Study On Influencing Factors And Mechanism Of Starch Digestibility Of Coarse Cereal Dried Noodles

Dried noodles,as traditional staple foods of industrial processing in China,are very favored among people.With the gradual improvement of living standard and the increasing attention to nutrition and health,coarse cereal noodles?CCN?with low GI have exhibited huge market development potential.However,researchers mainly focused on produce high quality CCN,and rarely considered their nutritional value during improvement process.Especially,the problem of high postprandial blood glucose after noodles excessive consumption was ignored.The study explored the influencing factors of starch digestibility of CCN,and investigated the effects of common improvement methods on in vitro starch digestibility of cooked noodles.Firstly,the effects of divers cereal raw materials on in vitro starch digestibility of cooked noodles were studied,and the main reasons for their differences were analyzed.The results showed that starch hydrolysis and eGI of CCN from the highest to the lowest were as follows:barley>highland barley>tartary buckwheat?TB?>oat>sorghum.The physicochemical characteristics of starch indicated that sorghum starch had significant?P<0.05?higher gelatinization temperature and enthalpy value than others.Oat and sorghum noodles had higher amylose content and starch relative crystallinity,which also had more short-range ordered structure and stronger hydrogen bond strength.The dephenolization,defatting and deproteinization of coarse cereal increased the starch digestion of CCN,which had most obvious effect on TB,oat and sorghum noodles.Microstructure observation showed that starch granules inside oat noodles had a lower swelling degree,and the protein network structure of sorghum and oat noodles was denser and more continuous than others.Secondly,the effects and mechanisms of sorghum kernels thermal treatments on in vitro starch digestibility of sorghum dried noodles?SDN?were investigated.Results showed that all treatments obviously decreased the starch hydrolysis rate of cooked noodles.HMT treated noodles contained the highest amount of resistant starch?RS?and lowest amount of rapidly digestible starch?RDS?.It was mainly related to the changes of starch multi-scale structure.The treated starches had lower molecular weight and less proportion of short chains of amylopectin.Accompany the structural depolymerization,some molecular rearrangement and repolymerization occurred,and gelatinization temperature increased.The molecular short-range ordered degree and intra-molecular hydrogen bond intensity of noodle system were both enhanced by thermal treatments.These changes reduced the enzymetic hydrolysis rate at different levels.Moreover,the formation of denser microstructure of cooked noodles further hindered the contact between enzymes and starch.Thirdly,the effects of protein modifiers on in vitro starch digestibility of SDN were studied.The results showed that exogenous proteins could significantly?P<0.05?reduce the RDS content and increase the RS content of cooked noodles,with wheat protein influenced more.The extractable protein in SDS buffer?SDSEP?value significantly?P<0.05?decreased,which indicated that exogenous proteins promoted the degree of protein polymerization in noodles.Meanwhile,starch swelling power reduced and short-range ordered degree increased.CLSM observations showed that exogenous protein improved the integrity and tightness of the internal structure of cooked noodles.The starch morphological changes during digestion showed that the encapsulation effect of protein network on starch granules was strengthened,thus forming a physical barrier against enzymatic hydrolysis.Fourthly,the effects of protein cross-linking modifiers on starch digestibility of SDN were studied.Results showed that the modifiers were helpful to reduce the starch hydrolysis rate of cooked noodles in some way.Oxidation was the strongest when the addition of ascorbic acid was 0.02%,and cooked noodles had the lowest hydrolysis rate and eGI value.Under the action of TGase,the RDS content and hydrolysis kinetic constants significantly?P<0.05?decreased,and the amount of RS significantly?P<0.05?increased.A low concentration?<0.12%?of alkali could significantly?P<0.05?reduce the hydrolysis rate of noodles,which was related to the balance between protein polymerization and starch gelatinization.SDSEP results showed that three modifiers all promoted the cross-linking of proteins in cooked noodles,especially alkali had the most obvious effect.CLSM observations showed that the strengthening of protein network was helpful to lower enzymatic hydrolysis rate of noodles by preventing the accessibility of amylase to starch during digestion.Finally,the effects of hydrocolloid modifiers on the starch digestibility of SDN were investigated.The hydrocolloids significantly?P<0.05?reduced the RDS content of cooked noodles.The amount of RDS was minimal when the addition of konjac powder,sodium alginate,and curdlan was 0.3%,0.2%,and 0.2%,respectively.The amount of RS of noodles with hydrocolloids were significantly higher?P<0.05?than that of the control.Hydrocolloids could significantly?P<0.05?reduce the swelling degree of starch,with curdlan influenced more.Curdlan could significantly increase?P<0.05?the gelatinization temperature and reduce the breakdown value.FTIR showed that curldan could significantly?P<0.05?increase the peak intensity ratio of 1047/1022 cm^-1,which increasing the short-range order degree and the molecular crystalline.CLSM results showed that the hydrocolloids strengthened the interaction between starch and protein,which enhanced the internal structural of cooked noodles,especially curdlan and sodium alginate exhibited good effects.