| Lettuce is the green vegetable which has nutrition value and healthy function, and the taste is crispness. Because it has high moisture and exuberant metabolic activity,Lettuce is easy to dehydrate to wilting.It could cause browing and decay due to mechanical damage and germs infect in the process of harvest,storage and transportation. It restricted the Lettuce's storage,transportation,sales and consumption because it has only 2 days postharvest life under normal temperature.The paper discussed the kinetic character of polyphenol oxidase; Preventing the water loss,wilting and browning by adopting PE and MA packaging,inhibitors processing;Exploring and discossing parameter threshold in the cold chain logistics of pre-cooling,transportation,sales and so on.The results were as follows:1 Study of the characteristics of PPO activities of LettuceThe characteristics of polyphenol oxidase(PPO) activities of Lettuce were discussed by spectrophotometer,including catalytic characteristics,the optimum temperature,the optimum pH,and so on.It was also studied the differences of different parts of PPO activity.The results showed that the maximum absorption wavelength was 390nm,the optimum pH and temperature of Lettuce PPO were 8.0 and 40℃respectively,The reaction kinetic of enzymatic browning was accordant with Michaelis-Menten,the Km andνmax were 0.0691mol/L and 26.5957U/min respectively.The PPO activity was gradually declined from leaves,petiole to stem.The browning part of PPO activity in Lettuce stems incision were higher than no browning.2 Study different packaging and cooling methods on cooling rate of LettuceThe effect of pressure cooling and cold room cooling packaged with the plastic box and carton during 0℃pre-cooling refrigerator were studied.The results showed that the central temperatures of Lettuce with plastic box and carton using pressure pre-cooling and cold room pre-cooling were decreased from initial 22℃to 3℃,the pre-cooling times were 3,3.5,5 and 6.5 hours respectively,the weight loss were 0.82%,0.88%,0.98% and 1.04% respectively after pre-cooling.The pre-cooling rate with pressure pre-cooling faster than cold room pre-cooling, and its weight loss were lower. 3 Study PE packaging on the effect of Lettuce qualityLettuce contains 9496% moisture,11.4g protein,1.83.2g carbohydrates,1015mg Vitamin C and some minerals every 100g edible part.Two thicknesses PE packaging which were 0.02mm and 0.06mm on effect of Lettuce quality were studied according to the indices of weight loss,Vc content,soluble sugar content,soluble protein content,PPO activity,sensory evaluation in (0±0.5)℃.The results showed that weight loss rate with PE packaging were lower than controlled group obviously,it also keeped Vc,soluble sugar and soluble protein,0.06mmPE was better than 0.02mmPE packaging.4 Effect of inhibitors on enzymatic browning of Lettuce0.5% citric acid,0.5% ascorbic acid and 0.05% L-cysteine which were screened through advanced experiments used to process Lettuce storaged at (0±0.5)℃packaged with 0.06mmPE.the results showed that three inhibitors can inhibit the polyphenol oxidase(PPO) and peroxidase(POD) of Lettuce,and inhibition effect from strong to weak was 0.05% L-cysteine,0.5% ascorbic acid,0.5% citric acid respectively,reducing the phenolics oxidation. The optimal factors and levels were 0.6% citric acid,0.4% ascorbic acid and 0.06% L-cysteine through orthogonal experiment.5 Effect of MA packaging on quality of LettuceAdopting airtight plastic box as modified atmosphere packaging cases during (0±0.5)℃condition.The results were that CO2 contents reached to about 2.1% after 12 days,MA packaging can provide a relative high humidity environment and reduce moisture loss of Lettuce,it also can reduce browning and keeped nutrition such as Vc,protein,and so on.The sensory score was 3.3 and it still had commodity vale 28 days latter,but controlled group droped to 2.5,losing its commodity vale.6 Effect of different transportation methods on quality of LettuceThe central temperature of Lettuce droped to about 0℃by pressure pre-cooled,then transported by three methods which were expanded polystyrene cases with and without cold accumulator in normal temperature,cartons with 0.06mm polyethylene (PE) in 0℃refrigerated.During 22.5℃transportation,the central temperatures of Lettuce packaged by expanded polystyrene cases with and without cold accumulators were from 0℃up to 7.3℃and 21.2℃respectively after 1 day,up to 21.1℃and 22.3℃respectively after 2 days,the weight losses were 1.15% and 1.51% respectively,sensory evaluation were 3.9 and 3.5 after 3 days.The Lettuce packaged by cartons with 0.06mm PE during 0℃refrigerated transportation,the weight loss were 0.52% after 3 days and 1.73 after 15 days,the sensory evaluation were 4.7 and 4.2 anter 3 and 9 days. The quality of Lettuce packaged by cartons with 0.06mm PE were the best during 0℃refrigerated transportation.Its Vc,protein,soluble sugar contents were the highest,it could delay respiration peak and extend the shelf life of Lettuce.7 Effect of different sales on quality of LettuceThe Lettuce were saled packaged by shrinkable film after one day with three transportation methods which were expanded polystyrene cases with and without cold accumulator in normal temperature,cartons with 0.06mm polyethylene(PE) in 0℃refrigerated.The results were that different transportation had great effect on nutritional quality and commodity during saling under 4℃, 20℃80%RH and 20℃. The sensory score of Lettuce packaged by shrinkable film which expanded polystyrene cases with and without cold accumulator in normal temperature,cartons with 0.06mm polyethylene in 0℃refrigerated were 2.5,3.0,3.9 under 20℃saled one day latter.The Lettuce had no commodity with normal transportant but it still had commodity vale in 0℃refrigerated.It can reduce weight loss,keep nutrition such as Vc,protein,soluble sugar packaged by shrinkable film in 4℃saling.8 Study of cold chain logistics of LettuceIn order to know about parameter threshold during postharvest commercialization and cold chain logistics,the cold chain logistics were tracked from harvest,packaging, transportation before pre-cooling,pre-cooling,transportation in low temperature to destination. The surface temperature of Lettuce from origin to refrigerator were from 15.3℃to 15.7℃in 4 hours,rising by 0.4℃,and weight loss was 0.28%.The pre-cooling time was 7h20min and weight loss was 1.24% when central temperature of Lettuce were from 16.1℃to 3.1℃by pressure pre-cooling.It was about 9 hours from refrigerator to destination which was Husi Food Company,the surface temperature of Lettuce from 2.1℃to 5.0℃,and central temperature from 3.1℃to 5.7℃.The transportation time was 23h57min and weight loss was 2.57% during whole cold chain logistics. |
PDF Full Text Request