Ageing of starch based systems as observed with FT-IR and solid state NMR spectroscopy

Abstract

The retrogradation and physical ageing of model starch systems with respect to their glass transition temperatures Tg have been investigated by Fourier transform infrared Spectroscopy and solid state NMR spectroscopy. Diffuse reflectance Fourier transform infrared (DRIFT) spectra demonstrate the commencing retrogradation of starch materials stored above their Tg by changes in peak lineshapes and intensities in the characteristic area between 995 cm-1 and 1020 cm-1. Solid state NMR proton relaxation times in the rotating frame (proton T1) show a characteristic course in relation to the storage conditions (time, humidity), for which a distinction is made between physical ageing which occurs below the Tg, and recrystallisation (retrogradation) which takes place above Tg. The proton T1s of materials stored below Tg increase asymptotically in time due to physical ageing, whereas the proton T1s of materials stored above Tg increase until a moisture content is reached that rises them above Tg, decrease due to further water absorption and then increase due to recrystallisation (retrogradation).