effect on inactivation of PPO enzyme (p>0.05). The PPO residual were higher than60min treated (66.7%)(p<0.05).When treated under400MPa about20min,30℃and45℃treated had the same effect on inactivation of PPO enzyme (p>0.05). The PPO residual activity were higher than60℃treated (59.9%)(p<0.05).When treated under400MPa about30℃, the PPO residual activity in juice treated by one time about20min was higher than two times about10min (p<0.05), one time about40min was higher than two times about20min (p<0.05).After30d storage under10℃, the PPO activity of all UHP treatments were remained the same with0d respectively in juice. The PPO activity of UHT treatment was higher than its0d storage.4. All the treatments changed the Structure of protein in Strawberry juice. The strawberry juice protein structure turned to random coil, but the relative contents of β-sheet had not significant change. Higher pressure, higher thermal synergy UHP, longer treatment time and more times resulted more changes of the secondary structure of Strawberry juice protein. After30d storage under10℃, the protein secondary structure of all treatments were restored some degree.5. All the treatments changed the aroma compared with the control juice. The treatments of10minx2times and20min×1time were really the same with the control juice (without treated).From the above, the juice treated by400MPa synergy30℃about20min could correspond to commercial sterile. Higher pressure, higher thermal synergy UHP, longer treatment times and more treatment times resulted in more killed for the total bacteria count. Higher pressure and longer treatment time resulted more inactivated for the PME enzyme, but higher thermal or more treatment times had no obvious effect on PME. Higher pressure, higher thermal synergy UHP, longer treatment time and more treated times resulted more inactivated for PPO and more changed of the protein secondary structure. When treated by different ultra higher pressure, the aroma of juice was better than treated by UHT. After30d storage, the aroma was changed. The total bacteria count, PME, PPO activity in juice treated by all the high pressure treatments did not chang obviously. But total bacteria count, PPO activity in" juice treated by the UHT were became more or higher.