Rheological Properties Of Porphyra Yezoensis Polysaccharide And Its Effect On Gel Properties Of Silver Carp Surimi

Porphyra yezoensis is one of the most economically valuable marine algae in China,mainly distributed in the coastal areas of Jiangsu,Fujian and Zhejiang.It is rich in polysaccharides,proteins,peptides and other nutrients,and has biological activities such as antioxidant,antibacterial,anticancer,hypoglycemic,etc.It has a history of edible and medicinal use in China for many years.Polysaccharide is the main component of laver and the material basis of its biological activity.Currently,porphyra polysaccharides are mainly focused on extraction optimization,structural modification and bioactivity studies,but relatively few studies on rheological properties and gelling behavior have been conducted.Therefore,this thesis investigates the physicochemical properties,basic structure,rheological properties and gelling behavior of Porphyra yezoensis polysaccharides,and explores the effects of polysaccharides on gel properties,microstructure and antioxidant activity of surimi.The main research contents and results are as follows.1.Physicochemical properties,structural characterization and antioxidant activity analysis of polysaccharides.The polysaccharide yield,total sugar content,protein content and sulfate group content were obtained as 15.05±0.35%,66.12±0.28%,12.27±0.06%and0.33±0.16%,respectively,using ultrasound-assisted hydro extraction and alcoholic precipitation to extract Porphyra yezoensis polysaccharides.The structures of the polysaccharides were identified and analyzed using PMP derivatization-high performance liquid chromatography,high-performance gel permeation chromatography,Fourier infrared spectroscopy,NMR spectroscopy,and scanning electron microscopy techniques.The results showed that porphyra polysaccharide was an acidic heteropolysaccharide with galactose and glucose as the main components,and consisted of four molecular weight polysaccharides（7.07×10⁵ Da,4.67×10⁴ Da,376 Da and 78 Da,respectively）containingαandβconformational glycosidic bonds exist in the polysaccharide structure.The structure of polysaccharide is irregular flocculent with rough surfaces accompanied by cavities.The in vitro antioxidant results showed that porphyra polysaccharide exhibited excellent antioxidant capacity in DPPH,ABTS and hydroxyl radicals and increased with the increase of concentration.2.To investigate the rheological behavior,textural properties and microstructure of polysaccharides.The effects of different concentrations,temperature,p H and Ca²⁺on the rheological properties of polysaccharides were determined.The experimental results showed that in the concentration range of 0.5-5%,there was a positive correlation between apparent viscosity and mass fraction,which has good pseudoplastic fluid characteristics.The apparent viscosity decreased due to the weakening of the interaction between polysaccharide molecules by increasing temperature.Polysaccharide gels under acidic or alkaline conditions break the hydrogen bonds,change the molecular chain conformation and decrease the apparent viscosity.The addition of a low concentration of Ca²⁺（0-6 m M）contributes to the increase of apparent viscosity of polysaccharide gels.The textural properties showed that the polysaccharide gels showed the best hardness and chewiness with the addition of 6 m M Ca²⁺.The microstructure of the polysaccharide gels was characterized by Scanning electron microscopy,Fourier infrared spectroscopy,and X-ray diffraction,and the results demonstrated that the addition of Ca²⁺from 0 to 6 m M gradually smoothed and ordered the surface of the polysaccharide gels,while the addition of higher concentration（9 m M）of Ca²⁺disrupted the network structure of the polysaccharide gels,resulting in the delaminated and porous gel structure.Compared with the gels without calcium ion addition,the samples with calcium ion addition all exhibited stronger hydrogen bonding.The addition of calcium ions can change the crystalline area of the gel and increase the intensity of the diffraction peaks.Finally,thermogravimetric analysis curves showed that the addition of an appropriate amount of Ca²⁺could improve the thermal stability of the polysaccharide gels.3.The effects of porphyra polysaccharide on the gel properties,water status,microstructure,protein secondary structure and antioxidant activity of silver carp（Hypophthalmichthys molitrix）surimi were elucidated.The results showed that the addition of 0.4%porphyra polysaccharide significantly improved the gel strength,textural properties and water holding capacity（WHC）of silver carp surimi,and the whiteness was within the acceptable range.The result of the water state illustrated that the addition of porphyra polysaccharides could shorten the gel relaxation time T₂ of surimi and convert part of free water into bound water.Scanning electron microscopy observed that surimi added with 0.4%polysaccharide exhibited the most uniform and dense three-dimensional gel network structure,while excessive polysaccharide weakened the intermolecular forces of proteins and damaged its network structure,resulting in lower gel quality.FTIR analysis confirmed that theα-helix content decreased significantly and theβ-turn content increased（P<0.05）in the range of 0-0.4%of polysaccharide addition.When the addition amount exceeded 0.4%,theα-helix content gradually rebounded and theβ-turn showed the opposite trend.Antioxidation experiments indicated that the surimi gels supplemented with porphyra polysaccharides exhibited the more excellent scavenging ability of DPPH radicals,hydroxyl radicals and ABTS radicals and inhibited protein oxidation in a dose-dependent manner compared with the control group.