Optimization of an image analysis system for the determination of the crystal quality in sucrose crystal suspensions

Based on an off-line image analysis system consisting of a microscope, a scanning stage, a monochrome camera, a frame-grabber (analog-digital converter), a PC and the image analysis software ImageC® an analytical method was developed at the Berlin Sugar Institute to meet the requirements for the characterization of the crystal quality in sucrose crystal suspensions. The quantitative analysis is realized by the contour detection of objects (edge tracking with fixed gray threshold values) and the measurement and calculation of object characteristics. By carrying out several automation and adaptation steps, quality criteria such as crystal size, crystal size distributions (frequency as well as mass distribution), conglomerates and false grain content can be calculated automatically by a single course of analysis. The possibility of manual interaction guarantees the flexibility of the system. The accuracy of detection of objects was improved by mathematical filter operations which, for example, result in an increase of the contrast of object boundaries against the background or in cutting the overlapping crystals.

On the basis of the projected object area and the minimum Feret dia-meter, shape factors of sucrose crystals for the mass calculation were determined, which are to a great extent independent of the crystal shape and the amount of conglomerates. Therefore the calculation of mass distributions and the absolute numbers of crystals in the magma is possible. Significant object characteristics were compiled, which permit the classification into single crystals, conglomerates, false grain and foreign particles (air bubbles).