Characteristics And Application Of Sorghum Starch From Saline-alkali Land

As a kind of saline-alkali tolerant crop,sorghum exhibits outstanding planting adaptability and growth ability.There are a large number of saline-alkali lands were used to grow sorghum in China.However,most of sorghum have been used in wine making or as animal feed.To develop the diversity of sorghum applications,further investigation needs to be conducted on sorghum.Starch content accounts for 68-75%of sorghum grains.The further investigation on the properties of sorghum starch is beneficial to broaden the application of sorghum and sorghum starch.Therefore,the properties of sorghum starch were comprehensively characterized in this thesis,and according to the characteristics of low digestibility and high gelatinizing viscosity,sorghum starch was used as thickener in fat-free yogurt to prepare healthier yogurt drinks.Sorghum starch with high amylose content was used to make degradable starch film.The main research contents and results of this thesis are as follows:（1）Starch was extracted from 30 varieties of sorghum grown in saline-alkaline areas and analyzed their particle morphology,amylose content,amylopectin chain length distribution,crystalline structure,gelatinization characteristics,thermodynamic properties,and digestion characteristics.The results showed that sorghum starch appears as irregular spherical or polygonal granules,with an average particle size of 14.5μm.The apparent amylose contents（AAC）of sorghum starch were varied between 7.42%and36.44%,and the relative crystallinities were ranged from 20.5%to 32.4%.The range of rapidly digested starch（RDS）,slowly digested starch（SDS）,and resistant starch（RS）was 13.46%-39.89%,40.98%-58.08%,and 8.50%-36.69%,respectively.The A chain content of branching starch ranged from 22.85%to 27.50%,the B₁ chain content was54.97%-59.58%,the B₂ chain content was 12.32%-14.75%,and the B₃₊chain content was 4.34%-6.84%.According to the Pearson correlation analysis,gelatinization enthalpy（ΔH）,peak viscosity（PV）,relative crystallinity（RC）,double helix degree（DD）,ordered degree（DO）,and digestion rate（DS）had negative correlation with AAC（p<0.05）,while final viscosity（CPV）and setback value（SB）showed positive correlation with AAC（p<0.05）.The thermal and gelatinization properties were strongly correlated with RC,DO,and DD（p<0.05）.（2）Four sorghum starches（S2,S4,S7 and S9）with higher amylopectin content,higher resistant starch content and higher pasting viscosity were selected among 30sorghum starches for application in skimmed yogurt.The effects of sorghum starch on yogurt were analyzed by measuring the rheological properties,texture properties,water holding capacity,particle size and Zeta potential.The results illustrated that the addition of sorghum starch increases the values of G’,G’’,and G*in yogurt.Consequently,the gel network was enhanced.Before the shear strain reaches approximately 30%,the yogurt without starch showed a larger strain under the same shear stress,and at a larger stress,the deformation of the yogurt without starch was the largest.The texture characteristics,water-holding capacity,and particle size indicated that the addition of sorghum starch increased the hardness,cohesiveness,viscosity,water-holding capacity,and particle size of the yogurt.Confocal microscopy showed that the protein network remained the dominant continuous phase at different sorghum starch addition levels.The yogurt without starch had a sparse network structure,while the yogurt with sorghum starch had a denser structure.According to the Pearson correlation analysis,the cohesiveness,water-holding capacity,and average particle size of yogurt with sorghum starch showed a positive correlation with starch expansion power（p<0.05）,while cohesiveness and average particle size showed a negative correlation with starch solubility（p<0.05）.（3）Six kinds of sorghum starch（S6,S14,S17,S28,S29 and S30）were selected from30 kinds of sorghum starch to make starch-base film in comparison to corn starch film and potato starch film.The water content,water solubility,crystal structure,short-range order,hydrophobicity,water vapor permeability and surface morphology of sorghum starch films were studied.The correlation analysis was carried out according to the research results of various properties.The results showed that except for S6,the water content of sorghum starch films was lower than that of corn starch and potato starch films.The water solubility of sorghum starch films was lower than that of corn and potato starch.The plasticization of sorghum starch destroyed the original A-type crystallization and formed V-type crystallization.The degree of crystallinity（RC）of sorghum starch films showed a positive correlation with linear starch content（p<0.01）.The contact angles of S29 and S30 starch films were greater than 90°,and their water vapor permeability was better than that of corn and potato starch films.The hydrophobicity and water vapor permeability of sorghum starch films showed a positive correlation and a negative correlation with linear starch content,respectively（p<0.05）.The tensile strength and elongation at break of the six sorghum starch films were between those of potato and corn starch films,which indicates that the starch film prepared by sorghum starch can give consideration to the toughness and load-bearing capacity at the same time.