Differential Scanning Calorimetry Study of Glass Transition in Frozen Starch Gels KANITHA TANANUWONG AND DAVID S. REID* Department of Food Science and Technology, University of California, Davis, 1 Shields Avenue, Davis, California 95616 The effects of initial water content, maximum heating temperature, amylopectin crystallinity type, and annealing on the glass transition of starch gels were studied by differential scanning calorimetry (DSC). The glass transition temperatures of the frozen gels measured as the onset (T g,onset *) or midpoint temperature (T g,midpoint *), heat capacity change during the glass transition (ΔC p ), unfrozen water of starch gels, and additional unfrozen water (AUW) arising from gelatinization were reported. The results show that T g,onset * and T g,midpoint * of the partially gelatinized gels are independent of the initial water content, while both of the T g * values of the fully gelatinized gel increase as the initial water content increases. These observations might result from the difference in the level of structural disruption associated with different heating conditions, resulting in different gel structures as well as different concentrations of the sub-T g unfrozen matrix. The amylopectin crystallinity type does not greatly affect T g,onset * and T g,midpoint * of the gels. Annealing at a temperature near T g,onset * increases both T g,onset * and T g,midpoint * of the gels, possibly due to an increase in the extent of the freeze concentration as evidenced by a decrease in AUW. Annealing results in an increase in the ΔC p value of the gels, presumably due to structural relaxation. A devitrification exotherm may be related to AUW. The annealing process decreases AUW, thus also decreasing the size of the exotherm. KEYWORDS: Starch; glass transition; differential scanning calorimetry; gelatinization; annealing INTRODUCTION The processing of starch-based foods usually involves heating starch in the presence of water to a temperature above the gelatinization temperature, causing disruption of the starch granule structure. During gelatinization, the semicrystalline polymer structure in native granular starches is gradually transformed into an amorphous state, which is metastable and subject to time-dependent physical change (1, 2). An important example is recrystallization of amylopectin in starch gels, which greatly affects the textural properties of starch-based foods (2). Sufficient cooling of an amorphous polymer can induce a phase transformation of the rubbery amorphous matrix to a glassy, solid matrix. This transition, denoted as a glass transition, is evidenced by both a large increase in the viscosity and an immobilization of the polymer chains (1). In general, the glass transition largely relates to the changes in quality and storage stability of food products (1, 3, 4). Depending on the storage temperature and the composition of the system, the amorphous phase can exist in the glassy state, rigid and stable, or become rubbery and prone to physical and chemical changes (4). For a high moisture system, the glass transition temperature of a homogeneous amorphous matrix, T g,C (given the initial solute concentration of C c ) is predicted to be below the freezing temperature of the system, T m,C (Figure 1). During cooling, ice crystallization can occur before the system reaches T g,C . The system is then separated into an ice phase and an unfrozen phase. As the temperature lowers, more ice is formed, with a resulting increase in the concentration of the unfrozen matrix. At a sufficiently low temperature, this freeze-concentrated unfrozen phase solidifies into the glassy state and ice formation ceases because of kinetic restrictions (5, 6). In a system in which the maximum amount of ice is allowed to form, the glass transition of this maximally freeze-concentrated phase occurs at T g ′, which is independent of the initial solute concentration (7). T g ′ may be an important parameter for the quality and stability of frozen food systems, as a long-term stability may be anticipated for the product stored at a temperature below T g ′ (4, 8). If the maximum amount of ice is not formed in the system, the resulting unfrozen matrix will be more dilute. The glass transition temperature of this partially freeze-concentrated phase, denoted as T g *, is lower than T g ′. Along the T g curve, T g * will fall between T g,C and T g ′, depending on the concentra- tion of the unfrozen phase (the shaded gray area in Figure 1). The exact value of T g * will depend on the imposed conditions (9). For low moisture starch systems (13-30% moisture) after heating to over 100 °C, the glass transition temperature (T g ) decreases as the moisture content increases due to the plasticiza- tion effect of water. At a moisture content greater than 22%, T g