Effect of storage temperatures on the head rice yield in relation to glass transition temperatures and un-freezable water

• Glass transition temperature (Tg) and un-freezable water affected head rice yield.
• Storage of rice especially above Tg could improve the head rice yield.
• Fissured rice should be stored at the temperature above Tg.
• This can reduce the broken rice or increase head rice yield during milling.

Head rice yield (HRY) in relation to glass transition temperatures and un-freezable water of waxy (San-pah-tawng) and non-waxy (Phitsanulok 2) rough rice were studied. Freezing points and glass transitions were measured by differential scanning calorimetry (DSC) as a function of solid fraction. Both waxy and non-waxy rice samples exhibited similar trends in glass transition when solid fraction was decreased. The transition temperature and enthalpy of ice melting decreased as the solid fraction increased. The on-set (i.e. initial) glass transition was increased from 27 to 35 °C when solid fraction increased from 0.80 to 0.97. Un-freezable water contents were found to be different for both rice samples. The rate of HRY (i.e. rate constant) increased differently (i.e. higher above glass transition) when rice samples were stored above (i.e. 35 and 45 °C) and below glass transition temperatures. It was found that storage of fissured rice (low HRY) at the temperatures especially above glass transition could relax the strains inside a rice kernel and subsequently improved HRY.