Effect Of Frying Conditions On Starch Properties And Its Relationship With The Quality Of Instant Noodles

Fried instant noodles are booming in the global market with the advantages of special fried flavor,easy to carry,convenient cooking and long shelf life.However,the high oil content might lead to chronic diseases such as obesity,hyperlipidemia and hypertension.At present,the research on fried instant noodles mainly focused on the frying process optimization or the addition of improvers to improve its sensory quality and nutritional value.Starch was the most important component of instant noodles.However,the studies on starch molecular degradation,starch-lipid complexes formation and its stability during the frying process were limited.In this paper,wheat starch was used as the material.The effects of different processing methods and frying conditions on the physicochemical properties of starch and the formation of starch-lipid complexes in the complex food system were investigated by means of differential scanning calorimetry,X-ray diffraction and rapid viscosity analysis.The changes of starch microstructure and physicochemical properties during frying were systematically studied by constructing the fried starch system.In addition,the changes on the quality of instant noodles during frying process were studied,and the correlations between the changes of starch physicochemical properties and the quality of instant noodles were explored,which provided scientific basis for further improvement of the quality of instant noodles.The main research contents are as follows:Firstly,studies of the pre-steaming and then deep frying process in the complex system have shown that when the frying time was 75 s,the amylose content of sample increased rapidly from 23.84%to 31.86%,and the endothermic enthalpy(?H)of starch-lipid complexes increased significantly from 0.91 J/g to 1.40 J/g,as the frying temperature increased from100?to 160?.When the frying temperature was 160?,with the frying time prolonging from 35 s to 115 s,the amylose content of the sample increased,meanwhile,the content of starch-lipid complexes and the relative crystallinity of the sample firstly increased and then decreased.An excessive frying time might result in partial disintegration of the complex.By comparing different processing methods,it could be found that the double helix structure of amylopectin disintegrated and the starch molecular structure became loose during the pre-steaming process,which resulted in the degradation of amylopectin and the formation of starch-lipid complexes in the pre-steaming and then deep frying process was more significant than that in the direct frying process.Secondly,wheat starch was extracted and studied in the pure system.The microstructure of starch showed that after being pre-steamed,the starch granules swelled and only a small part of the granules disintegrated.During the frying process,the disc-shaped starch granules continued to swell and rupture,then formed a continuous and dense gel structure gradually.When the frying time was 75 s,the starch molecules underwent violent degradation with the increasing frying temperature,and the weight average molar mass(M_w)of starch decreased from 3.04×10~7 g/mol to 0.80×10~7 g/mol.The increasing content of starch-lipid complexes increased the relative crystallinity and shear stability of the fried starch,inhibited the swelling of the starch,and reduced the solubility,peak viscosity and digestion rate of the starch.When the frying temperature was 160?,the M_w of starch molecules decreased from 2.13×10~7 g/mol to 0.10×10~7 g/mol with the prolonging frying time,the content of starch-lipid complexes and the relative crystallinity of the starch increased firstly and then decreased,meanwhile,the swelling power,solubility,peak viscosity and digestion rate of the starch decreased.Finally,the quality changes of instant noodles during the frying process were studied and the correlations between physicochemical properties of the starch and the quality of instant noodles were established.With the increasing frying temperature and prolonging frying time,the brightness(L~*)of the instant noodles decreased,the redness(a~*)and yellowness(b~*)increased,the cooking yield and cooking loss decreased,and the hardness of the instant noodles after rehydration increased.However,when the frying time was longer than 75 s(frying temperature was 160?),the hardness of the instant noodles after rehydration was reduced.Correlation studies showed that the hardness and resilience of instant noodles after rehydration had a significantly positive correlation with the?H of the starch-lipid complexes.The values of L~*,a~*and b~*had a significant correlation with the total amylose content,M_w and consistency coefficient(K).The cooking loss of instant noodles showed a significantly positive correlation with the area of the thixotropic ring of the starch paste.Besides,the cooking yield of instant noodles had a significantly positive correlation with the swelling power and M_w of the starch,while had a significantly negative correlation with the total amylose content.