Study On Effect Of Pretreatment On Physicochemical Properties Of Buckwheat Flour Fractions And Quality Of Buckwheat Noodles

Noodles are one of China’s traditional staple foods,and long-term consumption of noodles produced from refined white wheat flour rich in fast-digesting starch in the daily diet can increase the risk of type II diabetes,obesity and other chronic diseases.Therefore,rich in slow digestion and anti-digestive starch,buckwheat noodles with hypoglycemic effect was increasingly popular in the market.Due to the lack of gluten protein in buckwheat flour,the processing process was difficult to form a continuous gluten network structure,with pure buckwheat flour produced by the quality of the dried noodles difficult to meet consumer expectations.Therefore,buckwheat flour and wheat flour as the raw material of high added buckwheat noodles become the focus of corporate research and development.In order to improve the quality of high-addition buckwheat noodles,buckwheat flour usually needs to be pre-treated for processing,such as micro-crushing and heat treatment.However,there are great differences in nutritional components in different parts of buckwheat seeds,such as cortex rich in fiber,aleurone layer rich in protein and endosperm rich in starch.Pretreatment of whole buckwheat flour may not only improve the functional properties of one component,but also deteriorate the functional properties of another component.Therefore,classifying buckwheat flour into fractions rich in different nutrients,and then selecting appropriate processing methods according to the characteristics of each fraction could give full play to the characteristics of each component,thus improving the quality and nutritional characteristics of buckwheat vermicelli.From the point of view of grading and processing,the buckwheat system flour(bran,semolina and coarse flour)and whole buckwheat flour,rich in fibre,starch and protein,were prepared by roller grading.Targeted pretreatments were carried out according to the characteristics of each grade to investigate the effects of various pretreatments on the physicochemical properties,dough rheology,dough quality and starch digestibility of the buckwheat flour grades,and to obtain optimal treatment methods for each grade.Initially,the mechanism of the effect of various pretreatment methods on the quality and starch digestibility of the noodles was revealed.Then the unmodified whole buckwheat flour was used as a control to study the effect of ungraded pretreatment(microwave,steam,dry heat),grading and post-processing reorganization on the processing performance of whole buckwheat flour to obtain the optimum processing method for whole buckwheat flour.The aim was to provide support for improving the edible quality and nutritional and functional properties of buckwheat noodles.The main findings of this paper are as follows:(1)Firstly,buckwheat bran was used as the raw material for coarse,medium and fine buckwheat bran of three different particle sizes(CB,MB,SB)for microwave treatment(MT),steam treatment(ST)and ultrasonic treatment(UT)to investigate the effect of pretreatment on the physicochemical properties of different particle sizes of buckwheat bran and the quality of buckwheat dried noodles.It was found that: the untreated group of samples had a longer dough stabilization time and less protein weakening as the particle size decreased.The protein weakness of the dough was further improved by the pretreatment.Among the quality of dried noodles,the mechanical properties and cooking quality of MB dried noodles in untreated group were the best.With the increased of wheat bran particle size,the hardness,chewiness and tensile properties of dried noodles increased significantly.After pretreatment,the cooking quality of dried noodles was improved,and the cooking loss rate of MB was reduced by 5.25% after microwave treatment.Ultrasonic treatment had the best effect on improving the hardness of dried noodles.With the increase of particle size,the gelatinization temperature and gelatinization enthalpy of untreated samples decreased,and the extractable protein content decreased.At the same particle size,the pasting temperature of steam and ultrasound treatment was higher than the control,protein aggregation-dissociation occurred,microwave treatment reduced the extractable protein content and ultrasound treatment did the opposite,MB had the lowest starch digestibility after microwave treatment.To sum up,medium particle size combined with microwave treatment(MB-MT)can significantly improved the quality of buckwheat noodles,reduced the starch digestion characteristics of dried noodles.(2)Secondly,buckwheat semolina was used as the raw material for steam treatment(100°C,30 min)of buckwheat semolina with different moisture contents(15%,25%,35%)to investigate the effect of steam treatment on the physicochemical properties of buckwheat semolina and the quality of buckwheat noodles.It was found that steam treatment increased the solubility and cold water swelling of buckwheat semolina,reduced the paste swelling potential,and limited the water absorption and swelling of buckwheat starch granules during the cooking process.Steam treatment increased the pasting temperature of the buckwheat semolina,with the most pronounced effect on MC35.The steam treatment extended the formation and stabilisation time of the dough,reduced the weakness of the proteins and improved the thermal stability of the dough.The steam treatment improved the steaming quality and tensile properties of the noodles and improved the hardness and chewiness of the noodles,with the best effect on the mechanical properties and tensile strength of MC25 noodles.The effect of steam treatment on the starch digestibility of buckwheat semolina was dependent on the moisture content,with the starch digestibility of buckwheat semolina increasing as the moisture content increased.Steam treatment significantly improved the internal denseness of the noodles,with MC25 showing the largest reduction in starch digestibility of 17.13%.The effect of steam treatment on the starch digestibility of freeze-dried cooked flour followed the same trend as that of raw flour,with MC35 showing the highest starch digestibility of 74.00%.In summary,appropriate steam treatment can improve the structure and quality of buckwheat noodles,thus significantly reducing its starch digestibility,of which MC25 noodles quality and starch digestibility of the best.(3)Then,buckwheat flour was used as the raw material for dry heat(130°C,1 h-3 h)and wet heat treatment(130°C,1 h-3 h)to investigate the effect of heat treatment on the physicochemical properties of buckwheat flour and the quality of buckwheat dried noodles.It was found that heat treatment changed the pasting parameters of buckwheat flour,the longer the wet heat treatment,the higher the pasting temperature and the lower the paste viscosity.Dry heat treatment time ≥ 2h,lower battering temperature and higher batter viscosity.After heat treatment,the protein weakening degree of dough decreased,and the quality of dough improved.Among the qualities of the dried noodles,the heat treatment improved the mechanical properties,the cooking quality,the TPA structure and the tensile properties of the noodles.After 2 hours of dry heat treatment,the cooking loss of dried noodles decreased by 9.29%,the hardness increased by 12.10%,the breaking strength increased by 13.86%,the tensile strength increased by 35.73% and the flexibility increased by 47.10%,which had the best effect on improving the quality of dried noodles.In flavor analysis,the flavor of dried noodles was obviously enhanced when the dry heat treatment time was ≥ 2 h.In starch digestibility,heat treatment reduced the starch digestibility of noodle segments,and the starch digestibility decreased by 27.17% when dry heat treatment was carried out for 2h.With the extension of treatment time,the starch digestibility of freeze-dried cooked flour increased.To sum up,the heat treatment significantly promoted the formation of the gel structure of the noodles and the increase in gel strength reduced their starch digestibility.The best quality and functional properties of buckwheat noodles were obtained after 2 h of dry heat treatment of buckwheat coarse flour.(4)Finally,untreated whole buckwheat flour was used as a control to study the effects of ungraded processing(microwave,steam,dry heat),graded processing and reconstitution of whole buckwheat flour added to the quality and starch digestibility of buckwheat dried noodles.Noodles made from modified whole buckwheat flour showed higher mechanical strength,tensile properties and sensory scores,with a reduced cooking loss,with the lowest cooking loss of 6.48% for noodles made from graded and reconstituted whole buckwheat flour(CBF)added.For starch digestion characteristics,both CBF noodle segments and freeze-dried cooked flour had the lowest digestibility,and modified buckwheat flour promoted the formation of compact structures within the noodles,reducing the accessibility of starch-digesting enzymes and slowing the rate of enzymatic digestion of starch.To sum up,targeted processing of different fractions of buckwheat flour could maximize the functional characteristics of its nutritional components,promote the formation of compact structure of buckwheat vermicelli,improve its quality and reduce its starch digestibility.Compared with buckwheat coarse flour and buckwheat bran,refined buckwheat flour was more conducive to forming a more compact vermicelli structure,improving the quality of vermicelli and reducing its starch digestibility.When the noodle structure remains intact,the food structure played a leading role in the digestibility of starch,and the tight food structure will reduce the digestibility of starch.Only when the food structure was seriously damaged,the molecular structure of starch begins to play a major role.It is of great significance to guide the development of functional buckwheat noodles.