Crusher ual particles and dry preagglomerates bound mainly by van der Waals forces. The effectiveness of this method depends on a variety of process parameters influencing in-terparticle collision frequency, relative velocities of the particles, interparticle contact forces between wetted particles, and strain on the agglomerates (8).
The second main group of processes for agglomerating food powders comprises special drying processes. Two examples are shown schematically in Figure 3, a spray drier with fluidized bed and a freeze drier.
Spray drying is one of the most widely used processes in food powder technology. The concentrate/slurry, either in the form of a suspension or a solution, is finely distributed using a nozzle or an atomizer disc, dried, and cooled in a connected fluidized bed from which the product is withdrawn in agglomerated form. The fluidized bed also serves the purpose of removing the fines, which are collected in a cyclone and recycled into the spray drier. Agglomeration occurs in the vicinity of the nozzle or atomizer, where the fine dry particles collide with the slurry droplets. In many cases the product is also after-dried in the fluidized bed, a process referred to as two-stage drying.
Freeze drying is relatively expensive but especially useful for products sensitive to high temperatures. Another advantage is the possibility to vary the porosity of the agglomerates over a wide range by foaming the concentrate before freezing (this can be achieved in a spray drier, too, by gassing the slurry immediately before atomization). As a novel technology, microwave drying in a vacuum chamber can be used instead of freeze drying.
Two examples for combined agglomeration methods are presented in Figure 4. These are special spray driers, the
Figure 2. Principles of moist agglomeration processes for the production of instantized powdered foods, (a) Batch fluidized-bed agglomerator, (b) continuous mixing chamber with integrated fluidized-bed agglomeration, (c) batch mixer agglomerator, (d) continuous mixer agglomerator with a perpendicular balling region ("SCHUGI" mixer), (e) continuous steam fusion process ("jet agglomeration"), (f) continuous belt aggomerator. A, agglomerates; G, gas; L, liquid; P, powder; S, steam.
"Filtermat" by GEA Niro AIs (Soeborg, Denmark), with integrated perforated belt drier (Fig. 4a), and spray drying with integrated fluidized bed and fines recycling (Fig. 4b).
In the "Filtermat" process, the product is partially dried in the spray-dry section of the machine to a moisture content of approximately 16 to 25%, whereupon it is deposited by gravity onto the perforated belt and further dried to approximately 5 to 10% moisture. Sufficient moisture is present in the intermediate dried product so that agglomerates are formed on the belt. These are then classified and/or size-reduced to the desired size (9).
These methods allow production of very loose, but still sufficiently strong agglomerates (5) with good instant properties and have been widely used in the food industry.
Pressure agglomeration is rarely applied to the production of redispersible products, because the resulting agglomerates are so compact that they show insufficient disper-sibility. Substances dispersing readily—due to a bursting effect of special additives—can be agglomerated using die pressing or low pressure extrusion processes. If no great demands are set upon the redispersibility, roll pressing is a useful and inexpensive process. Mostly low-pressure ring-roller presses are used in which the moistened powder is pressed through holes and thereby shaped into agglomerates.
A special kind of agglomeration is coating, to improve the wetting behavior of the particles. Usually the particles are coated by pure liquids, solutions, or suspensions that harden on the surface of the solid material. A novel technology is coating with submicron particles by "mechano-fusion" (10) to improve the wettability and for other applications (11).
Figure 3. Principles of production of instantized powdered foods by spray drying (a) and freeze drying (b). A, agglomerates; C, concentrate; F, fines; G, gas.
Figure 4. Principles of combined agglomeration processes for the production of instantized powdered foods: "Filter-mat" spray drier with perforated belt drier (a) and spray drier with integrated fluidized bed and fines recycling (b). A, agglomerates; F, fines; G, gas; SL, slurry.
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