Rheology of Boiled Rice Grain and Starch Granules in the Gelatinization Process

Abstract

The taste of boiled rice depends primarily on various mechanical properties of boiled rice grains and the internal structure is responsible for these mechanical properties.(1) The time dependency of strain in compressed boiled rice, a Japanese delicacy known as“oshi-zushi”, during its formation process was studied by applying a given compressive stress. Most of the properties of compressed boiled rice grains could be explained by a mechanical nine-element model, and the components of the strain in each element were determined by the thickness of boiled rice grains before compression, the compressive stress, and compression time.(2) Twenty-four types of domestic rice were examined for their mechanical properties. There was a strong correlation between the taste category and the experimental results, including the initial compressive modulus, adhesive energy, total strain, and relaxation time.(3) Deterioration of the taste of boiled rice with the retrogradation time and its physical factors were studied. Generally, the change in the degree of crystallinity obtained from an X-ray diffraction intensity diagram correlated well with the change in the compressive energy obtained from compression experiments.(4) Changes of structure and mechanical properties were studied in the gelatinization process of suspensions of starch granules from white potatoes, non-glutinous rice, and non-glutinous rice pretreated by NaOH, and that of gel of starch granules from non-glutinous rice. Before the heat treatment, four-leaf clover images were seen in the light scattering photographs. As the gelatinization proceeded, a maximum appeared in the viscosity-time relation. Streaks and speckles were observed in the succeeding light scattering measurements.(5) Furthermore changes in the structure and mechanical properties of starch granules from white potatoes during the gelatinization process were studied. The light scattering photographs and polarization microscopy during the heating process demonstrated that there was a relationship between the disintegration characteristics of granules and change in viscosity. Also, there was a definite difference in the thermal resistance between the surface layers and the inner part of the same starch granule.