Thermal transitions in freeze-dried carrot and its cell wall components

Differential scanning calorimetry (DSC) was used to measure the glass transition temperature, T-g and a sub-T-g event in freeze-dried carrot material and different carrot cell wall components, at moistures ranging from 0% to 20% (w.w.b.). Two glass transition temperatures were detected in the freeze-dried carrot material associated with two phases: a sugar-rich phase and a cell wall-rich phase. The water distribution in the different phases is also important and sorption isotherms of freeze-dried carrot and different cell wall components were determined and compared with published results produced on sugar, pectins and cellulose. For the insoluble carrot cell wall materials, a single T-g was detected which increased in the order: a Ca2+ bound pectin-free residue, an esterified pectin-free residue and a cellulose-rich residue. In all cases T-g increased with decreasing moisture. Additionally, a sub-T-g endothermic event was observed in all the materials which disappeared on rescanning, consistent with results published on ageing of other biopolymers. The T-g of the different cell wall residues was modelled, using the Gordon-Taylor and Kwei equations. The latter showed that water-biopolymers interactions are less prominent in the cellulose-rich cell wall residue, shown by a negative value of q. (C) 1999 Elsevier Science B.V. All rights reserved.