•Modified starches with different branch chain length were prepared.•Branch chains of starch were mostly elongated before the precipitation occurred.•The modified starches containing longer chains had a better thermostability.•High RS content was attributed to the formation of long-ranged double helices.•The produced very long chains were highly sensitive to the enzymatic hydrolysis.Recombinant Amylosucrase (AS) from Neisseria polysaccharea was used to modify waxy corn starch, and the reaction process was monitored by the turbidity and transglycosylation degree (TD). Results showed that the modification by AS significantly elongated the chain length of the starches, resulting in the formation of precipitate. Based on regression equation, AS-modified starches with defined TD values were prepared, and chain length distributions of AS-modified starches indicated that the non-reducing ends of the branch chains were mostly elongated before the occurrence of precipitation. Differ from the native starch, all of the AS-modified starches showed a B-type crystal structure, and thermal analysis suggested that the AS-modified starches containing longer chains possessed a higher thermostability. High resistant starch content (33.8%–73.6%) of AS-modified starches was attributed to the formation of long-ranged double helices. Moreover, the very long chains, produced after the precipitation, might not generate the double helical structure during the retrogradation.Download high-res image (169KB)Download full-size image

•Properties of amylosucrase-modified native and acid-thinned starches were studied.•Crystallinity of amylosucrase-modified starch decrease when reaction time increase.•Stability of amylosucrase-modified starch increase when reaction time increase.•Resistant starches was isolated from the modified starches.•Resistant starch content was affected by the fraction of intermediate and long chains.Recombinant amylosucrase from Neisseria polysaccharea was utilized to modify native and acid-thinned starches. The molecular structures and physicochemical properties of modified starches were investigated. Acid-thinned starch displayed much lower viscosity after gelatinization than did the native starch. However, the enzyme exhibited similar catalytic efficiency for both forms of starch. The modified starches had higher proportions of long (DP > 33) and intermediate chains (DP 13-33), and X-ray diffraction showed a B-type crystalline structure for all modified starches. With increasing reaction time, the relative crystallinity and endothermic enthalpy of the modified starches gradually decreased, whereas the melting peak temperatures and resistant starch contents increased. Slight differences were observed in thermal parameters, relative crystallinity, and branch chain length distribution between the modified native and acid-thinned starches. Moreover, the digestibility of the modified starches was not affected by acid hydrolysis pretreatment, but was affected by the percentage of intermediate and long chains.

•Solubility of rice proteins was boosted (42 times) by freeze–milling.•Mechanism underlying solubilization was proposed.•pH profoundly influenced the effectiveness of freeze–milling.The solubilization of rice protein isolates (RPIs) has been regarded as one of the critical and challenging processes affecting commercial availability. Simultaneous treatment with freezing and milling (freeze–milling) combined with alkali pretreatment can remarkably increase the maximum achievable amounts of soluble RPIs by up to 42 times. This study investigates the mechanism of solubilization of RPIs by freeze–milling (RPIfm). Structural analyses reveal that milling causes proteins to unfold with ice crystals formed inside protein bodies. Fluorescent and Fourier transform infrared spectra show that RPIfm possesses disrupted hydrophobic surface and exposed hydrophilic inner groups. Size exclusion chromatography results reveal that RPIfm exhibits disaggregation and strong water–protein interactions. These results indicate that freeze–milling may be a promising manufacturing technique in food industry.

To investigate the action mode of amylosucrase (AS) on amylopectin, waxy corn starch (WCS) was selected as an acceptor. The effects of WCS dissolution method, reaction temperature, sucrose concentration and AS activity on transglycosylation degree (TD) were investigated. Under the selected reaction condition, the enzymatic reaction process was divided into two stages, i.e. before and after 0.25 h, of which the relations between TD value and reaction time were successfully described using a linear and a logarithmic function, respectively. Then, the elongated WCSs with different TDs were produced according to the theoretical reaction time calculated based on the regression equations. The chain length distribution of the elongated WCSs indicated that all of the branch chains of WCS were greatly elongated by AS before occurrence of starch precipitation. Afterwards, however, AS merely elongated the branch chains whose non-reducing ends were exposed on the surface of the precipitate.