Biopolymers polysaccharide gels

In a small-scale separation, especially for biopolymer separation such as for proteins, polysaccharide gel-type ion exchange materials can be used for many possibilities. At the HPLC level separation, the use of an organic polymer gel is most effective. 

TSK-GEL PW type columns are commonly used for the separation of synthetic water-soluble polymers because they exhibit a much larger separation range, better linearity of calibration curves, and much lower adsorption effects than TSK-GEL SW columns (10). While TSK-GEL SW columns are suitable for separating monodisperse biopolymers, such as proteins, TSK-GEL PW columns are recommended for separating polydisperse compounds, such as polysaccharides and synthetic polymers. 

Dextran gels have been utilized since the late 1950s (1) for the separation of biopolymers. First attempts on Sephadex (2-5) and Sephadex/Sepharose (6-8) systems are documented for hydrolyzed and native starch glucans. Up until now, particularly for the preparative and semipreparative separation of polysaccharides, a range of efficient and mechanically stable Sephacryl gels (9-14) have been developped. 

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]. 

Ti values may occur with such native biopolymers as ribonuclease A, deoxyribonucleic acid, and collagen, whose molecular motions are restricted, but, as yet, high values have not been observed for polysaccharides in solution, or for gels, in which these motional-restriction effects may be equivalent, or less marked. However, an extensive relaxation-study by Levy and coworkers68 on poly(n-alkyl methacrylates) may serve as a model for future experiments on polysaccharides, as this type of molecule has a main chain and side chains, albeit more mobile than those in polysaccharides. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

The list of the new gels for which phase transitions are possible is supplemented in the paper by Amiya and Tanaka, who discovered discrete collapse for the most important representatives of biopolymers - chemically crosslinked networks formed by proteins, DNA and polysaccharides [45]. Thus, it was demonstrated that discrete collapse is a general property of weakly charged gels and that the most important factor, which is responsible for the occurrence of this phenomenon, is the osmotic pressure of the system of counter ions. 

The typical biopolymers used for the preparation of micro- and nanogels are polysaccharides (Scheme 11) cellulose (CL), chitosan (CS), hyaluronan (HA), heparin, pullulan (PuL), dextran, and gelatin - a proteinaceous polyamolytic gel obtained by partial hydrolysis of collagen. Gelatine microgels are addressed by Landfester and Musyanovych in another chapter of this issue [5] and will thus not be discussed further here. 

More detailed discussion of food polymers and their functionality in food is now difficult because of the lack of the information available on thermodynamic properties of biopolymer mixtures. So far, the phase behaviour of many important model systems remains unstudied. This particularly relates to systems containing (i) more than two biopolymers, (ii) mixtures containing denatured proteins, (iii) partially hydrolyzed proteins, (iv) soluble electrostatic protein-polysaccharide complexes and conjugates, (v) enzymes (proteolytic and amylolytic) and their partition coefficient between the phases of protein-polysaccharide mixtures, (vi) phase behaviour of hydrolytic enzyme-exopolysaccharide mixtures, exopolysaccharide-cell wall polysaccharide mixtures and exopolysaccharide-exudative polysaccharide mixtures, (vii) biopolymer solutes in the gel networks of one or several of them, (viii) enzymes in the gel of their substrates, (ix) virus-exopolysaccharide, virus-mucopolysaccharides and virus-exudative gum mixtures, and so on. 

Biopolymer incompatibility seems to provide phase-separated liquid and gel-like aqueous systems. In highly volume-occupied food systems aggregation, crystallisation and gelation of biopolymers and their adsorption at oil/water interfaces favour an increase in the free volume, which is accessible for other macromolecules. Denatura-tion of proteins during food processing decreases their solubility and co-solubility of proteins with one another and with polysaccharides and induces phase separation of the system. 

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