This paper investigates the layer formation in spray coating processes. Based on a Monte-Carlo simulation, a stochastic model of the coating layer thickness distribution was derived. It couples the stochastic process of droplet deposition on the particle surface with the droplet shape constructed from a spherical cap model and the droplets wetting properties (contact angle). The model was successfully shown to be able to replace the simulation. A parameter study revealed recommendations for designing a coating process, which were in agreement with the works from other authors. The model was then used to investigate the influence of overspray on the coating quality in comparison with experiments. It was found that the presence of overspray not only reduces the process efficiency but also increases the coefficient of variation of the resulting layer thickness distribution. This was caused by an increase in droplet size due to a predominant drying of small drops. It was also found, that a higher solid content of the spray solution increases the coefficient of variation, not only due to a decreased number of droplets, but also due to a greater variability in the layer thickness each droplet introduces.

A design of experiments approach was used to investigate the impact of bed temperature, volumetric air flow rate, spray rate and concentration of the coating solution on fluidised bed coating of particles. The process was analysed in terms of agglomeration tendency, efficiency and coating quality. Response surfaces were fitted to the experimental data from which optimal factor combinations were calculated in order to improve the coating quality. The coating quality was assessed by a previously developed method based on a dissolution test to efficiently measure the thickness, the uniformity and the completeness of the coating. It was demonstrated that the method provides a reliable way to assess the various measures for coating quality.

We present a rapid and gentle drying method for the production of high-grade tomato powders from double concentrated tomato paste, comparing results with powders obtained by foam mat air drying and freeze dried powders. The principle of this method consists of drying tomato paste in foamed state at low temperatures in vacuum. The formulations were dried at temperatures of 50, 60, and 70 °C and vacuum of 200 mbar. Foam stability was affected by low serum viscosity and the presence of solid particles in tomato paste. Consequently, serum viscosity was increased by maltodextrin addition, yielding optimum stability at tomato paste:maltodextrin ratio of 2.4:1 (w/w) in dry matter. Material foamability was improved by addition of 0.5% (w/w, fresh weight) egg white. Because of solid particles in tomato paste, foam air filling had to be limited to critical air volume fraction of Φ = 0.7. The paste was first pre-foamed to Φ = 0.2 and subsequently expanded in vacuo. After drying to a moisture content of 5.6% to 7.5% wet base (w.b.), the materials obtained were in glassy state. Qualities of the resulting powders were compared with those produced by freeze and air drying. Total color changes were the least after vacuum drying, whereas air drying resulted in noticeable color changes. Vacuum foam drying at 50 °C led to insignificant carotenoid losses, being equivalent to the time-consuming freeze drying method. In contrast, air drying caused lycopene and β-carotene losses of 18% to 33% and 14% to 19% respectively. Thus, vacuum foam drying enables production of high-grade tomato powders being qualitatively similar to powders obtained by freeze drying.

Fluid bed coating of powder is widely applied in the industry. Analysis of the production batch in terms of coating thickness and its distribution within the population cannot be performed easily. Microscopic analysis to accurately measure the shell thickness of a particle lacks the ability to give statistical information of the population within a reasonable amount of time. In this work, a novel method based on measuring the effect of the coating and solving the involved inverse problem is proposed as a fast way to measure the shell thickness distribution in the sample. The method was tested with sodium chloride particles coated with varying amounts of maltodextrin. It could be shown that coating thickness distributions between 0.2 and 20μm can be measured with significant differences between samples.