Abstract
The effects of mineral water hardness on the softening of kidney beans and Japanese radish roots during cooking were investigated. Mineral water samples with various levels of hardness were prepared by adding calcium sulfate or/and magnesium sulfate, and the firmness of the vegetables cooked in the water samples was measured by means of a rheometer. The microstructure of the parenchymal tissue of each vegetable was also observed by a low-vacuum scanning electron microscope. The mineral contents in the beans and roots after cooking were measured by the inductively coupled plasma method to evaluate the infiltration of minerals into the cooked vegetables. The greater the amount of minerals in the water sample, the more, minerals infiltrated into the vegetables. The higher the concentration of calcium sulfate, the greater was the suppression of softening of the kidney beans, whereas magnesium sulfate did not suppress softening even when used at a high concentration. The microstructural images of kidney beans cooked in calcium sulfate-added water clearly showed that deformation of the cotyledonary cells had been restrained. When the Japanese radish roots had been cooked, neither of the sulfates suppressed softening, regardless of the hardness of the mineral water sample, and there was no difference in the microstructure of any sample cooked for 20 min at low magnification.
