Fruit freeze concentration

Phillips (2) A fractional crystallization process used to freeze-concentrate beer and fruit juices. Formerly used in the production of p-xylene. 

Thijssen and Spicer1 1191 has given a general review of freeze concentration as an industrial separation process and Bushnell and Eagen(63) have discussed the status of freeze desalination. The potential of freeze crystallisation in the recycling and re-use of wastewater has been reviewed by Heist 120, and the kinetics of ice crystallisation in aqueous sugar solutions and fruit juice are considered by Omran and King(121). 

The development of the freeze concentration process for fruit juices has been hampered by the fact that solute concentrate is entrained by the ice crystals. This incomplete separation of the entrained concentrate from the ice results in a considerable increase of the cost of the process. In this investigation sucrose solutions were concentrated by the formation of an ice layer on the externally cooled walls of the crystallizer. The formation of the layer was initiated by secondary nuclei induced by rotating ice seeds, at subcoolings smaller than the critical subcooling needed for spontaneous nucleation. A minimum in the amount of sucrose entrapped in the ice layer was observed at a subcooling smaller than the critical subcooling for spontaneous nucleation. The effect of soluble pectins on the minimum was also studied. 

Freeze dryers meat, seafood, vegetables, fruits, coffee, concentrated beverages, pharmaceuticals, veterinary medicines, and blood plasma... 

Although a combination of product quality and cost considerations will dictate the methods used for bulk processing of fruit juices, there are instances where the flavour components present in the juice at e vulnerable to any form of heating din ing concentration. Strawberry juice is perhaps the best example of this, being one of the most heat sensitive of fruits, and it works well with alternative processes for concentration such as freeze-concentration and hyperfiltration. 

Crystallization can be used to remove solvent from a liquid solution. For example, concentration of fruit juice requires the separation of solvent (water) from the natural juice. The common procedure is evaporation, but the derived juices may lose flavor components or undergo thermal degradation during the evaporative process. In freeze concentration, the solvent is crystallized (frozen) in relatively pure form to leave behind a solution with a higher solute concentration than the original mixture. Significant advantages in product taste have been observed in the application of this process to concentrations of various types of fruit juice. 

Concentration of fruit juices should not result in marked loss of ascorbic acid if the pressed juice is deaerated and and evaporated at low temperatures (100). Ascorbic acid retentions in excess of 90% have been reported for concentration and freezing processes (38,101) and can be expected for freeze concentration processes (100). 

Since initial flavor is preserved, the most important consideration for freeze concentration is that feed juices be of very high quality. For example, juice from immature or overmature, or a few rotten fruit, may contain off-flavors which will still be present in the finished product. Also, juice handling practices prior to actual concentration will affect final product quality. 

Orange Juice. In a study of some parameters important to freeze concentration of orange juice, horticultural factors related to the fruit, juice handling and the extent of thermal treatment were more important to product quality than the concentration process (30). 

A definite advantage of freeze crystallization, important in many food industry applications, is that volatile flavour components that are normally lost during conventional evaporation can be retained in a freeze-concentrated product. In fact, at present, freeze crystallization finds its main application in the food industry, for the concentration of fruit juices, etc. Indirect-contact freezing processes are normally used, e.g. the liquid feedstock is crystallized in a scraped-surface heat exchanger (section 8.2.2) and the resulting ice slurry passes to a wash column where the crystals are separated and washed to recover valuable product. The wash column is the key item in the process. Figure 8.56 shows an example of the Grenco system of freeze crystallization. 

Figure 1.4.2. Freeze-concentration process for fruit juices.

Freeze Crystallization. Freezing may be used to form pure ice crystals, which are then removed from the slurry by screens sized to pass the fine sohds but to catch the crystals and leave behind a more concentrated slurry. The process has been considered mostly for solutions, not suspensions. However, freeze crystallization has been tested for concentrating orange juice where sohds are present (see Fruit juices). Commercial apphcations include fmit juices, coffee, beer, wine (qv), and vinegar (qv). A test on milk was begun in 1989 (123). Freeze crystallization has concentrated pulp and paper black hquor from 6% to 30% dissolved sohds and showed energy savings of over 75% compared with multiple-effect evaporation. Only 35—46 kJ/kg (15—20 Btu/lb) of water removed was consumed in the process (124). 

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