Study On Improving Instant Properties Of Yam Flour By Extrusion Treatment And Its Related Mechanism

Yam is a medicinal and food homologous with rich nutrients and functional factors.Yam flour is a kind of dehydration product of yam.It can not only maintain original nutrients mostly,but also be directly brewed and eaten.However,Yam flour is rich in starch and insoluble in water,which limits its application in the field of instant food.Therefore,it is necessary to change the properties of yam flour through appropriate processing technology,so as to improve its instant solubility.Extrusion technology is a kind of treatment technology which can improve the instant solubility by changing the chemical composition and physicochemical properties of materials,and it is widely used in the field of food processing.In order to provide guidance and theoretical basis for the application of extrusion technology in the processing of yam instant flour,this paper studies the effect and mechanism of extrusion treatment on the instant solubility of yam flour as raw material.(1)The process of yam flour extrusion treatment was studied.Taking the water solubility index(WSI)as the investigation index,by single factor and orthogonal test,the optimum process conditions were obtained as follows: water addition for 14%,extrusion temperature at 135 °C,screw speeds at 180 rpm.At the same time,the order of main factors affecting WSI of yam flour by extrusion treatment were water addition >extrusion temperature > screw rotation,of which water addition is the most significant factor.(2)The effect of extrusion treatment on the instant solubility of yam flour was studied.The results showed that after extrusion treatment on the yam flour,the solubility,WSI and gelatinization degree increased from 11.34%,9.68% and 38.12%to 62.05%,68.53% and 96.86%,respectively,and the static contact angle changed obviously.Simultaneously,extrusion treatment led to depolymerization and degradation of macromolecular substances,and increased contents of water-soluble substances such as amylose,dextrin,soluble sugar,soluble dietary fiber(SDF)and reducing sugar.The starch granules were strongly gelatinized,the gelatinization properties and thermodynamic properties were changed,and the water absorption capacity was enhanced,thus improving the instant solubility of yam flour.Correlation analysis showed that there was a significant relevance between the instant solubility of yam flour and its chemical composition together with physicochemical properties.And with the decrease of water addition,the instant solubility of yam flour gradually enhanced,indicating that the improved instant solubility of yam flour by extrusion treatment was not only related to its chemical composition and physicochemical properties but also closely linked to the water addition.(3)The mechanism of the effect of extrusion treatment on the instant solubility of yam flour was studied,and it was found that the damage of the particle structure of yam flour by extrusion treatment was gradually strengthened with the decrease of water addition,which led to the rupture of intact yam starch granules and the formation of irregular small fragments together with the decrease of particle size.Meanwhile,the crystal structure of yam starch was damaged by extrusion treatment,which changed the short-range crystal structure and long-range crystal structure,in the meantime,shifted the chemical bonds and functional groups,and the ordered crystalline regions were turned into disordered and water-binding-prone amorphous regions,as a result the relative crystallinity reduced.Besides,under the extrusion action of high temperature,high pressure and high shear,the molecular structure of yam flour was destroyed,therefore the molecular weight changed,and the small molecular substances aggrandized,then reflecting the advanced instant solubility.