Freeze concentration process

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. 

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. 

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 processes are based on the difference in component concentrations between solid and liquid phases that are in equihbrium. Most minerals and many organics grow less soluble in water as the temperature decreases. When an aqueous solution is cooled, ice usually crystallizes as a pure material, and dissolved components in the aqueous waste stream are concentrated in the remaining brine, thereby reducing the volume of waste. 

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). 

High heat transfer coefficients, up to 1 kW m K, and hence high production rates, are obtainable with double-pipe, scraped-surface heat exchangers. Although mainly employed in the crystallization of fats, waxes and other organic melts (section 8.2.2) and in freeze concentration processes (section 8.4.7), scraped-surface chillers have occasionally been employed for crystallization from solution. Because of the high turbulence and surface scraping action, however, the size of crystal produced is extremely small. 

Figure 8.56. Grenco freeze concentration process single stage). 1, scraped surface heat exchanger, 2, agitated recrystallizer, 3, wash column with bottom perforated piston and top ice scraper, 4, circulating pumps , 5, concentrated product expansion vessel, 6, ice melter...

Figure 1.4.2. Freeze-concentration process for fruit juices.

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. 

The freeze-drying process is initiated by the freezing of the biopharmaceutical product in its final product containers. As the temperature is decreased, ice crystals begin to form and grow. This results in an effective concentration of all the solutes present in the remaining liquid phase, including the protein and all added excipients. For example, the concentration of salts may increase to... 

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). 

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-... 

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. 

The DirCon freeze crystallization process is a technology that can be used to purify aqueous waste streams and concentrate liquid waste by the freezing and subsequent melting of the liquid. This technology is a type of direct-contact secondary-refrigerant freeze crystallization and operates on the principle that when water freezes, the crystal structure that forms naturally exudes contaminants from its matrix. The terms freeze crystallization and freeze concentration are often used interchangeably. 

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