| Precooling is an important part of the cold chain logistics of agricultural products.In order to realize rapid precooling of litchi production area,a lychee spray precooling test platform was set up.The spray precooling method was taken as the research object,and the effects of different precooling methods on the cooling characteristics and storage quality of litchi were studied.The effects of process parameters such as spray temperature,spray flow rate and litchi layer on the precooling process of litchi were analyzed.The removal method of residual water on the surface of litchi after spray precooling was discussed.Combined with the market's"small investment,high efficiency"demand,the cold storage spray precooling vehicle is designed to accurately reach the rapid precooling of the production area.The main research contents and results are as follows:(1)To grasp the influence of different pre-cooling methods on the cooling characteristics and storage quality of litchi,the"Huaizhi"litchi was used as the test material,and the temperature changes of litchi fruits during the precooling process were recorded by soaking,spraying and cold storage precooling respectively.Then the 25-day storage test was carried out,and the effects of three precooling methods on the cooling characteristics and storage quality of litchi were compared and analyzed.Spraying the precooled upper litchi fruit has the fastest temperature drop,the middle level,the lower layer is the slowest.The temperature of litchi under the precooled lower layer was the fastest,and the precooling rate was significantly higher than that of the middle-upper layer(P<0.05).The precooling rate of litchi fruits in the middle of cold storage precooling was significantly higher than that of the left and right sides(P<0.05).The 7/8 precooling time of spray,immersion and cold storage pre-cooling were 7.54 min,11.77 min and 119.51 min,and the?of 7/8 pre-cooling time were0.12,0.081 and 0.305.Spray precooling can maintain a high good fruit rate,peel moisture content,storage for 10 d,good fruit rate and peel moisture content are 82.5%,75.49%,better than other precooling methods,spray,soak precooling The quality loss rate and the browning index of litchi fruits were significantly lower than that of cold storage(P<0.05).The precooling of spray was beneficial to maintain the bright red color of litchi,and it was stored for 25 days.L*,a*and b*were 29.80,24.09,13.76,the precooling method had no significant effect on TSS and TA content(P>0.05).(2)In order to explore the internal heat transfer mechanism of litchi spray precooling,the heat transfer theory was used to analyze the litchi precooling process,and the Fourier series solution was derived by analytical method.The precooling time was predicted and tested by the model.The litchi branches with initial fruit temperature of 26.94°C were precooled under the conditions of spray temperature(5±0.5)°C and spray flow rate of 6 L/(s·m~2).The heat transfer coefficient was 250.6 W/(m~2·k)and the Bi was 7.96;precooling Compared with the experimental values,the prediction time required to 25,22,19,16,13,10,7°C of average error is 9.66%,and the root mean square error is 12.53%.The prediction results are in good agreement with the experimental results.(3)To investigate the effects of spray precooling parameters on the cooling characteristics of litchi fruit,a spray precooling test platform was established.“Huaizhi”litchi fruit was chosen as raw materials for this study.The effects of spray temperature,spray flow rates and litch layer on the cooling coefficient,7/8 cooling time,and cooling uniformity,as well as the characteristics of multilayer litchi spray cooling,were studied.For spray precooling of single layer litchi,the lower the spray temperature was,the bigger the cooling coefficient was,the shorter the 7/8 cooling time was,and the worse the temperature uniformity of the fruit was.When the spraying temperature was less than(5±0.5)?,the 7/8 precooling time was shortened,and the precooling unevenness increased significantly.The effects of reducing the spray temperature to achieve rapid precooling was limited.With the increase of pray flow rate,the cooling coefficient initially increased and then stabilized,and the 7/8 precooling time initially shortened and then leveled gently,and the fruit temperature uniformity increased.The 7/8 precooling time had a quadratic function relationship with the spray flow rate.The 7/8 precooling time decreased slowly when the spray flow was higher than 5.9L/(s·m~2).When multilayer litchi fruits were stacked,the closer the spray nozzle was,the bigger the cooling coefficient was,the shorter the 7/8 precooling time was,and the better the temperature uniformity was.The relative precooling time had a quadratic function relationship with the number of layers,and the number of critical precooling layers was 4.1.The parameters of the single layer litchi spray precooling were used to precool the multilayer litchi,and the number of layers of the best stack of litchi was found to be 4.In the orthogonal test,the optimal parameter combination is spray temperature 5°C,spray flow rate 5 L/(s·m~2),and the number of litchi layers is 3.(4)In order to study the effect of drying method on the quality of litchi storage,a ventilation and drying test platform was built.The"Huaizhi"litchi was used as the test material to study the influence of ventilation speed on the drying process.The air drying and cold storage were compared and analyzed.The effect of dry and naturally dried three drying methods on the quality of litchi storage.The air drying process is divided into four stages,phase one,to remove large droplets on the surface of litchi;phase two,to remove large droplets on the surface of litchi and the evaporation of water on the surface of litchi;phase III,mainly evaporation of water on the surface of litchi;In stage four,the evaporation of water on the surface of litchi is synchronized with the loss of water from the peel.The drying rate has a quadratic function relationship with the drying time.Considering the water loss of litchi peel and the energy consumption of the fan,the ventilation wind speed is 6 m/s and the ventilation time is 24.53 min.Ventilation and drying can maintain the high moisture content of litchi,maintain the structure of litchi peel,inhibit fruit water loss,reduce fruit quality loss rate and browning index;storage for 10 days,ventilation and drying and cold storage can guarantee more than 85%The fruit rate was good,and there was no significant difference(P>0.05).The air-drying and cold-drying in the early stage of storage could reduce the quality loss rate of litchi fruits and maintain a high rate of good fruit,but the effect of the later ventilation and drying method is more significant.(5)According to the characteristics of litchi production and concentration time,combined with the effects of lychee spray pre-cooling and cooling and air drying on the residual water on the surface of litchi,a cold-cooling spray pre-cooling vehicle was designed to provide optimization for litchi pre-cooling technology and equipment.reference. |
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