Glass transitions in aqueous carbohydrate solutions and their relevance to frozen food stability

Abstract

During the freezing of aqueous low molecular weight carbohydrate (sugar) solutions, the solute is freeze-concentrated into a supersaturated solution of high viscosity that is capable of undergoing a transition into the vitreous (glassy) state. If the freezing process follows the equilibrium liquidus line, maximal ice formation results in a glass transition temperature known as the T′ g. However, it is apparent from freezing rate studies that the concentration of solute in the glass may be lower than expected as a result of less than maximal ice formation, resulting in a T g < T′ g. The addition of polysaccharides to sugar solutions has no significant effect on T′ g, but increases the mechanical properties of the solution at T > T′ g. Sugar and sugar/polysaccharide solutions can be used as model systems to determine the behaviour of several categories of frozen foods, such as fruits, frozen dairy desserts, or other types of sugar-added food systems. Storage of such frozen foods under conditions where the unfrozen, freeze-concentrated phase is in the vitreous state may greatly improve stability and shelf-life. An examination of the low-temperature thermal behaviour of carbohydrate solutions also provides considerable insight into the action of polysaccharides in improving the stability of frozen foods to ice recrystallization.