Structural and rheological properties of normal rice and potato starches with amylosucrase-treated starch added

Abstract

Rice and potato starches with low digestibility were prepared using amylosucrase(AS) from Neisseria polysaccharea and mixed with raw starches in different ratios (0%, 25%, 50%, 75%, and 100%). These mixtures were compeletely gelatinized and retrograded for 14 days at 4℃. The blending of unmodified raw starch and modified starch caused the changes of structural characteristics such as branch chain length distribution, X-ray diffraction and thermal properties, leading to the changes in digestibility and texture properties. The increase in the chains of DP ≥ 13 was observed in the AS-treated starches and starch mixtures, while the short chains of DP ≤ 12 decreased compared with raw starches. Differential scanning calorimetry showed that the onset, peak and conclusion temperatures increased with an increase in the proportion of AS-treated starches. Especially, the enthalpy of starch mixtures was lower than that of native starch and laid between those of the respective raw and AS-treated starches, suggesting that the starch mixtures had a low tendency towards retrogradation compared with individual raw and AS-treated starches. After AS treatment, X-ray diffraction pattern of rice starch changed from A to B-type, whereas potato starch maintained the original B-type pattern. All starch mixtures exhibited B-type pattern, and their relative crystallinity increased with an increase in the proportion of AS-treated starch. The contents of SDS and RS were proportional to the amount of AS-treated starch. In texture analysis, the starch mixtures showed lower or similar gel hardness compared with the respective individual starches, unmodified starch and modified starch. The chewiness exhibited a similar tendency to by hardness and cohesiveness. Especially, chewiness and cohesiveness were related to microstructure of starch mixture gels. The samples with low values of cohesiveness and chewiness exhibited more loose structures and contained some empty spaces. In summary, the starch mixtures showed different structural and rheological characteristics according to the amount of AS-treated starch added. These mixtures maintained low digestibility and reduced the rheological disadvantages of AS-treated starches.