Water binding to biopolymers in different cereals and legumes: proton NMR relaxation, dielectric and water imbibition studies

Abstract

Proton NMR relaxation time (T1), dielectric properties by means of the thermally stimulated depolarization currents (TSDC) method, and water imbibition were measured in cereal and legume grains (wheat, triticale, maize, pea, chick pea, horsebean, white lupin, lentil and beans) having different chemical composition (proteins, carbohydrates, lipids). T1 versus water content in the range 0.05–1.40 g water/g dry matter showed characteristic V-shaped curves with a sharp or a broad minimum depending on the species. Water content at T1 min was in high positive correlation with protein content of the grains (r=0.90) and in high negative correlation with soluble carbohydrates (r=−0.92), while lipids gave a very low correlation (r=0.38). The water content at T1 min (0.18–0.47 g water/g dry matter) was assigned to a primary hydration sphere around the macromolecules, since, when T1 was plotted versus per cent maximum hydration, the T1 min values for all grains fell between 25–30% of maximum hydration. The extrapolated value for zero protein content was 0.08 g water/g dry matter, which coincided with data in the literature for the water monolayer on starch. The TSDC measurements enabled us to determine the amount of tightly (irrotationally) bound water at primary hydration sites to 0.18±0.02 g water/g dry matter for beans, pea and chickpea, and, tentatively, to about 0.10 g water/g dry matter for wheat. Water imbibition data for 11 cereal and legume species gave total water hydration capacity in the range a=0.44–1.82 g water/g dry matter. This value divided by the water content of the primary hydration sphere (swelling index) was also in high positive correlation with the protein content of the grains (r=0.84).