The role of mannitol in affecting the thermal transitions in carrot tissue

Dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) were used to observe thermal events in osmotically manipulated carrot. A glass transition of the concentrated amorphous solution (CAS), recrystallisation of water and ice melting were shown in the tissue by DMTA. DSC showed the role of mannitol as a plasticiser in freeze-dried osmotically manipulated carrot tissue, lowering the cell wall rich and sugar rich phase glass transitions. This cautions against the use of mannitol as an osmoticum for texture measurement since a decrease in stiffness could be due to shift towards a more rubbery state or a reduction of turgor. Processing treatments of air-drying-rehydrating, freezing-thawing or heating-cooling without prior osmotic treatment, induced consistently lower stiffness of carrot tissue compared to that of fresh specimens. The order of stiffness decrease was: fresh>drying-rehydrating∼heating-cooling>freezing-thawing. When osmotically manipulated carrot tissue was processed, stiffness decreased in comparison with osmotically treated specimens alone for molarities less than isotonic. Stiffness could not be restored further to any process events after osmotic changes, demonstrating the irreversible effect of processing on stiffness and contrary to some earlier reports that a prior decrease in turgor would enable the mechanical properties to be recovered.