Freeze-concentration

Concentration of juice by freezing is less economical than evaporation. Hence, it is utilized mostly for products containing sensitive aroma constituents, e. g., orange juice. The juice is cooled continuously below its freezing point in a scraper-type cooler. The ice crystals are separated from the resultant ice slurry by pressing or by centrifugation. The obtainable solid content of the end product is 40-50%. This content is a function of freezing temperature, as illustrated with apple juice in Fig. 18.11. 

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. 

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Free-Wilson method Freeze concentration Freeze drying... 

Ice formation is both beneficial and detrimental. Benefits, which include the strengthening of food stmctures and the removal of free moisture, are often outweighed by deleterious effects that ice crystal formation may have on plant cell walls in fmits and vegetable products preserved by freezing. Ice crystal formation can result in partial dehydration of the tissue surrounding the ice crystal and the freeze concentration of potential reactants. Ice crystals mechanically dismpt cell stmctures and increase the concentration of cell electrolytes which can result in the chemical denaturation of proteins. Other quaHty losses can also occur (12). 

Extract is stored in insulated tanks prior to drying. Because high soluble soHds concentration is deskable to reduce aroma loss and evaporative load in the driers, most processors concentrate the 15—30% extract to 35—55% prior to drying (33). This may be accompHshed by vacuum evaporation or freeze concentration. Clarification of the extract, normally by centrifiigation, may be used to assure the absence of insoluble fine particles. 

Concentration. The concentration of fmit juice requites removal of solvent (water) from the natural juice. This is commonly done by evaporation, but the derived juices may lose flavor components or undergo thermal degradation during evaporation. In freeze concentration, solvent is crystallized (frozen) in a relatively pure form to leave behind a solution with a solute concentration higher than the original mixture. Significant advantages in product taste have been observed in the appHcation of this process to concentration of certain fmit juices. 

Evaporation is the oldest process for the concentration of liquid foods. Temperatures are higher compared to those of the more modern membrane filtration or freeze concentration processes. Tocopherols, carotenes, ascorbic acid, flavonoids and other phenolic antioxidants are partially destroyed by heating. Therefore, it is necessary to minimise the time needed for evaporation, and heating to the evaporation temperature should be carried out very rapidly. The temperature may be decreased if the pressure is reduced. The process is then more expensive, but losses of antioxidants become substantially lower. 

Freeze concentration has been used for the concentration of aqueous solutions of organic volatiles and substances that are heat labile using an apparatus similar to that shown in Figure 8.4(B) (24,55,56). For successful results, the contact layers... 

Figure 8.4 Apparatus for sample preparation using physical separation. A - fractionation tube and trap for assisted distillation (A septum injector, B carrier gas inlet, C Florisil trap for collecting volatile pesticides) B, Shapiro-type freeze concentrator and C, apparatus for solvent sublation.

The aqueous extract is usually vacuum-concentrated although freeze concentration has been described.103... 

Even after all ice has been removed by sublimation, the product phase, or freeze concentrate, contains a large amount of dissolved water that must be removed to produce a stable product. This water is removed by desorption during secondary drying. Secondary drying is usually carried out at elevated product temperature to achieve efficient water removal. 

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

This process is of special interest if a product has to be frozen more quickly than is possible on belts or in trays A pellet of 2 mm diameter is cooled from 0 °C to -50 °C in approx. 10 s, or at a rate of approx. 300 °C/min. The advantages are minimum freeze concentration, free-flow product, small ice crystals (which are acceptable in this case of small transport distances for energy and water vapor). It is likely that some pellets (those too large or too small) will need to be removed by sieving. 

The second location of interest is the Tg associated with a maximally freeze-concentrated solute matrix, T (point E), which is important to the processing and stability of frozen foods. The freezing of most foods results in the formation of an amorphous freeze-concentrated phase that is plasticized... 

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). 

Following years of glacial slow development, freeze concentration processing is suddenly heating up" says J. Chowdhury in 1988 (1). He underlines that point by finding more than ten new freeze concentration plants installed in the USA in that year. About the same number of companies offer new or improved processes in the field of directed crystallization as a separation technology. 

Compared to the evaporation processes crystallization has a lot of advantages like the energy advantage of about 7 to 1 (for water) and other advantages like a smaller plant volume, less corrosion problems and easier treatment of heat sensitive materials due to lower temperature level, advantages in the field of environment safety due to no possible gas leakages. However it is still the case, that freeze concentration has to look for the niches and is not competing in fields where evaporation or distillation is estab-... 

The freeze concentration process is based on the partial solidification of water into ice in a fluid food product followed by the removal of the solid ice phase from the concentrated liquid phase. This process has some inherent advantages over evaporation and reverse osmosis for concentrating fluid foods as well as other process streams (1). One advantage is that essentially none... 

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 concentration involves the concentration of an aqueous solution by partial freezing and subsequent separation of the resulting ice crystals. It is considered to be one of the most advantageous concentration processes because of the many positive characteristics related with its application. Concentration processes such as evaporation or distillation usually result in removal of volatiles responsible for arom in addition the heat addition in these processes causes a breakdown in the chemical structure that affects flavor characteristics and nutritive properties. In contrast freeze concentration is capable of concentrating various comestible liquids without appreciable change in flavor, aroma, color or nutritive value (1.2.3) The concentrate contains almost all the original amounts of solutes present in the liquid food. 

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