The first step in any agglomeration process (except drying methods starting with a slurry) is to make the primary particles contact each other, which is frequently achieved by external force. Powders for instant products usually consist of primary particles smaller them 200 //m to facilitate solubility. In a second step, permanent adhesion forces stronger than any possibly existing disruptive forces must be established between these particles. For food powders, this is usually achieved by wetting (which causes partial dissolution and the development of liquid bridges) and subsequent drying (which leaves solid bonds in place of the liquid bridges).
The duration and intensity of the forces acting among the particles during agglomerate formation and stabilization have an important influence on agglomerate porosity and stability. For example, an agglomeration process in which high forces act on the particles and agglomerates will turn out dense, smooth, and stable agglomerates that are easy to handle and dispense. However, instant properties would be poor owing to low agglomerate porosity and strong bonds between the primary particles. Such a process, like compaction, would be inappropriate for in-stantizing.
The final product should have the following properties:
• sufficient agglomerate porosity for fast liquid suction by capillary action, although a critical porosity must not be exceeded (5);
• sufficient agglomerate strength to withstand handling and transportation.
Agglomeration processes suitable for producing instant-ized food powders can be divided into three groups:
• moist agglomeration,
• agglomeration by drying, and
• combined methods.
Moist agglomeration, using capillary and liquid bridge forces to achieve sufficient interparticle adhesion during agglomeration, is the most important process for the production of instantized powders. This method starts from dry powder, which is moistened either by condensing vapor, atomized liquid, or a mixture of both. The material is then dried and solid bridges between the primary particles provide the necessary strength. A large variety of equipment is available for moist agglomeration. Schematics of some typical examples are shown in Figure 2.
Except for the static process Figure 2f, all methods are "dynamic"; that is, agglomerates are formed due to the collision and subsequent adhesion of the particles. Common features of all processes shown in Figure 2 are the moistening of the dry powder, the size enlargement of the wet particles, and subsequent drying and cooling, if required. The drying rate and temperature considerably influence the strength of the dry agglomerates because of different crystal structures and distributions of the interparticle solid bridges formed by crystallization of dissolved substances during drying (6).
Fluidized-bed agglomerators for batchwise or continuous production, Figure 2a and 2b, are provided by numerous manufacturers. Units for continuous operation (eg, APV Anhydro) utilize either a moisture product feed or a rewet system so that sufficient moisture is present to agglomerate the product. A typical unit has three fluidized zones. These include the entry or wetting zone, a drying zone, and a cooling zone.
Also, several mechanical agglomerators utilize mechanical mixers to provide the liquid addition, product interaction, and mixing to facilitate the agglomeration process.
The "SCHUGI" mixer (Bepex Corp.), shown in Figure 2d, has extremely short residence times (approximately 1 s) and has considerable flexibility in the types and amounts of feedstock. The system employs a flexible housing so that product buildup on the interior walls of the agglomerator is minimized, if not eliminated (7).
Jet agglomeration (Fig. 2e) has been used in the food industry for several years to produce agglomerates with favorable instant properties from fine powders. In a jet agglomeration plant, freely moving, wetted particles are made to collide with each other to form agglomerates. The solid material fed to the agglomerator consists of individ-
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