Preparation And Study Of Highly Effective Adsorbent For Steviol Glycosides In Stevia

Stevia glycosides(SGs) extracted from stevia are a kind of high-sweet natural dietary supplement and sweetener.It is known as the third largest sugar source after sugar cane and beet sugar.Among the SGs,rebaudioside A(RA) and stevioside(STV) have the highest content and sweetness.STV exhibits a bitter or licorice aftertaste,which affects the taste quality characteristics of SGs.Therefore,obtaining the high-purity RA products is the key to the separation and purification of SGs and to improve the quality of Xinjiang natural products.In this paper,STV and RA were regarded as the separation system.Based on the adsorption method,the preliminary separation adsorbent was selected to purify the crude stevia extract to obtain mixed glycosides.According to the difference of complexing strength between boric acid and STV or RA,the functional adsorbents were prepared,and selectively separated them to improve the taste quality of SGs.The specific research content are as follows:The mesoporous carbon material were selected according to the structure of the adsorbents and the target molecular size of the separator system,the optimal structure was selected and the crude extract was initially separated.By comparing the adsorption performance of mesoporous carbon adsorption materials with different structures,the OMC with the largest specific surface area(1011.760 m²·g^-1) and appropriate pore size(4.59 nm) was selected as the optimal adsorbent.OMC's high specific surface area and ordered open channel structure make it possible to quickly adsorb STV and RA in aqueous solution.As a crude separation adsorption material,the crude extract is preliminarily separated to obtain mixed glycosides.By optimizing the separation process,the adsorption capacities of OMC for STV and RA were 212.18 mg·g^-1 and 359.64 mg·g^-1,and the selectivity(?_STV/RA) was 1.4.The purity of STV and RA was increased from 8.01% and 16.73% to 17.51% and 45.32%,and the yields were 89.22% and 87.73%,respectively.According to the structure of the adsorbent and the function of the functional group,boric acid functionalization was carried out on the basis of the resin with good adsorbability to improve its selectivity and further separate STV and RA.The resin BJ7501 with strong adsorption capacity,active functional groups and easy modification was selected,and specific functional group modification was carried out to prepare the resin BJ7501 C with boric acid functional groups.FT-IR and XPS characterization showed that B-O groups appeared after modification.The peak and element B indicated that the modification was successful.By optimizing the separation process of the resin,the adsorption capacities of crude sugar solution were STV: 35.22 mg·g^-1,RA: 62.00 mg·g^-1,respectively.The selectivity(?_STV/RA)reached 4.34.And refined the crude sugar solution,BJ7501 C improved the purity of STV from 17.51% to 29.83%,and increased purity of RA from 45.32% to 70.17%.According to the structure of the adsorbent and the role of functional groups,on the basis of metal organic frameworks(MOFs),the boric acid functionalized adsorbents(MOF-BA) was synthesized in situ through a simple microwave method by the organic ligands containing the required boric acid functional groups.It can avoid the conjugation effect between functional groups during post-modification and improve the usability of introducing functional groups,and compare with modified resin performance.A simple one-step microwave method was used to synthesize boric acid-functionalized Zn-based MOF-sorbent(MOF-BA) in situ.The prepared MOF-BA was isomorphic to the parent framework.FT-IR characterization verified the coordination of carboxyl group with Zn,indicating successful preparation.ICP and XPS characterization showed the presence of element B.By changing the ratio between the two ligands,the MOF-BA-1.0 prepared when the feed ratio of BBDC to BTC is 1:1 has good crystallinity and a pore size(4.69 nm) that is more suitable for separating STV and RA.By optimizing the separation process of the MOF-BA-1.0,the adsorption capacities of crude sugar solution were STV: 42.93 mg·g^-1,RA: 22.96 mg·g^-1,respectively.The selectivity(?_STV/RA) reached 4.35.Refining the crude sugar solution to obtain MOF-BA-1.0 improved the purity of STV from 17.51% to 31.83%;increases the purity of RA from 45.32% to 68.17%.