Study On The Interaction Between Modified Insoluble Soybean Fiber(ISF) And Milk Protein In Gel System And Its Application

The dairy industry plays an important role in maintaining the health of the population,and yogurt products stand out for their unique taste,rich flavor and high nutritional value.In recent years,the growing health consciousness has driven the yogurt industry to innovate towards value-added products such as non-additive and multifunctional products.As a result,the search for naturally derived functional food ingredients has become key to the development of yogurt formulations.Insoluble soy fiber（ISF）,a major component of okara,a by-product of soybean processing,has received much attention for structural and physicochemical fortification of yogurt.However,the native structure of ISF is dense and has a large number of hydroxyl groups in the molecular structure,leading to poor physicochemical properties and limited applications.In this dissertation,the physical-chemical treatments related to structural modifications were screened with the aim of improving the physicochemical properties（hydration capacity and rheological properties）of ISF,and the modification mechanism was elucidated.The interaction between ISF and milk proteins in the gelation process of skim milk induced by glucono-δ-lactone（GDL）was clarified.Furthermore,the role of ISF on the structural properties and water stability of fermented set-type yogurts was investigated and the mechanism of its influence was clarified.Finally,based on a systematic evaluation of the rheological and sensory properties of set-type yogurt with ISF and a comparison with commercially available products,the potential of ISF for commercial application in set-type yogurt was investigated.This dissertation provides technical support for the production of high-quality set-type yogurts enriched with dietary fiber and the development of ISF as a multifunctional food ingredient.The main research and findings of this work are as follows:The okara-derived ISF was modified by ultrasonication（50℃,400 W,60 min）and/or autoclaving（121℃,20 min）combined with alkaline treatment（p H 12）.The best means of modification was screened according to the structural and physicochemical properties of ISF.The results showed that the water holding capacity（WHC）,swelling capacity（SWC）and rheological properties of the modified ISF were enhanced,which may be due to the disruption of the dense structure,the wrinkled and porous surface,and the exposure of polar groups due to the breaking of polymer molecular chains.Among them,the ultrasonic autoclaving alkaline treatment（UA-AT）was more effective,and the WHC and SWC were 3.84 and 9.04 times higher than those of untreated ISF,respectively.The UA-AT-ISF suspension had ideal viscosity and gel-like viscoelastic properties.The UA-AT-ISF was used as the main object of the following study.The intrinsic change patterns of the structural and physicochemical properties of ISF under ultrasonic autoclaving alkaline treatment of different intensities were investigated to elucidate the modification mechanisms.The results showed that when the treatment effectively disrupted the compact structure of ISF from the amorphous region to the crystalline region,and broke the hydrogen bond from intramolecular to intermolecular hydrogen bonds,the looseness of the structure was greatly increased,as shown by the appearance of branching morphology in the crystalline region and the increase in particle size of ISF;in addition,a large number of polar groups on the surface of ISF were exposed.Based on the above characteristics,the viscosity and viscoelasticity of ISF suspensions increased significantly.In conclusion,breaking the accessibility barrier in the crystalline region and breaking the intermolecular hydrogen bonds may be the key to the modification by ultrasonic autoclaving alkaline treatment.The effect of ISF addition（0.5%and 1.0%,w/v）on the structure and final gel properties during the skim milk gelation process was investigated using a GDL-induced skim milk acidification model.The results showed that ISF promoted milk protein dissociation at p H 6.6,6.0 and 5.5,as evidenced by an increase in the surface hydrophobicity of milk protein,the concentration of free Ca²⁺in the system and the concentration of casein in the supernatant with the addition of ISF,which was more pronounced at 1.0%.As acidification progressed to p H 5.0,protein aggregation in skim milk increased,as evidenced by an increase in the size of the aggregates.As acidification progressed to p H 4.8 and 4.5,ISF was involved in the formation of skim milk gels through its own steric hindrance,filling and electrostatic interactions with milk proteins,and the network structure was more homogeneous at 0.5%ISF addition.Meanwhile,ISF imparted higher gel properties such as viscoelasticity,viscosity and water holding capacity to the skim milk gel.Different concentrations of ISF were added to reconstituted milk by the fermentation of lactic acid bacteria to produce set-type yogurt,and the effects of ISF on the structural architecture,water stability and basic rheological properties of yogurt,as well as the mechanisms involved,were investigated.The results showed that 0.25%and 0.50%（w/v）ISF induced the formation of a homogeneous network structure dominated by protein,while 0.75%,1.00%and 1.25%（w/v）ISF and protein formed a non-homogeneous network structure mediated by protein network structure,ISF-protein aggregate electrostatic complex and gel-like network structure of ISF.All of the above structural features resulted in the increased strength of the yogurt gel structure.Meanwhile,the viscosity and viscoelasticity of yogurt increased ISF concentration-dependently,and improved the stability of yogurt by enhancing the water binding of the gel matrix.The rheological properties of ISF-set-type yogurt were systematically compared with those of eight commercially available products,and the potential application of ISF in set-type yogurt was further evaluated in conjunction with sensory evaluation.ISF improved the rheological properties of yogurt such as creep recovery,thixotropic properties,and elastic structural stability under external environmental stimuli.Further comparison of the rheological properties of ISF-set-type yogurt with those of commercially available yogurt revealed a highly similar rheological property profile at ISF concentrations between 0.25%and 0.75%,indicating similar network structure and oral texture perception.Subsequently,sensory evaluation results showed that 0.50%was found to be the optimal concentration to add to the yogurt,mainly in terms of improved texture and overall preference.