Study On Quality Change And Its Mechanism Of Frozen Raw Noodles During Frozen Storage

Fresh raw noodles are widely popular owing to the smoothness and chewiness,but are prone to quality losses in normal temperature conditions.The application of freezing technology and development of cold chain logistics realize the industrialized and large-scale production,storage and transportation of frozen raw noodles.Frozen raw noodles are similar with fresh raw noodles in taste and flavor,showing a growing development trend in Chinese catering market.However,the quality change of frozen raw noodles during frozen storage has been far from investigated.In light of this,the present paper studied the quality change of frozen raw noodles during frozen storage,in which three kinds of wheat flour with different protein contents were used to prepare frozen raw noodles.And the characteristics of gluten and starch isolated from frozen raw noodles were further discussed on the underlying mechanism.Firstly,the effects of freeze-thaw cycles on the quality change of frozen raw noodles were investigated.With increasing freeze-thaw cycles,the tensile distance increased and the break force decreased in frozen raw noodles.Higher water mobility and less compact microstructure with more exposed starch granules in frozen raw noodles were detected.Cooking properties showed increases in the optimal cooking time,cooking loss and percentage of broken noodles with increasing freeze-thaw cycles.The hardness and chewiness of the cooked noodle texture and sensory scores decreased.Frozen raw noodles made from the wheat flour with the lower protein content were easier to fracture under tensile force.Besides,cooking loss and broken noodle ratios up to 8.92%and 61.3%,respectively,as well as significantly（p<0.05）lower chewiness were observed in frozen raw noodles with lower protein content when subjected to8 freeze-thaw cycles.Meanwhile,the effects of frozen storage time on the quality change of frozen raw noodles were investigated.During the first 6 weeks of frozen storage,increasing tensile distance and decreasing break force were detected in frozen raw noodles.Cooking properties showed higher cooking loss and lower water absorption.Cooked noodle hardness increased and sensory scores decreased.Frozen raw noodles made from the wheat flour with lower protein content showed a significant（p<0.05）increase in cooking loss up to 9.98%after 6 weeks.During the last 6weeks of frozen storage,lower tensile distance and higher break force were detected.The cooking loss decreased and water absorption increased.Cooked noodle hardness tended to fall.For frozen raw noodles with prolonging frozen storage,longer optimal cooking time was required,and higher water mobility and less compact microstructure with more exposed starch granules after 12 weeks were detected.By correlation analysis,the wheat flour with higher protein content and dough strength was more suitable for preparing frozen raw noodles with greater stability during freeze-thaw cycles and long-term frozen storage.Then,the gluten was isolated from frozen raw noodles and studied on its characteristics.The composition of gluten subunits did not show obvious change with repeated freeze-thaw cycles.While decreasing proportions of the total glutenin,HMW-GS and LMW-GS were determined with prolonging frozen storage time.The quality maintaining of frozen raw noodles was positively correlated with the proportions of the total glutenin,HMW-GS and LMW-GS,while negatively correlated with those of the total gliadin,α-andγ-gliadin according to the correlation analysis.By uniaxial extensional rheological tests,with increasing freeze-thaw cycles,significant（p<0.05）decreases in break force and tensile distance of the gluten were determined.Creep-recovery rheological properties showed significantly（p<0.05）greater compliance and lower zero viscosity（η）.These results indicated that repeated freeze-thaw cycles induced the weaker interaction between gluten protein molecules,leading to the reduced extensibility and mechanical strength of the gluten.As a consequence,the gluten was unable to provide sufficient encapsulation to the starch granules or structural support to the noodles,which contributed to the quality loss of frozen raw noodles.With prolonging frozen storage time,the break force declined significantly（p<0.05）,and the tensile distance increased first and then decrease.Creep compliance showed an increase before decreasing,while the recovery retardation time（λ_r）increased significantly（p<0.05）during frozen storage.These results suggested the different influence by long-term frozen storage that the weaker mechanical strength of the gluten dominated during the first six weeks,while the increasing stiffness and lack of flexibility were responsible for the deterioration during the last six weeks.Finally,the starch was isolated from frozen raw noodles and studied on its characteristics.Both continuous freeze-thaw cycles and prolonging frozen storage time decreased the swelling power of the starch and increased the short-range order of the swollen starch,indicating the more contact structure of starch granules in frozen raw noodles during frozen storge.As a result,the starch granules were more difficult to break down,contributing to the longer optimal cooking time for frozen raw noodles.During the swelling progress,the ratio of leaching amylose was reduced by freeze-thaw treatment but increased by prolonging frozen storage time.By steady rheological tests,decreasing yield stress（σ₀）and flow behavior index（n）were observed in the starch gel during freeze-thaw cycles,indicating the lower structural strength.With prolonging frozen storage,σ₀ and n first increased and then decreased,implying a decrease after an increase in the structural strength of the starch gel.The steady rheological properties of the starch gel were consistent with the cooked noodle texture.