Effects Of Modification Methods On Physicochemical Functional Properties And Structure Of Insoluble Dietary Fiber From Flax Seeds

Flax is rich in China,but for a long time,it is usually used for oil extraction.Linseed cake as the by-product of linseed oil production,which is currently mainly used as animal feed or fertilizer.However,IDF were the predominant fiber fraction in byproducts,resulting a great waste of resources.In addition,the research on dietary fiber mainly focuses on the extraction and preparation,and its modification is still in the primary stage.Therefore,the modification of dietary fiber has special and further significance.The objective of this study is to evaluate the effects of different modification（the alkali treatment,the dilute sulfuric acid treatment,the enzyme treatment,the carboxymethyl treatment as well as the graft treatment）on the physicochemical functional properties and structure characterization of IDF extracted from defatted flax seeds meal.The findings will furnish reference for the use value of insoluble dietary fiber of flax residue so as to make better development and utilization of flax residue.The dominating contents and results of this paper are as follows:（1）The physicochemical properties of insoluble dietary fiber before and after modification were determined.The results were as follows:compared with the unmodified flax IDF,after the acid modification and grafting modification,the water holding capacity and oil holding capacity increased,while after the alkali modification,enzyme modification and carboxymethylation modification,the water holding capacity and oil holding capacity decreased.The water retention of IDF modified by acid,carboxymethylation as well as graft modification was improved.After modification,swelling force and fluidity were improved.Cation-exchange capacity of IDF was improved by enzymic modification and grafting modification,but not realized by alkali,acid and carboxymethylated modification.The modification reduced the a^*value.After alkali modification,L^*value and b^*value became larger.The other modification methods reduced L^*value and b^*value to different degrees.After grafting modification,the color difference value was the largest and the acid modification was the smallest.（2）The functional properties of insoluble dietary fiber before and after modification were determined.The results were as follows:no matter before or after modification,the adsorption capacity of NO₂^-on the samples was stronger under the condition of pH 2 than that at pH 7 in short time.As for the maximum adsorption capacity,the results of acid modification and grafting modification were better.Except unmodified IDF,the cholesterol adsorption capacity of the rest of the samples were greater in the small intestine environment than that in gastric environment,among them,the IDF modified by acid,enzyme and grafting had better effect.At pH 7,the modified dietary fiber had a higher adsorption capacity for cholesterol than that of the unmodified dietary fiber,nevertheless at pH 2,the alkali modified dietary fiber had the best cholesterol adsorption effect.The adsorption capacity of sodium cholate was not improved greatly after modification.The effects of different modification methods on the glucose adsorption capacity of flax IDF were not significant.The glucose dialysis retardation index was different with the time of dialysis.（3）The structural characterizations of insoluble dietary fiber before and after modification were determined.The results were as follows:it could be seen from the scanning electron micrograph that the IDF contained blocks of different sizes,with slight folds and flake structure before modification,while the structure changed slightly,and the surface was rough with fine cracks and gullies after modification.Analysis of the infrared spectrogram showed that the infrared spectrum of IDF had the characteristic absorption peak of cellulose polysaccharides.The position and shape of the characteristic absorption peak were generally similar before and after modification.The main absorption peak has no obvious change between samples.As seen from the X-ray diffraction figure,the X-ray diffraction figure before and after modification was alike in shape,with similar 2θvalue of crystallization peak,and the position of characteristic diffraction peak basically unchanged.It could be seen from the DSC curve,the peak was relatively close,besides the enzyme modification and carboxymethyl modified,the rest of the modified IDF from 150oC to 500oC was relatively stable and showed no significant exothermic peak.The thermogravimetric analysis curves of dietary fiber obtained by different modification methods were basically the same.Different modification methods had no significant or minor influence on the thermal stability and phase change characteristics of dietary fiber,and the whole heating process included four stages（such as drying water loss,pre-carbonization,carbonization and combustion）.