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1.3.1 Obtaining Polysaccharide Fraction or Acemannan
ОглавлениеA variety of methods are used for polysaccharide purification and separation, such as alcohol precipitation [4, 7, 9, 14, 20, 22, 37, 66, 67], ion-exchange chromatography [5, 34], gel permeation chromatography [7, 20, 27], dialyses [5, 7, 20, 27, 30, 68–70], and membrane separation [71] (see Table 1.1).
Ethanol precipitation appears to be a simple method for obtaining crude polysaccharides, but a further process must be performed to obtain a high purity product. The ion-exchange chromatography approach is time-consuming, and requires a high amount of organic solvent. Gel-permeation chromatography is an efficient purification method but requires costly and complex operations, limiting its large-scale application. Membrane separation is an effective method for purifying polysaccharides. However, it is essential to consider, gels from A. vera is very high viscous, and they can easily saturate the membrane without previous treatment [69].
Polysaccharide extraction is often carried out by homogenizing the pulp, filtration or centrifugation to remove insoluble fibers, and subsequent alcohol precipitation or direct chromatographic fractionation. When the alcohol precipitation method is applied, subsequent operations are performed such as centrifuging, dialysis [27] and drying. These fractions can also be obtained by immersing the samples in boiling ethanol [9, 27].
The aqueous two-phase system (ATPS) extraction constitutes an efficient pretreatment method applied to separate and purify proteins, natural products, enzymes, aminoacids, many comprising a single-step procedure [69, 71]. In this regard, Xing & Li [71] developed an ATPS as a pre-treatment method using poly (acrylonitrile-acrylamide-styrene) membranes prepared using the phase inversion method. The polysaccharide was separated by a combination of aqueous two-phase extraction and membrane separation, yielding a polysaccharide of high purity. Tan et al. [69] developed a simple efficient, and emergent technique for the simultaneous extraction, and isolation of polysaccharides and proteins from A. vera. The polysaccharides migrated into the salt-rich phase, composed of (NH4)2SO4 and NaH2PO4, whereas the major impurities of protein, minerals, and phenolic compounds were extracted into the ionic-liquid rich phase using 1-butyl-3-methylimidazolium tetrafluoroborate. Based on the investigation of the partitioning behavior of polysaccharides and proteins in the Ion Liquid Aqueous Two-Phase System, the extraction conditions obtained were optimal, confirming the efficiency of this method for obtaining high purity polysaccharides. The polysaccharides were submitted to dialysis and after this lyophilized.