Comparison in glass transition and enthalpy relaxation between native and gelatinized rice starches

Native and gelatinized rice starches were compared in their glass transition and enthalpy relaxation at various water contents using a differential scanning calorimetry (DSC). In a low moisture content range (8–18%), the glass transition temperature (Tg) of native starch was higher (up to 20°C) than that of gelatinized starch, and the difference became greater as the moisture content decreased. Heat capacity change (ΔCp) at Tg became substantially higher by gelatinization. Plasticizing effect of water on the glass transition in the low moisture content range followed the Couchman–Karasz equation. The glass transition temperature (Tg′) of native starch with sufficient moisture (40 or 60%) also appeared higher (−6.8 or −6.0°C) than that of gelatinized starch (−10.0 or −7.7°C), but ice-melting occurred in broader temperature range with smaller ΔH when the starch was gelatinized. Upon extended storage up to 14 days at 4°C, the gelatinized starch showed increased Tg′ but decreased ice-melting enthalpy due to the water incorporation in recrystallization of starch. Enthalpy relaxation appeared only when the moisture was <20% regardless of gelatinization. The relaxation peak increased in magnitude as the moisture content increased, and appeared as ‘Tg overshoot’ at a moisture content above 12% due to superimposed glass transition, whereas at a moisture content below 12%, it located in a temperature range far below glass transition, showing a ‘sub-Tg endotherm’.