| Starch is the essential component of most food raw materials,which is widely used in food industry.However,natural starch is prone to aging,and poor stability during production,circulation and storage,and its application range is limited.In recent years,The combination of hydrophilic colloid and natural starch to adjust the properties of natural starch and improve the quality of starch based food has become a research hotspot.Pectin is an excellent hydrophilic colloid,commonly used as food thickener and stabilizer.In this thesis,pectin was compounded with waxy rice starch to study the effect of pectin on the gelatinization,rheological and retrogradation properties of waxy rice starch,preliminarily explore the interaction between pectin and waxy rice starch,and explore the effect of pectin on the quality of quick-frozen glutinous rice dumplings.The main research contents and results are as follows:Firstly,the effects of pectin on the gelatinization and rheological properties of waxy rice starch were studied.The median size(D50),transparency,swelling power,solubility,texture,freeze-thaw stability,and thermal properties of waxy rice starch paste with pectin（0%,2%,4%,6%,8%and 10%,w/w）were also studied.The results of rapid viscosity analyzer showed that when the pectin content increased from 0%to 10%,the peak viscosity of waxy rice starch increased from 3415.5 c P to 3548.5 c P,and the gelatinization temperature increased from75.2°C to 78.7°C.At the same time,the breakdown value and setback value decreased significantly.The static rheological results showed that the waxy rice starch pastes with and without pectin showed shear thinning behavior.The dynamic rheological results showed that the storage modulus（G’）and loss modulus（G’’）of the composite system increased with the increase of pectin content.At the same time,the D50,transparency,swelling power and solubility of the composite system decreased with the increase of pectin addition.The hardness,adhesiveness and chewiness of the composite gel decreased,but there was no significant change in springiness and cohesiveness.Pectin could inhibit the dehydration of starch gels during freeze-thaw cycles.After five freeze-thaw cycles,the dehydration rate of composite gel significantly decreased.Differential scanning calorimetry（DSC）results showed that the addition of pectin could compete with waxy rice starch particles for water absorption,thereby reducing the hydration of starch granules and enhancing the thermal stability of starch.Secondly,the effects of pectin on the retrogradation and digestibility of waxy rice starch were investigated.The prepared natural waxy rice starch paste and waxy rice starch-pectin complex were stored at 4°C for 7 d,14 d and 21 d.The effects of pectin on the retrogradation properties of waxy rice starch were studied by DSC,texture analyzer,X-ray diffraction（XRD）and low-field nuclear magnetic resonance（LF-NMR）.DSC results showed that pectin could significantly reduce the retrogradation enthalpy of waxy rice starch.The addition of pectin measured by texture analyzer could inhibit starch retrogradation and recrystallization,thereby reducing the hardness of the gel.XRD results further showed that the addition of pectin could significantly reduce the relative crystallinity of the composite system,and the higher the pectin content,the lower the relative crystallinity.When the pectin content was 10%,the relative crystallinity of the composite system was only 10.55%after 21 d of cold storage.The results of LF-NMR showed that after 21 d of cold storage,the T23 free water signal appeared in the natural waxy rice starch paste,while only a small free water signal appeared in the compound system with 2%pectin addition,which further confirmed that pectin could inhibit the long-term aging of waxy rice starch.Digestion experiments showed that pectin could improve the digestibility of waxy rice starch to some extent,and reduce the starch digestion rate and digestibility.When the pectin content increased from 0%to 10%,the digestibility decreased from 82.31%to71.84%.Thirdly,the interaction mechanism between pectin and waxy rice starch was further studied.The SEM results of the samples demonstrated that pectin was wrapped on the surface of starch granules,thus inhibiting the expansion and subsequent gelatinization of waxy rice starch.The hydrogen bond interaction between pectin and waxy rice starch was analyzed by Fourier transform infrared spectroscopy.The results proved that the hydrogen bond interaction between pectin and waxy rice starch was mainly hydrogen bond interaction.With the increase of pectin content,the intermolecular force between pectin and waxy rice starch increased,which inhibited the intermolecular interaction of starch and slowed down the aging of starch.The effect of pectin on the molecular weight distribution of waxy rice starch was analyzed by high performance size exclusion chromatography.The results indicated that the interaction between pectin and waxy rice starch increased the number-average molecular weight Mn,weight-average molecular weight Mw and rotation radius Rz of the composite system.Finally,the effects of different pectin additions（0%,2%,4%,6%,8%and 10%,w/w）on the quality of quick-frozen glutinous rice dumplings were investigated.The results showed that with the increase of pectin content increased from 0%to 10%,the water loss rate and freezing cracking rate of frozen dumplings decreased significantly from 2.12%and 23.61%to 1.06%and 9.72%,respectively.The addition of pectin to some extent delayed the quality deterioration of dumplings during freezing.At the same time,the addition of pectin significantly increased the transmittance of the soup after cooking,and inhibited its depulverization and collapse.The results of texture test showed that the hardness,adhesiveness,springiness,cohesiveness,and chewiness of quick-frozen glutinous rice dumplings decreased after adding pectin.Sensory evaluation showed that the addition of pectin improved the edible quality of quick-frozen glutinous rice dumplings,reduced the cohesiveness of quick-frozen glutinous rice dumplings,and made them taste good without sticky teeth,thus having a better sensory experience. |
