Mechanochemical Effect In Wheat Milling And Its Influence Mechanism On Wheat Flour Quality

Wheat milling is a process in which wheat grains are crushed,milled,graded and blended into wheat flour.Early studies suggested that the process was generally physical and that differences in wheat flour quality were caused by differences in the particle size and damaged starch content of the flour formed by mechanical milling of the kernel and wheat endosperm granule.However,in recent years,it has been found that the variation of damaged starch content in actual production is no longer sufficient to fully explain the quality differences of wheat flour with different particle sizes.Therefore,the evaluation system of wheat flour quality changes during wheat milling needs to be continuously developed and improved.Mechanochemistry is the study of chemical or physicochemical changes in solids due to mechanical forces,including particle fragmentation,size reduction,crystal structure changes,conformational changes,chemical bond changes,etc.,and the resulting changes in quality characteristics.Using the field of mechanochemistry as a reference,this project is based on the study of wheat seed and endosperm grains during the milling process.The laws of influence of different structures on the fracture behaviour of the grains under the action of mechanical forces,the influence of the combined mechanochemical types on the action of the substances,the changes in the content of the basic components and the structure of the main components of the endosperm grains of flour made by different milling methods,and the quality characteristics of wheat flour made by different milling methods were investigated in detail,respectively.At the same time,the influence of mechanical forces on the processing quality and nutritional quality of wheat flour and the mechanism of mechanical forces acting on wheat flour were analysed based on the mechanochemical theory and the changes in the crystalline state and structural characteristics at the molecular level.The aim of this study is to provide new perspectives and key data in innovating and complementing the theory of wheat flour milling.The main research components and conclusions are presented below.Firstly,the physical property analyzer was used to conduct rupture tests on single-grain wheat with different deformation modes to investigate the rupture characteristics of the kernels.The microstructure of the kernels in the rupture test and the break system material and sizing system material of the milling on-line was combined to analyze the differences in the formation of internal cracks after rupture of wheat kernels with different endosperm structures.The fractal theory was used to construct the fractal dimension of the kernels of vitreous wheat and floury wheat,respectively.The vitreous wheat mostly fractured along endosperm cells with fractal dimension of 1.262,while floury wheat mostly fractured along the inner endosperm cells with fractal dimension of 1.365.In a study of the mechanical contribution of the bran layer to the mature wheat grain,it was found that the epidermis and epicarp layers played a major role in protecting the integrity of the grain.And the rupture force of both types of wheat grain decreased by approximately 40%after these two layers removed.The rupture force of the floury grain continued to decrease with the removal of the structural layers of the bran layer,while the rupture force of the vitreous grain was mainly influenced by the endosperm structure and did not change significantly.Secondly,the basic composition content,physicochemical properties and structural changes of the endosperm particles were investigated,under mechanical shear,compression and impact milling conditions,respectively.Kinetic equations for each milling method on flour samples of endosperm granule were developed to elucidate the correspondence between milling time and sample particle size(D90,the particle size value corresponding to the cumulative particle size distribution number reaching 90%).1)The kinetic equation of endosperm granules milled by mechanical shear is:y=57.1003+868.9069×0.9122^x(R~2=0.9999)(x(s)is the shear milling time,y(μm)is the sample particle size(D90)).The reduction in particle size,the increase in specific surface area and the reduction in both apparent and compaction densities of the samples are indicative of damage to the starch granules as a result of the shearing action.The type of comminution was consistent with a combination of"volume comminution"and"surface comminution",and there was no sequential breakdown of the main components,starch and protein,throughout the process.Shear significantly disrupted the relative crystallinity and short-range order of the starch in the samples.And shear also caused the breakage of some peptide bonds and interchain disulfide bonds(S-S),resulting in changes in the free amino acid content of wheat flour,a decrease in glutenin macropolymers(GMP),an increase in free sulfhydryl(-SH),and an increase in the hydrophobicity of the protein surface,as well as a decrease in the content of random coil andα-helical structures,and an increase in the content ofβ-sheet andβ-turn in the secondary structure of the protein.2)The kinetic equation for mechanical compression endosperm granules is:y=50.0595+875.9379×0.8889^x(R~2=0.9999)(x(min)is the crushing time and y(μm)is the sample particle size(D90)).Compression results in damage to the starch granules,a reduction in particle size,an increase in specific surface area and a reduction in the apparent and compaction density of the sample.Although this type of comminution is consistent with the’volumetric comminution’model,the starch granules embedded in the large endosperm mass are preferentially’squeezed out’at the onset of compression.Analysis of the samples showed that compression significantly disrupted the crystalline regions of the starch,resulting in a reduction in the relative crystallinity and short-range orderliness of the starch,and a consequent reduction in flake thickness after reaching comminution equilibrium.Compression also broke the peptide bonds and increased the free valine content of the sample,leading to an increase in the surface hydrophobicity of the sample proteins,as well as break the S-S bond between the chains resulted in a decrease in the GMP content and an increase in the-SH content of the sample wheat flour,but had almost no effect on the secondary structure of the proteins in the sample.3)The kinetic equation for the mechanical impact crushing of endosperm granules is:y=62.7650+863.2191×0.9313^x(R~2=0.9999)(x(min)is the impact time and y(μm)is the sample particle size(D90)).Impact resulted in damage to the starch granules,reduction in particle size,increase in specific surface area,and decrease in both apparent density and tap density of samples;the comminution process was consistent with the"surface comminution"model and preferentially"flaked"the surface proteins by the impact.Impact significantly disrupted the crystalline region of the starch in the sample and contributed to its amorphization,and this transformation was proportional,as evidenced by a continuous decrease in relative crystallinity and short-range order,with the thickness of the lamellae remaining below raw material after impact equilibrium.Impact disrupted the peptide bond and interchain S-S in the sample,causing an increase in the valine content and a decrease in the tryptophan content in the system,but does not affect its surface hydrophobicity.Impact can induce the interconversion of S-S and-SH between chains,leading to changes in GMP content.It also led to a decrease in protein spatial order and flexibility,and a partial transformation ofα-helix and random coil intoβ-sheet andβ-turn.Finally,the differences in processing quality and nutritional quality properties of wheat flour of samples milled in three modes:shear,compression and impact were explored.In tests of hydration properties,the sheared samples showed the lowest hydration capacity(in the range of 0-80%moisture);the study showed that high temperatures during compression reduced the hydration capacity of the samples.The peak viscosity of the three mechanically milled samples in the pasting properties decreased as the starch granules damaged and the breakdown values increased.The peak viscosity value(2915.50 c P)and the breakdown value(920.50 c P)of the sample obtained by low temperature compression for 5 min were the highest.Thermal performance tests showed that the compressed samples reached nearly twice the raw material after 5 min of compression at cooling temperature,demonstrating that the compressed samples had the strongest molecular thermal motion.Also,the glass transition temperature of the compressed samples increased with increasing milling time,decreased for the impacted samples and was lowest for the sheared samples,indicating that the three milling methods had different effects on the magnitude of intermolecular forces within the sample system.In vitro digestion showed that all three crushing methods increased the enzymatic sensitivity of the samples and increased the rate of starch hydrolysis;specifically,shear activation saturated after 40 s(twice the rate of raw material hydrolysis),compression activation saturated after approximately 15 minutes(1.2 times the rate of raw material hydrolysis)and impact activation increased with increasing duration(5-35 minutes).In other words,all three milling methods activated theα-amylase site.The damage of starch structure caused by continuous comminution increased the degree of starch hydrolysis.Interestingly,there was no significant difference in the in vitro digestion results between normal and cool temperature compression samples;given the large temperature difference between the two compression processes(15-20°C),the mechanism of this phenomenon may need to be explained by the hot spot effect.