Caking of sucrose crystals: Effect of water content and crystal size

White crystalline sugar after crystallization, centrifugation and drying is generally stored in silos or conditioned in bags. It happens, relatively often, that caking is observed during silo storage or in the packages. To solve the problem of caking, a systematic study of the parameters affecting it should be undertaken. Among these parameters, water content, equilibrium relative humidity (ERH) and crystal size are the most important. The water content in sucrose crystals of different crystal sizes was determined using a Karl Fischer method adapted to the analysis of total water content by dissolving a first sample of sugar at high temperature (50 °C) in a modified solvent. The surface water was determined by continuing the analysis in the same solvent saturated with sugar, using a second sample of sugar. Analysis of the water content made possible the observation that internal water in the crystals is constant whereas surface water content increases with relative humidity. The water content of sugar increases rapidly with decreasing crystal size up to a particle size of 250 µm. To account for the caking phenomenon, a method of measuring the friability angle was used. It made it possible to show that fine particles (250 µm) cake at a relative humidity as low as 33%. Likewise adding 10% of fine particles to a standard sugar sample decreased the relative humidity of the beginning of caking from 80 to 45%. It appears, then, that the prevention of caking involves avoidance of the production of fine particles and to paying attention to the water content and relative humidity during storage.