Outer Shells And Blocklets Of Starch Granules As Indicated By Insufficient Gelatinization

Sufficient gelatinization of starch usually refers to granules swelling, rupturing, collapsing, and losing recognizable shape after heated to a certain temperature with sufficient moisture. Starch granules can also swell at limited level, rupture partly and thus keep identifiable shape by controlling the condition of gelatinization. This is called insufficient gelatinization. Structure of starch includes four levels: molecule, blocklet, shell and granule. Information for shell and blocklet of starch granules is limited. Tuber(sweet potato and tapioca), legume(pea and mung bean) and cereal(corn, wheat, and buckwheat) starches were investigated on their outer shells and blocklets of granules by using method of insufficent gelatinization. Then, the effect of acetylation on outer shells and blocklets of potato starch granules was studied. Two types of acetylating agents, vinyl acetate and acetic anhydride, were used and compared.The main conclutions were followed:Firstly, based on the pasting temperature determined by rapid visco analyzer(RVA), starch granules were insufficent gelatinized by lowering p H and temperature during treatment. Method for observing outer shells and blocklets of granules was established for these seven starches. The temperature of treatment was different. It was 5℃ and 10- 15℃ higher than the pasting temperature for tuber and legume starches, respectively. There was great difference among cereal starches. It was the same as the pasting temperature for corn, while 20℃lower and higher than their pasting temperature for wheat and buckwheat starches, respectively. The degree of gelatinization varied for seven starches after treatment. It was higher than 45% for both tuber and legume starches, and 40% for wheat starch, around 15% for corn and buckwheat starches. The lower degree of gelatinization of cereal starches was due to the higher toughness of outer shells for tuber and legume starches.Secondly, The outer shells and blocklets of seven types of starch, after insufficient gelatinization of granules, were observed by using scanning electron microscopy(SEM). The thicknesses of outer shells were about 50-570 nm, and the size of blocklets were about 30-150 nm. These values were diverse among seven types of starch. The shell thickness varied among granules for the same starch, and among positions for the same granule. The thickness of outer shells and size of blocklets did not show significant correlation with granule size, amylose content, and relative degree of crystallinity.Thirdly, based on the results of native starches, insufficient gelatinization method for observing outer shells and blocklets of granules was also established for acetylated potato starch. As compared with native potato starch, temperature of treatment was lower for acetylated potato starch. Acetyl group is hydrophobic, and gives steric hindrance. The addition of a few acetyl groups(DS<0.1) weakened the hydrogen bond between starch molecules. Outer shells and blocklets were observed by using SEM for acetylated potato starch granules after insufficient gelatinization. When compared with the native starch granules, in general, acetylated ones showed thinner outer shells and larger blocklets. For acetylated ones, the thickness of outer shells was 90-710 nm(native ones, 140-920 nm), the size of blocklets was 30-190 nm(native ones, 30-120 nm).Fouthly, the outer shells and blocklets of acetylated potato starch granules prepared by two types of acetylating agents(vinyl acetate and acetic anhydride) were compared. There was difference in thickness of outer shells, but not in size of blocklets. For the thickness of outer shells, those prepared by vinyl acetate was 90-710 nm, and those prepared by acetic anhydride was 140-600 nm. The results may be due to the different reactivity of two acetylating agents. Vinyl acetate has lower reactivity, the acetyl groups are homogeneously distributed among interior and periphery of the granules. While acetic anhydride has higher reactivity, more acetyl groups are distributed at periphery of the granules.