The Research On Vacuum Pre-cooling Technology And Fresh-keeping Impact For Broccoli

Vacuum pre-cooling technology is mostly exploited to lower the temperature of materials by vacuumizing in which low pressure helps to evaporate the moisture,and then remove the heat.The uniformity and rapidity in cooling is the plus point,and no cross-contamination occurs.Vacuum pre-cooling technology is getting great popularity and broad development horizons in the post-harvest technology of fruits and vegetables.Broccoli,a cruciferous cabbage brassica species,has enjoyed,since first brought into China in the early 20th century, the expansion in the cultivated area and the export volume to Japan,Europe and America.It has become one of the major Chinese export vegetables,but after harvesting with a lot of field heat and strong post-harvest respiration;broccoli easily becomes senescence during transportation and storage.The flower balls tend to change from green into yellow,dehydrated, and wilted.The nutritional value degraded seriously in a short span of time,which ultimately results in lowering its commercial value.Therefore,it is of great practical significance and high economic value to carry out research work on the application of vacuum pre-cooling technology to the storage of fresh broccoli.In this article,the vacuum pre-cooling technology was applied to the broccoli for keeping fresh.The main achievements of this research and conclusions are given as follows:Firstly,the best vacuum pre-cooling process of Broccoli was studied.On the basis of single-factor pre-trial,four factors and three levels of the orthogonal pairs of broccoli were used.The weighted analysis identified a favorable broccoli vacuum cooling process conditions were:200 Pa,40min,200g,5%.Secondly,the effects of different pre-cooling modules on both the cooling velocity and various quality index of broccoli were studied.The comparison of vacuum pre-cooling in combination of ice water cooling,refrigeration cooling,and modified atmosphere packaging (MAP) with common refrigeration without vacuum pre-cooling and MAP singled out vacuum pre-cooling as the most efficient in lowering the temperature and lowering respiratory rate. Vacuum pre-cooling treatment could cool broccoli from 24.3℃to 5.5℃within 30min. Combined with MAP storage,this group demonstrated the best storage efficiency:after the 30^th day of storage,it contained 11.28 mg·(100g)^-1 chlorophyll and 27.88 mg·(100g)^-1 sugar,which is significantly higher than the other groups.Moreover this group showed comparatively low weight loss rate,and low respiration rate,and consequently had the best sensory quality and commercial value.The results also showed that broccoli without pre-cooling and MAP had the highest respiration rate,weight loss and the degradations of chlorophyll,vitamin C and sugar. At the same time,they lost their commodity values completely after 18 days of storage due to the rapid decrease of nutritional and sensory quality.The weight loss rate was as high as 15.38%.Finally,the influence of vacuum pre-cooling on the physiological activities of broccoli was studied during the entire storage period.The results showed that the respiration rate,ethylene production and weight loss were lowered.After vacuum pre-cooling,the respiration rate dropped from 210.84mg·(kg·h)^-1 to 191.41mg·(kg·h)^-1;it also reduced the peak ethylene production by 28.19%;The weight loss rates of the vacuum pre-cooling group were still less than 5%after 24 days.At the same time,the degradation of chlorophyll and vitamin C was delayed.Chlorophyll content of the vacuum pre-cooling group fell about 50%after 18 days; Vitamin C content was 48.09 mg·(100g)^-1 which is higher than the control group.Vacuum pre-cooling also delayed senescence of broccoli by regulating the activity of antioxidant enzymes.The SOD,CAT activity of the group which used the vacuum pre-cooling treatment was increased during the storage:CAT activity of the vacuum pre-cooling group was 1028.09 U·g^-1 which was higher than the control group(744.10 U·g^-1) after 6 days;the SOD activity is 22.67 U·g^-1 in the 6 day which is higher than that of initial storage(22.27 U·g^-1),and since then the SOD activity decreased slowly,but always remained higher than that of the control group. After vacuum cooling POD activity was decreased significantly;during 6-24 days,the POD activity of control group was significantly higher than that of the vacuum pre-cooling group (25.40%～98.92%higher).