Structural, physicochemical and digestive properties of rice starch modified by preheating and pullulanase treatments

The structural, physicochemical and digestive properties of rice starch modified by the combination of different temperature (60, 70, 80, 90 and 100 °C) preheating and pullulanase (PUL60, PUL70, PUL80, PUL90 and PUL100) treatments were investigated. The PUL60 treatment mainly modified the surface layer of starch granules, which increased the amylose content and damaged some ordered structures, resulting in slight decreases of gel strength and estimated glycemic index (eGI). With the increase of preheating temperature, PUL could act on more enzymatic sites to release a large amount of linear chains, reduce the ordered degree, and transform the A-type crystalline structure into B-type. The low molecule interaction strength between linear chains weakened the gel network structure, and some stable crystal structures formed by longer chains resisted the enzyme digestion. The gel strength and eGI value of PUL70 starch decreased significantly, and the properties of PUL80-100 starches tended to be stable, showing a further significant decrease of gel strength and a slight reduction of eGI value. Therefore, the preheating treatments at 60, 70 and 80 °C were suitable for the PUL modification of rice starch to obtain strong, medium and weak gel strength respectively, and the digestibility decreased with increasing preheating temperature.