Theoretical And Experimental Study On Hypobaric Storage Of Fruits And Vegetables

Hypobaric storage of fruits & vegetables include vacuum pre-cooling and low pressure storage.Compared with the traditional pre-cooling methods,vacuum pre-cooling have advantages of fast cooling rate and uniform cooling temperature and pollution-free during the whole pre-cooling process.Relative to ordinary cold storage and ice-temperature storage and controlled air storage,the low pressure storage of fruits & vegetables return to normality and after-ripening process is slow after leaving the storage environment,thus low pressure storage can prolong the shelf life of fruits & vegetables.However the difficulty of manufacturing technique and the higher cost of operation have been the bottleneck of hypobaric storage application widely.Therefore,it’s important to explore and improvement operation condition of hypobaric storage system,to improve the storage quality of fruits & vegetables,reducing the input-output ratio effectively,which will has great significance for the promotion of hypobaric storage.This paper take strawberry as experimental objects,the cooling rate and quality has been analyzed under different pressure respectively,and comparative analysis the change of strawberry indicators under the two ways of low pressure storage and ice-temperature storage.The main contents and conclusions are as follows.1.Through theoretical analysis of fruits & vegetables thermophysical property,concluded that the thermophysical property is a function of its components,pressure and temperature,but the thermophysical property of fruits & vegetables hardly change along with the pressure and temperature in areas where the temperature changed small.In the certain area of storage temperature,for the moisture content with 80% fruits & vegetables,its thermophysical property can simply regard as a function of moisture and other components.2.The expression of water chemical potential in the storage environment and fruits & vegetables has been deduced by using phase equilibrium thermodynamics.And the analysis of the factors influencing the evaporation of water of fruits & vegetables including vacuum tank pressure,trap temperature,surface activity of fruits & vegetables,gas-liquid interface state,variety of fruits & vegetables,etc.3.The physical model and mathematical model of vacuum pre-cooling have been established,and theoretical analysis of the moisture migration inside the fruits & vegetables and the moisture diffusion surface the fruits & vegetables.Through analyzing the surface mass transfer resistance layer,learned that the mass water vapor diffusion from the surface of fruits & vegetables is inversely proportional to the thickness of the mass transfer resistance layer,while is directly proportional to the square root of the fruits & vegetables surface temperature.Learned that the water vapor partial pressure of the fruits & vegetables surface is an increasing function to the mass of moisture diffusion,while the water vapor partial pressure in the storage environment is a decreasing function to the mass of moisture diffusion.Therefore improving the pre-cooling rate and increasing the moisture evaporating capacity of fruits & vegetables surface can be achieved by reducing the storage environment pressure,or bring down water vapor partial pressure in the storage environment,or decreasing the boundary thickness.4.The system structure of multi-function fresh-keeping device has been detailed presentation,and verified the validity of vacuum pump selection in the vacuum system,and verified the validity of humidifying pipe diameter and length.Through low pressure storage experiments of strawberry,verified the stability of pressure and temperature and RH in storage environment.5.Finished vacuum pre-cooling of strawberry under the pressure of 0.5kPa,0.8kPa and 1.0kPa respectively.The experimental results showed that the lower pressure pre-cooling,the faster the cooling rate,and the lower the water loss rate.The cooling rate of strawberries at 0.5kPa pressure is 5.1 times than that of 1.0kPa,and the water loss rate is 2 times than the former after pre-cooling completes,and the strawberry temperature drop slowly when pre-cooling pressure is constant.Compared to no pre-cooling strawberry,the respiration rate is to reduce 53.8% after vacuum pre-cooling in 0.5kPa pressure.The soluble solids content of strawberry is hardly change before and after pre-cooling,which has nothing to do with the cooling pressure.The cooling rate of strawberry epidermis is always higher than the quarter and the center of strawberry.6.Low pressure storage experiments of strawberry shows that weight loss and temperature fluctuations of strawberry are very small during the low pressure storage stage,thus verified the conclusion that there is on heat and mass transfer between storage environment and fruits & vegetables in the stage of low pressure storage.By contrast with ice-temperature storage,after 10 days low pressure storage,its sensory evaluation,color difference,respiration rate,firmness,weight loss and shelf life were 5.6,42.1,22.1 mgCO2/(kg·h),0.47 N,2.0%,3d,while after 10 days ice-temperature storage,the same index were respectively 5,53.5,35.9mgCO2/(kg·h),0.42 N,3.8%,2d.From the comparison of two groups of experimental data,known that low pressure storage is superior to the ice-temperature storage if only consider the preservation quality of strawberry.