Thermally induced dynamical and structural changes in lactose – maltodextrin system: insights from neutron scattering, thermal analysis and Raman spectroscopy

Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and Raman spectroscopy, augmented by neutron transmission (NT) and Compton scattering (NCS), were employed to investigate thermally-induced changes in local structure and dynamics in the amorphous and crystalline matrix in a lactose, maltodextrin and lactose-maltodextrin mixture.

Incubation at low relative humidity (33 %) produced a dehydrated sample containing a crystalline form of lactose characterised by enhanced thermal stability, as confirmed by DSC and TGA. Strikingly, these structural changes were accompanied by unaltered local dynamics, as characterised by the widths of hydrogen nuclear momentum distribution (NMD widths) obtained from the NCS.

Under higher humidity conditions (65 %), deviations in the shapes of the NT curves from the prediction of the Average Functional Group Approximation (AFGA) indicated melting of lactose crystallites above 120 °C in a lactose-maltodextrin mixture, which was also supported by DSC. Moreover, the constant stoichiometry inferred from the NCS confirmed the absence of sample decomposition (e.g., caramelization) in this temperature range.

Glass transition upon sample heating above 30 °C was monitored by Raman spectroscopy, DSC, and NCS in a lactose-maltodextrin mixture. Spectral shifts and peak broadening were observed in Raman spectra at the temperature region where DCS detected the glass transition. In the same temperature region, NCS measured NMD widths of protons that the AFGA predictions could perfectly model, indicating that the vibrational density of states lacks contributions from intermolecular vibration due to the loosening of the secondary structure.