Thermal transitions of rice: Development of a state diagram

Abstract

A state diagram of basmati rice was developed by measuring the glass line; glass transition temperature vs. solids content, freezing curve; initial freezing point vs. solids content and maximally freeze concentrated conditions by using the modulated differential scanning calorimetry (MDSC) method. In addition, a cooling curve method was used to measure the initial freezing point (TF) and the end point of freezing (Tm ′). The glass transition line of the state diagram was modeled using the Gordon-Taylor and linear equations, and the pattern of freezing curve was modeled using the Chen equation which incorporated unfreezable water. The condition of maximal-freeze-concentration corresponded to Xs ′ (characteristic solids content) = 0.60 and Tm ′ (end point of freezing) = -3.6 °C obtained from freezing curve. The characteristics glass transition Tg ′ = -7.1 °C and corresponding solids content Xs ″ = 0.66 were obtained by extending the freezing curve up to glass line modeled using linear equation. The Tg ′ = -11.6 °C and Xs ″ = 0.82 were obtained when the glass line was modeled using Gordon-Taylor equation. Adsorption isotherms of rice were measured at room temperature by isopiestic method and data were modeled with BET and GAB equations. The BET and GAB monolayer values were found as 0.083 and 0.136 kg H2O/kg dry solids, respectively. The state diagram and water sorption properties of basmati rice can be useful in optimizing the drying and freezing processes as well as studying the physicochemical changes during storage. © 2008 Elsevier Ltd. All rights reserved.