An investigation of the water-binding properties of protein + sugarsystems

Abstract

The water-binding properties of sucrose + β-lactoglobulin andtrehalose + β-lactoglobulin prepared by freeze-drying, spray-dryingand evaporation from solution have been studied using gravimetric methods. Thehydration characteristics of the individual sugars are dependent on the methodof drying, and different isotherms have been recorded for each of the samplepreparations. However, the initial hydration isotherms produced for the sugar+ protein samples appear to be very similar for each sample typeirrespective of drying method, with the sugar present in the amorphous glassyform in all cases. There is evidence of interaction between the sugar in thisform and protein, specifically in the hydration region where singlehydrogen-bonded water is expected to be bound. The magnitude of theinteraction 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 thesugar adopting a crystalline form and phase separation of the sugar andprotein. In this crystalline form there is no evidence of interaction betweenthe sugar and protein. For sugar + protein samples prepared by evaporationfrom solution, a small amount (~3% by weight) of water is trapped inthe complex even under extreme conditions of dehydration.