Preparation Of Low Dissolution-point Agar And High Water-retaining κ-carrageenan

Agar and κ-carrageenan extracted from the red algae are water-soluble polysaccharides,and they are widely used in food,biological and medical fields.However,the dissolve temperature of the agar extracted by the traditional method is high,and the syneresis rate of the κ-carrageenan gel is large,which limited their applications.In this paper,the modified agar and κ-carrageenan were studied by the method of screw extrusion and carboxymethylation.The aim was to develop and prepare low dissolution-point agar and high water-retaining property ofκ-carrageenan,and to explore the modification mechanism.The prepared products save the energy which dissolve the agar in industrial production,make the common application of substances possessing sensitive temperature and agar possible,and improve the water-retaining property of κ-carrageenan.This study broadens the application scope of agar and κ-carrageenan and provides reference for industrial production.The results of the research are summarized below:(1)Preparation of low dissolution-point agarThe influence of the screw extruding and carboxymethylation method on the agar was studied by using the commercially available agar as the raw material,the dissolve temperature as the main index and dissolve time and the gel strength were used as auxiliary indexes.In the process of screw extrusion,the processing temperature,the ratio of water to material and the screw speed have influence on the solubility of agar,but are not obvious.Therefore,the technological parameters of screw extrusion are as follows:processing temperature 115℃,the ratio of water to material 2.5:5 mL/g and screw speed 10 Hz.The supplementation of sodium chloroacetate,the reaction temperature of etherification and the concentration of ethanol were significant during modification.However,the influence of agar by single screw extrusion or carboxymethylated modification is not ideal,so the joint modification of screw extrusion and carboxymethylation is necessary.Determining the screw extrusion parameters firstly(processing temperature 115℃,ratio of water to material 2.5:5 mL/g,screw speed 10Hz),and then the carboxymethylation modification was carried out The results showed that the primary and secondary factors affected the carboxymethylation followed the order:ethanol concentration>etherification reaction temperature>supplementation of sodium chloroacetate.The optimum conditions for the modification of carboxymethylation were as follows:the addition amount of sodium chloroacetate was 0.021 mol(To 10g raw materials as an example),the etherification reaction temperature was 50℃,and the ethanol concentration was 77%.The dissolution temperature of the low dissolution-point agar was 55.6±0.51℃,the gel intensity was 112±1.10 g/cm2,the dissolution time was 5～6 min.The dissolution temperature decreased by 41.5%,the gel intensity was in accordance with the processing requirements,and the dissolution time of agar did not change significantly.From the results of FT-IR spectra and NMR,it was found that the carboxyl group structure was introduced into the sugar unit structure of the low dissolution-point agar,but the original structure was not destroyed.The surface of the unmodified agar particles was smooth,and the particles were in a thick flaky form;agglomerated agar particles were not smooth and presentsed more fracture surface by scanning electron microscopy.(2)Preparation of high water-retaining κ-carrageenanThe effect of κ-carrageenan modified by screw extrusion and carboxymethylation was studied by using the commercially availableκ-carrageenan as the raw material and moisture extraction rate as the main index,the gel strength and dissolution time were used as auxiliary indexes..In the process of screw extrusion modification,the processing temperature,the ratio of water to material and the screw speed have the influence on the moisture extraction rate of κ-carrageenan,but they were not sensitive.Therefore,the technological parameters of screw extrusion were determined as follows:75℃,the ratio of water to material was 2.0:5mL/g,screw speed was 15Hz;sodium chloroacetate addition,etherification reaction temperature and ethanol concentration and other factors had sigificant influence on κ-carrageenan during carboxymethylated modification.But the influence of single screw extrusion or carboxymethylation was not perfect,so the joint modification of screw extrusion and carboxymethylation is necessary.Determining the screw extrusion parameters firstly(processing temperature 75℃,ratio of water to material 2.0:5 mL/g,screw speed 15Hz),and then the carboxymethylation modification was carried out.The results were as follows:Ethanol concentration>etherification temperature>addition of chloroacetic acid.The optimized conditions for the modification of carboxymethylation were as follows:0.02 mol of sodium chloroacetate(To 10g raw materials as an example),65.10℃ of etherification reaction temperature and 69%of ethanol concentration.The moisture extraction rate of the high water-retaining κ-carrageenan prepared by joint modification process was 0.16±0.001%and the gel intensity was 205±1.22g/cm2.,which moisture extraction rate decreased by 96.3%comparing with the natural κ-carrageenan,and the gel strength was in accordance with the processing requirements.From the results of FT-IR spectra and NMR,it can be seen that the carboxymethyl functional group has been introduced into the sugar unit structure of the high water-retaining κ-carrageenan,but the original structure has not been destroyed.By scanning electron microscopy,the unmodified κ-carrageenan were flat and the surface was relatively flat,and the surface of high water-retaining κ-carrageenan particle morphology appeared many cracks and depressions.