Agglomerated food powder is a unit operation during which native particles are assembled to form bigger agglomerates, in which the original particle can still be distinguished. Agglomeration can be achieved through processes that use liquid as a binder (wet methods) or methods that do not involve any binder (dry methods).
Description
The liquid used in wet methods can be added directly to the product or via a humid environment. Using a fluidized bed dryer and multiple step spray drying are two examples of wet methods while roller compacting and extrusion are two examples of dry methods.
Advantages of agglomeration for food include:
Dust reduction: Dust reduction is achieved when the smallest particles (or "fines") in the product are combined into larger particles. Improved flow: Flow improvement occurs as the larger, and sometimes more spherical, particles more easily pass over each other than the smaller or more irregularly-shaped particles in the original material. Improved dispersion and/or solubility: Improved dispersion and solubility is sometimes achieved with instantization, in which the solubility of a product allows it to instantly dissolve upon its addition to water. For a powder to be considered instant it should go through wettability, sinkability, dispersibility, and solubility within a few seconds. Non-fat dry milk and high quality protein powders are good examples of instant powders. Optimized bulk density: Consistent bulk density is important in accurate and consistent filling of packaging. Improved product characteristics Increased homogeneity of the finished product, reducing segregation of fine particles (such as powdered vitamins or spray-dried flavors) from larger particles (such as granulated sugars or acids). As a powder is agitated, smaller particles will fall to the bottom, and larger raise to the top. Agglomeration can reduce the range of particle sizes present in the product, reducing segregation.
Disadvantages of food agglomeration:
Extra cost. The benefits of handling an agglomerate often outweigh the extra cost involved in processing. Additional processing time. Agglomeration of a finished blend is an additional step after blending.
Particle size distribution is an important parameter to monitor in agglomerated food products. In both wet and dry agglomeration, particles of undesired sizes must be removed to ensure the best possible finished product performance. High-powered cyclones are the most common way to separate undesired fine particles (or "fines") from larger agglomerates (or "overs"). Cyclones utilize the combination of wind power and the different densities of the two products to pull the fines out of the mix. The fines can then be reworked through the agglomeration process to reduce yield loss. In contrast, shaker screens are often used to separate out the overs from the rest of the product. The overs can be reworked into the process by first being broken into smaller particles.
Wet agglomeration methods
Wet agglomeration is a process that introduces a liquid binder to develop adhesion forces between the dry particles to be agglomerated. Mixing disperses the liquid over the particles evenly and promotes growth of the aggregate to the desired size. A final drying step is required to stabilize the agglomerates.
In all wet agglomeration methods, the first step is wetting the particles. This initiates adhesion forces between the particles. The next step, nucleation, is the process by which the native particles come together and are held with liquid bridges and capillary forces. Then, through coalescence or the growth phase, these small groups of particles come together to create larger particles until the particles are the desired size. Consolidation occurs as the agglomerates increase in density and strength through drying and collisions with other particles. Mixing as the powder dries also causes some particles to break and erode, creating smaller particles and fines. To achieve the correct particle size, erosion and growth must be balanced. The last step in wet agglomeration is the final stabilization through drying. The agglomerated particles are dried to less than 5% water content, and cooled to below their glass transition temperature.
Wet agglomeration falls into two categories based on method of agitation: Mechanical mixing and pneumatic mixing.