Alkaline Extraction Of Alginate From Lessonia Trabeculata

Lessonia trabeculata was considered as one of the potential raw material for the extraction of alginate.However,When the traditional extraction procedure was designed for the extraction of alginate from Laminaria digitata,when employed to extract alginate from Lessonia trabeculata,the extraction efficiency was so low that most of alginate retained in raw material even the extraction time was prolonged to several times of Laminaria digitata.In order to regulate the extracion process and improve the extraction efficiency,the analysis method of raw material and monitoring methods of extraxtion process were firstly studied in this paper,the traditional stew-stirring extraction procedure was ameliorated,and a new extraction procedure taking a twin-screw extruder as reactor to enhance surface renewal was explored.To avoid the complexity of traditional analytical methods of alginate content and uronic acid composition of alginate in the raw material,a metal element analysis method was established to determine the content of alginate,metal and uronic acid composition of alginate in Lessonia trabeculata.After metal elements unbound to alginate in algal tissues removed by exhaustive deionized water leaching,the content ofmetal elements bound to alginate were determined and the content of alginate was calculated according to the stoichiometric relationship between metal elements and alginic acid（C5H7O4COO-）.The results showed that the content of alginate obtained by the metal element analysis method was similar to Poly（hexamethylenebiguanide）hydrochloride（PHMB）method,slightly higher than the gravimetric method.By the method of metal element analysis,the metal constituent of alginates in the raw material can be obtained.In addition,as the multivalent metal elements in raw materials were mainly associated with G-blocks,the content of G-blocks were calculated according to the content of multivalent metal elements in the material determined by metal element analysis.Compared to NMR method,it was simple and easy to determine the G-blocks content of alginate in raw materials by metal element analysis method.It was convenient to obtain the content and the constituent of alginate in raw materials without the extraction of alginate.A method to monitor the alkaline extraction process was developed to determine the end point of alkaline extraction and optimize the extractioncondition.It indicated that the extraction course and the end point of alkaline extraction can be judged by changes of the extraction rate of alginate,since the variation of extraction yield and that of product yield during the alkaline extraction process were synchronous.That the product yield and extraction yield followed a good linear relationship manifested the extraction condition can be evaluated according to extraction yield.Since there was a good relationship between the extraction viscosity（After the liquid supernatant separated from alkaline extraction paste and diluted to a specific alginate concentration,the viscosity of the alginate solution was measured and marked as extraction viscosity）and the product viscosity,the changes of product viscosity can be characterized by extraction viscosity in the process of the alkaline extraction.When the traditional alkaline extraction procedure was applied to extract alginate from Lessonia trabeculata stipe,the stipe was not completely hydrated after soaked by water for 48 h and the extraction yield of alginate was only 15 g/100g raw materials after alkaline extraction for more than 10 h.The traditional alkaline extraction procedure was improved from the aspects of raw material pretreatment.At first,the smashing step was introduced.After the stipe smashed to d50=5 mm,they were hydratedfully when soaked by water for 6 h.When the soaked stipe smashed to d50=2 mm,the extraction yield was 43 g/100g raw material after alkaline extraction for 6 h.the production period could be shortened by 46 hours.Further,taking the procedure of "squeeze-soaking-smashing-alkaline extraction",when the stipe was squeezed under pressure =149 MPa,the stipe can be hydrated sufficiently within 6 hours.After the soaked stipe smashed to d50=2 mm,the extraction yield was 44.6 g/100g raw material after alkaline extraction for 4 h,and the extraction cycle could be shortened by 48 hours.It was found that alkaline extraction was limited by the mass transfer of alginate at the solid-liquid interface during the latter stage of alkaline extraction.The twin-screw extruder was taken as reactor to carry out the alkaline extraction,in which the smashing,extrusion and surface renewal function can be integrated to enhance the alkaline extraction efficiency.It indicated that the screw configuration of reverse screw element in the end of the positive conveying element showed good smashing ability and efficient extraction capability.Taking this configuration,the extraction yield at high temperature and alkali concentration（temperature 80℃,23.3 g/L Na2C03）was more than 46 g/100g raw material after alkaline extraction for 25 minutes,where the extraction time was shortened greatly,and the production efficiency was enhanced.However,alginate was degraded in advance,and the product viscosity was low.At low temperature and high alkali concentration（temperature 50℃,30 g/L Na₂CO₃）or high temperature and low alkali concentration（temperature 80℃,13.8 g/L Na₂CO₃）can be improved,and meanwhile the degradation of alginate was alleviated to guarantee high viscosity of the product.