:(Part V. The Properties of Insoluble Residues from Irradiated Amylomaize Starch)
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Both amylomaize and normal corn starches of 3% and 12% moisture contents were irradiated with gamma rays up to 60 Mrad to determine the changes in the chemical and physical properties by irradiation. Irradiated starches were washed with cold water untill the soluble carbohydrates were free and then dried. The insoluble residues were characterized by the determination of the mean degree of polymerization (DP), β-amylolysis and Sephadex G-50 permeation chromatography of the pullulanase hydrolyzates . The results were compared with those of NÄGELI amylodextrins prepared from the non-irradiated starches by steeping in 16% sulfuric acid at 38°C. 1) A. mylomaize and normal corn starches on irradiation with gamma rays became soluble in cold water. Under a dose of 60 Mrad, amylomaize starches of 3% and 12% moisture contents were solubilized 62% and 56%, respectively. Normal corn starch was more sensitive to this treatment than amylomaize starch, and the solubilities reached up to about 90% . Both irradiated starches showed a little changes in X-ray diff ractograms even at a dose of 60 Mrad. 2) The DP of insoluble residues from irradiated starches were 30-40. But those residues contained considerable amounts of high molecular weight components elut ed at the void volume on Sephadex G-50 column. In the elution patterns of the pullulanase hydroly zates of those residues on Sephadex G-50 column, a marked decrease in the amylose fraction of long chains (Fr. I) was observed, and Fr. III was eluted at lower molecular weight regions. In addition, the residues from amylomaize starch consisted of longer chain -length components than those of no rmal corn starch. X-ray diff ractograms of the residues did not show a signifi cant change as compared with those of the original starches . 3) On conversion of starches to the amylodextrins by acid tre atment, the sharpness and intensity of the X-ray diffractograms were both strengthened . Amylodextrins from both starches showed a very close molecular weight distribution patters on Seph adex G-50 column. 4) It is well known that acid hydrolysis occurs preferentially on the amorphous parts of starch granules. On the other hand, the properties of insoluble residues from irradiated starches significantly differed from those of amylodextrins. These findings suggest that the degradation of starch granules by gamma irradiation occurs at random in both crystalline and amorphous parts.
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