| China's cold chain logistics supply capacity is insufficient,and the added value of most of vegetables is low,cold chain's using also exists the problem with high costs.Fresh vegetables are lacking in effective means of preservation in the circulation process,and there is usually a serious loss in that process.The insulating package uses the PCMs to absorb heat so as to control the temperature in the box,and it can maintain the quality of vegetables in course of logistics.Insulating package as a add to cold chain system is not only widely used on current market,but one of the most economical and practical logistics preservation means of vegetables.Lettuce tip mainly refers to the edible leaves of the lettuce,commonly known as Pteris.Compared with lettuce stems,it has richer nutrition,better health efficacy and higher food value.Lettuce tip is a leafy vegetable with high respiration rate,organized crisp,its tissue is very crisp,easy to dehydrate wilting and yellowing,and it has great difficulties in normal temperature-logistics.However,it is cheap and easy to obtain,it also has certain tolerance to CO2.Thus,lettuce tip as a experimental material to study postharvest vegetables' physiology and normal temperature preservation technology is strongly representative.In this paper,the lettuce tip was used as the research object to study the preservation mechanism of insulating package in logistics process of lettuce tip at normal temperature.And the experiment was through the technique of coordinated pretreatment,the amelioration of the structure and the establishment of a shelf life estimtion model to improve the preservation effect of insulating package and save logistics cost.Firstly,it analyzed the environmental changes in the box and the changes of lettuce tip's physiological activities and nutritional quality by adding different weight of the PCMs to different boxes,as well as exploring the preservation mechanism of insulating package and the main limiting factors of its preservation effect.Then,the combination of pre-cooling treatment and heat treatment was carried out to make compensations for the disadvantages of a single insulating package,and it studied the effect of compound treatment of insulating package on the quality of lettuce tip in normal temperature-logistics preservation.Once more,the different size of small holes were drilled through the cover for a certain gas exchange capacity of insulating package.Thus it could form modified atmosphere for a long time,we studied its preservation effect.At the final stage,we analyzed the heat transfer process inside and outside the insulating package,and established a estimtion model of the shelf life according to the PCMs' dosage to verity the result mentioned above.The main conclusions are as follows:1.This chapter studies lettuce tip's preservation mechanism of insulating package in course of nomal temperature-logistics by adding different weight of PCMs.The results showed that the effect of insulating package on the physiological activities and nutritional quality of lettuce had obvious stage.In the early stage of the logistics process,the temperature inside the box was low,the respiration of lettuce tip caused the decrease of O2 content in box and it formed a modified atmosphere.The treatments both could maintain low weight loss,good color(?E,h)and nutritional quality(chlorophyll content and Vc content).However,in the later stage of the logistics process,the consumption of O2 and the accumulation of CO2 caused by the respiratory process would eventually cause gas damage.The lettuce tip carried out anaerobic respiration and produced secondary metabolites such as ethanol.In the end,physiological metabolic was disordered and decay would accelerate.It was beneficial to maintain the quality of the lettuce tip in the early stage of the logistics process,but the effect of prolonging the shelf life was limited.The anaerobic respiration caused by the change of the gas composition in the box was the main factor that restricted the preservation effect of the temperature control package.2.This chapter kept lettuce tip fresh through the combination of pre-cooling treatment and heat treatment.It studied the influence of compound pretreatment on the quality of lettuce tip's preservation of normal temperature--logistics with the box airtight.The result showed that pre-cooling treatment was conducive to extend the control time of insulating package,it had a good performance in lettuce tip's color and nutritional quality at early stage,but accelerated aging in later period of logistics.At that time,its effect was similar to the blank treatment's or worse.The heat treatment would exacerbate the damage of original cuticle surface and wax coat of fruits and vegetables,it promoted transpiration and created a brief increase in respiration and resulted in the increase of weight loss ratio during early period of logistics;meanwhile,the heat treatment could maintain the stability of cytomembrane,reduced superoxide anion generation rate,improved the activity of PPO enzyme and POD enzyme,consequently inhibited the decay of the lettuce tip at the end of the logistics process due to the stress of low O2 and high CO2 content.The combination of pre-cooling treatment and heat treatment was better than any one of them,which showed a preferable preservation effect.That compound treatment was effectively able to delay the physiological metabolism of lettuce tip as well as postponing the occurrence of anaerobic respiration and the arrival of the peak of lettuce tip' breathing.And it enhanced the preservation effect of insulating package on the lettuce tip and extended the shelf life time.3.In this chapter,sponge-MA and microporous-MA were used to improve insulating package's structure.The effects of sponge-MA and microporous-MA on the logistics preservation quality of lettuce tip were studied.The results showed that sponge-MA and microporous-MA both could adjust the gas composition in the box: it was the way to prevent excessive accumulation of CO2 to ensure the lettuce tip' normal respiratory metabolism and avoid the occurrence of anaerobic respiration.But the gas exchange would be accompanied by the loss of water and heat exchange,and that two kinds of MA would increase the weight loss ratio of lettuce tip,while slightly weakening the effect of insulating package.Sponge-MA and microporous-MA had little difference in maintaining relative conductivity,decreasing superoxide anion generation rate and improving the activity of PPO and POD(P <0.05),but the sponge-MA could maintain a lower weight loss ratio and a higher content of vitamin C and chlorophyll.The sponge-MA also could keep lettuce tip the better color,texture,shape.The preservation effect of sponge-MA was overall better than microporous-MA.4.In this paper,we studied the typical double-layer insulating package with a polystyrene box in a corrugated box.We calculated the thermal resistance of the insulating packaging system,as well as considering the heat absorption process of the fruits and vegetables and the temperature rising process after the phase change of PCMs,respectively analyzed the steady-state heat transfer process of the PCMs' phase change and the unsteady heat transfer process with the increase of temperature in the box,finally established a shelf life estimtion model of the insulating packaging.Meanwhile,different amount of PCMs were used to simulate the experiment of lettuce tip' logistics preservation at normal temperature,then we monitored the temperature change in the box and verified the model we established.The experimental results showed that the relative deviation between the estimated value of the model and the actual shelf life was between-9.68 and 6.45% and the deviation factor(Bf)was 1.021 with the different dosage of PCMs.And it was an acceptable range in practical applications.Thus,the estimtion model could accurately predict the shelf life time of insulating package in normal temperature-logistics with the ambient temperature basically constant.In summary,in course of normal temperature-logistics,the effect of insulating package on the physiological activities and nutritional quality of lettuce tip had obvious stage.At the later stage of the logistics process,the gas damage caused by O2 consumption and CO2 accumulation in the box was the main reason to restrict fresh-keeping effect of insulating package.Coordinated pretreatment technology and the improvement of modified atmosphere had good effect on the fresh-keeping of lettuce tip at normal temperature.Pre-cooling treatment combined with heat treatment had both advantages of two pretreatment methods,its fresh-keeping effect was better than any treatment alone.And it could effectively delay the physiological metabolism of lettuce tip but could not avoid the occurrence of anaerobic respiration the box.Modified atmosphere could effectively regulate the gas composition in insulating package,ensure the normal respiration metabolism of the lettuce tip and avoid the occurrence of anaerobic respiration.But,on the other hand,it would also slightly weaken the temperature control effect of insulating package,improve the weight loss ratio of lettuce tip.The perforation filled nano sponge(sponge-MA)had better overall fresh-keeping effect.The estimtion model of temperature controlling time was established based on the form of typical double-layer insulating package.This model could accurately estimate the temperature controlling time of insulating package in course of normal temperature-logistics.These three kinds of improvement methods had certain application value,and them also had some reference for the logistics preservation of the other leafy vegetables or other agricultural products with high respiration rate. |
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