Biopolymers colloidal nature

The colloidal nature of biopolymers has been overlooked. In waterlogged wood, lignin must be in the form of a colloidal hydrated gel, which on drying must cause some dimensional changes. We should be looking into colloidal chemistry for answers to dimensional stability of this colloid (9). 

Nature itself gives us a spectacular example of a biopolymer-based delivery system in the form of the native casein micelle of mammalian milk (Lemay et al, 2007). This is primarily a colloidal delivery system for calcium, where the micronutrient is in the form of calcium phosphate, which does not give a bitter taste, and which provides good bioavailability owing to its colloidal size, amorphous state and quick dissolution in gastric conditions (pH 1-2). Nevertheless, the casein micelle structure is unique there are no other readily available natural delivery systems for most nutraceuticals. Therefore some new designs are clearly required (Velikov and Pelan, 2008 McClements et al, 2008, 2009). 

Owing to the diverse chemical nature of functional groups in proteins and polysaccharides, they are prone to a variety of types of molecular interactions, both in bulk aqueous media and at air-water or oil-water interfaces. To a first approximation one may consider an adsorbed layer of biopolymers at the interface as simply a special type of highly concentrated biopolymer solution. Thus, the same variety of interactions that are typically found for biopolymers in a bulk aqueous media also occur in biopolymer adsorbed layers at the interfaces in food colloids. Moreover, these same molecular interactions are also involved in the close encounters between pairs of colloidal particles covered by adsorbed biopolymer layers. In the rest of this chapter we shall briefly remind ourselves of the main basic types of intermolecular interactions readers requiring more detailed background information are directed to other sources (Cantor and Schimmel, 1980 Lehninger, 1982 Israelachvili, 1992 Dickinson, 1998 Finkelstein and Ptitsyn, 2002 McClements, 2005, 2006 Min et al., 2008). 

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). 

Part Three describes a range of important specific examples of the interactions of individual biopolymers in the bulk aqueous medium of food colloids. Chapter six is devoted to the subject of the self-assembly of food biopolymers, and how this self-assembly is affected by conditions such as pH, ionic strength, divalent ions, cosolutes, etc. It is indicated how biopolymer self-assembly can form the basis of the bottom-up nano-biotechnological approach, which attempts to mimic Nature in the creation of new and varied structures with potential applications. It is... 

The dimensionality of the lattice depends on the physical nature of the phase. For transitions in linear biopolymers and associative colloids, a one-dimensional lattice is used because these materials become ordered in a onc-dimcnsional manner even though they actually exist in three dimensions. For absorption of gas onto a surface, a two-dimensional lattice is sufficient. For bulk phase changes, however, a three-dimensional lattice must be used. 

Most food systems are of a colloidal as well as a polymeric nature. The presence of a nonadsorbing polymer in a skim milk dispersion induces an effective attraction between the casein particles, called depletion interaction, resulting in phase separation at sufficiently high polymer concentration. Tuinier et al. (2003) discussed the influence of colloid-polymer size ratio, polymer concentration regime, size, poly-dispersity and charges in colloid/biopolymer mixtures, demonstrating the challenging complexity of the subject. 

More recently, the Faraday eflfect (particulariy its dispersion and absorption) has been found to provide new ways of studying the structure of molecules and, even more so, of macromolecules and biopolymers. This is because the Faraday eflfect is primarily due to the direct influence of the electric field on the optical polarizability of the microsystems (it thus differs essentially from natural optical activity). The theoretical and experimental results obtained in these studies are presented in the articles of Buckingham and Stephen, and Schatz and McCaflfery. Existing laser and high-magnetic-fidd techniques permit studies of non-linear variations in the Faraday eflfect, particularly with regard to macromolecular and colloidal substances. ... 

Self-assembled nanostructures of biopolymers play an important role in nature. For example, extracellular branched polysaccharides decorate bacterial surfaces and therewith mediate cell adhesion [11], aggrecans (protein-polysaccharide complexes) control mechanical stresses in synovial joints [12], whereas neurofilaments (neuron-specific protein assemblies) support the elongated cell shape and participate in the maintenance of the axonal caliber [13]. It is believed that these biological functions rest on the ability of bioassemblies to provide adequate responses to variations in the local environment. Therefore, a better understanding of the physical mechanisms that govern conformational rearrangements in (bio)nanostructures, is of key importance, not only for colloid and material sciences, but also for cell biology. 

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