TEMPERATURE SENSITIVITY OF NEAR-INFRARED SCATTERING TRANSMITTANCE SPECTRA OF WATER-ADSORBED STARCH AND CELLULOSE

Near-infrared spectroscopy was used to examine unmodified wheat starch and microcrystalline cellulose (MCC) at temperatures of -80 to 60-degrees-C and water activities of 0.22-0.94 a(w). A commercially available liquid transmittance cell was modified to permit collection of scattering transmittance spectra of powdered samples. Changes to the shape, peak location, and intensity of the 1400-1500 and 1900-2000 nm water absorption bands were measured as temperature varied. Results indicated that frozen water was not detected at any of the temperatures or a(w) examined. All water was strongly associated with the carbohydrate matrix so that bulk water was absent. Sharpening of spectral features was not evident at low temperature. However, temperature changes resulted in slight changes to the water absorption bands in the 1400-1500 nm and 1900-2000 nm regions. From -80 to 60-degrees-C, the 1900-2000 nm band shifted approximately 10 nm and 15 nm toward shorter wavelengths in starch and MCC, respectively. The direction of shift is consistent with the hypothesis that an increase in temperature results in a decreased proportion of hydrogen bonded water-to-water molecules.