Abstract
Cooked rice boiled in hard water is considered to be harder than when boiled in soft water. In order to clarify this phenomenon, the visco-elastic and morphological properties were compared between rice grains cooked in water with and without calcium ions. The calcium concentration used for the Ca rice was 0.05 M of calcium lactate. Texturometric measurements indicated the hardness of the Ca rice to be significantly higher (20%) than that of the non-Ca rice. The averaged sizes of both the Ca rice grains and the endosperm cells were statistically smaller than those of the non-Ca rice from microscopic measurements. A histochemical comparison was carried out on the sections of cooked rice grains, focusing on the localization of proteins, pectin and Ca ions. Protein, pectin and calcium in the non-Ca rice were mostly localized in the endosperm cell wall forming the surface layer of the rice grain, being sparsely distributed at the center of the grain. Calcium was present in the locci of Ca rice containing protein and pectin and in the starch granules within. It is considered that the hardness of Ca rice was due to the prevention of swelling of the rice grains, thus reducing the water absorption, which may have been caused by interaction of the calcium and calcium-binding components such as proteins and pectin in the endosperm cells and starch granules.
