Observation of Molecular Relaxation Behavior of Powdered Carbohydrates Using Low Field Nuclear Magnetic Resonance (NMR)

Molecular relaxation behaviors which explain changes in molecular mobility, for six different carbohydrate powders with various water activities were observed by measuring the spin-spin relaxation time constant $T_2$ at the temperature range of $-20\;to\;110^{\circ}C$ using temperature controlled low field nuclear magnetic resonance spectroscopy. According to the investigation of $T_2-temperature$ curves with changing water activity and scanning electron microphotographs in each sample, it was found that the sample structure basically causes characteristic differences in relaxation behaviors of carbohydrate powders. No change in the molecular mobility of crystalline fructose, crystalline glucose monohydrate and crystalline sucrose was observed at the ambient temperature when the sample water activity was higher than 0.5. On the other hand, molecular mobility seemed to increase in amorphous glucose with the water activity lower than 0.5 at the ambient temperature. The caking phenomenon was observed above the glass transition temperature $(T_g)$ for all tested monosaccharides. The molecular mobility $(T_2)$ of polysaccharides such as wheat flour and chemically modified com starch started to increase dramatically below $0^{\circ}C$, but decreased with increasing temperature without caking after reaching a maximum value around $40\;to\;70^{\circ}C$.