Study On The Preservation Technology Of Typical Leafy Vegetables In Low Temperature

Due to the leafy vegetables its large surface areas, crisp organization, and it isprone to be yellow, off the leaves and decay. So the loss of them is very serious instorage, transportation and sale. In order to solve its intolerance storage and shortshelf-life, the greengrocery and lettuce were selected as the research object. Sensoryevaluation, chlorophyll, Vc, nitrite, total bacterial and other physical indexes weredetermined to evaluate the effects of the different preservation methods on the leafyvegetables. The technologies of low temperature storage, modified atmospherepackaging and ozone were used in keeping the leafy vegetables fresh. Besides, theprocesses of vacuum pre-cooling and fresh-cut vegetables were also studied in thispaper. This research can provide some theoretical and technical support for keeping theleafy vegetables fresh. The main contents and conclusions are as follows:(1) Effects of low temperature storage and plastic membrane package on thepreservation of greengrocery were studied. Quality changes of greengrocery packagedwith plastic membrane were studied under the cold storage with0℃,4℃and8℃inthe constant temperature and humidity boxes. Sensory evaluation, weight loss rate,titratable acidity, chlorophyll, Vc, nitrite were determined to evaluate the effects ofpreservation. The results showed that the low temperature could decrease the weightloss rate and nutrition elementary losses effectively. It’s good for preserving leafyvegetables in the low temperature. The shelf-life of greengrocery was about50days at0℃, while under4℃and8℃, that was only10days and20daysrespectively.(2) Effects of modified atmosphere packaging on the quality of the greengrocerywere studied. Greengrocery after natural cooling was packaged by nylon vacuum bagscombined with different proportion of CO2、O2、N2, and was storage at4℃. Sensoryevaluation, total soluble solids, nitrite and conductivity were analyzed to evaluate theeffects of the different preservation methods. The results indicated that the aircompositon of10%CO2+5%O2+85%N2was selected as the best one, which can extendthe shelf-life of greengrocery up to30days while the air matched group only less than 12days. This research can provide some theoretical and technical support for thepreservation of small packaging leafy vegetables.(3) Vacuum cooling process on the preservation of leafy vegetables was studied.The greengrocery from the field was supplied with water in accordance with4%of thequality of greengrocery after selection, precooled to4℃and stored at4℃. The resultsshowed that vacuum cooling could remove the field heat quickly after the harvest, andabout10kg greengrocery were precooled to4℃which took less than6min. Contrast tothe controlled group, vacuum cooling could increase the quality by inhibiting sensorydecline, and delay the decrease of VCand chlorophyll, restrain the increase of nitrite,and keep MDA at lower level. The shelf-life of greengrocery was more than21daysafter vacuum cooling, but the controlled was only less than15days. So vacuum coolingis a good pre-treatment process of the preservation of vegetables.(4) Effects of low temperature storage and plastic membrane package on thepreservation of lettuce were studied. In order to make sure the effects of lowtemperature storage and plastic membrane package on the preservation of leafyvegetables, lettuce packaged with plastic membrane were studied at0℃,4℃,9℃, and20℃. Sensory evaluation, weight loss rate, chlorophyll, colour and nitrite weredetermined to evaluate the effects of preservation under different temperatures. Theresults showed that the low temperature could reduce the rate of metabolism of lettuce,and reduce the rate of decline in its quality. Plastic membrane packaging was good forreducing the weight loss rate of lettuce, and could inhibite the lettuce wilting.(5) Fresh-cut vegetable process on the preservation of the leafy vegetables wasstudied. The fresh lettuce after sorting, cutting, cleaning, packaging and other series ofprocesses was stored at4℃. The total bacterial count and other physical indexes weredetermined to evaluate the effects of the different preservation. Static electricity, ozone,sodium hypochlorite, brine electrolysis and tap-water were used to clean the fresh-cutlettuce, and the quality changes during storage were studied. The results indicated thatozone, sodium hypochlorite, brine electrolysis were helpful for the storage of fresh-cutlettuce, and the others were not significant. The ozone was the best treatment of all,which could extend the shelf-life up to12days while the tap-water treatment only6days. This research can provide some scientific theories for fresh-cut leafy vegetables.