Application Fundamental Research Of Suspension Freeze Concentrated Apple Juice

Freeze concentration is a method which freezes part of water in a solution,converts it into a relatively pure ice crystal,and then removes it so that the remaining solution can be condensed instead of evaporation concentration.This paper takes apple juice as the research object,and explores the comparison of the effects before and after freeze concentration and vacuum evaporation concentration,the comparison of the effects of program-controlled centrifugation and washing separation,and the development and performance analysis and testing of intelligent freeze concentration devices.Based on the suspension freeze concentration（SFC）,a trinity intelligent multi-pass freeze concentration instrument was developed.Taking the concentrated apple juice as an example,the performance parameters of the freeze concentrator were tested experimentally.The heat transfer coefficients of the cooling stages of the first-,second-,and third-pass freeze concentration scraped surface heat exchangers are 0.23±0.00 k J·m^-2·℃^-1,0.11±0.02k J·m^-2·℃^-1 and 0.22±0.02 k J·m^-2·℃^-1;the heat transfer coefficients in the freezing stage are0.54±0.03 k J·m^-2·℃^-1、0.44±0.02 k J·m^-2·℃^-1 and 0.29±0.01 k J·m^-2·℃^-1;ice production rates are 28.66±1.59 kg·h^-1·m^-2,20.38±3.50 kg·h^-1·m^-2 and 9.55±1.91 kg·h^-1·m^-2.Compared with the complicated equipment and cumbersome operation of traditional SFC process,and the system is simple and efficient.It can also transfer the complicated operation control of freezing and concentration to the machine.Apple juice is concentrated by suspension freeze concentration.Its concentration ratio in a single pass is about 1.8（i.e.from 10%to 18%）.After three-pass freeze concentration,the concentration of apple juice increased from 10.5°Bx to 34.5°Bx,and the retention rates of vitamin C and aromatics（butyl acetate,for example）were above 90%.At the same time,it has little effect on the damage of reducing sugar and color of the juice.In addition,the distribution coefficients of soluble solids ranged from 0.0081 to 0.067.The recovery yield of soluble solids is 98%with the around 2%of soluble solids after three-pass freeze concentration.When the apple juice concentrate was 33.6°Bx,the vitamin C retention rate of 84.5%with the aromatics content of 43.9%can be achieved during the parallel comparative experiments of apple juice concentrate by vacuum evaporation.Obviously,in terms of nutrition and flavor retention,freezing concentration is much better than evaporation concentration.The study provides a reference and basis for the development and application of freeze concentration in the field of liquid food processing and industrial chemistry.In the crystallization-washing column,the maturity of the ice bed,the washing rate and the temperature of the washing water are relatively high,and the tunnel effect is very likely to occur,so the washing operation which is complicated and prone to tunneling effect in the process of freeze concentration is replaced by the method of centrifugal filtration.The effect of centrifugation time on the separation of ice crystals and concentrate was investigated.Its concentration ratio in a single pass is about 1.92（i.e.from 10%to 19.2%）.After three-pass freeze concentration,the concentration of apple juice increased from 10.5°Bx to 40.3°Bx.After the first-,second-and third-pass freeze concentration,the concentration ratios of 1.92,1.45 and 1.41,the solute concentration in ice crystal of 0.2°Bx、1.0°Bx and 4.5°Bx,The solid solutes recovery of 0.996,0.985 and 0.954,the solid solute partition coefficients of0.0102,0.0351 and 0.1117 in ice crystals and mother liquor.The crystallization rates of0.4376,0.4168 and 0.4084,can be obtained.At the same time,a mathematical model of centrifugal filtration was established.The permeability,coefficient of permeability and resistance of ice bed were 4×10^-8 m²,0.16 m·s^-1 and 1.84×10⁷ m^-1,respectively,and the specific resistance of the cake was 7.47×10³ m·kg^-1.The method of suspension crystallization freeze concentration combined with centrifugal separation is obviously superior to the traditional block freeze crystallization with centrifugal separation in the apple juice concentration and separation.