| Hami melon juice is a kind of typical heat-sensitive juice, the changes of volatile compounds and its off-odors formation that caused by thermal processing has not been avoided. The nonthermal processing technologies are the critical and effective means to sterilize the bacteria and inactivate the undesired enzymes in freshly squeezd juices, which can preserve their flavors, color and nutrients well. At present, the research related to Hami melon juice processed with the nonthermal methods was less reported home and abroad. Both GC-MS ananlysis method and the solid phase microextraction (SPME) were used to systematically analyze the flavors of four Hami melon varieties and the flavor changes caused by different nonthermal processing methods such as freezing, irradiation and ultra high pressure processes. The residual activities of enzymes and survival bacteria were detected as indices in Hami melon juice for evaluating its different sterilization effects. The mechanisms of its off-odors formation were also explored in the paper. Our aim is to choose the proper nonthermal processing methods and the optimum parameters. The main results were summarized as follows:1. The dominant flavor compounds in the four Hami melon varieties were esters, alcohols and aldehydes. Golden Empress melon had a good flavor quality and was chosen as trial material from them, its main volatile compounds were ethyl acetate, propyl acetate, butyl acetate, ethyl hexanoat, hexyl acetate, methyl 2-methyl propionate, methyl 2-methyl butyrate, butyl 2-methyl acetate, ethyl phenylmethyl ester, cis-3-hexenol and cis-3-nonenol.2. The loss of esters in acidified Golden Empress juice heated at 80℃ for 1 min was the smallest, the raw melon juice heated at 80℃ for 1 min was the larger, and the acidified melon juice heated at 120℃ for 1 min was the largest. The heated melon juice generated a lot of new C9 unsaturated aldehydes and alcohols, which strengthened the melon juice green note. Trans-2,4-decadienal and dimethyl sulfide were possibly responsible for the important off-odors .3. The volatile compounds of the frozen melon had changed, but the flavor quality of fast frozen melon was higher than that of slow frozen melon. The melon frozen by liquid nitrogen can preserve its flavor well. The esters and their areas of the slow frozen melon reduced, while the concentration of C9 unsaturated aldehydes and alcohols increased with the long time frozen storage , and they gradually became the main flavor compounds.4. The esters and their areas of Golden Empress juice irradiated at 1, 3 and 5kGy decreased obviously, the main esters such as ethyl acetate, butyl acetate and ethyl phenylmethyl ester were not detected. The melon juice at 2 kGy irradiation smelled off-odor, the amounts of C9 unsaturated aldehydes and alcohols induced by irradiated unsaturated fatty acids oxidation were greatly responsible for the off-odors formation.5. The esters and their areas of Golden Empress juice pressurized at 400MPa and 500MPa for 20 min decreased, but their main esters were the same as unpressurized melon juice, the flavor quality of the pressuried juice was good. The area of esters in the juice pressurized at 500MPa for 20 minwas larger than that of the juice at 400MPa for 20 min, but their difference was small. Increasing the pressurization level for the juice did not affected its flavor compounds and its profile. The pressurized juice produced 6Z-nonenal, 6Z-nonenol, 2Z,6E-nonadienal and 3Z,6E-nonadienol, which enhanced its juice green note.6. The losses of POD, PPO and LOX activities in the frozen melon were small, their residual activities were high during long time frozen storage. The loss of POD activity heated at 65℃ for 3 min was small, while its activity was lost completely when heated at 90℃ for 3 min. The activities of POD, PPO and LOX in the frozen melon juice at 1 kGy irradiation reduced obviously. When irradiated at 5 kGy, their activities were not lost totally. POD in the melon juice was pressure-tolerant, its residual activity was 81% when pressurized...
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