| With the speeding up of life rhythm, the demand of fresh-cut fruit and vegetable products are increasing. Leafy vegetables, such as lettuce and Chinese cabbage which have rich nutrition like chlorophyll and Vitamin C, are important parts of fresh-cut vegetables. However, fresh-cut vegetables face microbial contamination, nutrient loss, sensory quality deterioration and other issues during processing, storage and sale. So it is necessary to take some means such as cleaning, sterilization and packaging to maintain the quality of fresh-cut vegetables.Pulsed light can increase sterilization to a certain extent. Modified atmosphere packaging can maintain quality of fruit and vegetables. Therefore, this article mainly uses the synergism of pulsed light combined with MAP to preserve fresh-cut lettuce and Chinese cabbage. The results of the study are as follows:(1) Pulsed light has a positive effect on killing microorganisms(Echerichia coli salmonella and mixed bacteria) on the surface of fresh-cut lettuce and Chinese cabbage. The fatality rate is positively correlated with voltage, when the voltage is 2500 V, the fatality rate can reach 98%. The fatality rate is positively correlated with pulsed light times, when the pulsed light frequency is 30, the fatality rate can reach 98.5%. The fatality rate is negatively correlated with pulsed light distance, when the distance is 7 cm, the fatality rate can reach 98%.(2) Based on the results of the sterilization experiment, the sterilization processing procedures of fresh-cut lettuce and Chinese cabbage by pulsed light are studied. Response surface methodology (RSM) can experiment the best condition. The impacts of voltage, frequency, and distances on sterilization efficiency are investigated. Results show that for fresh-cut lettuce, the optimal pulsed light parameters are determined as 2400 V,26 times and 9 cm for voltage, pulsed light frequency and distance, the total number of bacterial colonies is reduced by 99.78%. For fresh-cut Chinese cabbage, the optimal pulsed light parameters are determined as 2400 V,24 and 10 cm for voltage, pulsed light frequency and distance, the total number of bacterial colonies is reduced by 99.45%. In the 10 days storage period, the bacterial colonies on fresh-cut lettuce and Chinese cabbage keep low level, which are 2.73 log(cfu/g) and 2.88 log(cfu/g).(3) Pulsed light has no negative effect on the quality of fresh-cut lettuce and Chinese cabbage. It is concluded that sensory quality, soluble solids, chlorophyll content, Vitamin C, weight loss rate, nitrate content, have no obvious difference after the sterilization process. For fresh-cut lettuce, the change rate of nitrate content is the biggest, but the growth rate is only 3%. For fresh-cut Chinese cabbage, the change rate of weight loss is the biggest, but the rate of descene is only 5%.(4) Under different gas composition, results show that the best gas composition for fresh-cut lettuce is 3%O2+4%CO2+93%N2, the best gas composition for fresh-cut Chinese cabbage is 3%O2+4%CO2+93%N2. MAP can reduce the loss of sensory quality, soluble solids, chlorophyll content, weight loss rate, delay the increace of nitrate content. But MAP has no obvious effect on inhibition of microbial growth. For fresh-cut lettuce, the number of bacterial colonies is above 8.79 log(cfu/g). For fresh-cut Chinese cabbage, the number of bacterial colonies is above 9 log(cfu/g).(5) The synergism of pulsed light combined with MAP, not only can effectively kill microorganisms on the surface of fresh-cut lettuce and Chinese cabbage, but also improve the quality of fresh-cut lettuce and Chinese cabbage, prolong the shelf life. During the 10 days storage, sensory quality, chlorophyll content, weight loss rate, nitrate content, soluble solids, Vitamin C and bacterial colonies are measured on a regular basis. In the 10th day, the number of bacterial colonies of fresh-cut lettuce is only 2.08 log(cfu/g). For fresh-cut Chinese cabbage, the number of bacterial colonies is only 2.29 log(cfu/g). Besides, physical quality such as chlorophyll and Vitamin C have been improved significantly.Results show that the synergism of pulsed light combined with MAP has the best preservation effect compared with using pulsed light or MAP alone. The reason is that the effect of instantaneous sterilization by pulsed light can make up the lack of MAP for microbial killing. The physical and chemical indexes are analyzed, the results show that MAP has obvious effect on keeping physical quality of fresh-cut lettuce and Chinese cabbage. So the combination can be applied to the preservation of fresh-cut lettuce and Chinese cabbage. |
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