An investigation of the water-binding properties of protein + sugar systems

The water-binding properties of sucrose + beta-lactoglobulin and trehalose + beta-lactoglobulin prepared by freeze-drying, spray-drying and evaporation from solution have been studied using gravimetric methods. The hydration characteristics of the individual sugars are dependent on the method of drying, and different isotherms have been recorded for each of the sample preparations. However, the initial hydration isotherms produced for the sugar + protein samples appear to be very similar for each sample type irrespective of drying method, with the sugar present in the amorphous glassy form in all cases. There is evidence of interaction between the sugar in this form and protein, specifically in the hydration region where single hydrogen-bonded water is expected to be bound. The magnitude of the interaction appears to be of the same order for both sugars. Irrespective of the method of preparation, rehydration of the sugar + protein complexes above a critical value causes a transition resulting in the sugar adopting a crystalline form and phase separation of the sugar and protein. In this crystalline form there is no evidence of interaction between the sugar and protein. For sugar + protein samples prepared by evaporation from solution, a small amount (approximately 3% by weight) of water is trapped in the complex even under extreme conditions of dehydration.