Molecular Structures and Properties of Starches from Different Cultivars of Rice (Oryza sativa L. japonica) Produced in Japan

Abstract

This study was conducted to investingate the molecular structures and the properties of starches prepared from six different kinds of rice (Oryza sativa L. japonica) cultivars produced in Japan. These six kinds were Yukihikari, Kirara397, Koshihikari, Nipponbare, Koganebare and Reiho. The results can be summarized as follows: 1) Analysis of RVA viscogram revealed that three kinds of cultivars, Yukihikari, Kirara397 and Koshihikari, reached peak viscosity in a shorter period of time and showed higher break-down values and lower set-back values compared to the other three kinds. The former tended to gelatinize more quickly and to be more resistant to retrogradation than the latter. 2) An examination of endothermal temperature (To, Tp, Tc) showed that Koshihikari and Nipponbare had the highest value among the six kinds tested. For all the cultivars, a negative correlation was found between the blue values of starches and the levels of To and Tp. A similar relationship was also found between the enthalpy levels of the cultivars and the blue values of amylopectins. 3) Through X-ray diffractometry experiments, starches of Yukihikari, Kirara397, Koshihikari and Nipponbare turned out to have the A-type pattern. Starches of the other two had a similar pattern, but they also exhibited some characteristics of the C-type pattern. 4) Analysis of iodine affinities of starch, amylose and amylopectin revealed that all the rice starches experimented, except for Koshihikari, contained 1617% of amyloses, whereas Koshihikari starch contained only 14.6%. 5) The d.p.n of amyloses from the six kinds of cultivars ranged from 630 to 980, and these amyloses were found to form some branches. The d.p. of larger amylose molecules varied in weight in a wider range than that of smaller molecules. Particularly, Koganebare amylose contained the largest d.p.w molecules, while its c.l. was short and the n.c. large. 6) An experiment of β-amylolysis demonstrated that Koganebare amylopectin had the highest β-amylolysis limit. The c.l. of Koganebare amylose stood at 14 by the rapid SMITH degradation method as opposed to 28 by isoamylolysis. This difference appears to suggest that this cultivar is likely to have a different structure from the others. 7) The DSC data lead us to conclude that the length and the amount of the linear region of amyloses and amylopectins may influence the gelatinization properties of starches.