| Vacuum cooling has been known as an rapid cooling method,of which principle is based on the phenomena that the free water evaporates rapidly from the surface or inside of the product,which absorbs heat of the food product.The sensible heat of the product converts to latent heat required for the evaporation,as the product does not contact with other medium except air.Consequently this process leads to the dropping of temperature and the product is cooled.And vacuum cooling has shown great potential in cooling for energy saving and fast cooling rate.In the meantime,food can obtain uniform temperature distribution during vacuum cooling method.Now vacuum cooling has been widely use in solid food like leafy vegetables,cooked meat,bakery food.However,there are few related researches and applications on liquid food in the field of vacuu cooling.At present,the cooling method for liquid food in the food industry mainly adopts shell-and-tube heat exchangers or plate-frame heat exchangers,which possess fast cooling rate.But the energy consumption of them is large,as well as the risk of refrigerant leakage.Therefore,pure water,apple juice and milk were selected as different kind of liquid food to study the effect of both vacuum film flash evaporation cooling(FVC)and vacuum spray flash evaporation cooling(SVC)on them.The experiments are conducted from different initial temperature,different superheat and different pressure reduction rates of the liquid.By observing the effects of different working conditions on the temperature curve,boiling process of CCD pictures,particle size change,weight loss,soluble solid content,volatile compounds,the experiment made better evaluation of the effects of vacuum cooling on liquid foods.The main research results are as follows:1.The higher the initial temperature,the greater the temperature change rate of the liquid during the vacuum film flash evaporation cooling,and the faster the equilibrium temperature is reached.When starting with different initial temperature of 20?,40? and 60? in the same superheat degree,the maximum temperature change rate of pure water during vacuum film flash evaporation was 0.45?/s,1.25?/s and 2.4?/s,respectively;the time of pure water reaching equilibrium temperature was 200 s,43 s and 22 s,respectively.With the image taken by the high-speed camera,it was found that the higher the initial temperature,the more intense the liquid film was boiling,and the corresponding maximum expansion height value was larger.In the case where the initial temperature was 60?,the maximum expansion height of pure water was 1.08 times and 2.22 times of the initial temperature of 40 ° C and 20 ° C,respectively.2.For liquid foods of water,apple juice and milk,the maximum temperature change rate increases sequentially during vacuum film flash evaporation with the same condition.And according to the high-speed camera images,the maximum expansion height of milk was 1.95 times and 2.03 times than that of apple juice and pure water,respectively,under the condition of initial temperature of 20?.The surface tension of water,apple juice and milk decreased in turn.When the surface tension was smaller,the easier it was to produce bubbles in the boiling flash process,and the disturbance caused by the flashing process was more intense.And finally the heat transfer was strengthened,so its maximum temperature change rate increases in turn.3.During vacuum spray flash evaporation cooling process,as the pressure of the vacuum chamber decreases,the atomization effect was more sufficient,and the obtained liquid particle size was smaller.For the three working fluids of pure water,apple juice and milk with an initial temperature of 20?,the droplet size from 10 s(pressure P = 24.3 kPa at this time)to 90 s(pressure P=0.7 kPa at this time)was reduced by 13.4%,10.3% and 10.3%,respectively.Combined with thermal infrared imaging images and temperature profiles,it was found that the liquid obtained a large temperature drop during the spraying process.4.Whether it was vacuum film flash evaporation cooling or vacuum spray flash evaporation cooling,it will cause loss of volatile aromatic substances in apple juice and milk.For apple juice,some volatile aromatics can be recovered through the condensation trap.There were 25 kinds of volatile aromatic substances detected in the apple juice before vacuum cooling,and the alcohol content accounted for 56.71%.There are 10 kinds of volatile aromatic substances detected in the condensation water which was collected after vacuum film flash evaporation,in which the total content of alcohols accounted for 83.33%;12 kinds of volatile aromatic substances were recovered in the condensed water cooled by spray vacuum flashing,among which total content of alcohol was accounted for 80.54%.Eighteen volatile aromatic substances were detected in the milk before vacuum cooling,of which ketones accounted for 72.61%,and no volatile aromatic substances were detected in the condensed water of the milk.5.As the depressurization rate decreases,the maximum expansion height of pure water with an initial temperature of 20? does not change significantly,while the maximum expansion height of apple juice with an initial temperature of 80? and milk with an initial temperature of 40? was significantly reduced.During vacuum spray flash evaporation cooling,the lower the depressurization rate,the larger the droplet size;for the droplets of vacuum spray flash evaporation cooling at 10 s,50 s and 100 s,the droplet size at the P-1 depressurization rate was 6.73%,16.78% and 2.33% smaller than the droplet size at the P-3 depressurization rate. |
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