Mechanical Dissociation And Electric Separation Of Aleurone Cell-cluster From Wheat Bran

Wheat aleurone, which contains most of the nutritional substances in wheat bran, is a good potentialfood resource. However, aleurone layer is adhered to other layers tightly in wheat bran, and is difficult to beseparated using the traditional process. The aim of this paper was to separate aleurone cell-cluster fromwheat bran using centrifugal impact milling and electrostatic separation, and to study the potentialmechanisms.Firstly, the methods were built to conform and quantify the constitution of bran layers. It was foundthat, the bran tissues can be identified using microscope. After staining by fluorescence brighter andfuchsine, aleurone cell wall and its cell contents appeared light blue and dark red, respectively, while theouter pericarp and intermediate layer (IL) appeared as yellow-green. The enrichment and dissociationsituation of the bran particles during milling can be ensured by identifying the microstructures of the brantissues. The results of biomarker for the bran layers indicated that, alkylresorcinols (ARs), which were richin IL with the content of11.90mg/g, can be used as the biomarker of IL. Phosphorus (P) which were rich inin aleurone layer with the content of28.7mg/g, can be used a biomarker of aleruone layer. In addition,starch only exists in endosperm with the content of78.3g/100g. Although the contents of dehydrotri-ferulicacid, p-coumaric acid and ferulic acid in each bran layer were different, they only can be used to identifythe tissues. The biomarker method could be used to qualitify the proportion of wheat bran layers with arelative deviation less than10%.The elastic-plasticity properties of wheat and bran layers were investigated. The result showed that,the elastic-plasticity property wheat bran on longitudinal orientation was stronger than that on radialorientation. As the increasing of water content, the elastic limit, ultimate stress, and Young modulus of brandecreased, while the elastic strain, ultimate strain and fracture energy increased. Among the three branlayers, the intermediate and aleurone layers mainly demonstrated elastoplastic behaviour, while outerpericarp exhibited stronger elastic property. In addition, outer pericarp and IL exhibited some anisotropism,which was not found in aleurone layer. In order to obtain a suitable starting material for the electrostaticseparation, vibratory milling and centrifugal impact milling (CIM) was utilized to dissociate wheat branconstitutive tissues. The broken behaviour of bran tissues during the millings suggested that, aleurone cellscould be broken easily by vibratory milling, while CIM was beneficial for the isolation between the branlayers. During CIM, the aleurone layer was completely dissociated as cell-cluster when the particle sizedecreased to about200μm. The detached behaviours of bran tissues were well explained by therelationship of their elastoplasticities. According to the elastoplasticities of bran layers, it can be obtainedthat, the critical size at which the aleurone layer been dissociated was194~220μm, and the critical impactspeed was97m/s.The electrostatic properties of the bran tissues showed that, outer pericarp, which was a hollow fibretissue, exhibited the similar dielectricity to air, while the dielectric constant of the thick aleurone layer was5times more than that of the outer pericarp. Moreover, the specific resistance of aleurone layer was2300MΩ·cm, which was5times higher than that of outer pericarp.After corona charging, the charges obtainedby bran tissues were positive correlation with voltage, but were negative correlation with the water contentand particle size of the bran particles. In addition, the aleurone tissues could obtain about1.7times morecharges than outer pericarp under the same corona charging condition. Tribocharging properties of differentbran tissues in the pipes made of Tefon, Nylon and steel were investigated. The results showed that, afterrubbing with insulator, the total charges obtained by bran fragments changed as the times of tribochargingincreased. The more aleurone particles in bran fragments, the more their total charge changed. Aftertribocharging with Teflon, the total charge obtained by the bran fragments was positive, and decreased withthe number of tribocharging. While, after tribocharging with Nylon, the total charges obtained by the branfragments was negative firstly, then increased to be positive. After tribocharging with steel pipe, the totalcharge of bran fragments was positive and almost constant. The diversities of total charges obtained by branfragments may cause by the aleurone cells, which captured the opposite charges accumulated on the pipe wall. According to the theory of tribo-electric series, the position of bran tissues can be deduced as (+)Nylon> aleurone>outer pericarp>Steel (-). Furthermore, the charges obtained by the bran tissues increasedas the air flow rate increased and as the particle size decreased.Triboelectric separation (TS) of bran fragments in a uniform electric field showed that, different brantissues had the distinct deflected behaviours. After rubbing with Teflon, aleurone and IL particles deflectedto the negative electrode, while outer pericarp particles were deposited in the “middle fraction”. Afterrubbing with PE tube, the deflections of aleurone and IL particles were similar, but the yields of theelectrode fractions changed. When the tribo-material was changed from Teflon to Nylon, the deflectionbehaviours of aleurone and outer pericarp were reversed. Based on the results above and theory of thetribo-electric series, the relative positions of bran layers and the tribo-materials used in the tribo-series canbe deduced as:(+) Nylon> aleurone layer> intermediate layer> outer pericarp> PE>Teflon (-).In the nonuniform electric field (NUEF), wheat bran tissues exhibited the distinct deflected behaviourswith that in the uniform electric field. After rubbing with Teflon, more aleurone cell-clusters and ILparticles deflected to the positive electrodes, while the outer pericarp fragments were enriched in themiddle fraction. After rubbing with Nylon, the deflection behaviours of aleurone and outer pericarp werealso reversed. While the enrichment of aleurone was not significant after TS using steel as tribo-tube.Moreover, decreasing the water content of bran fragments would improve the separation quality of branlayers. While, decreasing the particle size of bran fragments only increased the yield of the charged branfractions after TS, but did not affect the separation quality of the bran layers significantly. The results oftwo steps of TS indicated that, after tribocahrging with Nylon, the aleurone layer in the bran with7%watercontent can be enriched from52%to about80%in the uniform electric field (45kV), with the extractionefficiency of78%and yield of51%. While after tribocharging with Teflon, the aleurone layer in the branwith7%water content can be enriched from52%to about77%in non-uniform electric field (45kV), withthe extraction efficiency of63%and yield of42%.In order to guide the TS in the industry, Agros2D, a simulation software, was used to simulate thedeflection of charged particles in a uniform electric field. The relationship among the minimum charges(Qmin) that need to deflect on the electrode, air-flow rate (V), voltage (U), scale of electric field (H) andparticle size of bran fragments (Dp) were deduced as:.The enriched aleurone fraction, which contains80%aleurone layer, has better colour and nutritionproperty than wheat bran and other bran tissues. The amino acid composition of enriched wheat aleurone isnearly equal to the proportions recommend by WHO/FAO, and is a good protein source for supplement thewheat flour protein. Comparing the antioxidant ability of enriched aleurone, wheat bran and non-aleuronelayers, it was found that the enriched aleurone fraciton contained about1.5and2.2times more totalphenols than heat bran and non-aleurone layers. Its reducing power and ability to remove free radicals werealso better than wheat bran and non-aleurone tissues. The enriched aleurone fraction can be used to forobtaining aleurone flour, which had the better colour, nutrition property and dough features than wholewheat flour. Moreover, the aleurone flour has the better application property than whole wheat flour insteam bread.