Vacuum Precooling Experiment And Theoretical Analysis

Chinese vegetable production and consumption are the first around the world.Due to insufficient attention to the cold chain and insufficient investment,Chinese vegetable wastage is more than 20% in logistics,resulting in huge waste.Compared with the other developed countries,the cold chain system is very complete,and the loss of logistics can be controlled below 5%.A large number of foreign studies have suggested that pre-cooling is the most effective method to reduce corruption in vegetable storage and is an important part of the entire cold chain logistics system.Vacuum pre-cooling can quickly eliminate the field heat after vegetable picking,and suppress the respiratory intensity of vegetables.It is currently the fastest precooling method for precooling and cooling.At present,researches on vacuum precooling by foreign scholars mainly focus on four aspects: different precooling final pressure,precooling time,product type and other vacuum precooling effects;numerical simulation of different types of fruits and vegetables vacuum precooling,more in-depth Explore the vacuum pre-cooling mechanism and its influence methods;improve the quality of vacuum pre-cooled products;solve the problems of high water loss caused by vacuum pre-cooling and local frost damage;expand the application of vacuum pre-cooling in different fields such as flowers,fruits,cooked meat products,baking,etc.Many vacuum precooling experiments involve the selection of several kinds of fruits and vegetables for research.However,there are many kinds of fruits and vegetables.Studying them one by one is too cumbersome,which is not conducive to the summary of vacuum precooling rules.In this paper,most common vegetables are divided into three categories according to the precooling rate of vacuum precooling: pre-chilled leafy vegetables,pre-cooled waxes,and pre-chilled roots.The specific research content is to study the optimal vacuum pre-cooling final pressure of the three types of vegetables;three kinds of typical vegetables in each category are selected to study the temperature change law and the water loss rate;the best amount of water for vegetables and the three types of vegetables in different water supply modes The precooling effect;changes in the cell gaps of the three types of vegetables under different final pressures;modeling of the heat and mass transfer coupling in the porous medium was simulated to predict the temperature field trends of meat and prechilled vegetables.The pre-cooling final pressure of three types of vegetables was determined by experimental measurement of water loss rate and sensory analysis.The precooled leaf vegetables were 800 Pa,the pre-cooled waxes were 700 Pa,and the pre-chilled roots were 600 Pa.In view of the problem of high pre-cooling water loss in vacuum,we use spinach tomake up the water recharge rate experiment.Through the calculation of the mass loss rate,we determined that the optimal water recharge rate should be 4% of its own weight.The effect of direct replenishment and cooling of precooled root vegetables was still not obvious.Using the method of wrapping water with gauze,the methods of direct precooling,direct replenishing,and replenishing water were compared to improve the water loss rate and precooling effect of the three types of vegetables.The experimental results show that the effect of coating replenishment is best,and the water loss rate can be significantly reduced for prechilled leafy vegetables,and the cooling rate can be significantly improved for precooled rootstocks.Under the condition of pre-cooling and final pressure of 600 Pa,the water loss rate of lettuce,Chinese cabbage,and potatoes is 3.7%,2.3% and 0.4%.The precooling time for precooled roots represented by potatoes was shortened by 30 minutes and the precooling temperature was reduced by 5.5°C.To study the changes in the cell gaps of vegetables after local frostbite,it was found that the decrease of pre-cooling final pressure will lead to an increase in the average cell diameter of vegetables.This rule is most obvious in pre-chilled leafy vegetables.The cell gap increased by 24 ?m during the reduction of pre-cooling final pressure to 600 Pa.The loss of moisture in vegetables is reflected in the increase in the gap between vegetable cells.A heat and mass transfer coupling model for porous media was established.Comsol was used to simulate the temperature field variation of single piece of spinach.The calculated results were compared with the experimental results.The maximum error in the vacuum pre-cooling process was 3°C.The smaller the error in the precooling process to perform the model,the more the trend tends to coincide.In this paper,according to the method of vacuum precooling rate classification,the precooling law experiment can save some repeated studies.The precooling law of each type of vegetable can be used in commercial applications to improve production efficiency.To verify the effect of gauze coating water supplementation on improving the water loss problem,improve the vacuum precooling numerical simulation model,and provide improved ideas for further optimization of vacuum precooling technology.