Studying relaxation phenomena in fruit and vegetable pectin-containing cell wall material: a focus on methodology and...

This study characterizes the relaxation phenomena, particularly the α-relaxation, in the semi-dry alcohol-insoluble residue (AIR) of apple, carrot, and tomato across a relevant moisture content (MC) range. It examines changes in thermal properties via differential scanning calorimetry (DSC), compressibility through thermal mechanical compression testing (TMCT), and viscoelastic properties using dynamic mechanical thermal analysis. Chemical characterization was conducted to link relaxation behavior to polymer composition. DSC showed minimal changes at the relaxation temperature (T_r), but significant differences in relaxation behavior, represented by the T_r-line, among botanical origins were observed. A higher ratio of amorphous polysaccharides to crystalline cellulose and greater pectin branching correlated with lower T_r-values. Mechanical techniques provided clearer signals than DSC due to their direct link to molecular mobility changes with increasing MC or temperature. Except for the carrot AIR T_r-line obtained by TMCT, the AIR T_r-lines were not significantly different among the matrices when assessed by mechanical techniques. This study highlights fundamental aspects and challenges in determining relaxation phenomena in AIR and identifies general trends related to composition. The methods and insights can be used in future studies on the role of cell wall material composition in relaxation phenomena and in predicting associated physical collapse phenomena.