Study On Preparation Of Dietary Fiber From Residue Of Nannochloropsis And Its Functional Properties

Nannochloropsis is rich in protein,fat,crude fiber and trace elements,the crude fiber content is about 20%.The large amount of residue produced after extracting lipids on an industrial scale from Nannochloropsis is usually discarded as industrial waste,causing problems such as environmental pollution and resource waste.In order to exploit and utilize this resource,the residue of Nannochloropsis was used as a crude materials to prepare DF in this research.Ultrasonic assisted alkali method was used to extract DF from residue of Nannochloropsis,and the extraction process was optimized by single factor and response surface optimization experiments.In order to explore the effect of ultrasound on the performance of SDF and its mechanism,the structure,physicochemical properties,in vitro hypoglycemic and hypolipidemic activities of U-SDF prepared by ultrasonic assisted alkali method and A-SDF prepared by alkali method were compared;the structure,physicochemical properties,and in vitro hypoglycemic and hypolipidemic activities of U-SDF and U-IDF prepared by ultrasound assisted alkali method were compared to analyze the differences in physiological functions of different kinds of DFs.The main conclusions of this research are as follows:(1)Nannochloropsis contained a large amount of nutrients such as protein,fat,crude fiber,as well as various mineral elements such as iron,calcium,magnesium.The results indicated that Nannochloropsis was rich in nutrition and had high nutritional value.The single factor and Box-Behnken response surface analysis method were used to optimize the extraction conditions of SDF from residue of Nannochloropsis by ultrasonic assisted alkali method.The optimumconditions were as follows: NaOH concentration was 1 mol/L,ultrasonic time was 31 min,ultrasonic power density was 5 W/m L,and material-liquid ratio was 1:40(W/V)and the extraction temperature was 40°C.Under this condition,the yield of U-SDF was 17.50± 0.26%,and the purity was 90.16%.Compared with A-SDF prepared by the alkali method,the yield and purity of U-SDF were increased by 150.72% and 7.56%respectively.(2)The determination of heavy metal content of DFs prepared by ultrasonic assisted alkali method indicated that the contents of chromium,copper,arsenic,cadmium,lead and mercury in U-SDF and U-IDF all met the national limit standard of China.Results of XRD and FT-IR showed that DFs from residue of Nannochloropsis were in a semi-crystalline state in which crystalline and amorphous domains coexisted,and have a typical polysaccharide structure.The results of SEM and particle size analysis showed that the surface of U-SDF was loose and honeycomb-shaped,and had a smaller particle size,which contributed to improving the physicochemical properties and adsorption performance.The results of DSC and TGA showed that ultrasound could improve the thermal stability of SDF;the thermal stability of IDF was better than that of SDF.Compared with A-SDF,U-SDF had higher solubility,water holding capacity,swelling capacity and oil holding capacity,indicating that ultrasound might improve the physicochemical properties of SDF.Water holding capacity,swelling capacity,water retention capacity and oil holding capacity of U-IDF were 6.65 g/g,6.47 g/g,6.25 m L/g,1.90 g/g,respectively,which were significantly(p<0.05)higher than that of U-SDF.U-SDF had better nitrite ion removal rate than A-SDF,reaching 83.08% at 120 min,which might be due to the increase of phenolic acid and uronic acid in U-SDF caused by ultrasound.The nitrite ion removal rate of U-IDF was significantly(p<0.05)lower than that of U-SDF,only60.07% at 120 min.(3)When the glucose concentration was 200 mmol/L,the glucose adsorption of U-SDF reached 3.33 ± 0.08 mmol/g,which was significantly(p<0.05)higher than that of A-SDF and U-IDF,and was concentration-dependent.The ability of DFs from residue of Nannochloropsis to inhibit glucose diffusion decreased with time.Compared with A-SDF or U-IDF,U-SDF had a higher glucose dialysis delay index,reaching a maximum(10.08%)at 40 min.U-SDF,A-SDF and acarbose exhibited various inhibition of ?-amylase: acarbose>U-SDF>A-SDF,and the inhibition capacity increased with the increase of sample.U-IDF had poorer ability to inhibit ?-amylase activity than U-SDF.At the same p H,the cholesterol adsorption capacity of U-SDF was significantly(p<0.05)higher than that of A-SDF and U-IDF;and the cholesterol adsorption capacity under p H 7.0 was significantly(p<0.05)higher than that of p H2.0,indicating that cholesterol adsorption mainly occured in the small intestine.The ability of binding sodium cholate of U-SDF in vitro was higher than that of A-SDF and U-IDF.When the concentration of sodium cholate was 3 mg/m L,the adsorption capacity of U-SDF reached 36.96 mg/g.U-SDF had higher pancreatic lipase inhibition ability than A-SDF and U-IDF,and the inhibition rate of U-SDF was 55.64± 0.30%.Therefore,the hypoglycemic and hypolipidemic activities of U-SDF in vitro were higher than that of A-SDF and U-IDF.