Vibrational spectra analysis of amorphous lactose in structural transformation: Water/temperature plasticization, crystal formation, and molecular mobility

•Vibrational signature of crystalline lactose structure is affected by water molecule.•Water insertion, H-bonding site exposure and structure disruption make plasticization.•Crystal formation is affected by aw and temperature based on Raman spectra analysis.•Spectral changes in OH vibrational region can link to Tg-related molecular mobility. Lactose is a common component found in many foods and dairy products. In this study, the vibrational signatures in the crystalline structure of α-, β-, and α-lactose monohydrate were calculated based on quantum chemistry calculation (QCC), whilst the vibrational spectra in freeze-dried lactose equilibrated at various aw and pre-humidified amorphous lactose (0.33 aw) stored from 25 to 95 °C were determined by using Raman and FT-IR spectroscopies. The vibrational signatures of crystalline lactose were affected by the presence of water according to QCC results. Water plasticization, involving water insertion, exposure of H-bonding sites, and structure disruption, was accelerated by storage temperature based on Raman and FT-IR spectra analysis. Raman spectra indicated that the crystal formation of lactose was affected by aw and storage temperature. Moreover, the spectral changes assigned in OH group provided useful information for determining the critical aw or temperature when Tg-related molecular mobility occurred in lactose-containing products.