Construction And Characterization Of Glucomannans Based Gel Systems Suitable For Development Of Food For Dysphagia

The dysphagia,caused by disease and aging,has gradually became a social issue that cannot be ignored,which makes the design of food for dysphagia a new trend to meet market demand.Currently,plant derived polysaccharides are widely used in food,medicine,materials,cosmetics,and other fields due to their excellent physicochemical properties and biological activities.Glucomannans（GMs）are mainly composed of mannose and glucose in different ratios byβ-（1→4）linkage.Although the structural types of GMs from different plant sources are the similar,there are significant differences in structural characteristics.Considering the structural variability in molecular weight,mannose/glucose ratio,and degree of branching substitution as well as functional activities of GMs,it is necessary to apply GMs in the food field,especially in foods for dysphagia.This thesis mainly explored the construction methods of GMs（extracted and purified from the Amorphophallus rivirei,Dendrobium officinale,Bletilla striata,and the leaves skin of Aloe Vera,respectively）based gel systems and compared the characteristics of gels.In addition,the formation mechanisms of gel were preliminarily investigated.In order to enhance the nutritional value of these gel systems,minerals and proteins were added into gels.Then,the effects of gel concentrations,ion species and ion strength,and soy protein isolate（SPI）on the rheological properties of GMs based gel systems were investigated.The effects of storage and processing（storage at low temperature and redissolution after freeze-drying）on the rheological properties of these GMs based gel systems were also determined.Finally,the rheological properties of these GMs based gels in oral environment（37℃,p H,saliva）in vitro was also detected.The main contents and related results are as follows:（1）A gel system with konjac glucomannan（KGM）as the main raw material,was successfully constructed,which can be applied to foods with dysphagia,and the rheological properties of gels were characterized.The results showed that the apparent viscosity of mixture（0.5%,w/v）increased significantly when adding xanthan gum（XG,content≤50%）to KGM solution.The results of dynamic rheology,appearance and microstructure all showed that the thermoreversible hydrogel with three-dimensional network structure was formed between KGM and XG by synergistical interaction.The strength of the gel was the largest when KGM/XG was 5:5.The viscosity and gel strength of mixtures decreased as adding salts,and the effect of Ca²⁺was greater than that of Na⁺or K⁺.The processes of adding SPI,or storing at-20 or-80℃,or dried gel being resolved in water had almost no effect on gel properties.Therefore,the characteristics of the gel systems can be controlled by altering the KGM/XG ratios and gel concentrations,and these gels can be further applied to foods with less minerals and rich SPI for dysphagia.（2）Gel systems with polysaccharide from Dendrobium officinale（DOP）as the main raw material were successfully constructed,which can be applied to food with dysphagia.The gel properties of gels were also characterized.The results showed that the apparent viscosity of mixture（0.5%,w/v）increased almost 100 times when adding XG（content≤50%）to DOP solution.The results indicated that DOP and XG synergistically formed thermoreversible hydrogels with three-dimensional network structure,which were determined by dynamic rheology,texture analyzer and scanning electron microscope.The strength of the gel was the largest when DOP/XG was 7:3.The viscosity and gel strength of mixtures decreased as adding salts,and the effect of Ca²⁺was greater than that of Na⁺and K⁺.The rheological properties of gels were not affected by adding SPI,or storing at-20 or-80℃,or dried gel being resolved in water.Therefore,the characteristics of the gel systems can be controlled by altering the DOP/XG ratios and gel concentrations,which can be applied to foods with less minerals and rich SPI for dysphagia.（3）Gel systems with polysaccharide from Bletilla striata（BSP）as the main raw material,which can be applied to food with dysphagia,were successfully constructed.The gel properties were also further characterized.The results suggested that the apparent viscosity of mixtures（0.5%,w/v）significantly higher than XG or BSP solution.The results showed that the thermoreversible hydrogels with three-dimensional network structure was formed between BSP and XG by synergistical interaction.The strength of the gel was the largest when BSP/XG was5:5.The viscosity and gel strength of mixtures decreased with the presence of salts,and the effect of Ca²⁺was greater than that of Na⁺and K⁺.The process of adding SPI,or storing at-20 or-80℃,or dried gel being resolved in water almost had no effect on gel properties.Therefore,the gel properties can be controlled by altering the BSP/XG ratios or gel concentrations,which can be applied to foods with less minerals and rich SPI for dysphagia.（4）ASP-4N,polysaccharide from Aloe vera skin,was a thin liquid near Newtonian fluid.Partially acetyl groups were removed by weak alkali method（sodium carbonate）and then preparing gel foods for people with dysphagia through adding XG.The results showed that the strength of ASP-4N decreased at 1740 cm^-1 in FTIR spectra,the peak width decreased in X-ray diffraction patterns,and the thermal stability decreased with the decreasing acetyl content.The microstructure of polysaccharides gradually changed from flaky to filamentous and spherical.When the acetyl content of ASP-4N was in the range of 26.6%～19.3%,it can form gel with XG.In addition,the apparent viscosity,gel strength as well as gel formation temperature increased as the acetyl content decreased.The results of microstructure showed that partially deacetylated ASP-4N could form thermoreversible gels with three-dimensional network structure when blending with XG.（5）The rheological properties of KGM/XG,DOPXG and BSP/XG were determined in oral environment（37℃,p H,commercial saliva and human saliva）in vitro.,The results indicated that the viscosity and gel strength of all gels decreased under the oral environment and the viscosity of KGM/XG and BSP/XG was higher than that of DOP/XG.The viscosity of all gels,which was not affected by the source of saliva,was similar at 37℃presence or absence saliva.The presence of ions greatly reduced the viscosity of all gels under the oral p H condition.the decrease of gels viscoelasticity in the oral environment in vitro indicated that gels have good palatability which are easy to swallow.Therefore,KGM/XG,DOPXG and BSP/XG gels are suitable to apply in foods for dysphagia.In summary,natural GMs have good solubility,and the apparent viscosity of aqueous solutions increases with increasing concentration.The acetyl content of GMs directly affects whether GMs can form gel with Xanthan gum.GMs and Xanthan gum synergistically formed thermoreversible gels with three-dimensional network structure through hydrogen bonding.The surface of gels was smooth and wet,and the whole was soft and fragile.The apparent viscosity of gel(less than 2.5 Pa·s as the shear rate is 50 s^-1)and the strength of gel（elastic modulus is less than 60 Pa）can be accurately obtained by controlling the GMs/XG ratios,or the concentrations of gels,or adding salt ions.In order to meet the needs of patients with different degrees of dysphagia or for food during swallowing training,GMs based gels can be applied in foods,especially liquid foods,for dysphagia.