Mechanism Study Of Different Pasting Properties On Wheat Starch With Different Damage Levels

During wheat milling, mechanical forces of mill roller such as shear, extrusion and friction lead to mechanical damage on a portion of the starch granules, which are called damaged starch. Mechanical damage profoundly changes the structure and properties of starch, thus affecting the starch processing and food quality. This paper focused on the pasting properties of wheat starch, studying the structure and molecular dissolution behavior of wheat starch with different damage levels. The influences of the mechanical stirring speed and hydrophilic colloid on pasting properties of wheat starch were also evaluated. The aim of this study is to further understand the nature of mechanical damage effect on starch properties, and provide theoretical reference to control the effect of the mechanical damage on food quality.The effect of mechanical damage on pasting and thermal properties of wheat starch was studied by Rapid Visco Analyser(RVA) and Differential Scanning Calorimetry(DSC). The results showed that mechanical damage could significantly affect pasting properties of wheat starch. Peak viscosity, trough viscosity, final viscosity, breakdown, setback measured by RVA and gelatinization enthalpy, gelatinization temperature measured by DSC decreased with the enhanced level of damage. The above parameters were significantly negatively correlated with damaged starch content except gelatinization temperature(p<0.05).The structures of granule, crystal and molecular were studied to understand the effect of mechanical damage on wheat starch structure. The results showed that in terms of granule structure, with the increase of damage degree, the surface of wheat starch granule was more rough, the shape was more irregular, and more pieces of small particles appeared; when starch granules were in the water, granules had greater water absorption capacity and granular size showed a greater heterogeneity caused by damage effect; under the action of shear force, with the increase of damage degree, the viscosity of wheat starch slurry increased and the extent of thixotropy enhanced. In terms of crystal structure, when wheat starch was subjected to mechanical damage, there was no change in crystal type while the relative crystallinity of starch was significantly negatively correlated with damage degree(r=-0.970, p<0.05), and was very significantly positively correlated with DSC gelatinization enthalpy(r=0.994, p<0.01), it follows that mechanical damage caused a decline in the amount of the crystal structure of starch, leading to lower gelatinization enthalpy, besides, lower gelatinization temperature could be correlated with the decrease of thermal stability of the crystal induced by mechanical damage. In terms of molecular structure, within the extent of the damage degree studied in this paper(1.80%-28.8%), minor molecular degradation of amylopectin(AP) was probably induced by mechanical damage. In summarization, the mechanical damage could affect starch granule structure and crystal structure, while had little effect on molecular structure.AM-AP ratio of soluble starch at different time points in the pasting process on RVA, solubility and swelling power of wheat starch in the cold(30℃, before pasting) and hot water(90℃, after pasting), molecular weight distribution of fully branched and debranched soluble starches in the cold and hot water were determined to understand the effect of mechanical damage on dissolution behavior of wheat starch. The results showed that, in the pasting process on RVA, compared to AM, the dissolution rate of AP increased with the level of damage. AM-AP ratio of soluble starch in the supernatant was significantly positively correlated with RVA peak viscosity, trough viscosity and final viscosity(p<0.01); solubility of wheat starch in the cold and hot water, swelling power in the cold water increased gradually, but swelling power in the hot water had no obvious changes; AM-AP ratio of soluble starch in the cold and hot water decreases with the increase of damage degree. The explanation of the decrement of RVA viscosity could be concluded as follows: mechanical damage led to a more disrupted crystalline structure and granular surface together with more loosely granular structure, therefore, AP was easier to dissolute, which had weakened the rigidity of the residue starch and decreased the viscosity of starch system.Differences of pasting viscosity on wheat starches with different damage levels could be corrected by increasing mechanical stirring speed and adding hydrophilic colloid such as xanthan gum. With the increase of stirring speed, RVA peak viscosity, trough viscosity, final viscosity decreased, while breakdown and setback increased at first and decreased later, the differences of pasting parameters of samples decreased at the same time, but not to zero. After adding xanthan gum(0.5%), viscosity parameters had a significant increment while breakdown, setback and pasting temperature decreased, at the same time, the extent of the changes of the viscosity values increased with enhanced damage degree. This paper provided theoretical reference for the reduction of negative effect on the product quality caused by mechanical damage.