Quantitative determination of bound water in wheat starch by time domain NMR spectroscopy

Abstract

Above a water content of about 0.42 g per g of dry starch (DS), the Carr–Purcell–Meiboom–Gill (CPMG) relaxation-curve analysis of wheat starch–water suspensions shows two components. The intensity of the short component ( T 2≈1.5 ms) corresponding to ‘bound’ water increases over the full hydration range studied from 0.55 to 0.9 g of water per g of DS. Assuming an intermediate exchange rate between ‘weakly bound’ and ‘bound’ water at 20 °C, a constant ‘bound’ water content of 0.170±0.008 g of water per g of DS has been obtained. The existence of a two-site exchange process has been argued to validate this amount of ‘bound’ water. This result takes into account the fact that about 7% of the starch gives a ‘liquid-like’ signal. A complete treatment of the FID signal (solid and liquid parts) allowed us to determine the signal in arbitrary units per atom g of hydrogen present in the samples. Thus, we have been able to transpose the signal obtained by a relaxation curves analysis (in arbitrary units), to grams of water. This ‘bound’ water, determined through NMR CPMG sequence, would correspond to a bilayer of water. The concept of ‘potentially free’ water has been introduced to explain the lack of noticeable ‘weakly bound’ water by a CPMG sequence in the range of 0.17–0.42 g water content. The NMR data are discussed in relation to similar measurements made on sorption curves and in relation to plasticization. Overall, the concept of several populations of water molecules presenting different mobilities is reinforced.