Solidification of molten hard fats in dependence on the chemical composition and thermal pretreatment

The solidification rate of molten hard fats strongly depends on the chemical composition and thermal pretreatment. This has been investigated by means of rheological oscillation measurements by subjecting the fatty substances to definite temperature cycles differing in their maximum temperature. This last-mentioned parameter strongly influences the beginning and the rate of the solidification. The different behaviour of several hard fat types can be attributed to the content of tri-, di- and mono-glycerides and to the composition of the fatty acids. Anisotropic particles have been found up to 56 °C in clearly molten vehicles with high hydroxyl values by polarization microscopy. Crystals with relatively high contents of partial-glycerides could be separated from these melts by ultracentrifugation. Further, the crystallization kinetic is influenced by drug substances incorporated as a suspension. The consequences of these results with respect to the optimization and validation of the manufacturing processes as well as to the quality assurance of drug preparations are discussed. Oscillation rheology has proved to be a sensitive method for controlling the solidification of hard fats.