THERMODYNAMIC ACTIVITY OF WATER AND SUCROSE AND THE STABILITY OF CRYSTALLINE SUGAR

Practical problems like caking and stickiness of crystalline sugar aft er drying and curing of sugar cubes need to be solved with a minimum o f theory. As each of the crystals is surrounded by a thin film of syru p, the thermodynamic and kinetic conditions for this film to reach sat uration stability after recrystallization of sugar in excess are usefu l to define. Such conditions are dependent on water activity and sucro se activity in the film submitted to varied temperatures and water vap our pressures during storage. After recall of water-sugar interactions in saturated solution and the relation giving water activity of sucro se solutions, taking into account the hydration number = 5, the author s summarize the work of the late Professor Andrew VanHook on the activ ity of sucrose in the film surrounding the crystal. The thermodynamic and kinetic aspects of crystal growth relevant to the treatment of the subject, stability of crystalline sugar, are recalled, mainly based o n papers by Andy VanHook. Experimental results of study of the curing of sugar lumps are reported. Interpretation of the evolution of moistu re and water activity during the storage of the cubes is made. It is s uggested that simultaneous crystallization and release of water are ne eded to obtain stability of the product. This requires that the heat o f crystallization be used to evaporate the residual moisture at about 60-degrees-C, the glass transition temperature of amorphous layer that might stick on the surface of the crystal, proves to be helpful in ac hieving a good curing at normal temperature and short time.