Research On The Freeze Concentration Technology Of Litchi Juice

In recent years, as a new processing technology, freeze concentration has been paid more attention. In this paper, multistage freeze concentration was successfully used to produce concentrated litchi juice. The influence of freeze concentration stages on the nutrients, the growth of ice and the heat transfer coefficient in the process were studied.The relationship between the soluble solid content of litchi juice (C) and freezing point (Tf) was investigated by temperature thermocouples. The correlation of C with Tf was worked out with the method of curvilinear regression as below: Tf = -0.00654C2+0.0855C -1.0981(R2=0.9998). Therefore, using the freezing-point-depression method, the concentration of litchi juice during the freeze concentration process can be monitored through determining the temperature.By applying an independent crystal growth zone in the freeze concentration rig, the total soluble solid content of litchi juice was increased from 16.3°Brix up to 47.6°Brix. The best single stage operation can increase the solids content by 12°Brix with the minimum loss of entrained ice crystals about 1.5% and the efficiency was 94%.The nutrients of recovered juice including pH value, total acid, reducing sugar, protein, amino acid as well as vitamin C were detected and compared with the original juice. The results showed that the main compositions of the recovered juice were similar to that of original. Meanwhile, the content of vitamin C was apparently decreased. Under the fourth stage, the retention rate of vitamin C was 85% while this number was only 9% after thermal concentration treatment. The ratio between various compositions with different stage concentrated juice remained an almost constant value comparing with the original juice.The particle size of ice were detected by laser granulometer in the freeze concentration process. The results showed that the best average grain diameter, about 50μm, was obtained when the concentration of juice was range from 15 to 20°Brix. Meanwhile, the ice mass fraction was decreased by increasing the concentration of litchi juice.Through the analysis of the litchi juice temperature during the freeze concentration, a mathematical model was developed and a unified expression of the heat transfer coefficient in an scraped-surface heat exchangers with/without phase change was presented. Meanwhile, the model predicted the heat transfer coefficient decreased with the increase of the ice thickness.