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

The water-binding properties of sucrose + beta -lactoglobulin and trehalose + beta -lactoglobulin prepared by freeze-drying, spray-drying and evaporat ion from solution have been studied using gravimetric methods. The hydratio n characteristics of the individual sugars are dependent on the method of d rying, and different isotherms have been recorded for each of the sample pr eparations. However, the initial hydration isotherms produced for the sugar + protein samples appear to be very similar for each sample type irrespect ive of drying method. with the sugar present in the amorphous glassy form i n all cases. There is evidence of interaction between the sugar in this for m and protein. specifically in the hydration region where single hydrogen-b onded water is expected to be bound. The magnitude of the interaction appea rs to be of the same order for both sugars. Irrespective of the method of preparation, rehydration of the sugar + prote in complexes above a critical value causes a transition resulting in the su gar adopting a crystalline form and phase separation of the sugar and prote in. In this crystalline form there is no evidence of interaction between th e sugar and protein. I;or sugar + protein samples prepared by evaporation f rom solution, a small amount (similar to3% by weight) of water is trapped i n the complex even under extreme conditions of dehydration.