Polysaccharides phase separation

The main hurdle for the use of starch as a reinforcing phase is its hydrophillicity leading to incompatibility with polymer matrix and poor dispersion causing phase separation. Two strategies have been adopted to improve the performance of polysaccharides. 

It did not give rise to phase separation or precipitation. Similar behavior was observed when other types of polysaccharides were examined [53,54]. By now all the commercially important polysaccharides have been applied to the fabrication of hybrid silica nanocomposites in accordance with Scheme 3.2. What is more, various proteins have been entrapped in silica by the same means. In all instances the THEOS demonstrated good biocompatibility with biopolymers, even though its amount in formulations was sometimes up to 60 wt%. Biopolymer solutions after the precursor admixing remained homogeneous to the point of transition into a gel state. 

The ethylene glycol-containing silica precursor has been combined, as mentioned above, with most commercially important polysaccharides and two proteins listed in Table 3.1. In spite of the wide variety of their nature, structure and properties, the jellification processes on addition of THEOS to solutions of all of these biopolymers (Scheme 3.2) had a common feature, that is the formation of monolithic nanocomposite materials, proceeding without phase separation and precipitation. The syner-esis mentioned in a number of cases in Table 3.1 was not more than 10 vol.%. It is worthwhile to compare it with common sol-gel processes. For example, the volume shrinkage of gels fabricated with the help of TEOS and diglyceryl silane was 70 and 53 %, respectively [138,141]. 

Manipulation of mobile phase and temperature parameters can have some unusual effects on chiral separations. Variation of temperature and mobile phase composition has been reported to reverse the elution order on protein phases and polysaccharide phases (Persson and Andersson, 2001). 

In addition to classical reverse phase separation of peptides on octadecyl derivatized silica monoliths, sugars and peptides as well as proteins and nucleosides have been analyzed on a 20-cm-long silica-based poly(acrylic acid) column (ID. 200 pm), employing HILIC and weak cation-exchange chromatography, respectively [194]. Furthermore, HILIC fractionation of polysaccharides delivered remarkable and promising results [84,194]. 

Microspheres are particles ranging between 1 and 100 pm. They are typically formed from degradable polymeric materials such as albumin, polysaccharides, or poly(a-hydroxy acids) by precipitation or phase-separation emulsion techniques [6, 332]. The relatively large diameters of microspheres make their extravasation into the tumor mass difficult and the uptake of microspheres by the RES is very rapid. 

Tuinier, R., de Kruif, C.G. (1999). Phase separation, creaming, and network formation of oil-in-water emulsions induced by an exocellular polysaccharide. Journal of Colloid and Interface Science, 218, 201-210. 

Nowadays it is well established that the interactions between different macromolecular ingredients (i.e., protein + protein, polysaccharide + polysaccharide, and protein + polysaccharide) are of great importance in determining the texture and shelf-life of multicomponent food colloids. These interactions affect the structure-forming properties of biopolymers in the bulk and at interfaces thermodynamic activity, self-assembly, sin-face loading, thermodynamic compatibility/incompatibility, phase separation, complexation and rheological behaviour. Therefore, one may infer that a knowledge of the key physico-chemical features of such biopolymer-biopolymer interactions, and their impact on stability properties of food colloids, is essential in order to be able to understand and predict the functional properties of mixed biopolymers in product formulations. 

In Part Four (Chapter eight) we focus on the interactions of mixed systems of surface-active biopolymers (proteins and polysaccharides) and surface-active lipids (surfactants/emulsifiers) at oil-water and air-water interfaces. We describe how these interactions affect mechanisms controlling the behaviour of colloidal systems containing mixed ingredients. We show how the properties of biopolymer-based adsorption layers are affected by an interplay of phenomena which include selfassociation, complexation, phase separation, and competitive displacement. 

Polysaccharide dispersions phase-separate spontaneously, a phenomenon called aging. Phase-separation may be induced in special systems, under controlled conditions (e.g., encapsulation), to industrial and commercial advantage. 

The mixture of phenol and water (45 50, v/v) can be used to extract LPS (Westphal and Jann, 1965). This mixture is a single phase above 65°C but separates into two phases below 65°C. LPS and proteins can be extracted from bacteria by this mixture above 65°C. When cooled down, phase separation occurs. The phenol phase mainly contains proteins, while the water phase contains LPS, polysaccharides, and nucleic acids. The following protocol is the most used for phenol-water extraction of LPS (Johnson and Perry, 1976). 

On mixing solutions of a protein and a polysaccharide, four kinds of mixed solutions can be obtained. Figure 3.1 shows that two single-phase systems (1 and 3) and two-types of biphase systems (2 and 4) can be produced. The two-phase liquid systems 2 and 4 differ in the distribution of biopolymers between the co-existing phases. The biopolymers are concentrated either in the concentrated phase of system 2 because of interbiopolymer complexing, or within separated phases of system 4 because of incompatibility of the biopolymers. The term biopolymer compatibility implies miscibility of different biopolymers on a molecular level. The terms incompatibility or limited thermodynamic compatibility cover both limited miscibility or limited cosolubility of biopolymers (i.e., system 2) and demixing or phase separation... 

The phase separation threshold is lower for systems containing a branched polysaccharide than for systems containing a linear polysaccharide of the same molecular weight. It is higher for globular proteins compared to proteins of unfolded structure. An increase in excluded volume means a decrease in the free volume of the solution accessible for biopolymers. Thus, the excluded volume of biopolymer molecules implies that water in real foods can be nonsolvent water relative to macromolecules. 

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