Effects Of Mixing Modes On Dough Physico-chemical Structure And Noodle Quality Property

In this study, the performance of different types of industrialized dough mixers and effects on noodle quality was studied. The study involved four mixing units, including two horizontal mixers with double shafts and two continuous high-speed mixers. The mixing process of the selected mixer was optimized using a response surface design, and the effects of vacuumdegree, water addition and mixing time on the cooking and sensory quality of Chinese white noodles were studied. Then, the effects of vacuum degree and mixing time on physical properties, microstructure, water state and protein structure of noodle dough were investigated using TPA、SEM、FTIR-M、NMR/MRI、DSC、SE-PHLC、SDS-PAGE and FT-IR etc.The relationships between physical properties and water state and protein chemical structure were analyzed. The main conclusions were as follows:(1) Compared to the other three modes, a horizontal pin-mixer with a constant mixing speed gave poorer performance. The horizontal mixer with angled, large surface area-blades and adjustable-speed mixing gave a best performance. The mixing uniformity and stability of the horizontal high-speed mixer with mixing speed of 1440 rpm was relatively poorer than that of the vertical continuous mixer with mixing speed of 970 rpm.(2) Vacuum mixing significantly improved the sensory quality of both fresh and refrigerated noodles and decreased the cooking loss. However, extremely high vacuum degree(0.08 MPa) could decrease noodle quality. The optimal mixing conditions were vacuum degree, 0.06 MPa; added water, 35.3%; and mixing time, 7.2 min. The fresh noodles quality was most affected by the water addition, followed by vacuum degree. However, for refrigerated noodles, the contribution rate of vacuum degree effect on sensory score exceeded water addition, and the effects of vacuum degree on appearance, firmness and stickiness of noodles obviously increased. Further study showed that increasing vacuum degree could slow down the quality deterioration of raw noodles in the process of refrigeration.(3) The dough samples produced from three cultivarswith vacuum mixing, when sheeted all gave significantly higher levels of adhesiveness, elasticity and chewinessthan from non-vacuum mixing. The optimum vacuum applied during dough mixing was judged to be 0.06 MPa for the samples of Ningchun 4 and Jimai 22, and 0.08 MPa for the sample of Zhengmai 366. Inspection of dough sheets prepared after mixing showed that, the cross-section of sheeted dough mixed at 0.06 MPa had a more continuous and compact microstructure with fewer holes and gaps, as well as more even protein distribution and a more coherent gluten network at the surface. However, an extremely high degree of vacuum was detrimental to the developed dough network for weak-gluten flour. Vacuum mixing at 0.06 MPa may promote the interaction of water and gluten protein, resulting in a decrease in the molecular mobility of water in noodle dough, retarding water migration and texture deterioration in sheeted dough. And non-vacuum or excessive vacuum could both increase the molecular mobility of water in dough. Compared to non-vacuum mixed noodle dough, the degree of polymerization of protein was higher in vacuum mixed dough, as evidence by results of SE-HPLC and laser diffraction. The more large protein aggregates was observed at 0.06 MPa in Jimai 22 dough, while 0.08 MPa for Zhengmai 366. Vacuum mixing did not significantly affect the protein subunits. In noodle doughs, β-turn was the predominant secondary structural feature, followed by β-sheets. Vacuum mixing at 0.06 MPa imparted an increase in β-turn content of Jimai 22 dough at the cost of a reduction in β-sheets content, while induced an increase in α-helix content at the cost of a decrease in β-turn content for Ningchun 4 and Zhengmai 366.(4) Sheeted dough mixed for 8 min presented better textural properties and a more compact, coherent and even microstructure as evidenced by SEM and FTIR-M. Insufficient mixing resulted in an uneven distribution and an inadequately developed gluten network, especially for Jimai 22 flour with weak gluten. Excessive mixing was detrimental to the developed dough network and decreased the uniformity of component spatial distribution. The mobility of water was low in noodle dough mixed for 8 min, and deficiency or excessof mixing time could lead to significantly higher water mobility as evidenced by NMR and DSC. During the initial stages of mixing, the monomeric proteins increased, the polymeric proteins proportion decreased, and the content and particles size of GMP decreased. During mixing from 4 to 8 min, the polymerization degree of protein increased as evidence by results of laser diffraction and SE-HPLC. Excessive mixing(12 min) led to a significant decrease in the polymerization degree and molecular weight of protein as evidence by changes of GMP content and particles size, protein fractions and free-SH content. Compared to wheat flour, in noodle doughs, the percentage of α-helix and β-sheets declined and β-turn and unordered structures increased. Mixing at 8 min imparted a higher percentage of β-turn and less β-sheets and unordered in Jimai 22 dough, while induced more α-helix structure in Ningchun 4 dough. For Zhengmai 366, the β-turn increased from 4 to 8 min; then, the β-turn significantly decreased and α-helix increased after a longer mixing time.(5) Wheat flours with different protein characteristics behaved differently in changes of physical properties and protein structure of noodle dough induced by vacuum mixing. The development of the gluten network for weak gluten flour was more sensitive to the degree of vacuum. High vacuum degree may result in the rupture of some large glutenin polymers in dough of weak-gluten flour, while increased the degree of polymerization of protein and induced more ordered secondary structure in dough of high- and strong- gluten flour. The TPA parameters, microstructure, GMP particle size and free-SH content of noodle dough of Zhengmai 366 was less affected by mixing time than that of Jimai 22, suggesting that strong-gluten flour has better noodle dough mixing tolerance.(6) Better noodle dough texture might correspond to enhanced water combination, increased polymerization degree and molecular weight of protein, increased SS bond formation, and more α-helix or β-turn structures.