| Starch is an important raw material in food and industry.Starch mainly comes from corn,cassava,potato,and wheat.The production of these traditional sources of starch is affected by the natural environment and the performance of natural starch is not sufficient to meet the application requirements,which limits the development of the starch industry to a certain extent.Therefore,it is necessary to develop new starch resources with low agricultural production costs and little influence from the natural environment,explore their performance and expand their application.Spirodela polyrhiza ZH0196 is a new starch resource with great potential.There are few studies on the starch of Spirodela polyrhiza,especially the extraction method,structure,properties and application of the starch that has not been reported.In this study,Spirodela polyrhiza ZH0196 was used as a starch resource to investigate the effects of the structure and composition of fronds and turions on starch extraction.The turions were selected as raw materials for starch extraction,and the starch extraction process was determined according to the characteristics of the turions.The fine structure,properties and relationship between the structure and properties of the Spirodela turions starch were systematically studied.The retrogression evolution of the starch at-20℃was studied,and the mechanism of the stability of freeze-thaw was explained from the perspective of the change of starch structure.Finally,the application effect of the starch in surimi gel was realized.The main research contents and results are as follows:Firstly,according to the characteristics of the fronds and turions,the raw material of starch extraction was determined to be turions,and the raw material collection method was collected in one time on the 45th day.According to the structure and components of the turions,starch was extracted by wet grinding and dispersing.The influence of parameters such as grinding fineness of the turions,dispersing speed,dispersing solid-liquid ratio and dispersing time on starch extraction was investigated,and the extraction process was optimized.The results showed that after wet grinding,the fineness of the resting body was≤80 mu,the ratio of dispersing material to liquid was 15 g/100 m L,the dispersing speed was 6000 r/min,and the dispersing time was 10 min.The purity of starch was 98.85%（w/w）and the extraction rate was82.34%（w/w）.Secondly,the fine structure of the Spirodela turions starch was analyzed and compared with that of common corn starch and rice starch.Chain structure results showed that the amylose content in the Spirodela turions starch was 27.48 g/100 g.Among the three kinds of starch,the extreme degree of polymerization（DPw>10500）was 46.67%.The size of amylopectin molecules in the Spirodela turions starch varied greatly,while the size of amylose molecules was relatively uniform.The branching mainly occurred in the part with a long chain length.The helical structure results showed that the order degree of amylose and amylopectin in the Spirodela turions starch was between the other two kinds of starch,the content of amorphous form was the highest(45.93%,13C CP/MAS NMR),and the total hydrogen bond strength was the lowest.The results of crystal structure showed that the Spirodela turions starch was C-type and the crystallinity was 18.41%（XRD）,which was significantly lower than that of rice starch and common corn starch.The particle structure of the Spirodela turions starch was elliptic,with an average particle size of 11.30μm and tiny holes on the surface.In conclusion,the starch structure of the Spirodela turions starch is"loose".Thirdly,the physicochemical properties of the Spirodela turions starch were analyzed,and the relationship between the structure and property of starches was established by principal component analysis.In this paper,the following starch properties were studied:（1）water absorption,solubility,swelling power,thermal stability,gelatinization,enzyme sensitivity toα-amylase,in vitro digestion;（2）the transparency,settling volume,drop length and rheological properties of starch paste;（3）Starch gel properties（gel strength,elasticity and freeze-thaw stability）.The results show that the first type of properties is mainly related to the"loose"structure of the Spirodela turions starch,which leads to higher binding of starch with water/enzymes.The results showed strong water absorption capacity（101.45%）,superior solubility and swelling power,low initial gelatinization temperature（66.9℃）,extreme gelatinization viscosity（the concentration was 8%,w/w,and the peak viscosity was 2664.50mPa·s）and high enzymatic hydrolysis rate.Specifically,the strong water-binding ability of the Spirodela turions starch may be related to the high content of the Afp（DP 6-8）chain and the high branching degree of the starch.The Spirodela polyrhiza turions starch is easy to gelatinize and has higher gelatinized viscosity,which may be related to loose semi-crystalline flakes and the high content of the B2 chain in the starch.The second kind of properties is mainly affected by amylose content,starch molecular chain length,branching degree and so on.The Spirodela polyrhiza turions starch showed good transparency with light transmittance of 11.45%.The rheological study showed that the starch paste of the turions had a dense paste structure and stronger elastic structure.In addition,the Spirodela turions starch also showed decent wire drawing performance.In the third type of properties,the stronger gel elasticity of the Spirodela turions starch may be related to the thickness of the crystal nucleus,the thickness of the interfacial layer and C1 in the glucose unit.The good freezing-thawing stability of Spirodela polyrhiza turions starch may be related to the long branched chain length and the branched chain mainly occurs in long chains.However,since freezing-thawing stability involves the regeneration of starch in a freezing state,its mechanism still needs to be further studied.To thoroughly explain the mechanism of good freeze-thawing stability of the Spirodela turions starch,the changes in starch gel hardness and the evolution of starch microstructure,molecular network and condensed state during the regeneration process were investigated.The results show that the starch gel hardness and recovery enthalpy increase slowly at 0-4 h,suddenly increase at 8 h,and then enter the process of fluctuation.The results of TGA,SEM,and AFM showed that the mass heat loss,microstructure,molecular network,condensed matter structure and helical structure of starch fluctuated with the increase in storage time.This indicated that the starch structure was not a directional change process under the condition of high moisture content（92%,w/w）and freezing.The molecular network results showed that the regeneration of common corn starch and rice starch was a process of continuous particle aggregation and continuous network connection,while the Spirodela turions starch first formed a network of sheets,and then the network developed to a fine and uniform direction.The condensed matter structure showed that the starch paste of the Spirodela turions formed a large gel network（8.37-14.82 nm）at the initial freezing stage（0-4 h）,and the water was bound in the network but not entirely frozen.With the increase in storage time,the water was distributed twice.The smaller size of the network freezes the water in the smaller size network（2.69-3.26nm）.Compared with common corn and rice starch,the starch gel of the Spirodela turions has higher strength,better water holding capacity and porous and dense microstructure,and the secondary distribution of water in the gel network during the storage process is beneficial to the improvement of freeze-thaw stability.Finally,the Spirodela turions starch was added to the surimi gel and compared with potato starch and acetylated distarch phosphate surimi gel.The freeze-thaw stability of surimi gel and the gel strength,texture,morphology,water distribution,water holding capacity and sensory properties of surimi gel during the freeze-thaw process were investigated.The results showed that the water extraction rate of SSG was considerably lower than that of potato starch surimi gel（PSG）,but higher than that of acetylated distarch phosphate surimi gel（ASG）during the repeated freeze-thawing process.The strength of raw SSG gel did not change with the increase of freeze-thaw times,while the strength of cooked SSG gel increased with the increase of freeze-thaw times.The hardness,elasticity and chewability of cooked SSG were higher than that of PSG and lower than that of ASG.After cooking,the whiteness of SSG was 74.62-76.45,which was significantly higher than that of PSG and ASG.The taste of SSG was significantly better than that of PSG and ASG,but there was no significant difference between SSG and ASG in smoothness and taste.The sensory scores of SSG were 73.50-81.17.In conclusion,Spirodela polyrhiza turions starch is a potential raw material for frozen food. |
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