The increasing market for convenience food with its need for speciality products and tailor-made additives demands new production solutions. Here, Gudrun Ding looks at the increasing role and benefits played by the pelletisation of food additives.
Some years ago, the detergent industry introduced with megapearls a new type of washing powder. Since then, the spherical and compact structure of the pellets has attracted more and more attention for use in product innovations.
Dust formation of enzyme products causing allergic reactions can be significantly reduced.
Food additives as vitamins, minerals or plant extracts in pellet form provide better flowability and simplify capsule filling.
The compact structure and dense surface of the pellets facilitate later encapsulation by spray-coating to achieve controlled release products. Extrusion spheronisation allows liquids, oily or aqueous, to be embedded in a matrix.
These are a few examples of the wide range of applications where pellet formulations provide technological advantages.
Furthermore, the regular and spherical shape also meets high marketing claims for appearance and convenience.
The process
Pelletisation is a multi-stage process, starting with mixing, followed by extrusion, spheronisation and then a final drying step.
The extrusion process requires a wet mass which can be pressed into a rod. Therefore, formulation and moisture content are the key factors to achieve an optimal pellet quality. A certain plasticity of the product is required and can be improved by adding starch or cellulose.
The liquid is applied by intensive mixing in a high shear granulator to achieve a uniform distribution of the moisture. The dough is then passed through a punched screen forming a spaghetti-like rod.
Two types of extruders are mainly used for extrusion processes: screw and rotary extruders. As screw extruders operate with higher pressure, the temperature of the product increases significantly so that cooling of the extruder head might be necessary.
Using a rotary extruder the dough is passed with low pressure through the screen. The product temperature does not exceed 40°C a temperature which is tolerable even for heat sensitive products like enzymes. The throughput of the extruder depends on the diameter of the holes in the screen. Pellets with a diameter of less than 1mm can be produced up to 0.6t/h.
Centrifugal forces
To achieve the regularly shaped pellet, the rod must be broken into regular cylindrical pieces. The rotating disk inside the spheroniser applies the necessary centrifugal forces to crack the rod into pieces and round them off. Rotor speed, time and moisture are important parameters influencing the sphericity and quality of the pellet.
Drying of the pellets is ideally carried out in a fluid bed unit (Fig.1). Intensive mass and heat exchange in a fluid bed system warrants the gentle and effective drying. The pellets are fluidised by heated unsaturated air of 80°100°C whereas the product temperature drops to about 3040°C due to the cooling effect of evaporation. After drying, fines and oversized particles are removed in a final sieving step.
The losses are between 13per cent and can be recycled to the mixing step after milling. This provides an economical production process which is not only interesting for high price products.
Flowing properties
Pellets are non-dusty, regularly shaped particles of a size between 0.72mm providing perfect flowing properties.
The formulation can consist of different ingredients which are stabilised in one dosage form.
For multivitamin products capsules can even contain several pellet formulations. Due to the particle size and the regular surface, additional film coating can be applied economically. Temperature controlled release or moisture protective coatings are only a few possible examples.
For these applications, pellet formulations will gain more and more importance.
enquiry no 23
Gudrun Ding is with IPC Process Center, Schweinfurt, Germany. For further information email: ipc@ipc-dresen.de
