Freeze concentration, fruit juice

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). 

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

Crystallisation by freezing, or freeze crystallisation, is a process in which heat is removed from a solution to form crystals of the solvent rather than of the solute. This is followed by separation of crystals from the concentrated solution, washing the crystals with near-pure solvent, and finally melting the crystals to produce virtually pure solvent. The product of freeze crystallisation can be either the melted crystals, as in water desalination, or the concentrated solution, as in the concentration of fruit juice or coffee extracts. Freeze crystallisation is applicable in principle to a variety of solvents and solutions although, because it is most commonly applied to aqueous systems, the following comments refer exclusively to the freezing of water. 

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. 

These results show that it may be more efficient to depectinize fruit juices before their concentration by freezing because this would give minimum losses at lower heat removal rates and thus at conditions of more economical operation. The implication of these results for the design of a scraped surface crystallizer are currently being examined. 

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). 

Under proper processing conditions, fruits lose less than 30% of their original vitamin C content through the entire freezing and frozen storage period (106). Frozen concentrated fruit juices can retain over 90% of their ascorbic acid (38,106). Packing in syrup is generally protective of ascorbic acid. 

Examples of mass transfer under high vacuum are distillation of thermally unstable organic compounds, high-vacuum freeze drying, vacuum concentration of fruit juices, vacuum drying of coffee concentrate, vacuum purification of molten metals, etc. The mechanism of mass transfer in the concentration of fruit juices can be adequately described by the Gilliland-Sherwood equations (Gl), since the mean free path is negligible relative to the dimensions of the vessel at the pressures used. These equations show that the mass transfer coefficient is inversely proportional to the pressure. By extrapolation to very low pressures, it should be possible... 

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. 

To handle dehydration of fruit juices, a technology called slush drying was proposed and tested with apple juice for the potential loss of volatile flavor and aroma substances (Chandrasekaran and King 1971 Lowe and King, 1974). The principle of this method stems from the dependence of the freezing point on the concentration of dissolved solids (Figure 20.1). It boils down to the fact that drying takes place from an ice-liquid mixture (slush) in which 20% to 70% of the water present in the fruit juice is frozen (the... 

This phenomenon, called reverse osmosis, is used in a number of processes. An important commercial use is in the desalination of seawater or brackish water to produce fresh water. Unlike distillation and freezing processes used to remove solvents, reverse osmosis can operate at ambient temperature without phase change. This process is quite useful for processing of thermally and chemically unstable products. Applications include concentration of fruit juices and milk, recovery of protein and sugar from cheese whey, and concentration of enzymes. 

Fruit pulps and purges as well as juices and concentrates may be preserved with sulfur dioxide. This practice is quite widespread abroad, as was mentioned before, although it is not used in the United States, since preservation freezing of fruits and fruit pulps is preferable to sulfiting for subsequent use by jam and preserve industries. The preservation of fruit pulps with sulfur dioxide is described by Atkinson (1941), Atkinson and Strachan (1941), and Charley (1934). Both cold fruit pulps and hot fruit pulps may be barrelled with SO2 the latter usually require a lower concentration of SO2 for preservation. 

Figure 1.4.2. Freeze-concentration process for fruit juices.

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