Concentration of liquid foods is a fundamental operation in many food processes; it is completely different from dehydration. Usually, foods, which are concentrated, remain in the liquid state; while drying produces solid or semisolid foods with significantly lower water content. The concentration of liquid foods has three different methods; evaporation, membrane concentration, and freeze concentration. Evaporation usages gas liquid phase separation. It has the lowest capital cost and the maximum concentration for freeze concentration is more than 50 Brix.
Membrane concentration can be used for separating components of foods on a molecular basis, where the foods are in solution and where a solution is separated from one less concentrated by a semi-permeable membrane. The maximum concentration reachable is comparatively lower than 30 Brix. It requires least energy for separation but the cost of membranes is high. Freeze concentration is centered on solid liquid phase separation at low temperatures so for that reason, there is expectation for a good retention of flavors and thermally sensitive components.
It gives the best quality and the total costs are higher than evaporation or membrane concentration about three to four time (van Mil and Bouman, 1990). The nutritional and sensory quality of freeze concentrated fruits juices is higher than those concentrated conventionally by means of evaporation due to the low processing temperature which avoid undesirable chemical and biochemical changes, and minimize the loss of sensory properties. Freeze concentration is a technology that can be used in the food processing industry to concentrate fruits juices (Rahman et al. 006).
The basic mechanism of freeze concentration is simple. While a solution or a liquid food is frozen, water is separated from the rest of the solution as crystals of pure water ice. Afterward, the mixture can be separated into ice and a concentrated solution. There are three basic mechanisms for ice crystal formation in solutions of liquid foods. The first method is suspension crystallization. It is containing on an initial phase of nucleation, followed by a second phase that includes the growth of ice nuclei in the solution.
The second method is the crystallization of water that present in the solution in the form of an ice layer on a cold surface. After that, the whole frozen solution is thawed and the concentrated fraction is separated from the ice fraction by means of gravitational thawing assisted or by other techniques to enhance the separation efficiency. Regarding these three mechanisms of ice crystallization, there are three techniques for freeze concentration have been established, classified as suspension freeze concentration, progressive freeze concentration and block freeze concentration.
Freeze concentration processing Freeze concentration has been qualified as a potentially attractive method for the concentration of aroma-rich liquid food, including fruit juices, coffee, and tea (Deshpande, S. , Cheryan, M. , Sathe, S. , Salunkhe, D. , & Luh, B. (1984). It is the removal of pure water in the form of ice crystals at sub-zero temperature. “IceconMT is the latest innovations of freeze concentration design. In figure1, it is shown the complete process for the innovation; the single stage process consists of one crystallizer (1) and one wash column (2).
The crystallizer is a container with a cooling cover. The inner wall of the container is scraped and the outer wall is cooled by a circulating refrigerant. Ice production and ice crystals growth take place inside the crystallizer. By creating ice crystals grow during the time of staying, creating an optimal crystal size distribution for efficient separation. While in the wash column, the concentrated liquid is separated proficiently from the ice crystals.
A compressed ice crystals bed is washed with melted ice to remove all traces of concentrated liquid. Freeze concentration ensures that all original product characteristics remain in the concentrate. ” 3. Advantages and disadvantages of the freeze concentration process The advantages of the freeze concentration technique is based on the quality of the product obtained due to the low temperature used in the process, which makes it very suitable technology for the processing of fruit juices.
So, there are the advantages, 100 % aroma retention in the freeze concentration, extremely high quality of the freeze concentration, no thermal degradation of the food during its freeze concentration, high hygienic standards due to the low temperature closed system, no intermediate cleaning of the plant required, the volume reduction also allows for considerable saving in packaging, storage, and transportation, frozen concentration juice with a stabilizer has a longer shelf – life (when stored at -20 OC) up to 9 -10 months (Pruthi, J. 999), frozen concentration juice can be produced with less consumption of energy, and the ice produced can be the reused for the cooling of the incoming juice. While, the disadvantages of freeze concentration are when juice is frozen in continuous slush freezers and subsequently centrifuged, losses of soluble solids up to 5 percent are common and loss of suspended solids during freeze concentration has been more troublesome than loss of soluble solids. 4. Basic methods of freeze concentration
There are two basic methods for ice crystal formation in solutions of liquid foods as shown in Fig 2, according to Aider et al. (2009), The first method is suspension crystallization figure 3a; it is containing on an initial phase of nucleation, followed by a second phase that includes the growth of ice nuclei in the solution. The second method is the film crystallization of water that present in the solution in the form of an ice layer on a cold surface (Flesland, 1995). 5. Suspension Freeze Concentration Suspension freeze concentration is the most important technique at the industrial level.
Research has focused on two issues: control of nucleation and growth of ice crystals to obtain large ice crystals, preferably of uniform size, and to separate ice crystals selectively from the concentrate. According to Hartel and Espinel (1993) this system requires separate stages for nucleation and growth of ice crystals since the optimal operating condition requirements for these distinct crystallization phenomena can be significantly different. It comprises three stages (Sanchez et al. , 2009): crystallization, ripening, and separation of ice crystals.
The solution enters into a scraped surface heat exchanger, then small crystals are used to form a suspension and these crystals grow in the ripening stage, and in the end the ice crystals are separated through filtrating and washing columns. 6. Film Freeze Concentration Method There is an inclusive variety of liquid foods have been studied in film freeze concentration and in the studies the maximum concentrations attained are around 30 oBx and 54 oBx with sugar solutions figure 4. In film freeze concentration techniques there are two films which more common to use: progressive freeze concentration and Layer freeze concentration.
The difference between these two techniques lies in the equipment used for the formation of ice layers. 6. 1. Progressive Freeze Concentration In the progressive freeze-concentration, the mechanism of concentration is the exclusion of solute molecules from the moving ice front, the interface between the ice and solution phases. In progressive freeze concentration, an ice film is produced on a cooling surface while the fluid is moving on the growing ice layer (Miyawaki et al. , 2005). Progressive freeze concentration has been carried out using vertical equipment and tubular equipment.
6. 1. 1 Vertical Progressive Freeze Concentration A vertical progressive FCS consists of a cylindrical receptacle, a cooling bath, a system for immersing the receptacle in the bath, an agitator for the solution in the ice- solution interface and an external heating blanket to control the level of ice formed in the receptacle and regulate the growth of the crystal Figure 5. “The variables of the process studied using this type of equipment are the type of solution, rate of immersion or rate of ice growth, rate of agitation, and mechanisms to reduce supercooling.
In progressive freeze concentration, the distribution coefficient (K) of solute between the ice and the liquid phase is most important. It is defined as K=C_S/C_L where CS and CL are the solute concentration in ice and liquid phases, respectively. The value of the partition coefficient K changes between 0 (ideal freeze concentration) and 1 (no concentration)”(Fernandes, F. (2012) 6. 1. 2 Tubular Progressive Freeze Concentration Tubular progressive freeze concentration proposed by Shirai et al (1998) and it consists of two concentric tubes that are connected.
The solution to be concentrated and the refrigerant fluid circulate through the inner and the external tube, respectively. Ice is produced on the internal wall of the inner tube. The velocity of the fluid reduces solute occlusion on the ice layer. Relevant operational variables include initial concentration and type of solution, cooling temperature, and flow rate of the solution (Miyawaki et al. , 2005; Shirai et al. , 1998). Figure 5. Tubular Progressive Freeze Concentration (Miyawaki et al. , 2005) 6. 2.
Layer freeze Concentration In layer freeze concentration, the solution to be concentrated is in contact with a cold surface which consists of a cooled vertical plate on which the fluid descends; ice forms a single layer on the cold surface, and the solution is concentrated continuously throughout the whole procedure. Muller and Sekoulov (1992) proposed that layer-freezing process is easier to manage, but when the crystal grows on a cooled surface, this stimulate a rapid rate of crystallization, and under these conditions the unclean ice crystals can be produced.
Conclusion Suspension crystallization attains levels of ice purity that are clearly superior to those attained with film crystallization. For this reason, the future of film crystallization technology will depend on reduction of levels of occlusion in the solid phase. One advantage of the film crystallization system is its simplicity, in terms of both the construction and operation of the equipment; nevertheless, in order to optimize operation, a continuous operation system will have to be devised.