1. Introduction
To improve handling and product properties, enzymes are often incorporated into granules which are commonly produced by fluidised bed processes. As presented by prior work by e.g., Härkonen et al. [1] and Jørgensen et al. [2], inert carrier particles in the fluidised bed are typically coated with a formulated enzyme concentrate sprayed onto the particles through nozzles. The use of conventional top-sprayed fluidised bed systems finds wide use in pharmaceutical and food coating operations in pilot scale as well as in larger production scale, as presented by Rubino [3] and reviewed by Teunou and Poncelet [4]. The desired product is a product consisting of unagglomerated individual carrier particles each coated homogeneously with a layer of the enzyme. If formulation or process conditions such as fluidisation velocity, bed temperature, humidity of the air, spray rate and droplet size etc., are incorrectly chosen, either excessive agglomeration or excessive spray drying of the spray feed will happen. In both cases a poor product quality and loss will be the result. In general the optimum coating conditions with respect to product quality and capacity are close to the agglomeration conditions. Despite the wide use of fluidised bed processing, it is still not possible to predict this limit based on the formulation properties and the basic process conditions. Extensive experiments are needed in order to find the optimum process conditions and at the same time optimise the product properties.
Regarding product properties, the mechanical strength of the final coated granule is of vital importance. Due to handling, transportation, packing and further mixing with e.g. detergents, the enzyme granule is exposed to many types of mechanical stresses, causing degradation and breakdown of the coating layer [2]. This is unwanted not just for health and safety reasons but also because of reduced enzyme activity. Especially impact strength is important regarding mechanical properties of granules made in fluidised beds. Consequently, it is desired to optimise the impact strength by adjustment of the formulation and process conditions. It is the aim of this paper to address the tendency of agglomeration and impact strength quantitatively in terms of models build from statistical valid experimental data based on an unreplicated double 24 − 1 fractional factor design. In this paper four parameters were screened being the fluidisation velocity, the atomisation pressure, the coating solution salt concentration and the bed temperature