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

Abstract

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 Ca 2+ 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.