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Phase change in amylose-water mixtures as seen by Fourier transform infrared.

The phase content of amylose-water mixtures (0.7/0.3 w/w) has been analyzed by transmission Fourier transform infrared (FTIR) spectroscopy in the 1175-950 cm(-1) region. Spectra are obtained under three different conditions: (a) as a function of temperature (T) from 25 to 97 degrees C; (b) at room temperature (RT), after slow cycles of T; and (c) at RT after quenching. T(max), the maximum temperature in the cycle, ranges from 50 to 120 degrees C. The quality of the seven-band spectra allows for an unambiguous determination of each band area. Unexpectedly, slow cooling after different T(max) brings about wide changes in the spectra while quenching does not. Two jumps in the absorbance are found: one at 70 degrees C and the other above 105 degrees C. Previous work on slow calorimetry of amylose-water mixtures suggests that these temperatures correspond to the beginning and the end of the same physical phenomena that takes place slowly between these two temperatures-namely the dissolution of the strained network phase. The spectra have two distinct regions, the low wavenumber region (1078-950 cm(-1)) and the high wavenumber region (1175-1078 cm(-1)). A distinct gain in the integrated absorbance of the 1175-1078 cm(-1) region at the expanse of that of the 1078-950 cm(-1) region when T(max) increases is interpreted as a change from strained to unstrained environments. A nonequilibrium state between the chains is a strained environment. In light of the (13)C NMR evaluation of the change of molecular order with T, the observed changes of the ir spectra could correspond to a transformation of a network of double helices into freer chains, possibly single helices. The present in-depth analysis of equilibrium or near equilibrium states by FTIR can serve to understand, through in situ spectra, molecular mechanisms during the gelation/crystallization of amylose and other gel-forming polymers.