Effects And Mechanism Investigation Of Pullulan On The Properties Of Rice Starch

Starch is one of the most important and abundant materials in nature, and possesses extensive applications in food industry. However, native starch sometimes does not satisfy the requirements of production due to its retrogradation tendency and instability under shear and acidic conditions. Compared to starch system, starch-hydrocolloids combinations generally exhibit unique properties and are often used in food to provide proper texture, control moisture.In the present study, physical blending rice starch(RiS) with non-starch polysaccharides-pullulan(PUL) was investigated as a safe and effective method to modify the quality of native starch. The main objectives of this work were to investigate the effect of PUL on the pasting and rheological properties of RiS; investigate the effect of PUL on the retrogradation and digestible properties of RiS; and explore the possible interaction mechanism between RiS and PUL.PUL affected the pasting and rheological properties of RiS effectively and this influence was highly dependent on PUL concentration. Rapid viscosity analyzer(RVA) showed that PUL significantly changed the viscosity of rice starch-pullulan(RiS-PUL) mixtures. Generally, PUL caused a slight increase in peak viscosity at concentration of PUL ≤ 0.07%. However, a sharp decrease in peak viscosity occurred when the addition levels of PUL were higher than 0.07%. Dynamic rheological measurements revealed that the modulus(G′, G″) of the mixtures increased with the increase of PUL concentration from 0.01% to 0.07%, but decreased with the increase of PUL concentration from 0.07% to 0.50%. The pasting and rheological properties of samples indicated that PUL could blend well with RiS and promote the gelatinization of starch granules at low concentration of PUL, but suppress the gelatinization of starch granules at high concentration of PUL.Pullulan apparently influenced not only the short-term retrogradation of amylose, but also the long-term retrogradation of amylopectin. The effect of PUL on the retrogradation of RiS was investigated by means of RVA, rotational rheometer, differential scanning calorimetry(DSC), and X-ray diffraction(XRD), respectively. RVA results showed that the addition of PUL significantly decreased the breakdown and setback values, which meant that the short-term retrogradation of amylose was inhibited. The dynamic time sweep of samples also proved the retarding effect of pullulan on the retrogradation of RiS. DSC curves showed clearly that pullulan significantly reduced the retrogradation enthalpy of amylopectin. XRD results showed that recrystallinity of RiS was reduced from 10.421% to 8.841% with the addition of pullulan. The digestive properties of RiS as affected by pullulan were analyzed by using in vitro Englyst assay. The results indicated the conspicuous effect of PUL on the digestion of RiS.A follow-up investigation was performed in current study with the purpose of giving an interpretation about the interactions between PUL and RiS. The compatibility, hydrogen bond, and thermal stability of RiS-PUL mixtures were analyzed by means of DSC, fourier transform infrared spectroscopy(FTIR), and thermogravimetric analysis(TGA). Furthermore, the molecular weight distributions of amylose and amylopectin as affected by PUL were determined by HPSEC-MALLS-RI coupling system. When adding PUL, the weight-average molecular(Mw) of amylopectin was significantly increased, while no apparent changes in the molecular weight of amylose were observed. The Low Field-Nuclear Magnetic Resonance data indicated that the motility of water decreased with the addition of PUL. The pasting properties of RiS with high concentration(10%, w/w) were investigated in the presence or absence of PUL using a RVA, and an interesting phenomenon, i. e. a small viscosity peak appeared in the RVA curves of RiS-PUL mixtures, was observed. This peak became more visible and appeared at a later time with the increasing of pullulan concentration. This indicated that the addition of pullulan might suppress the gelatinization of starch granules by maintaining the integration of some granules. The scanning electron microscope(SEM) observation of samples suggested that starch granules could be wrapped by a thin membrane composed of pullulan and pullulan-amylose associations. The coating ability of PUL on starch granules, the competition effect of PUL on starch for available water, and the possible molecular interactions between PUL and starch were hypothesized to be responsible for these results.