Effect Of Hydrocolloids On The Physicochemical Properties Of Rice Starch

Gluten-free products based on rice flour can provide alternative foods that fulfill the needs of gluten allergic people. However, due to the lack of gluten in rice flour, the rice bread or other gluten-free products always have lower gas-holding capacity than that from wheat flour. On the other hand, these gluten-free foods are easy for retrogradation during storage. Hydrocolloids could interact with rice starch granules, which would promote the formation of viscoelastic network, thus improving product quality, gas-holding capacity and inhibit the retrogradation.The effects of various food hydrocolloids (high-acyl gellan gum, low-acyl gellan gum, curdlan, xanthan gum, carrageenan, guar gum, locust bean gum, gum arabic, pectin, HPMC, CMC) on the physicochemical properties of rice starch were systematically studied in this paper. Pasting properties, thermal properties, rheological properties, microstructure, retrogradation and other properties of hydrocolloids-rice starch system were all investigated. Moreover, the possible interactions between different hydrocolloids and starch granules were discussed according to the result of experimental data. Finally, this paper provides a preliminary probe into the possibility of food hydrocolloids being applied to the development and quality improvement of gluten-free rice bread.The main contents and conclusions of this work are listed as follows:At first, the effects of 11 kinds of food hydrocolloids on the pasting properties of rice starch were comparatively analyzed by Rapid Visco Analyser (RVA), swelling power and percentage of starch leaching, differential scanning calorimetry (DSC) et al. It was found that, the addition of all hydrocolloids significantly (P<0.05) alter the peak viscosity and setback value of rice starch. Most hydrocolloids significantly (P<0.05) affect the valley viscosity, except from HPMC and curdlan. Carrageenan and pectin significantly (P<0.05) increased the thermal stability of rice starch paste. CMC enlarge the swelling power of rice starch, and improve its thermal stability. The short-term retrogradation of rice starch was inhibited by all food hydrocolloids used in this study. The swelling starch granules can be wrapped in the CMC, hindering the rupture of its structure, and minimize the setback of rice starch (Compared to the blank, the setback value decreased 44%). High-acyl gellan and xanthan gum reduces the mobility of amylose granules in the process of gelatinization and prevent the rupture of swelling starch granules. The impact of guar gum on starch pasting properties was hydrogen bonding interactions and changes of relative concentration of hydrocolloids solution. Hydrocolloids molecules can interact with leaching amylose granules, so that the stretching of network structure in paste system is suppressed. In addition, strongly hydrophilic hydrocolloids reduce the free water content that was required by starch granules in the procedure of rearrangement and association. All the experimental hydrocolloids increase the peak pasting temperature (Tp) of starch in varying degree, and inhibit the gelatinization of starch.Secondly, the effects of 11 kinds of hydrocolloids on the rheological properties of rice starch gels were investigated by dynamic rheometer. Possible interaction between food hydrocolloids and starch granules were also discussed, combining with scanning electron microscopy (SEM). The addition of food hydrocolloids increase the G’,G" of starch gels, and all gels exhibits typical "weak gel" properties. CMC, high-acyl gellan and xanthan gum immensely increases the viscoelasticity of starch gel, and reduces the frequency dependence of G" in the range of low angular frequency. Steady flow curves were successfully simulated with power-law model. Mixed system that starch and hydrocolloids formed exhibited a typical pseudoplastic behavior and shear thinning characteristics. Experimental hydrocolloids decrease the thixotropic property of gels in varying degree. Xanthan gum, locust bean gum, guar gum and CMC considerably reduce the thixotropic loop area, and respectively reduced by 24.9%,25.4%,41.0%,33.2% compared with the blank thixotropic loop area. High-acyl gellan gum, xanthan gum, guar gum, HPMC and CMC were significantly (P<0.05) increased the internal structure recovery rate of the starch gels after high-speed shear. All the hydrocolloids used in this study alter the microstructure of starch gels in varying degree. Different surface morphologies closely related to the interaction between colloids molecules and starch granules.Finally, the influences of 11 kinds of hydrocolloids on the retrogradation degree, water distribution and textural properties of rice starch gels during the cold storage were studied. DSC results show that the different food hydrocolloids have a greater impact on the retrogradation enthalpy, and inhibited the long-term retrogradation of rice starch gels with varying degree. The retrogradation of rice starch was considerably inhibited by xanthan gum, guar gum and gum arabic, etc. Approximate temperature that crystals of starch gels melting was between 50-60℃. The long-term retrogradation has a dominant influence on the hardness changing of rice starch gels. CMC, xanthan gum, carrageenan and HPMC are preferable colloids for the improvement of elastic properties of rice starch gels during chilled storage. With the increasing of retrogradation degree, the combination between water and non-water ingredients become more and more closely. Hydrocolloids strengthen the water-holding capacity of starch granules in the retrogradation process, and hindered the migration of macromolecules.The application of different hydrocolloids in the making of gluten-free bread based on rice flour was preliminarily investigated by concerning the above-mentioned physicochemical properties and mechanism study. It was found that the order for the influence of experimental colloids on rice bread specific volume was listed as follow: HPMC>CMC>xanthan gum> locust bean gum> pectin> carrageenan> blank≥guar> low acyl gellan gum> curdlan≥arabic gum> high-acyl gellan gum. Finally, the optimum formulation of gluten-free rice bread was obtained by single factor experiments and response surface optimization method.