(109h) The Amorphous Glassy State Of Spray-Dried Carbohydrate: Temperature And Humidity Influence On Ageing

Glassy amorphous carbohydrate polymers are widely used in various industrial applications in foods, pharmaceuticals, textiles, paper, and environmental technology. For cost efficiency reasons, the industrial production processes of such amorphous solid materials differ from the conventional way of producing a glass (the cooling of the melt). Differences in the vitrification route have a strong impact on the physical state of the end-product. Indeed, the glassy material being out of equilibrium, its physical state strongly varies with its history: preparation process and ageing. For example, in the conventional cooling way to prepare a glass, the energy level where the end-product is trapped depends on the cooling rate itself. Industrial processes like spray-drying or freeze-drying are characteristic methods of fast dehydration which are prone to produce a very specific amorphous state. Indeed, structural relaxations in glasses being non-exponential and non-linear, amorphous systems prepared in this way are submitted to ageing regimes depending on the preparation mode. This particular history of amorphous product production is thus expected to have a strong influence on the later stability evolution of the sample. We present here a DSC investigation of the annealing behaviour of a typical widely used carbohydrate polymer: spray-dried glucose syrup. Ageing of this compound was studied in different conditions of temperature and relative humidity. Indeed, the glass transition temperature (Tg) of glucose syrup is very sensitive to moisture. The results of this study show the formation of a sub-Tg endotherm whose peaking temperature depends on the annealing conditions (temperature, time, and relative humidity). Such behaviour was already observed for hyperquenched materials, which indicates that the energy level reached after spray-drying is high. Enthalpy interpretation of the presence of such an endotherm at temperature below Tg reveals the complexity and the broadness of the relaxation spectrum of spray dried glassy glucose syrup. Further to this latest observation, it seems that the fictive temperature concept applied to spray-dried systems is not enough to describe them in a univocal way. In addition, a systematic study of the position of the pre-endotherm with regard to the conventional Tg allows to improve knowledge in the nature of the glassy state industrially produced, and to clearly see the difference between temperature and water plasticization.