An improved method for the measurement of colour uniformity in pellet coating

Sucrose pellets in the size range of 0.71--0.85 mm were film-coated in a bottom-spray Wurster fluidised bed coater with a colour coating suspension of 7.5% w/w solids. Hydroxypropyl methylcellulose was used as a film former, polyethylene glycol as a plasticiser and yellow iron oxide as a coloured pigment. The colour distribution on the film coat was analysed using a tristimulus colorimeter and the colour value was measured in CIELAB units. Uniformity in the colour coat was indicated by the standard deviation of the colour measurement values. Four different methods for measuring the colour distribution over the colour coated pellets' surfaces were carried out. In method I, colour measurements of the pellets' surfaces were made by placing the pellets directly on the stage of the tristimulus colorimeter. A specially designed pellet sample holder was employed to assist the collection of colour measurements in methods II--IV. Colour measurements from eight spots on each pellet were taken in methods I, II and IV while method III involved measuring 24 spots per pellet. A total of eight overlapping spot measurements were taken in method IV while the eight spot measurements in method II were non-overlapping. Method II was found to be the most efficient, accurate and sensitive method for the measurement of spot colour distribution on pellets.