Effects Of Curing Treatment On The Processing Quality Of Naked Oat And The Underlying Mechanism

Oat,as an internationally recognized healthy whole grain,has unique nutritional properties and health benefits,which is widely used as raw material or ingredients in the processing of gluten-free products and staple products.However,due to its high fat content and gluten free,oat products have some problems such as poor elasticity and weak cohesiveness,which limits the application of oat in staple food industry.Therefore,the processing of traditional oat food in China generally requires“stir-frying-hot water kneading-steaming”to improve the processing quality of oat products.However,there is little attention to traditional oat food,and it is unclear why it should undergo“three thermal”treatment,how is the synergistic effect between thermal treatment stages,and what is the underlying mechanism on the processing quality of oat products.Based on this,this study,combining with the“three thermal treatment’’of stir-frying,hot water kneading and normal pressure steaming,systematically explored the effects of curing treatment on the microscopic morphology and structure properties of oat kernels and oat flour,rheology behavior of oat dough and the processing quality of steamed oat rolls,aiming to illustrate the synergistic effect and underlying mechanism.The main research contents and results are as follows:（1）With starch gelatinization degree as the starting point,oat kernels with gelatinization degree of 15%-90%were prepared by the tempering-stir-frying controllable gelatinization technology.The mechanism of“the first thermal treatment stages”for stir-frying was clarified from three aspects of micromorphology,physicochemical properties and structural characteristics.It was found that the dense structure morphology of native oat kernels became loose when the gelatinization degree was less than 50%,and the starch granules were partially gelatinized.When the gelatinization degree reached65%-90%,the starch granules were gelatinized in large quantities,further cohering to form a dense starch gel network structure.Moreover,the proteins were aggregated,adhered to or wrapped on the surface of starch granules,and gradually formed large clumps aggregates.Compared with untreated oat flour,the water absorption index,swelling index,amylose content and damaged starch contents of stir-fried oat flour were significantly increased,while the breakdown viscosity and setback viscosity values were obviously decreased,further improving the thermal stability of oat paste and retarding the retrogradation of starch.Besides,the results of thermal and structural characteristics showed that stir-frying gelatinization treatment increased the heat conversion temperature,decreased the enthalpy（ΔH）from 5.12 J/g to 0.11 J/g,promoted the crystal structure of starch gradually changing from A+V type to V type,reduced the relative crystallinity and short-range order of starch,and increased theβ-sheet structure ratio（28.58%-38.04%）,and conformed the macromolecular aggregates with the molecular weight>100 k Da.（2）Using oat flour of different gelatinization degree after stir-frying treatment as raw material,the effects of“the first thermal treatment stages”stir-frying gelatinization on rheological behavior of oat dough were studied by the means of basic rheological tests under the condition of fixed adding water and kneading temperature（25℃）.The results showed that with the increase of gelatinization degree,the sticky structure of oat dough gradually formed a viscoelastic honeycomb starch gel network structure,and the presence of gelatinized oat flour promoted the water state in oat dough from intermediate state to bound state and free state.In addition,with the increase of gelatinization degree,the rheological and texture（hardness and adhesiveness）characteristic parameters of oat dough firstly increased and then decreased.At the gelatinization degree of 50%,oat dough had the highest elastic modulus（G′）,viscous modulus（G″）and stress relaxation percentage（SR,34.55%）and the lowest maximum creep compliance(J_max,1.28×10^-5Pa^-1)and adhesiveness（1.59 N·s）,showing better viscoelastic behavior,deformation resistance and operation performance.Meanwhile,with the increase of gelatinization degree,the main intermolecular forces in dough changed from hydrophobic interaction to hydrogen bond and covalent bond interaction.（3）On the basis of stir-frying gelatinization,the synergistic effect of“the first thermal treatment stages”and“the second thermal treatment stages”hot water kneading on the rheological behavior of oat dough was further investigated by adjusting the water temperature（25℃-100℃）under the optimal water absorption rate.The results showed that with the increase of the kneading water temperature（25℃-65℃）,the structure of oat dough gradually became compact,and the honeycomb structure of the high gelatinization group was more obvious.The water distribution of the control group changed from intermediate state to free state,while the gelatinization group changed to bound state.The G′,G″,relative elastic ratio(J_e/J_max),zero shear viscosity（η₀）,maximum stress(F_max),hardness and gumminess of oat dough increased gradually,indicating that the elasticity,resistance fluidity,strength and cohesiveness of oat dough were further improved by stir-frying gelatinization and hot water kneading,and the synergistic effect was gradually enhanced.At the kneading water temperature of 65℃to 85℃,the structure of oat dough was more stable,and the proportion of bound state water in gelatinization group increased to the maximum.Meanwhile,the G′,G″,J_e/J_max,η₀,F_max,hardness and gumminess value of oat dough reached the highest,presenting the best viscoelastic behavior and texture properties with the best synergistic effect.Among them,the dough prepared with oat flour of 50%gelatinization degree and kneaded by 65℃water had the highest J_e/J_max（80.66%）.Further rise of water temperature（100℃）,the network structure of oat dough was damaged,the stability and continuity of dough structure were decreased and the viscoelasticity was significantly decreased,which showed the weakened synergistic effect.（4）Based on the synergistic effect of“the first thermal treatment stages”stir-frying gelatinization and“the second thermal treatment stages”hot water kneading,the effects of“third thermal treatment stages”steaming treatment on the quality characteristics of oat rolls were investigated under the optimum water absorption and optimum water temperature,further clarifying the underlying mechanism.The results showed that at the steaming treatment for 0 min-10 min,the steamed oat rolls of the control group gradually formed a continuous fibrous film structure from a relatively loose and discontinuous structure,while the oat rolls of the gelatinization group gradually developed a honeycomb three-dimensional gel network structure with more dense porosity and uniformity from a dense starch gel network.After the steaming treatment for 10 min to 13 min,the structure of oat rolls was damaged and the continuity and porosity decreased.Among them,the sample with gelatinization degree of 50%showed the best honeycomb three-dimensional gel network structure at steaming for 7 min,the stress relaxation percentage was 38.21%,the cohesiveness was 0.23,and the chewiness was 668.35 g,presenting better elasticity,cohesiveness and chewiness.These were attributed to the enhancement of water absorption and swelling,the increase of the bound water proportion,the amylose leaching and the polymerization of small molecular weight protein,the formation of starch-lipid complexes,the more conversion from free sulfhydryl to disulfide bond and the enhancement of covalent bond interaction during steaming process.Moreover,proper steaming treatment also improved the digestion hydrolysis rate of starch and reduced the content of resistant starch in steamed oat rolls.Based on the cohesiveness of gelatinized starch granules and thermal aggregation of proteins as well as the enhanced hydrogen bond,hydrophobic and covalent bond interactions under moderate thermal induction,curing treatment improved the viscoelasticity of gluten-free oat dough and promoted oat rolls to form a compact uniform like-honeycomb three-dimensional network structure with uniform pore,showing better elasticity,cohesiveness and chewability.In this study,the optimum curing treatment conditions for improving the processing quality of oat rolls were as follows:water tempering of 27.5%,stir-frying temperature of 160℃,stir-frying time of 25 min,oat flour gelatinization degree of 50%,water addition for kneading of 105%,water temperature for kneading of 65℃,thickness and diameter of oat rolls of 2 mm and 5 cm,steaming time of7 min.This study could provide a new theoretical basis for the application of pre-gelatinized oat flour in flour products,the quality improvement of gluten-free oat dough and the staple industrial production of traditional oat food.